Earlier this month, a beef-type cow in Florida was identified as having bovine spongiform encephalopathy (BSE), also called mad cow disease. For those old enough to remember, BSE was the cause of death (by disease and culling) of many thousands of cattle—most heavily in the United Kingdom—in the late 1980s through early 2000s. Worse yet, it caused the deaths of a couple hundred people who had consumed beef from infected cattle (in humans, this is called variant Creutzfeld-Jakob disease).
The caused-many-deaths BSE is referred to as classical BSE, where the route of infection was due to ingestion by cattle of the infectious agent: a nasty little misfolded protein called a prion, specifically a prion designated PrPSc. Infection with PrPSc came from affected animals that were recycled into meat and bone meal and fed to other cattle. When the molecules of PrPSc get into the body, they go around refolding the native, normal PrPC proteins into the abnormal PrPSc proteins (see figure). As the disease progresses (over years), normal brain tissue becomes decidedly abnormal, and the animal’s behavior follows suit. Key symptoms of BSE include nervousness or aggression, abnormal posture, and lack of coordination. Cattle exhibiting such behaviors are not allowed in the food chain and are automatically tested for BSE.
Due to bans on the recycling of higher-risk tissues into feed, identified cases of classical BSE have fallen to essentially zero worldwide. What surveillance programs have picked up are a very few cases of what is called atypical BSE. The prions detected in these cases are slightly different at the molecular level from that in classical BSE. Atypical BSE arises from a spontaneous mutation in the gene that encodes the native PrP protein with the result that they start to misfold into a PrPSc-like shape. Like the atypical BSE-affected cow recently identified in Florida, these cases are not caused by infection from the outside.
Discovery of this “mad” cow (and the five others over the last 28 years) demonstrates that the surveillance procedures conducted by USDA are effective. At this point, USDA is testing about 25,000 cattle a year, and those are largely sampled from older or ill animals. USDA estimates the prevalence of BSE in the US at 1 in 1 million cattle.
Bottom line: we will occasionally see cases of atypical BSE pop up due to nature (mutations happen!) and our well-functioning surveillance system, but risk to the health of people and other cattle is exceedingly low.