Mad Cow Disease : CJD and vCJD

by Food and Drug Administration (FDA)

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CJD and vCJD: Two Different Diseases

As of February 2004, 156 cases of the human form of mad cow disease, known as variant Creutzfeldt-Jakob disease (vCJD), have been reported worldwide, according to the Centers for Disease Control and Prevention (CDC). But there has never been a case of vCJD contracted in the United States. One resident of Florida was diagnosed in 2002 with a probable case of vCJD, but it is believed she acquired it in the United Kingdom, where she lived for more than 12 years during an epidemic of bovine spongiform encephalopathy (BSE), or mad cow disease. It is believed that the vCJD victims got the human variant by eating beef products that came from BSE-infected cattle.

The classic form of CJD has been found in the United States. Unlike the variant, the classic form is not known to be food-related. "CJD and vCJD are best thought of as two different diseases," says Lawrence Schonberger, M.D., M.P.H., epidemiologist and assistant director of the CDC's National Center for Infectious Diseases. "CJD was around long before the emergence of BSE in cattle." Both diseases are brain disorders, but the patterns of the brain lesions they leave are distinct.

Variant CJD is found in younger patients and the length of illness is longer. There is no treatment for either disease, and they always result in death. The average age for death of vCJD is under 30 years versus the mid- to late 60s for classic CJD.

Neither vCJD nor CJD is spread through direct contact with others with the infection. The classic form of CJD may be inherited (familial), transmitted by infectious surgical instruments or tissues (iatrogenic), or occur among people with no known environmental risk factors (sporadic). The sporadic form generally occurs at a rate of about 1 case per million people per year, and familial and iatrogenic cases are even rarer, according to the CDC.

Can My Pet Get Mad Cow Disease?

Cats are the only pets known to be affected by the infectious agent that causes bovine spongiform encephalopathy (BSE), known as mad cow disease, in cattle. No evidence of BSE has been found in dogs, horses, birds, or reptiles.

About 90 cats in the United Kingdom and several cats in other European countries have been diagnosed with feline spongiform encephalopathy (FSE). Before it was recognized that they could be affected by the BSE agent, cats were exposed to it through commercial cat food or meat scraps provided by butchers. The number of reported cases of FSE in Europe has been declining annually since 1994 after European countries implemented feed bans that lowered the risk of BSE transmission.

To date, no case of FSE has been found in the United States. The FDA believes that the safeguards it has put in place to prevent BSE in this country have also protected cats.

The Species Barrier

Infections are more likely to be transmitted within a species than from one species to another, a concept known as the species barrier. Bovine spongiform encephalopathy (BSE), or mad cow disease, was once thought to infect only cows. But now scientists know it jumps the species barrier to infect humans in the form of a brain-wasting disease called variant Creutzfeldt-Jakob disease (vCJD). BSE and vCJD belong to a family of degenerative diseases of the nervous system called transmissible spongiform encephalopathies (TSEs).

European authorities suggest that the cattle disease may be 10 to 100,000 times less infectious in humans. As of March 3, 2004, the United Kingdom reported 146 cases of vCJD, yet more than 180,000 cattle in Great Britain were diagnosed with BSE. But some scientists fear that many more people are incubating vCJD and signs of the disease just haven't shown up yet.

"It's not appropriate at this stage to totally rely on the species barrier to protect the population against BSE," says Lawrence Schonberger, M.D., M.P.H., epidemiologist and assistant director of the Centers for Disease Control and Prevention's National Center for Infectious Diseases. "We don't know for sure how well the population is protected. We must try to reduce exposures to the maximum extent possible."

Understanding which animals can get and transmit BSE and how likely they are to get the disease helps government agencies set policies to protect human and animal health. BSE has never been found to occur naturally in pigs or to be transmissible to pigs by feeding them the BSE agent. However, researchers have been able to infect pigs by injecting them with the BSE agent. Researchers have also infected sheep and goats by feeding them BSE-contaminated food. Chickens and turkeys have not been shown to develop the disease naturally or through injection.

But why are scientists trying to infect animals with BSE through non-natural routes, such as injecting it into the brain? "If a TSE is not transmitted by the most direct route — injection into the brain — one may have more assurance that the species is resistant through other routes, such as orally," explains Linda Detwiler, D.V.M., adjunct professor at the University of Maryland. "But I think we have to be so careful that by constant exposure we do not force infectivity into another species just because we think they have a resistance," she says. "The resistance may, in some cases, be able to be overpowered or the TSE agent might adapt."

About the Author

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