Demystifying Mad Cow in Humans

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Demystifying Mad Cow in Humans

 

Published on: June 6, 2003 By Christine Haran

Just as SARS began to drop off people's radar screens, the discovery of a single

cow with mad cow disease in Canada renewed fears about humans contracting a form

of this degenerative brain disease. It also prompted an immediate ban of

Canadian beef in the United States. Despite the hype, most people can offer few

details about this human brain disorder, which is called Creutzfeldt-Jakob

Disease.

Creutzfeldt-Jakob Disease is very rare, with about 6,000 cases occurring

annually worldwide. Following an outbreak of a variant form of Creutzfeldt-Jakob

Disease in the United Kingdom in the mid-90s, scientists learned that the

disease can be transmitted to humans when they eat contaminated beef. Cattle

contract mad cow disease, or bovine spongiform encephalopathy (BSE), when they

eat feed that contains tissue from an infected cow. But this fatal disease is

not new and has causes other than the consumption of contaminated beef. It also

can result from a genetic defect or may occur spontaneously. The disease is

marked by involuntary movements and psychiatric problems, and its symptoms are

similar to that of several other neurological conditions.

The UK outbreak led to a global ban of British beef, though the British

government has since banned the type of feed that became contaminated and

slaughtered cattle that may have eaten this feed. While no cases of mad cow, or

the variant Creutzfeldt-Jakob Disease, have been discovered in the United

States, both the US Department of Agriculture and the Centers for Disease

Control and Prevention has implemented national surveillance of both cattle and

humans.

Below, Creutzfelt-Jakob Disease researcher Michael Geschwind, MD, PhD, assistant

professor of neurology at the University of California, San Francisco (UCSF)

School of Medicine, discusses the causes, symptoms and treatment of this rare

human brain disorder.

What causes Creutzfeldt-Jakob Disease (CJD)?

Creutzfeldt-Jakob Disease is a brain disease caused by a protein called a

" prion. " This protein was identified and named by Dr. Stanley Prusiner, at the

UCSF Medical Center, who was awarded the 1997 Nobel Prize in Medicine and

Physiology for this work. In this disease, the prion protein takes on an

abnormal shape. That shape then becomes a template. When the normally shaped

proteins come near it, they take on that abnormal shape. It's like a domino

effect and soon many of the normally shaped proteins become abnormally shaped.

The prions create accumulations of fluid in compartments of nerve cells. When

you cut the brain, they look like holes in the brain. The old term was

spongiform because it looks like a sponge. The prions also lead to scarring and

eventually, the nerve cells themselves die.

What are the different types of CJD?

CJD primarily occurs in three forms in humans. The most common form is sporadic,

which means that it happens spontaneously, without any known cause. Those

compromise about 85 percent of all cases of CJD. About 15 percent of cases are

genetic. That means that there's an inherited mutation in the prion gene, so an

abnormal gene is making the prion protein.

Less than 1 percent of cases are induced inadvertently. Usually there's a

medical cause for it. For example, more than 25 years ago, some patients, mostly

in France, the United Kingdom and the United States, received contaminated human

growth hormone that was derived from cadavers. In these cases, the growth

hormone was contaminated because one of the cadavers happened to have had CJD;

the patients in the United States are being tracked.

Other possible sources of infection have involved electrodes that were implanted

in the brain; tainted neurosurgical instruments; corneal grafts; and grafts of

" dura mater, " a covering of the brain taken from cadavers and used in some

neurosurgical procedures. Most of these cases occurred before the mid-1980s when

improved patient screening and processing methods were implemented.

How do humans develop the variant form of CJD from beef?

The variant form of CJD has been linked to the consumption of cattle that are

infected with bovine spongiform encephalopathy, or BSE. In the variant form, the

abnormal prion in infected beef gets into the human digestive tract and slowly

makes it way up to the brain. The variant cause is still very rare. Only about

135 cases, mostly in Britain, have occurred.

How do we know that the human and animal forms are related?

There has been experimental evidence linking the variant form of the disease to

the BSE. Researchers found that the prion proteins from cattle with BSE and

patients with variant CJD essentially look the same. There was also a

relationship between the incidence of BSE and the incidence of the variant CJD.

The variant CJD, which was first discovered around 1995 in Britain, came after

the peak of the BSE epidemic.

Right now, the incidence of the variant CJD hasn't been increasing. We're not

sure how many patients are still at risk, but it could be as few as just a few

hundred or it could be in the hundreds of thousands, possibly.

How can you tell which form of CJD someone has?

People usually develop the variant form of CJD at a younger age than they

develop the sporadic form. The age range has been between 12 and 70, but the

average age is around age 29, whereas the sporadic form of the disease usually

occurs in the late 60s. From the first symptom, most patients with the sporadic

form of the disease live about seven or eight months. With the variant of the

disease, they live about 14 months.

The variant and sporadic forms of the disease have overlapping symptoms, but

there are some differences. With the sporadic form, there may be some early

behavioral disturbances and mild depression, but neurological problems, such as

dementia and movement problems, are more prominent early on. With the variant

form, there tends to be a much more profound psychiatric disorder early in the

course of the disease, and neurological problems generally appear later.

People with the sporadic and variant forms of the disease often have poor

control of balance or movement. Both types of patients also develop a movement

disorder, though it may manifest itself differently in each form.

People with the variant form may also develop painful sensations that tend not

to go away. It's often thought to be back pain, and the patients are worked up

for problems in their spine or neck.

Two tests, a type of brain imaging called Magnetic Resonance Imaging, or MRI,

and a brain wave test called an electroencephalogram, or EEG, can help to

distinguish the two forms of the disease as well. Additionally, biopsy or

autopsy can definitively distinguish between variant and sporadic forms of the

disease.

How is CJD diagnosed?

A definite diagnosis of CJD generally requires a biopsy or autopsy of the brain.

A clinical diagnosis can be made based on a combination of neurological symptoms

and specific findings on a brain wave test called an electroencephalogram, or

EEG. Recently, MRI has also been shown to be useful in making the CJD diagnosis.

How is CJD treated?

Right now, treatment is palliative. Dr. Prusiner's laboratory here at UCSF, in

collaboration with other laboratories, is trying to come up with agents that can

treat the disease. For example, we are trying to treat patients with the drug

quinacrine, an old antimalarial drug. The data are unclear at this point, but

are hopeful, and we need a formal study to determine if the quinacrine is

effective.

What kinds of research would you like to see?

We have to find better ways of identifying the disease, particularly early in

its course. We currently don't make the diagnosis of a patient until very late,

so even if we had a treatment, there's probably been too much damage done to the

brain.

Research that I'm working on is trying to come up with better clinical clues to

what differentiates CJD from other neurologic diseases. CJD can look like many

other neurological diseases, depending what part of the brain is affected first.

So we need to come up with better diagnostic methods, such as a definitive

blood, cerebrospinal fluid or brain-imaging test.

 

© 2003 Healthology, Inc.

 

 

 

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