On Tuesday April 24 2012, the U.S. Department of Agriculture (USDA) confirmed a case of bovine spongiform encephalopathy (BSE) in a dairy cow in California. This case marks the nation’s fourth case of the disease.
"Mad cow disease" has frightened the public since it emerged in the United Kingdom in 1986. People are alarmed because eating meat from a cow with BSE can be fatal. While the disease is not well understood, we know that BSE is transmitted by feeding cattle food with meat or bone products infected by the BSE prion. Prions are abnormal proteins that can cause rare progressive neurodegenerative diseases. Human infection from BSE is believed to be the cause of variant Creutzfeldt-Jakob disease (vCJD), a fatal human disease. In response to this new US case, a South Korean retailer (South Korea is the fourth largest importer of U.S. beef) has “temporarily” halted sales of U.S. beef for fear of spread of the disease in humans.
Given the potential human health consequences, the case in California may sound scary, but can actually be seen as good news. First, this is an atypical case, meaning that the prion disease occurred spontaneously in just this cow; there is no evidence to suggest that the cow was infected through feed. Second, according to USDA Chief Veterinary Officer John Clifford’s statement, the animal was “never presented for slaughter for human consumption, so at no time presented a risk to the food supply or human health.” Also, he reminds us, BSE cannot be transmitted through milk consumption. Third, and this point may be debatable, the meat industry suggests that control measures against BSE must be working as this is only the fourth identified case since 2003. These control measures, as described by Philip Yam for Scientific American, include: banning “downer” cattle, or cows that cannot walk on their own; making cows with any signs of neurological illness illegal for food use; forbidding “air-injection stunning” (a practice in which air is blasted into a cow’s brain prior to slaughter to stun them – however, it also risks blowing bits of the brain into the bloodstream, thus increasing the risk of spread of abnormal brain prions); and banning the inclusion of “risky material” from cattle in human food.
Some, such as Michael Hansen of Consumers Union, point out that the government only tests a fraction of the cows slaughtered so we really don’t have a good picture of the burden of the disease.
The BSE Surveillance program implemented in 2006 by the USDA tests approximately 40,000 cows each year. Yes, that is only a small fraction of cattle being slaughtered, however, it corresponds with the extremely low prevalence of BSE within the United States and represents a testing level that is ten times higher than the World Animal Health Organization (OIE) recommends.
As Yam points out, addressing the most recent case of BSE, even at the height of the European outbreak of mad cow disease, there were only 222 human cases of vCJD, despite the millions of people eating “risky beef.” Sporadic cases of BSE and CJD will occur and we should expect them, while maintaining careful surveillance on our cattle. However, Yam states, “[r]eactions such as a South Korean retailer suspending the sales of U.S. beef there are overblown.”