California mad cow case no reason for panic

Animal did not have foodborne form of disease

When a dairy cow in California was recently diagnosed with a rare form of mad cow disease, agriculture officials said the animal posed no danger to human health. The assurances are more than just platitudes, scientists and a new study in mice suggests.

On April 24, the U.S. Department of Agriculture announced that a 10-year-old dairy cow had been found to have bovine spongiform encephalopathy, or BSE, often called mad cow disease. The cow was the fourth animal ever diagnosed with the disease in the United States. One of those cows, an animal imported from Canada, had the classical form of the disease caused by eating feed contaminated with infected brain and nervous system material from other cows.

But the new case does not appear to have been caused by contaminated feed. This case proved to be a spontaneous type of BSE. And a new study using mice suggests that it may be harder for this atypical BSE to cause disease in people compared with the more common form of BSE even if people are exposed to it.

All forms of BSE are prion diseases, which result when a normal brain protein called PrP twists into a different shape and then converts other normal copies of the protein into the disease-causing form. Such diseases strike many different animals, including sheep, deer, elk and people. In atypical forms of BSE, the PrP protein probably folds into slightly different shapes compared with the ones caused by eating contaminated feed. Scientists have found two atypical forms, the H-type and the L-type. The California dairy cow had the L-type.

About one in a million people contract the human form of prion disease called Creutzfeldt-Jakob disease. That disease strikes sporadically, meaning that it appears seemingly at random and doctors and scientists don’t know why. The human disease that stems from eating beef infected with BSE, known as variant Creutzfeldt-Jakob disease, has been recorded in only 224 cases according to the World Health Organization.

So far, no humans are known to have contracted variant Creutzfeldt-Jakob disease from cattle infected with atypical BSE.

Whereas the classical form of BSE usually strikes cattle between 4 and 8 years old, the atypical form is found in cattle older than 8 years old. No one knows exactly how often the atypical form of the disease appears in cattle, but over a six-year period, French surveillance programs detected 13 cases of atypical BSE in 3.6 million cattle over 8 years old, says Thierry Baron, a neuroscientist at the French national food safety agency in Lyon.

Baron and his colleagues published a study in the January Emerging Infectious Diseases suggesting that mouse lemurs could contract the L-type of atypical BSE more easily than the classical version of the prion disease. The reason no humans have contracted variant Creutzfeldt-Jakob disease from cattle infected with L-type BSE may simply be a numbers game, he says: The disease occurs far too infrequently for people to get exposed to it.

But it may actually be harder for the L-type prion to corrupt human PrP than it is for the classical form of the prion, Rona Barron, a molecular biologist at the University of Edinburgh and colleagues, including Baron, report online April 11 in the Journal of General Virology. The researchers created mice that make the human version of PrP in their brains at normal levels. Atypical BSE, chronic wasting disease (a prion disease in deer and elk) and atypical forms of scrapie (a sheep prion disease) all failed to cause disease in the mice.

Her study is hardly the last word on the infectious potential of atypical BSE, Barron says. Other substances in the brain may have a hand in determining whether PrP will go rogue. Regardless of how infectious the atypical prion is, worldwide surveillance efforts and bans on including the brain and nervous system of cattle in food and feed are doing a good job of keeping all prions out of the food supply, she says. “I don’t think there is a cause for mass hysteria.” 

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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