Emerging tick-borne disease

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Emerging tick-borne disease

 

http://www.vetscite.org/publish/items/005812/index.html

 

23 March 2010

 

Emerging tick-borne disease

 

Stories of environmental damage and their consequences always

seem to take place far away and in another country, usually a

tropical one with lush rainforests and poison dart frogs. In

fact, similar stories starring familiar animals are unfolding all

the time in our own backyards -- including gripping tales of

diseases jumping from animal hosts to people when ecosystems are

disrupted. This time we're not talking hemorrhagic fever and the

rainforest. We're talking tick-borne diseases and the Missouri

Ozarks. And the crucial environmental disruption is not the

construction of roads in the rainforest, it is the explosion of

white-tailed deer populations. An interdisciplinary team at

Washington University in St. Louis has been keeping a wary eye on

emerging tick-borne diseases in Missouri for the past 20 years.

Team members include ecologists Brian F. Allan and Jonathan M.

Chase, molecular biologists Robert E. Thach and Lisa S.

Goessling, and physician Gregory A. Storch. The team recently

developed a sophisticated DNA assay, described in the March 2010

issue of Emerging Infectious Diseases, that allows them to

identify which animal hosts are transmitting pathogens to ticks.

" This new technology is going to be the key to understanding the

transmission of diseases from wildlife to humans by ticks, " Allan

says.

 

Missouri has three common species of ticks. The black-legged tick

(Ixodes scapularis) that carries Lyme disease is found here, but

is far less common than in other regions of the country. Missouri

also has American dog ticks (Dermacentor variabilis), which carry

Rocky Mountain Spotted Fever, but again this is a less frequently

encountered species.

The most common tick is Amblyomma americanum, called the lone

star tick because the adult female has a white splotch on her

back. It is a woodland species originally found in the

southeastern United States whose range now extends northward as

far as Maine. Until recently, this tick, which is an aggressive

and indiscriminate biter, was considered a nuisance species, not

one that played a role in human disease. Then in

1986 a physician noticed bacterial clusters called morulae in a

blood smear from a critically ill man that looked like those

formed by bacteria in the genus Ehrlichia (named for the German

microbiologist Paul Ehrlich). At the time Ehrlichia were thought

to cause disease only in animals.

 

The bacterium was later identified as a new species, Ehrlichia

chaffeensis, and the disease was named human ehrlichiosis. In

1993 E.

chaffeensis DNA was found in lone star ticks collected from

several states. Ehrlichiosis typically begins with vague symptoms

that mimic those of other bacterial illnesses. In a few patients,

however, it progresses rapidly to affect the liver, and may cause

death unless treated with antibiotics. In 1999, a second

Ehrlichia species was identified as an agent of human disease.

The DNA of the newly identified bacterium was also found in lone

star ticks. Gregory A. Storch, M.D., the Ruth L. Siteman

Professor of Pediatrics at the Washington University School of

Medicine in St. Louis, led the team that identified the second

Ehrlichia species. The discovery was described in the New England

Journal of Medicine in 1999. Blood samples from patients in the

St.

Louis area who might have a tick-borne disease are still sent to

Storch's lab for analysis.

 

But the erhlichioses weren't the only emerging diseases the tick

was carrying. In the 1980s, reports had started to trickle in

from Missouri, North Carolina and Maryland of an illness

accompanied by a bulls-eye rash. Called STARI, for southern

tick-associated rash illness, it resembled Lyme disease but

didn't seem to be as severe. The lone star tick was also

incriminated in these cases. STARI is thought to be caused by a

bacterium named Borrelia lonestari, after its tick vector. " The

question, " says Thach, Ph.D., professor of biology in Arts &

Sciences and of biochemistry and molecular biophysics in the

School of Medicine, " is where do infectious diseases come from? "

" Most seem to come from nature -- they exist in other animals --

and then make the leap from animals to people, Thach says. "

Assuming this model applies to the lone star tick diseases, what

is their animal reservoir and why are they jumping? Lone star

ticks need blood meals to power their metamorphoses (they go

through three stages: larva, nymph and adult) and egg laying.

 

They sometimes bite coyotes, foxes and other animals, but their

favorite hosts are wild turkey and white-tailed deer. Especially

white-tailed deer, which seem to be playing a major role in

maintaining large lone star tick populations and setting the

stage for tick diseases to jump to people. Fieldwork conducted by

Allan, Ph.D., a post-doctoral research fellow at Washington

University's Tyson Research Center in the oak-history forests

that grace the rolling hills of the Missouri Ozarks, was

reinforcing the team's suspicions about deer. In forests managed

by the Missouri Department of Conservation and by the Nature

Conservancy, Allan was looking at the effect on tick numbers of

management practices such as selective logging and prescribed

burns. Allan's results show that management practices sometimes

have counterintuitive effects on tick numbers. For example, he

reported in the Journal of Medical Entomology in September 2009

that prescribed burns increase tick numbers and human risk of

exposure to lone star tick diseases. To make sense of this

counterintuitive result all you need to do is follow the deer. A

prescribed burn leads to a flush of new plant growth. Deer, which

are selective browsers, are attracted by the tender greenery.

They flood into the burn sites, and drop blood-sated ticks as

they browse.

 

Although deer were looking shady, the case against them was still

largely circumstantial. Could the scientists get definitive

evidence?

Allan found a way. He read about an assay that had been developed

in Jeremy Gray's lab at University College Dublin to identify

animal reservoirs of Lyme disease. ( " There are twice as many

cases of Lyme disease in Western Europe as there are in the

United States, " says Thach, " and there is a lot of Lyme research

being done there. " ) Allan asked Thach whether his lab would be

willing to develop a similar assay for the lone star tick

diseases. " With my colleague Lisa Goessling, "

Thach says, " we developed the technique here and used it to

analyze the ticks Brian brought in from the woods. " " The

technology for identifying mosquito blood meals has existed for

some time, " Allan says, " because they take many blood meals over

a short period of time, so the blood is usually still fresh when

you capture them. And they keep coming back for another meal, so

it's very easy to capture them. It's much harder to get blood

from a tick, which usually takes only one blood meal per life

stage, " Allan continues. " By the time we capture the tick eight

months to a year may have elapsed. The tick has had a long time

to digest that blood, so there may be only a tiny amount of DNA

left -- if there's any. "

 

The team does two assays on the tick DNA: one to identify

pathogenic bacteria and the other to identify the animal that

provided the blood and with it the bacteria. The first step in

the assay is to pulverize the ticks to release the DNA, which is

then amplified using a procedure called the polymerase chain

reaction, or PCR. This provides enough DNA for identification.

Following amplification is a step called reverse line blot

hybridization. Probes, which are short sequences of DNA unique to

a bacterium or to a host animal, are deposited in lines on a

membrane. The membrane is then rotated, and the products of the

PCR step

-- tagged with a chemiluminescent (light-generating) dye -- are

laid down in lines perpendicular to the probe lines. Wherever two

lines cross, DNA from the tick sample mixes with probes for

either bacterial or animal DNA. If the two match, the molecules

will bond, or hybridize.

When the membrane is later washed, tick-sample DNA that has not

hybridized washes off. DNA that has hybridized sticks and shows

up as a chemiluminescent spot on the membrane. Reading the spots,

tells the scientists which bacteria the tick was carrying and

which animal provided its last blood meal.

 

Assay results showed that most of the nymphal lone star ticks

infected with E. chaffeensis fed upon a white-tailed deer in the

larval life stage. " So deer are definitely a primary reservoir

for this bacterium, "

says Thach. " But we also found some kind of squirrel -- which we

have more recently identified as the common gray squirrel -- and

what appears to be some kind of rabbit. " In general, the results

suggest deer are probably " weakly competent reservoirs " for the

tick diseases, meaning that ticks that bit deer stood only a

small chance of picking up one of the pathogens. On the other

hand, deer have huge " reservoir potential, "

because there are so many of them. The bottom line: a sprinkling

of deer is ok; crowds of deer are a problem. Are the bacteria

that cause the new tick-borne diseases truly new or have they

existed for a long time in wildlife reservoirs like the

white-tailed deer without causing human disease? " We don't know

the answer, " says Allan, " but my guess is these tick-borne

diseases are probably being unleashed by human-mediated

environmental change. "

 

By human-mediated environmental change he means deer protection,

the human behaviors that have led to an explosion in white-tailed

deer populations. " Some state agencies plant food plots for deer,

we've created deer forage in the form of crop fields and suburban

plantings, and we've taken away almost all of their predators --

except cars, "

Allan says. To be sure, white-tailed deer were once nearly

eliminated from the state. In 1925 there were thought to be only

395, according to the Missouri Department of Conservation. The

hunting season was closed that year and again from 1938 through

1944, and deer were re-located to help reestablish them in the

state. In 2009, Lonnie Hanson of the Missouri Department of

Conservation estimated the herd at 1.4 million.

Nationwide the pattern is similar. Nobody is sure how many deer

there are, but estimates range from 8 to 30 million, levels

everyone agrees are excessive. " If you had to point to one factor

that led to the emergence of tick-borne diseases in the eastern

United States, it would have to be these unnaturally large

populations of deer, " Allan says.

 

Science Daily

March 23, 2010