Bovine Spongiform Encephalopathy (BSE, or Mad Cow disease) has not been detected in the United States. The first major analysis of what would happen if BSE were introduced into the United States finds that there is little chance that the disease will be a serious threat either to the American cattle herd or to public health. The work was done for the U.S. Department of Agriculture by the Harvard Center for Risk Analysis (HCRA), part of the School of Public Health (SPH).
A team of researchers led by George Gray, HCRA’s acting director, spent three years studying the disease and its origins. Researchers Joshua Cohen and Silvia Kreindel, along with Massachusetts Institute of Technology doctoral student Keith Duggard, constructed a computer model to simulate the course of the disease should one or more sick animals be introduced to the U.S. herd. They ran several dozen scenarios through the model, generating 1,000 variations for each scenario.
They found that in all cases, the disease fails to take hold and dies out, usually within a matter of a few years. They also found that, even in the worst-case scenarios, the number of additional animals that might become sick would remain small, and the amount of contaminated tissue entering the human food supply and carrying the agent suspected of transmitting Mad Cow disease to humans, and causing variant Creutzfeld Jacob Disease (vCJD), would be minute.
The HCRA model found that U.S. government systems established to control the disease are critical, particularly the prohibition on rendering cattle parts into feed that is given to other cattle. This breaks the cycle by which the disease is believed to spread among animals, and keeps any outbreak in check. The model assumed that compliance with this feed ban is currently incomplete, and all scenarios conservatively assumed it would stay incomplete even after the first case is found. Of lesser but still significant importance in preventing the spread of the disease in animals is the government surveillance to detect BSE, should it show up.
Because the infected tissue in sick animals is concentrated in the brain, spinal cord, and some parts of the central nervous system, the most critical measures to limit risk to human health focus on steps to control how these parts are handled in meat processing.
The research team also investigated the theory that some of the 334 cattle imported from the United Kingdom between 1980 and the inception of a ban on such imports in 1989 might have carried the disease into the American herd. They found that 161 of these animals have been tracked and were disposed of in a way that couldn’t have spread the disease to other animals. The final disposition of the others is not precisely known. But they were breeding cattle, a type of cattle unlikely to get Mad Cow disease. Also, they came from British farms that were free of Mad Cow disease in the year in which those cows were born, and were believed to be disease-free when they died on American farms. Nonetheless, the model ran scenarios that presumed some of these animals brought BSE into America. It found that even if some of them were sick and it wasn’t detected, the number of additional animals infected would be very low and BSE would already be dying out.
The risk analysis of Mad Cow disease in the United States was performed by HCRA’s Program on Food and Agriculture. The Harvard Center for Risk Analysis receives financial support from government, corporate, academic, and private sources.