Health Impacts Of The Use Of Recombinant Bovine Somatotropin In Dairy Production

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Potential Health Impacts Of The Use Of Recombinant Bovine

Somatotropin In Dairy Production

by Michael Hansen, Ph.D., Jean M. Halloran, Edward Groth III, Ph.D.,

Lisa Y. Lefferts

 

Prepared for a Scientific Review by the Joint Expert Committee on

Food Additives

 

http://www.consumersunion.org/food/bgh-codex.htm

 

Introduction: Goals of the JECFA Review

 

The task of assessing the safety of widespread commercial use of

recombinant bovine somatotropin (rbST) is far more complex and

difficult than assessments of most food additive safety questions.

 

In this case there is no food additive involved, nor is the central

issue residues of rbST itself in meat or milk.

 

Establishment of a Maximum Residue Level (MRL) for rbST is not

appropriate.

 

The central human health questions are, to what extent does rbST use

increase the level of the hormone Insulin-like Growth Factor I (IGF-

I) in milk, and what possible risks to public health might be

associated with consuming milk with increased IGF-I levels?

 

We present below a review of what we believe is sufficient credible

evidence that average IGF-I levels are increased in milk from rbST-

treated cows.

 

We present further recent evidence that IGF-I survives

digestion and passes into the intestinal tract, and we review

evidence that associates exposure of the intestinal epithelium to

IGF-I at levels within the range found in milk with cellular growth

responses linked to the risk of colon cancer.

 

While this evidence is provocative, it falls short of providing an

adequate basis for a quantitative risk assessment.

 

 

Even if JECFA were to agree that there is sufficient evidence to

presume that increased IGF-I levels in milk do pose some risk to

public health, setting an MRL for IGF-I is neither appropriate nor

feasible.

 

If the outcome of JECFA's review of this issue is not to establish

an MRL, what should it be? There is a critical need for a

scientifically rigorous and objective, balanced and non-partisan

assessment of potential public health risks of rbST use.

 

Such an assessment needs to summarize lucidly for the many

policymakers

facing this issue both what is known scientifically and, equally

important, what is as yet not known, but needs to be known to better

assess possible risks.

 

Rather than make a " forced choice " between declaring rbST

use " unsafe " because there is sufficient credible

evidence of risks, or " safe " because the evidence of risks is judged

less than sufficient, JECFA should aim to produce a definitive

answer to the following questions:

 

 

 

What changes in the composition of milk are very likely or somewhat

likely to occur as rbST use becomes widespread?

 

(For example, increases in average IGF-I levels?

Increased likelihood of antibiotic residues?)

 

What are the nature (type of possible adverse effects) and magnitude

(number of people exposed to risk, existence of sensitive

subpopulations, quantitative estimates of risk) of possible public

health effects associated with those likely changes in milk

composition?

 

What other public health and economic risks, such as increased

antibiotic resistance, or increased susceptibility of rbST-treated

cows to bovine spongiform encephalopathy, may be associated with

rbST use

 

In this paper, we summarize new evidence, and interpret it in the

context of the evidence as a whole, on three key issues.

 

They are:

1) Do IGF-I levels in milk from rbST-treated cows pose a potential

human health hazard? 2) Does use of rbST increase mastitis incidence

and thereby lead to increased antibiotic residues in dairy products

and/or exacerbate problems of antibiotic resistance in bacteria? and

3) Does rbST use potentially exacerbate BSE risks?

 

 

 

1. Do IGF-I levels in milk from rbST-treated cows pose a potential

human health hazard?

 

IGF-I levels in milk are important because IGF-I is the hormone that

actually mediates much of the cellular response to growth hormones

in cows and humans.

JECFA previously concluded, and data published in 1986 demonstrated

that, while human and bovine growth hormones

differ by up to 35% in their amino acid sequences, human and cow IGF-

I are identical (Honegger and Humbel, 1986).

 

An assessment of potential public health impacts of IGF-I in milk

requires a review

of the evidence on three subsidiary questions:

 

Do IGF-I

concentrations increase in milk from rbST-treated cows? Does IGF-I

survive digestion? And, What are the possible adverse health effects

of increased IGF-I in milk?

 

Do IGF-I concentrations increase in milk from rbST-treated cows?

 

JECFA addressed this question in its 1993 report and concluded that

the best studies then available did not indicate an increase in

average IGF-I levels. However, a study by Monsanto did not become

available to the public until after JECFA completed its review. We

have conducted a new analysis of the data, considering both the

latest Monsanto study and all earlier evidence on this question.

 

We conclude that the weight of evidence indicates that rbST use

does

increase IGF-I levels in milk, substantially. In our judgment, the

most important question is no longer whether IGF-I levels increase,

but rather how much they increase, on average.

 

 

The latest Monsanto study, and five of the seven studies previously

reviewed by JECFA, found an increase in IGF-I levels. In five of

these six studies, the increase was statistically significant, and

the sixth study, involving a very small number of test subjects (six

rbST-treated cows and six controls), found an increase that was not

statistically significant. The three studies that did not find a

statistically significant increase used the lowest doses of rbST.

All four Monsanto studies, using the dose recommended by Monsanto to

farmers, did show a statistically significant increase. We discuss

all eight studies in more detail below.

 

The newly available Monsanto study was conducted at the Monsanto

Animal Research Center in O'Fallon, Missouri and reported in late

1993 in the US FDA's Freedom of Information Act Summary of the data

used to gain approval for POSILAC, Monsanto's rbST product.

 

This

study involved 18 cows, an rbST dosage of 500 mg injected every 14

days, with milk collected 7 days after each of three injections. IGF-

I levels were statistically significantly elevated in milk from rbST-

treated cows. Indeed, the milk IGF-I levels of treated and control

cows did not even overlap, i.e. the milk IGF-I level from the 9 rbST-

treated cows was higher than any of the levels found in the milk of

control cows: " During the study, milk IGF-I concentrations ranged

from 3.16 to 3.35 ng/ml for control cows and from 3.49 to 5.31 ng/ml

for treated cows. The difference in milk IGF-I between control and

treated cows was statistically significant at the 5% probability

level " (FDA, 1993: 121).

 

The first Elanco study (Schams and Karg, 1988a,b) involved 8 cows

and rbST dosages of 640 mg injected every 28 days, with milk

collected at 2-3 day intervals after the third and fourth

injections. The study found that " After somidobove injection, mean

IGF-I levels in the treated animals are always higher than those

found in controls. The average IGF-I milk concentration found in

control animals was 28.4 ng/ml, and the average IGF-I milk

concentration in the 640 mg somidobove-treated animals was 35.5

ng/ml. Therefore, in this study an increase of approximately 25% of

the mean was found in the somidobove-treated animals " (FAO, 1993:

120-121). The 25% increase was statistically significant (Juskevich

and Guyer, 1990).

 

A second Elanco study (Davis et al., 1989) involved 36 cows and rbST

dosages of either 320 mg or 640 mg injected every 28 days, with milk

collected 4 times (days 3, 10, 17 and 24) after the first injection.

They found that " the concentration of IGF-I in milk was higher by

day 3 in cows treated with 320 and 640 mg of somidobove relative to

the control cows. The values at day 10 and thereafter were not

statistically different between treatment groups. . . . After

somidobove treatment in this study, the levels of IGF-I in the milk

increased less than 50% relative to the milk IGF-I content in the

control cows " (FAO, 1993: 121). Thus, long-term average IGF-I levels

increased, but were not statistically significant.

 

A third Elanco study (Coleman et al., 1990) involved 12 cows, an

rbST dosage of 640 mg injected every 28 days, with milk collected 4

times (days 3, 10, 17 and 24) after the first and second injections.

There was not a statistically significant increase in milk IGF-I

concentrations in treated animals.

 

The sole American Cyanamid study (Schingoethe and Cleale, 1989)

involved 20 cows and an rbST dosage of 10.3 mg injected daily, with

milk collected weekly for 16 weeks.

 

This was a very small doseless

than a third the dose used in the Monsanto studies. Further

complicating the test was the fact that it also looked at the

effects of diet composition.

 

Animals were fed a normal diet or a

high energy and protein diet and got injections of either rbST or a

placebo, so there were four treatment groups. The study found

that " mean concentration throughout the study in control animals was

9.67 ng/ml and in the somagrebove-treated animals was 9.06 ng/ml.

 

 

The first Monsanto study (Torkelson et al., 1988) involved 18 cows

and an rbST dosage of 500 mg injected every 14 days, with milk

collected 7 days after each of 3 injections. The study found

that " After each of the 3 doses, mean milk IGF-I in controls was

3.22, 2.62 and 3.78 ng/ml and in treated cows was 3.80, 5.39 and

4.98 ng/ml, respectively.

 

Differences between treated and control

groups was [sic] significant after the second and third doses " (FAO,

1993: 121). Thus, the average IGF-I concentrations were increased by

18%, 106% and 31.7% for injection cycles 1, 2, and 3, respectively

in the treated groups compared to controls.

 

The second Monsanto study (White et al., 1989) involved 18 cows and

an rbST dosage of 500 mg injected every 14 days, with milk collected

7 days after each of 3 injections. As in the Torkelson et al. study,

mean milk IGF-I concentrations were statistically significantly

higher after the second and third doses. Mean milk IGF-I in controls

was 3.17, 3.34 and 3.35 ng/ml and in treated cows was 3.50, 5.33 and

4.68 ng/ml, respectively. Thus, the average IGF-I concentrations

were increased by 10%, 60% and 40% for the first, second and third

doses, respectively in the treated group compared to controls.

 

The third Monsanto study (Miller et al., 1989) involved 64 cows and

an rbST dosage of 500 mg injected every 14 days, with milk collected

7 days after each of 10 injections. This was both the longest of the

eight studies (140 days) and the largest (64 cows). We therefore

regard it as the most definitive and comprehensive of the eight

studies.

 

The study found that " milk concentration of IGF-I was

increased across the 10 injection cycles " (FAO, 1993: 126). For

primiparous cows the increase was 74%, from 3.5 ng/ml to 6.1 mg/ml

for control and rbST-treated cows, respectively. For multiparous

cows, the increase was 41%, from 3.9 ng/ml to 5.6 mg/ml for control

and rbST-treated cows, respectively. Both results are statistically

significant.

 

 

The four Monsanto-sponsored studies all found statistically

significant increases and used the highest dosage of rbST (500 mg/14

days), which was from 50 to 350 percent higher than the dosages used

in the other studies. Furthermore, of the eight studies, the Miller

et al. 1989 study had the largest sample size (64 cows) and the

longest duration of experiment (10 injection cycles, or 140 days)

(Table 1). On the basis of sample size, duration of experiment and

rbST dosage used, Miller et al.'s 1989 study is clearly the " most

definitive and comprehensive " of the studies. It found that rbST

treatment led to a statistically significant increase in milk IGF-I

levels (74% and 41% for primiparous and multiparous cows,

respectively). Thus, on balance, considering all currently available

evidence, the majority of studies, and the most definitive and

comprehensive studies, clearly demonstrate a significant increase in

IGF-I concentrations in milk from rbST-treated cows.

 

 

 

The US Food and Drug Administration reviewed most of the same

studies and concluded that rbST use does lead to statistically

significant increases in the IGF-I levels in milk (Juskevich and

Guyer, 1990; FDA, 1993).

 

Another study on the effect of rbST on IGF-I levels in milk, which

was reviewed by neither JECFA nor the US FDA, found an even larger

increase in IGF-I levels in the milk of rbST-treated cows (Prosser

et al., 1989). The study involved rbST treatment to six cows near

the end of the lactation cycle, when IGF-I levels are normally at a

minimum.

 

At the end of one week of daily rbST injections, the milk

of rbST-treated cows had IGF-I levels at least 3.6 times the levels

in the milk of untreated cows. The levels were still rising at the

end of the treatment period. How high the levels would have gone if

the rbST injections had continued is not known.

 

Does IGF-I survive digestion?

If levels of IGF-I increase in the milk of rbST-treated cows, the

question of whether IGF-I survives digestion in the stomach is

important.

 

New data, as well as earlier studies not considered by JECFA in

1992, suggest that IGF-I survives digestion in vivo. Three studies

involving neonate rats or calves suggest, either directly or

indirectly, that IGF-I does survive digestion and remain bioactive.

The first study involved orally administering labeled 125I-IGF-I to

suckling rats and found that more than three quarters (78%) of the

labeled 125I-IGF-I was retained in the stomach and intestinal

lining, where the authors proposed that it could have a local effect

(Phillips et al., 1990).

 

The second study involved orally

administering labeled 125I-IGF-I to calves.

 

It found a small amount

of 125I-IGF-I in the circulatory system, indicating that it not only

survived digestion, but was also taken up into the blood (Baumrucker

et al., 1992).

 

The third study investigated the effect of feeding calves for seven

days with either bovine milk replacer (which lacks IGF-I) alone or

supplemented with 750 micrograms/liter of IGF-I (Baumrucker and

Blum, 1993).

 

In the first three days no differences in serum IGF-I

levels were observed between the two diets. However, the milk

replacer with added IGF-I did lead to a transient decrease in serum

insulin (within 2 hours), a transient increase in serum prolactin

levels (within 4-8 hours), increased DNA synthesis (as measured by

thymidine incorporation) in jejunal and ileal intestinal explants,

and an increase in number of IGF-I receptors in jejunal and ileal

microsomal membranes (which may explain the increased DNA

synthesis).

 

 

 

These effects suggest indirectly that IGF-I did survive

digestion, was active in the lower gastrointestinal tract, and was

to some degree absorbed into the blood.

 

Collectively, these three

studies suggest that orally administered IGF-I at least partially

survives digestion, binds to IGF-I receptors in cells lining the GI

tract, stimulates synthesis of its own receptor, stimulates cellular

proliferation, and is absorbed into the blood where it can affect

levels of other hormones.

 

 

 

A new study, published in 1995, provides both clear evidence that

IGF-I survives digestion (Xian et al, 1995) and an explanation for

why the oral IGF-I feeding studies looked at by JECFA in 1992 had

ambiguous results, as is discussed below. Both the Monsanto-

sponsored and the Elanco-sponsored studies previously considered by

JECFA involved feeding free rIGF-I by itself to rats. Neither used

IGF-I associated with its binding proteins (IGFBPs). IGFBPs are

resistant to acidic conditions and may enable IGF-I to survive

digestion in the stomach (Corps and Brown, 1987; Donovan and Odle,

1994).

 

On theoretical grounds alone, one might expect IGF-I to survive

digestion. Milk has recently been shown to contain a number of

growth factors, including ones that stimulate growth of the gut

(Donovan and Odle, 1994).

 

 

Since newborns and young infants have the

fastest growth rates, one would expect that any growth factors that

a mother might give her infant would be found at their highest

concentrations in the earliest milk in the lactation, when the

infant is growing the fastest.

 

 

Furthermore, for the mother's milk to

be able to deliver growth factors that will affect the

gastrointestinal tract, those growth factors must survive digestion

in the stomach and reach the upper and lower intestines where they

can have local stimulatory/growth effects.

 

Both IGF-I and epidermal growth factor (EGF), a protein growth

hormone related to IGF-I, are known to stimulate intestinal

epithelial growth in vitro (Corps and Brown, 1987). IGF-I receptors

are found on the intestinal epithelium of rats (Laburthe et al.,

1988) which suggests that orally administered IGFs may exert

mitogenic responses in the gut, especially in newborn animals

(Koldovsky et al., 1992). In addition, both IGF-I and EGF occur in

both bovine and human milk, with the level highest at the start of

the lactational cycle. Particularly high concentrations are found in

the colostrum and early milk, which is just when the infant gut is

doing most of its growing, suggesting that IGF-I performs a

physiological function immediately after birth, promoting

development of the gut. Indeed, growth of the gut in infant animals

appears to be due in part to the presence of EGF and other growth

factors in the colostrum and milk.

 

Since EGF has long been known to survive digestion (Thornburg et

al., 1984), some authors have inferred that IGF-I probably also

survives digestion (Corps and Brown, 1987). A study published in

1993 showed that EGF survives digestion through the protective

effects of the major milk protein casein (Playford et al., 1993).

This suggests that the same mechanism might occur with IGF-I.

 

And this is just what a 1995 study found. The study was designed " to

investigate the potential of IGF-I peptides as therapeutics in the

gut " (Xian et al., 1995: 215). By therapeutics, the authors meant

the use of IGF-I to stimulate growth of the gut, which tends to

waste away in hospital patients on parenteral nutrition who eat

little solid food. The study involved feeding human IGF-I to

suckling rats in the presence of a number of proteins, such as

bovine casein, IGF-binding protein-3, bovine serum albumin (BSA),

lactoferrin and an antibody to IGF-I. The study found that free IGF-

I orally administered to rats was quickly digested in the stomach.

However, IGF-I in the presence of casein easily survived digestion

in the stomach and made its way to the intestine: " In stomach,

casein was the most effective protein, with near complete inhibition

of IGF-I degradation at casein concentrations of 10 mg/ml or

higher . . . All three proteins [bSA, casein, and lactoferrin] were

less protective in the duodenal flushings than in the stomach

flushings. Nevertheless, casein remained the most effective protein,

with 40 mg/ml conferring maximum protection, at which IGF-I remained

80% intact by TCA " (Xian, et al., 1995: 221).

 

Since casein levels in bovine milk average 25-40 mg/ml, the

experiment suggests that bovine milk has enough casein to partially

or fully protect IGF-I from digestion in the stomach, enabling it to

pass into the small and large intestine, where it might have a local

stimulatory effect on epithelial cells. Indeed, the authors of this

study concluded that using casein may make it possible to give

therapeutic oral doses of IGF-I: " It can be concluded that IGF-I

cannot be expected to retain bioactivity if delivered orally because

of rapid proteolysis in the upper gut, but the use of IGF antibodies

and casein could represent useful approaches for IGF-I protection in

oral formulae " (Xian et al., 1995: 215).

 

The demonstration that IGF-I survives digestion through the

protective effects of casein makes irrelevant the argument that

human saliva contains IGF-I at levels greater than the quantities

that would be consumed in milk. As the IGF-I produced by salivary

glands is free IGF-I, without the protective effect of casein, it is

unlikely to survive digestion.

 

Two earlier oral feeding studiesone sponsored by Monsanto and one by

Elancoconcluded that IGF-I does not survive digestion. Those studies

are not definitive because rats were given free IGF-I, without

casein or other protective proteins. Thus one would not expect the

IGF-I to survive digestion in these studies. Even so, a careful

review of the Monsanto study suggested that some small amount of the

IGF-I administered in this study survived digestion and affected the

rats' growth rate (Hansen, 1993).

 

In sum, we feel that the data clearly show that IGF-I survives

digestion in animal studies, makes its way into the gastrointestinal

tract, and has a mitogenic (i.e. promotes cell division) effect on

cells in the gastrointestinal tract. Although no data are available

on humans, there is ample reason to expect that we are like other

mammals in this regard.

 

 

 

What are the possible adverse health effects of increased exposure

to IGF-I in milk?

 

If IGF-I survives digestion and if IGF-I levels are increased in

milk from rbST-treated cows, it is important to examine the

potential adverse effects of increased exposure to IGF-I on humans.

Scientific understanding of how IGF-I works and its potential health

impacts in humans has grown considerably in the more than five years

since JECFA met to discuss rbST. The most important potential

adverse effects of IGF-I arise from the fact that it is a potent

mitogen for a number of cell types and has been associated with the

growth of numerous tumors, including colon (Tricoli et al., 1986),

breast (Rosen et al., 1991; Lippman, 1991), smooth muscle (Hoppener

et al., 1988) and others (Pavelic et al., 1986).

 

Most of the recent advances in the knowledge in this area were

discussed at a 1995 National Institutes of Health (NIH) Conference

on the insulin-like growth factor and cancer, and summarized in the

Annals of Internal Medicine in 1995. The advance in knowledge from

the NIH 1990 Technical Assessment Conference on BST, which suggested

further study on the local effects of IGF-I on the gastroin-testinal

tract, to the 1995 Conference, is quite striking.

 

The 1995 NIH Conference focused on the role that the insulin-like

growth factor (IGF) system plays in the development and spread of

cancer. The IGF system consists of three growth factors (IGF-I, IGF-

II, and insulin), a number of binding proteins (at least 6

characterized so far), and receptors for each of the growth factors.

It is clear that IGFs (e.g. IGF-I and IGF-II) are needed for normal

growth and develop-ment. However, it is now becoming evident that

IGFs may also play an important part in what goes awry in the

development and spread of many cancers. The summary of the NIH

conference concisely states this:

 

" As could be predicted from the importance of IGFs, their binding

proteins, and their receptors in normal cellular growth and

development, it has become apparent over the past few years that

IGFs are important mitogens in many types of malignancies. Although

these conclusions were initially derived from in vitro studies, IGFs

may enhance in vivo tumor cell formation, growth and even

metastasis. Insulin-like growth factors may reach tumors either from

the circulation (endocrine) or as a result of local production by

the tumor itself (autocrine) or by adjacent stromal tissue

(paracrine). Tumors also express many of the IGF-binding proteins,

which modulate IGF action, and IGF receptors, which mediate the

effects of IGFs on tumors " (LeRoith, 1995: 54)

 

Not surprisingly, most of the cancers that IGF-I is associated with

occur in tissues where IGF-I normally plays an important growth

role, including the mammary, cardiovascular, respiratory, and

nervous systems, the skeleton, and the intestinal tract. We will

briefly describe the evidence for the association in each of these

systems.

 

IGF-I appears to play a strong role in breast cancer. Not only do

breast cancer cells react strongly (i.e., grow and divide) in the

presence of low levels of IGF-I, but the tissue surrounding the

breast cancer cells produces IGF-I. Some strategies for combating

breast cancer involve reducing circulating levels of IGF-I. As summa-

rized at the NIH conference by Dr. LeRoith:

 

" IGFs have been shown to be involved in breast cancer. . . . Breast

cancer cells in vivo express low levels of IGF-II, whereas the

adjacent stromal tissue expresses IGF-I. . . . Estrogen receptor-

positive tumors will thus respond to antiestrogens such as

tamoxifen, which is widely used clinically. Initially, it was

thought to affect cancer cells primarily by blocking the activation

of estrogen receptors; it has also been shown, however, to decrease

circulating IGF-I levels in women with breast cancer and may thus

prove effective in treating both estrogen receptor-positive and

estrogen receptor-negative cancers. Another agent that inhibits the

proliferation of breast cancer cells is retinoic acid and its

derivatives. . . . Like tamoxifen, however, retinoic acid may also

reduce circulating IGF-I levels and may thus affect tumor growth in

vivo by more than one mechanism. The above data suggest that IGFs

are likely to be involved in breast cancer at the level of tumor

growth and perhaps at the level of initial development and later

metastases. Ongoing studies involve attempts to interfere with the

IGF system to develop additional therapeutic regimens " (Baserga,

1995: 55-56).

 

In the skeletal system, IGF-I has been associated with osteosarcoma.

The tumor seems to strike children with the most rapidly growing

bones, and has been shown in vitro to respond to IGF-I (i.e. grow

and divide in its presence and grow more slowly in its absence).

Suggested therapies include trying to reduce IGF-I levels. As

summarized at the NIH conference:

 

" Osteosarcoma is the most common bone tumor in children, usually

occurring during the adolescent growth spurt at sites of rapid bone

growth. Because IGF-I was initially described as the factor produced

that directly mediated the effect of growth hormone on skeletal

growth, there has been interest in a potential role of IGF-I in the

pathogenesis of osteosarcoma. Support for a role for IGF-I in

osteosarcoma growth comes from data showing that IGF-I is a potent

mitogen for human osteogenic sarcoma cells. Further, several reports

have shown that a rat chondrosarcoma (a closely related tumor) and a

murine osteosarcoma are growth inhibited in animals that have a

hypophysectomy [so they no longer produce GH], presumably through

the inhibition of the growth hormone-IGF-I axis. . . . It therefore

appears that the growth hormone-IGF-I axis may play a role in the

unregulated proliferation of osteosarcoma tumor cells and that

blocking this axis using somatostatin analogs that reduce

circulating levels of growth hormone and IGF-I may have therapeutic

potential " (Helman, 1995: 57).

 

In the respiratory system, IGF-I is known to play a role in normal

lung development and has also been implicated in lung cancer. As

pointed out in a 1992 British study, " IGFs seem important in lung

development, and are also implicated in growth regulation of lung

tumors. Primary lung tumors possess IGF-I binding sites as shown by

autoradiography, with the highest density of receptors in squamous

cancers and small cell lung cancer. . . . Thus there is good

evidence that lung cancer cells produce IGF-I and IGF BPs, express

IGF binding sites and exhibit a mitogenic response to exogenous IGF-

I, suggesting that IGF-I can function as an autocrine [tumor-

produced] growth factor for lung cancer " (Macaulay, 1992: 312).

 

In the circulatory system, IGF-I has been shown to have angiogenic

properties (i.e., to promote growth of blood vessels). These

properties are important to tumors as blood vessels supply food and

oxygen to cancerous tissues. Getting nutrients and oxygen to the

center of large tumor is difficult, and some tumors secrete growth

factors to promote angiogenesis.

 

Excessive angiogenisis can be dangerous by itself, i.e. in the

absence of a tumor. In fact, a recent study in mice found that

retinal neovascularization (excessive growth of tiny blood vessels

on the retina, which blocks vision), a major cause of untreatable

blindness, is inversely related to growth hormone and IGF-I levels,

and suggested that depressing IGF-I levels could be a potentially

important therapy to protect against neovascularization. As the

authors state, " Retinal neovascularization was inhibited in these

mice in inverse proportion to serum levels of GH and a downstream

effector, insulin-like growth factor-I (IGF-I). Inhibition was

reversed with exogenous IGF-I administration. . . . These data

suggest that systemic inhibition of GH or IGF-I, or both, may have

therapeutic potential in preventing some forms of retinopathy "

(Smith et al., 1997: 1706).

 

The potential effects of IGF-I on the gastrointestinal system are of

special concern. A 1987 study found that IGF-I stimulates growth of

intestinal epithelial cells, a sign that these cells are programmed

to respond to IGF-I (Corps and Brown, 1987). The authors noted that

epidermal growth factor (EGF), a protein growth hormone related to

IGF-I, has been shown to pass intact through the stomach to the

small intestine where it is absorbed into the bloodstream, and

wondered whether the same would be true for IGF-I. In part because

of these concerns and in part because they felt that the data showed

that IGF-I levels are statistically significantly increased in milk

from rbST-treated cows, the 1990 US National Institutes of Health

(NIH) Technical Assessment Conference on Recombinant Bovine

Somatotropin concluded, " Whether the additional amounts of IGF-I in

milk from [rbST-treated] cows has a local effect in the esophagus,

stomach or intestines is unknown. " One of the six recommendations

for further research was, " Determine the acute and chronic action of

IGF-I if any, in the upper gastrointestinal tract " (NIH, 1991).

 

More recent studies have suggested that IGF-I does have growth

effects on the gut. In 1988 French and Danish researchers showed

that IGF-I receptors are found throughout the intestines, with the

highest density occurring in the crypt cells in the epithelium of

the colon: " 125I-IGF-I and 125I-IGF-II binding is 4.0 and 1.8-fold

higher in crypt cells than in villus cells, respectively. Specific

125I-IGF-I binding is detectable throughout the gastrointestinal

tract. The level of IGF binding is similar in stomach, small

intestine, and cecum, but higher values are observed in the colon "

(Laburthe et al., 1988: G457).

 

A 1992 study not reviewed by JECFA found that intraluminal infusion

of IGF-I in rats at concentrations equivalent to those found in

bovine milk increased the cellularity of the intestinal mucosa

(Olanrewaju et al., 1992). A 1994 study found that epithelial crypt

cells in healthy human duodenal tissue proliferated at twice the

normal rate when exposed to recombinant human IGF-I at

concentrations of 400ng/ml, while " lower concentrations (100 and 200

ng/mL) also increased crypt epithelial cells in preliminary dose-

response studies " (Challacombe and Wheeler, 1994: 816). Although the

levels of IGF-I used were 100 ng/ml or higher, the main author, Dr.

David Challacombe, of the Somerset Children's Research Unit or

Taunton and Somerset Hospital, urged further study, stating of his

study that, " It could mean that if you have higher levels of IGF-I

in BST-treated milk, it could increase cell proliferation in the

small bowel, and there's always the possibility they could form

abnormally into a tumour of some kind " (Coghlan, 1994: 15).

 

Further evidence supporting this possibility is the finding that 5

of 8 human colorectal cancer cell lines were responsive to IGF-I

(Lahm et al., 1992). These cell lines were exquisitely sensitive to

IGF-I, with 30 ng/ml tripling cancer cell growth and with 1.9 - 6.5

ng/mllevels similar to those found in bovine milkincreasing cancer

cell growth 1.5-fold: " At 30 ng ml-1 both factors [iGF-I and IGF-II]

enhanced growth up to 3-fold. They induced half maximal stimulation

at 1.9 - 6.51 ng ml-1 " (Lahm et al., 1992: 341). Another study noted

that " Immunoreactive IGF-1 has also been shown to be increased in

primary human lung and colon carcinomas compared with adjacent

normal tissue. Specific IGF-1 receptors have also been characterized

on human T-lymphoblasts, neurogliomas, and colon carcinomas " (Ezzat

and Melmed, 1991). Furthermore, IGF-I mRNA has been found in colon

carcinoma, suggesting that these tumors produce IGF-I to stimulate

their own growth (Tricoli et al., 1986).

 

The lines of evidence cited aboveincreased level of IGF-I receptors,

the sensitivity of colon carcinomas to IGF-I, and production of IGF-

I by the carcinoma itself support the hypothesis that the colon

could be at special risk from increased IGF-I levels. More support

for this hypothesis comes from studies on acromegaly, a disease in

which patients have significantly elevated endogenous levels of

total and free serum IGF-I (Juul et al., 1994). Several studies have

found acromegaliacs are at increased risk of colon tumors and

precancerous colonic polyps. A recent review concluded that, now

that acromegaliacs are living to older ages, physicians " need to

remain aware of the potentially deleterious long-term consequences

of previous GH excess and, in this context, the possibility of

malignant transformation of the colonic polyps seems paramount "

(Tremble and McGregor, 1994: 10). In addition to cancer per se,

colon polyps, especially adenomatous polyps, are important as it

is " generally established that colonic adenomatous polyps are

premalignant lesions that have the potential to develop into

adenocarcinoma of the colon " (Klein et al., 1982).

 

An early study found that among 12 patients with acromegaly, 3 had

colon carcinoma and 2 had adenomatous polyps of the colon (Ituarte

et al., 1984). Since the expected number of colon cancers in a 12

person sample is less than one, the finding of 3 was very highly

statistically significant (p < 0.001). A study from 1982 of 17

patients with acromegaly found that 9 had colonic polyps (Klein et

al., 1982). Polyps were removed from 8 of the patients. In 5

patients, the polyps were adenomatous and in four of the five there

were multiple polyps. In addition, during the course of that study,

the authors " identified four cases of colon cancer in a total of 43

patients with acromegaly " and concluded that their " study identifies

a unique group of patients that are at risk of the development of

colonic polyps and perhaps colon cancer " (Klein et al., 1982: 29).

 

One retrospective study of gastrointestinal tumors in 48 acromegalic

patients found a statistically significant standardized incidence

ratio (SIR) of 4.6 for all GI cancers and 6.1 for colorectal

cancers; i.e., GI cancers were 4.6 times as prevalent and colorectal

cancers 6.1 times as prevalent among acromegaliacs as among the

general population (Pines et al., 1985). Another study of 52

patients with acromegaly found a prevalence ratio for colorectal

cancer of 6.9 per 100, which is considered quite high, with the

authors concluding that " this and most other recent investigations

have observed a trend of increased risk for colon cancer and polyps

among patients with acromegaly " (Brunner et al., 1990: 70). Finally,

a prospective study of 23 patients with acromegaly found that 8

(35%) had premalignant adenomatous colon polyps, which " exceeds the

12% (P < 0.01) frequency of polyps noted in normal persons " (Ezzat

et al., 1991: 754).

 

 

 

Discussion

Our focal concern is that increased levels of IGF-I in milk, by

stimulating growth of intestinal cells, could increase the risk of

colon cancer, the third most common cause of cancer mortality in the

US (Devesa et al., 1995).

 

The evidence seems sufficient to conclude that IGF-I is both a

paracrine and autocrine growth factor for colon cancers (Tricoli et

al., 1986, Lahm et al., 1992) This means it is appropriate to focus

on local levels of IGF-I in the intestine, rather than on levels of

IGF-I in the bloodstream. One study of human colorectal cancer cell

lines found that IGF-I made tumor cells grow 1.5-fold faster at IGF-

I levels (e.g. 1.9 - 6.51 ng ml-1) similar to those found in bovine

milk. Thus, an increase in average levels of IGF-I in milk, which

seems likely to occur with widespread use of rbST, could in theory

increase human colon cancer risks. We do not believe enough evidence

exists currently to assess this potential risk quantitatively. In

particular, assessing patterns of public exposure to increased IGF-I

in milk is an extremely complex process, requiring many assumptions.

However, this subject seems to demand intensive further research

toward the goals of further elucidating and quantifying the risk.

 

With no basis for quantitative risk assessment, there are

nevertheless several principles that can serve as guides for

inferences on this question. If a large increase in IGF-I exposure

in a small population (i.e. acromegalics) notably increases cancer

risk, then it is plausible that a small increase in IGF-I for a much

larger population could pose a significant public health risk. When

the substance of concern is present in milk, as is IGF-I, exposure

will be widespread. In Western countries, cow's milk is consumed by

virtually the entire population in childhood, and by much of the

population for a whole lifetime. Exposure during infancy and

childhood raises some additional concerns, because these age groups

drink the largest amount of milk on a body weight basis, and because

growth and development are most rapid during childhood. Thus, higher

exposures to IGF-I in childhood could set life-long processes in

motion that determine later risks.

 

Other authors, particularly Samuel Epstein, have argued that an

increase in IGF-I in milk could increase the risk of breast cancer

as well (Epstein, 1996).

 

 

 

2. Does use of rbST increase mastitis in cows and lead to increased

antibiotic use, contributing to drug resistance in bacteria and

residues in dairy products?

A second important concern is that rbST use increases disease rates,

especially mastitis rates, in treated cows, thereby increasing drug

use to treat those diseases, which, in turn, can contribute to

increased antibiotic resistance in bacteria and to an increased

likelihood of residues in milk and meat. The crucial evidence on

this issue falls into four areas: 1. Does use of rbST increase

mastitis rates in cows? 2. Are cases of mastitis more severe/less

easy to control in cows treated with rbST compared to controls? 3.

Does rbST use increase the amounts of drugs given to cattle to treat

diseases? 4. Does increased drug use in dairy cattle exacerbate the

problem of antibiotic resistance in pathogenic bacteria and/or

residues in milk? Much of the data analysis that follows is based on

the work of David Kronfeld, who has followed this issue very

carefully in the U.S. and has published some major analyses of these

data (Kronfeld, 1994, 1997).

 

Does use of rbST increase mastitis rates in cows?

 

Most of the evidence needed to answer this question has emerged

since 1992. According to the U.S. Food and Drug Administration,

which used data from eight Monsanto-sponsored trials in its decision

in 1993 to approve Monsanto's rbST product (POSILAC), the answer is

yes. The data from these eight trials, which involved 487 cows,

showed that during the period of rbST treatment, mastitis incidence

increased by 76% in primiparous cows (from 21 cases to 37 cases per

100 cows, for control and rbST-treated cows, respectively; p =

0.015), and by 50% for multiparous cows (from 36 cases to 54 cases

per 100 cows, for control and rbST-treated cows, respectively; p =

0.002) (See Table 2). Overall, the increase was 53% (from 32 cases

to 49 cases per 100 cows, for control and rbST-treated cows,

respectively; p = 0.0001) (FDA, 1993, Kronfeld, 1997).

 

The approval by US FDA of POSILAC, was conditioned, in part, on the

establishment of a post-approval monitoring program (PAMP). The PAMP

included monitoring cows from 28 herds for a variety of health

problems as well as accumulating data on violative drug residues in

milk. The data from the PAMP (see Table 2), which involved 1128

cows, showed that during the period of rbST treatment, mastitis

incidence increased by 22% in primiparous cows (from 27 to 33 cases

per 100 cows, for control and rbST-treated cows, respectively; p =

0.17, i.e., not statistically significant), and by 34% in

multiparous cows (from 44 to 59 cases per 100 cows, for control and

rbST-treated cows, respectively; p = 0.0001). Overall, the increase

was 32% (from 37 to 49 cases per 100 cows, for control and rbST-

treated cows, respectively; p = 0.0001) (Kronfeld, 1997). The large

differences in statistical significance between the results from

primiparous and multiparous cows can be explained by the fact that

the number of primiparous cows was much smaller.

 

In addition to these newer studies carried out for FDA, two earlier

studies, both published in 1991 (and involving some of the same data

that FDA looked at), found an increase in mastitis incidence in rbST-

treated cows compared to controls. One study looked at 15 commercial

herds and found an increase of 47%, which was not statistically

significant (p = .097) (Thomas et al., 1991), while the other looked

at 14 herds and found an increase of 35%, which was highly

statistically significant (p < 0.01) (Craven, 1991).

 

Kronfeld has analyzed the results of the various mastitis studies,

especially the 8 pre-approval trials and the data on the 28 herds

that were part of the PAMP, and has pointed out a number of things

about rbST-associated mastitis (Kronfeld, 1994, 1997). First, the

effect of rbST on mastitis is variable (or " inconsistent " in

Kronfeld's terminology), with an increased frequency of mastitis

being observed in only one-half to one-third of the rbST-treated

herds. Thus, in the 8 pre-approval herds, while there was an overall

increase in mastitis of 76% for the primiparous cows, this increase

was actually composed of no increase in 4 herds and a 152% average

increase in the other 4 herds. In the study of 15 commercial herds

that found an increase in mastitis incidence of 47% overall, there

actually was a 103% increase in 7 affected herds and little or no

increase (or a slight decrease) in the other 8 herds. This

variability of mastitis effect means that the global averages hide

the fact that some (one-third to one-half) herds are hit heavily by

mastitis (i.e. it more than doubles), while other herds are not hit

hard at all. Thus, the pooling of data or focusing on averages can

obscure the seriousness of mastitis that occurs in some rbST-treated

herds.

 

Are cases of mastitis more severe/less easy to control in cows

treated with rbST compared to controls?

RbST-associated mastitis appears to be harder to treat than " normal "

mastitis. In one trial from Vermont, the average length of treatment

for a case of mastitis was almost six times longer in the rbST-

treated cows compared to untreated cows (8.9 days vs. 1.5 days); the

authors attribute the greater length of treatment to infection with

Staphylcoccus aureus in the rbST-treated cows. S. aureus is

associated with particularly difficult cases of mastitis (Pell et

al., 1992). Data from a technical manual on rbST distributed to

veterinarians by Monsanto, and based on 10 U.S. rbST-trials (and

including all 8 trials that the US FDA used to grant approval),

showed that the percentage of bacterial isolates from clinical

mastitis cases that contained S. aureus increased by 62%, from 11.1%

in controls to 18.0% in rbST- treated cows (Monsanto, 1993).

 

In one of the original eight trials in the U.S., there was extensive

extra-label use of antibiotics not approved for use in dairy cattle

to treat mastitis, suggesting that the legal drugs (such as

penicillin) were relatively ineffective (Kronfeld, 1997). In another

study, the percent of mastitis cases harboring antibiotic-resistant

bacteria was one-third higher in the rbST-treated cows compared to

controls (65% and 49%, respectively) (Kronfeld, 1997).

 

Does rbST use increase the amounts of drugs given to cattle to treat

diseases?

Both increased incidence of mastitis and more severe or longer-

lasting cases of mastitis can lead to greater antibiotic use. In the

Vermont study cited above, there were more than seven times as many

cases of mastitis in rbST-treated cows compared to controls (29 vs.

4), while the average length of antibiotic treatment was almost six

times as long (8.9 days vs. 1.5 days), leading to a 43-fold increase

in the total duration of antibiotic treatment for rbST-treated cows,

compared to controls (Pell et al., 1992). In the study of 15

commercial herds that found a 47% overall increase in mastitis in

rbST-treated cows, antibiotic treatment doubled in rbST-treated cows

compared to controls. If we look only at the 7 herds which had

increased mastitis incidence, then total duration of antibiotic

treatment was 2.7 times as high in rbST-treated cows compared to

controls (Kronfeld, 1997). In the PAMP trial, which consisted of 28

herds and 1128 animals, total duration of antibiotic treatment for

mastitis was 2.3 times as high in primiparous rbST-treated cows

compared to controls, and 1.3 times as high, rbST-treated vs.

controls, in multiparous cows; both effects were highly

statistically significant (P < 0.01).

 

Finally, as a condition of POSILAC® approval in the U.S., the FDA

required that Monsanto include a package insert which explicitly

states that it will increase drug use: " Use of POSILAC is associated

with increased frequency of use of medication in cows for mastitis

and other health problems. "

 

Does increased drug use in dairy cattle exacerbate the problems of

antibiotic resistance in pathogenic bacteria and residues in milk?

 

Another part of the PAMP consisted of looking at the national

figures for violative drug residues in milk. These data were

presented at a meeting of the FDA's Veterinary Medicine Advisory

Committee (VMAC) in 1996. As Dr. Kronfeld notes, violative

antibiotic residues in milk tankers increased from 0.05% in 1992 and

1993, before rbST (sic) use, to 0.06% in 1994 and 0.09% in 1995. If

the 1.8-fold overall increase from 0.05 to 0.09% were confined to

the 10% of herds using rbST, then an 8-fold increase in milk

violations would be occurring in these herds (Kronfeld, 1997: 164-

165).

 

The US figures on violative antibiotic residues probably understate

the true incidence of residues. Bulk tanks of milk in the US are

routinely tested for residues of antibiotics in the beta lactam

family (which includes penicillin) and any milk found with violative

residues is discarded. However, many more antibiotics are used on

dairy cows. Under U.S. law, any drug approved for any use on humans

or animals, with just a handful of exceptions, can be used on dairy

cows, if used under a veterinarian's supervision. The FDA spot

checks 500 samples per year for 12 drugs, but this testing seems

likely to miss many drugs in use. Good, accurate tests that reliably

detect low-level residues simply do not exist for the bulk of the

drugs in use in dairy cows in the US. Thus, as the US General

Accounting Office (a watchdog arm of Congress) pointed out in 1990

and 1992 (GAO, 1990, 1992), the existing antibiotic testing program

cannot guarantee that illegal residues are not present in the milk

supply. This situation has not drastically improved in the last five

years.

 

Since data were not taken on the antibiotic residue levels from the

milk of treated and untreated cows, either in the 28 herds involved

in the PAMP or in the 8 herds used to gain approval in the U.S.,

there is no direct evidence on antibiotic residues in milk from rbST-

treated cows. However, the studies cited above clearly show that

mastitis rates do increase, and use of drugs to treat mastitis

increases.

 

Greater use of antibiotics in dairy cattle is of concern not just

because of residues, which some authorities believe may cause

adverse (i.e. allergic) reactions in a few sensitive individuals,

but also because it contributes to the growth of antibiotic

resistance in bacteria, an important public health problem.

Resistance can initially develop in both pathogens and harmless

bacteria in treated animals, and later be transferred to disease-

causing bacteria that infect humans, with the end result that a

given antibiotic may not be effective in treating disease.

 

In general, use of antibiotics contributes to the antibiotic

resistance problem by selecting for bacteria that are resistant to

the given antibiotic. Any factors that tend to increase antibiotic

use can contribute to or exacerbate this problem. The most effective

way to prevent and/or delay resistance is to use the drug as

selectively as possible. When resistance is present, however,

control of infections may require switching to other antibiotics,

which can be effective in the short term but may also contribute to

development of strains of bacteria that are resistant to multiple

drugs.

 

Antibiotic resistance is carried in bacterial genes called R

factors, which code for proteins that prevent specific antibiotics

from having toxic actions on the cell. R factors may be located on

the main chromosome(s) of bacteria, or on small genetic elements

called plasmids. R factors on the main chromosome are not very

mobile and can be transferred to other similar bacteria only through

conjugation, a sexual form of bacterial reproduction, or through the

fusion of different mating types. Plasmids, unlike chromosomes, are

highly mobile elements that can easily be transferred among bacteria

of all types, i.e. they can move between bacteria of different

genera or different families. Indeed, plasmids can even move from

dead bacteria into living bacteria. Furthermore, as plasmids move

from bacteria to bacteria they can accumulate R factors for

resistance to a number of different antibiotics on the same plasmid.

Plasmid-borne resistance is therefore a far more serious concern

than chromosomal resistance.

 

Antibiotic use in dairy cows can lead to resistance that could

potentially affect human health by several pathways. The most direct

path is if a resistant pathogen becomes established in dairy cows,

either because competing non-resistant bacteria in the cow are

decimated by repeated antibiotic use or because R-factors are passed

from non-pathogenic resistant bacteria in the cow. The pathogen

could then be present in meat or milk, which could produce human

exposure to infection that was resistant to antibiotics.

 

An indirect path also could exist if non-pathogenic bacteria with an

R factor were present in meat and/or milk. The R-factor-containing

plasmid could then move from non-pathogenic bacteria to pathogenic

ones in the human gut, creating the potential for a difficult-to-

treat infection. An even more indirect path would be for antibiotic

residues in milk to select for resistance in bacteria in the

intestine.

 

In the past, it has been argued that the antibiotic levels in milk

are far too low to select for antibiotic resistance. However, a

paper published in 1993 demonstrated that even FDA " safe levels "

(from 10 to 150 parts per billion) of antibiotic residues in milk

can select for disease resistance in Staphylococcus aureus (Brady et

al., 1993). In addition to its role in severe cases of mastitis, as

noted above, S. aureus is responsible for many serious human

infections in hospitals. The authors found that residues of one

antibiotic in the milk increased the rate at which S. aureus evolved

resistance to that antibiotic by 600 percent; with residues of three

antibiotics, the increase was 2,700 percent.

 

Data from a technical manual on rbST distributed to veterinarians by

Monsanto, and based on 10 U.S. rbST-trials (and including all 8

trials that the US FDA used to grant approval), showed that the

percentage of bacterial isolates from clinical mastitis cases that

contained S. aureus increased by 62% in rbST-treated cows (Monsanto,

1994). The authors of the S. aureus study concluded " that greater

emphasis should be placed on keeping the milk supply residue free

rather than reliance on maintaining the working residue levels

suggested by the term 'safe levels' " (Brady et al., 1993: 232).

 

In summary, convincing evidence from both pre-approval controlled

trials of rbST and from the PAMP in the US clearly indicates that

rbST use can dramatically increase mastitis rates in some treated

herds, requiring increased use of antibiotics.

The additional

antibiotic use due to rbST use cannot help but contribute to the

general problem of antibiotic resistance in pathogenic bacteria.

In

addition, there is some evidence (and an obvious need for further

research) to indicate that antibiotic residues in milk are capable

under some conditions of selecting for resistant bacteria in the

human intestinal tract.

 

To the extent that rbST use would lead to increased occurrence of

antibiotic residues in milk, which it

appears it has done in the U.S., widespread use of rbST could

exacerbate the resistance problem by this pathway as well.

 

 

 

3. Does rbST use increase BSE risks?

A third concern we have examined is whether rbST use could increase

the risk of bovine spongiform encephalopathy (BSE) in dairy cows.

Evidence to support this concern is very limited at this point, but

several lines of evidence suggest that it is a plausible question to

pose.

 

Given the potential economic implications for the dairy

industry, and public health implications if it is eventually proved

that BSE is causally connected to a new strain of the fatal human

Creutzfeldt-Jakob Disease (CJD), it seems important to pursue this

line of reasoning and to clarify priority issues for research.

 

There are two mechanisms whereby rbST use could potentially lead to

an increase in BSE in dairy cows.

 

First, increased circulating IGF-I

levels might increase a cow's susceptibility to BSE, should the

animal be exposed to the infectious agent. And second, the rbST-

treated cow's increased protein need could magnify the odds of

exposure to a BSE-infective agent.

 

 

 

Does IGF-I increase prion protein (PrP) production?

 

BSE belongs to a group of progressive neurodegenerative diseases of

humans and animals called the transmissible spongiform

encephalopathies (TSEs), which also includes scrapie in sheep,

chronic wasting disease (CWD) in mule deer and elk, transmissible

mink encephalopathy (TME), and Creutzfeldt-Jakob Disease (CJD) and

several other rare diseases in humans.

 

These diseases have been shown to be transmissible by eating parts

of infected animals. They

are also characterized by long incubation periods, invariably fatal

outcome, and infective agents that are unusually resistant to most

forms of sterilization (formaldehyde, 70% alcohol, heat, radiation).

 

The nature of the infective agent(s) is still imperfectly

understood. A widely held theory is that the infectious agent is a

protein. Prusiner (1982) coined the term " prion " to stand

for " proteinaceous infectious " particles that he believed

constituted the infectious agent.

 

Further work showed that the prion

protein is normally found in all animals and is encoded by a prion

gene, PrP. As research has unraveled the causal process, TSE

infectivity has been clearly associated with a protease resistant

protein (called PrP-res, or PrP-sc), which is a posttranslationally

modified form of the proteinase K-sensitive host-encoded prion

protein (PrP or PrPc, c for cellular) (Prusiner, 1991).

PrPc is a

membrane-bound protein found on the surface of all nerve cells, some

lymphocytes and some other tissues. Both isoforms of PrP (e.g. PrP-

res and PrPc) have the same amino acid sequence, but the molecules

differ in their three-dimensional structure.

 

According to the most widely held theory of how prion proteins

behave, the abnormal form (PrP-res), in combination with some other

factor, causes normal PrP to convert to the abnormal Prp-res form,

which in turn causes more PrPc to convert to PrP-res in a kind of

domino effect or crystallization process (Gajdusek, 1993). PrP

molecules on the surface of cells do not stay there for the life of

the cell; they are removed from the cell's surface, transported

through the cell membrane into the cell and digested, while new PrPc

is made within the cell and transported to the cell surface.

 

PrP-res cannot be digested by the cell, and builds up in the cell,

eventually causing cell death.

 

PrP-res molecules can form large oligomers that may eventually

congeal (or precipitate) into large

structures called prion rods or scrapie associated fibrils, which,

if abundant enough, result in the appearance of large plaques.

 

The role that PrP normally plays in nervous and lymph systems is

not fully known.

 

One way that rbST use might increase BSE risks is through effects of

IGF-I on prion gene expression. Lasmézas et al. (1993) demonstrated

that IGF-I dramatically increased production of PrP mRNA in a

laboratory system. That is, IGF-I leads to significantly increased

synthesis of prion protein. Increased IGF-I levels in cows might

therefore speed up the action of PrP-res in a BSE-infected animal,

shortening the incubation period for BSE.

 

Work with transgenic mice showed that increasing the amount of PrPc

in the brain (via increased PrP gene expression, as indicated

increased PrP mRNA levels) increased the speed of progression of

scrapie (Prusiner, 1991). In these experiments, mice were engineered

with varying numbers of copies of the hamster prion gene (HaPrP).

The mice were then exposed to a given dose of scrapie-infected

hamster brains. Results showed that " The length of the incubation

period after inoculation with Ha prions was inversely proportional

to the level of HaPrPc in the brains of transgenic mice " (Prusiner,

1991: 1519).

 

The transgenic mouse strain with the smallest amount of

brain HaPrPc (Tg69) had an incubation period of about 275 days,

while the strain with the highest amount of brain HaPrPc (Tg7),

approximately six times the amount found in brains of Tg69 mice, had

an incubation period of about 50 days, about one-sixth the

incubation period of the Tg69 mice (Prusiner, 1991).

 

Lasmezas et al. (1993) undertook their study to see whether either

human growth hormone (hGH) or human IGF-I affected the PrP gene.

 

If

such hormonal signals " switch on " the PrP gene, it would be good

evidence that IGF-I plays a role in progression of the human form of

the disease, CJD. They were interested in this question because 25

French children, who had contracted CJD after being treated with hGH

extracted from apparently CJD-contaminated cadavers, had exhibited a

particularly short incubation period.

 

The investigators wondered if

hGH or IGF-I may have hastened progression of the disease in these

children. They used, as an experimental system, a line of rat cells

(PC12 cells) that other studies had shown to be a good in vitro

model for studies of TSE agents (Rubenstein et al., 1990). They

found that while hGH had little or no effect on PrP gene expression,

IGF-I induced dosage dependent increases in PrP mRNA levels. PrP

mRNA levels increased 40% with IGF-I levels of 10 ng/ml and 100% at

IGF-I levels of 100 ng/ml.

 

The results are significant because, as

the authors point out, " IGF-I is found in human serum at

concentrations of up to 1 ug/ml, i.e. ten times more than the

highest tested dose. The effect of IGF-I on PrP gene expression in

our experiments therefore occurs within the 'physiological'

concentration range of this factor " (Lasmézas et al., 1993: 1167-

1168). They conclude that " an increase of the levels of PrP RNA

messengers, as a result of gene activation or of transgenesis, can

be deleterious for an [TSE] infected organism " (Lasmézas et al.,

1993: 1167). The deleterious effects could include a decrease in the

incubation period as well as a more rapid progression of the disease.

 

How is this work relevant to rbST use in cows? First, administration

of rbST clearly elevates the level of IGF-I in the cow's blood.

 

Injecting cows with rbST increases bovine serum levels of IGF-I by

at least 5- to 7-fold (Juskevich and Guyer, 1990). Indeed, the

elevated IGF-I is believed to mediate the increase in lactation.

 

However, from the work of Lasmézas et al. and Prusiner et al., one

would also predict that the increased IGF-I would lead to increased

prion protein (PrPc) production and possibly more rapid progression

of BSE in animals exposed to the infective agent.

 

Based on the work of a number of scientists, the

most likely route of the TSE infective agent in the human body would

be the same as in animals:

 

from stomach to intestines, through the peyer's patch (in the

intestine) into the lymphoreticular system

into peripheral nerves and then into the central nervous system

(Pattison and Millson, 1962, Blättler et al., 1997, Groschup et al.,

1996, Brandner et al., 1996).

 

 

 

The intestines are one of the earliest sites of TSE infection. They

are known to be among the infective parts of an animal, and since

studies have shown that normal prion protein is required for spread

of the infective agent (Brandner et al., 1996), intestinal cells

probably express PrP. They could conceivably produce more PrP in

response to IGF-I.

 

Milk levels of IGF-I are high enough to

potentially be of physiological relevance.

 

 

 

Increased levels of IGF-I in milk that reached the intestine (as we

have suggested, based on our review of current evidence, they are

likely to do) could conceivably stimulate PrP synthesis in

intestinal cells.

 

Thus, one could hypothesize that stimulating PrP

production in intestinal cells could increase the rate or ease with

which the infective agent could then move into the lymph system. As

noted, these concerns are highly speculative, and further research

is needed.

 

 

 

Does rbST use increase use of protein rich diets?

 

There is a second way that rbST use could contribute to BSE risk:

 

Changes in the diet of dairy cows associated with rbST use could

increase the risk that the animals might be exposed to the BSE

infective agent. Cows receiving rbST require more energy-dense food

than control cows, as higher milk output increases the amount of

protein needed.

 

One major source of energy-dense foods is protein

and energy supplements made from rendered animal remains. For

example, in the U.S., an official of the Center For Veterinary

Medicine stated in a 1991 memo, " There is a growing trend in the use

of meat and bone meal for calf rations . . . Most is used as a

protein source for high production dairy cattle and for feed lot

cattle " (Osborne, 1991: 4).

 

The European Union has prohibited use of mammal protein in feed for

dairy cows (poultry and fish protein is permissible) because of

concern that rendered protein might contain TSE-infective agents.

 

Cows in England are believed to have become infected by eating the

rendered remains of scrapie-infected sheep.

 

However, the U.S. allows

swine and horse protein in dairy feed, as well as poultry and fish

protein, despite some evidence suggesting presence of a BSE-type

disease in swine in the U.S. (Consumers Union, 1997).

Most other

nations have no current restrictions on the types of protein in

dairy feed. Although the Codex Committee on Food Hygiene is now

considering " Good Animal Feeding Practices, " the Codex process

typically takes years to produce consensus international guidelines.

 

Use of rbST will definitely increase the use of energy dense feeds

and protein supplements. Although this increased need for protein

could be met with feeds from plant sources, such as soybeans,

farmers will undoubtedly buy protein supplements based on their

relative cost. Indeed, as some countries control use of rendered

animal feeds, the relative costs of such materials seems likely to

decline in international markets, making rendered animal protein

more attractive to farmers in unregulated markets.

 

Animal protein feeds banned in Europe and the U.S. might also be

exported to developing countries.

 

When the U.K. banned use of specified bovine offals (SBOs) in animal

feed in 1989, because SBOs

carried the infectious agent for BSE, rather than destroy the SBOs,

the U.K. exported such materials to other EU countries that had no

regulations and to developing countries, such as Thailand.

 

There is

now a global ban on such material from UK cows, but exports from

other countries are unrestricted. The U.S., for example, recently

banned feeding ruminant protein to ruminants, as a precautionary

measure, but the regulation does not restrict feed exports.

 

There is no way to be sure rendered protein produced outside the

United Kingdom is safe. We currently have no assurance that BSE is

confined to the UK.

 

Monitoring elsewhere is poor to non-existent

(for example, the U.S. tests less than a tenth of a percent of

slaughtered cattle for BSE; many countries do no testing at all).

 

When European countries started more careful monitoring, they began

finding cows with BSE (MacKenzie, 1997). A recently published

statistical analysis of the cows exported from the U.K. during the

1980s predicts that a certain low percentage of cows would have been

infected, yet the number of BSE cases reported by the various

importing countries has been far smaller than the model predicts

(Wilesmith et al., 1997). This suggests that there is chronic

underreporting of BSE incidence.

 

On theoretical grounds, it is reasonable to expect a low natural

background level of mutant PrPs in virtually all mammals. Dr.

Clarence Gibbs of the U.S. National Institutes of Health states: " As

to the possibility that BSE may become endemic, I have proposed the

following hypothesis. Since we accept that sporadic CJD is the

result of a configurational change in a normal protein that occurs

at the rate of 1-2 cases per million population per year, and since

normal prion protein has been detected in all mammalian species thus

far tested, as well as in salmon fish and Drosophila, then the rare

occurrence of spongiform encephalopathy may certainly take place but

remain undetected due to its rare occurrence in nature " (Gibbs,

1997: 9). And there is both direct and indirect evidence to suggest

that TSEs occur in numerous species. In the U.S., TSEs have been

documented in sheep, mule deer and elk (chronic wasting disease),

and mink (transmissible mink encephalopathy). A recent report of a

cluster of 11 cases of CJD in rural western Kentucky, where all the

cases ate squirrel brains, suggests that a TSE is present in

squirrels (Berger et al., 1997; Blakeslee, 1997).

 

 

 

Swine may also be infected with a TSE in the U.S. Some 106 hogs in a

1979-80 USDA study at a packing plant in upstate New York showed

many of the same behaviors found in BSE animals (Doi et al., 1979;

Doi pers. com. 1997). The brain of one of the hogs also showed

spongiform degeneration and some other signs of a TSE (Langheinrich,

1979). Dr. William Hadlow, one of the foremost TSE pathologists in

the world, reviewed slides from the suspect pig brain. Dr. Hadlow

felt the slides were suggestive of a TSE, although not definitive

(Hadlow, 1997).

 

Case-controlled epidemiological studies of CJD patients in both the

U.S. and UK point to dietary consumption of a number of animal

products being associated with CJD. Two small case-controlled

epidemiological studies of U.S. CJD patients suggest the presence of

a TSE in swine.

 

A 1985 study, involving 26 CJD patients, found that

dietary consumption of six different pork products was associated

with an increased risk of CJD compared to a control group: " An

increased consumption among [CJD] patients was found for roast pork,

ham, hot dogs (p < .05), roast lamb, pork chops, smoked pork, and

scrapple (p < .10) . . .

 

The present study indicated that

consumption of pork as well as its processed products (e.g., ham,

scrapple) may be considered as risk factors in the development of

Creutzfeldt-Jakob disease. While scrapie has not been reported in

pigs, a subclinical form of the disease or a pig reservoir for the

scrapie agent might conceivably exist " italics added (Davanipour et

al., 1985: 443, 448).

 

The other study, from 1973, found that one

third of the 38 CJD patients studied ate brains, much higher that

the U.S. population overall, and that the patients had a preference

for hog brains compared to controls (Bobowick et al., 1973). Both

studies are small, but the results are highly suggestive.

 

The U.S. Food and Drug Administration's August, 1997 regulation

permits known TSE-positive material to be used in pet food, pig,

chicken and fish feed. FDA requires only that it be labeled " Do not

feed to cattle and other ruminants " when marketed in the United

States.

 

Finally, BSE-infected cattle typically do not exhibit symptoms of

BSE until the end stages of the disease, following a long incubation

period.

 

There is evidence that rbST use reduces the useful lifespan

of a dairy cow. Cows intensively treated with rbST from their first

lactation cycle could be removed from herds after just two or three

years rather than the now customary four to six years.

 

Given that

the incubation period for BSE is at least three to five years and

perhaps longer, rbST-treated cows could harbor " hidden " BSE. That

is, they might be infected but still asymptomatic when sent to

slaughter.

 

In summary, there is as yet no direct evidence that rbST use has, to

date, contributed to or in any way caused the BSE epidemic. Studies

comparing BSE incidence in dairy herds treated with rbST against

that of untreated cattle would be required to answer that question,

and no such studies have been done.

 

Indeed, rbST has not been in use

in the countries where BSE has been officially reported. But we

believe there is a very sound theoretical basis for the hypothesis

that rbST use could aggravate the risk of BSE. Cows treated with

rbST require more energy-dense feed, which increases the likelihood

they may be fed rendered animal protein.

 

Use of such feeds has been

causally linked with the spread of BSE, and current national and

international controls cannot guarantee that these materials are

free of TSE infective agents. Synthesis of PrP proteins has been

shown to respond in a dose-dependent manner to IGF-I levels within

the physiological range, and rbST use markedly increases serum

levels of IGF-I in cows. It appears that rbST use could therefore

decrease the incubation time for BSE in an infected animal.

 

 

 

Given the potential severity of the BSE problem, both for the

economic health of the

cattle industry and for public health, the possibility that rbST use

could aggravate the BSE risk appears to demand intensive

investigation.

 

http://www.consumersunion.org/food/bgh-codex.htm