Press Releases

2001 Releases

Analysis of Potential Mad Cow Risk in US Finds Little Chance of Disease Spread

For immediate release:  November 30, 2001 

Boston, MA – 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 U.S. 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 Harvard School of Public Health.
A team of researchers led by George Gray, HCRA’s Acting Director, spent three years studying the disease and its origins. The researchers investigated the experience in the United Kingdom, where millions of cattle in tainted herds have been put to death, and in other European countries where BSE has been found. They learned about the practices of American animal agriculture and its compliance or non-compliance with government regulations designed to reduce the risk of the disease. 

HCRA researchers Joshua Cohen and Silvia Kreindel, along with MIT doctoral student Keith Duggard, then constructed a computer model to simulate the course of the disease should one or more sick animals be introduced to the United States herd. They ran several dozen scenarios through the model, generating one thousand variations for each scenario. The researchers ran simulations based on the introduction into the American cattle herd of one, five, 10, 20, 50, 200 or 500 sick animals.

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.

Since 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 measure of infectivity used in the model is known as cattle oral ID-50’s, a dose expected to make half the cattle who ingest it sick. The HCRA model finds that at worst, the number of cattle oral ID-50’s that would go into the human food supply would be approximately 170 per year, each year for a maximum of 20 years. To put the risk to Americans in context, the exposure in the U.K. was at least several million cattle oral ID 50’s, leading to only approximately 100 cases of vCJD, so far.  The HCRA model cannot predict the number of human cases that might result in the U.S. from the scenarios they ran, but it is certainly either extremely low, or zero.

The research team also investigated the theory that some of the 334 cattle imported from the U.K. 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 which 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.

The complete report may be downloaded at:


For further information, please contact:

Robin Herman 
Office of Communications 
Harvard School of Public Health 
677 Huntington Avenue 
Boston, MA 02115 
Phone: 617-432-4752 

George Gray
Acting Director
718 Huntington Avenue
Boston, MA 02115
Harvard Center For Risk Analysis
Phone: (617) 432-4341

David Ropeik
HCRA Director of Risk Communication
718 Huntington Avenue
Boston, MA 02115
Phone: (617) 432-6011