Nutrition Hotline
By Reed Mangels, PhD, RD
QUESTION: Can BSE, or mad cow disease, be transmitted to humans through dairy products such as cheese and milk?
Via e-mail
ANSWER: It's possible, though the risk is probably much less than with beef.
Bovine spongiform encephalopathy (BSE), also called mad cow disease, is a fatal disease of the brain and nervous system in cattle. It is caused by an infectious agent known as a prion, a type of protein that is neither a bacterium nor a virus and is still poorly understood by scientists. A form of the disease, scrapie, has existed in sheep in England for over 200 years. Creutzfeld-Jakob disease (CJD) is a human version of the disease. In England, BSE was first confirmed in 1985 and became epidemic, eventually affecting half of all herds in that country. Numerous studies have shown that spongiform encephalopathies are transmissible between species. In England, BSE is linked with a new variant of CJD in humans, known as nvCJD.
British beef is now banned in the US, but cattle and beef products were exported to the US until 1989. England continued to export cattle feed containing rendered cattle remains throughout the rest of the world through 1996. According to Richard Lacey, MD, PhD, professor emeritus of clinical microbiology at the University of Leeds, England, and an expert on BSE, it is reasonable to assume that some of the animals or products exported to the US were infected with BSE. Lacey expects to see BSE turn up in the US within the next ten years, the length of time it takes for the disease to show up in cattle, assuming the US is looking hard enough and adequate policies are in place for identifying the disease. The human variant of the disease doesn't become symptomatic for another 20 to 30 years.
Eating meat contaminated with infectious prions is only one way that spongiform encephalopathies are transmitted. Bovine by-products are also used in medical products such as vaccines and other injected medications, blood products, grafts, and organ transplants. Bone marrow from cows is used to make gelatin, which finds its way into candy, marshmallows, and capsules used for vitamin supplements and over the counter medications. It is possible that there are many routes of transmission.
Unfortunately, there is still a great deal that is unknown about the manner in which BSE is transmitted, and it is impossible to quantify the risk to humans. If nonvegetarians choose to eat beef or beef products, they can assume that they are incurring some risk of contracting CJD. Vegetarians who use dairy products are probably at much lower risk than if they ate beef, but it is impossible to quantify the relative risks. According to Dr. Lacey, milk cannot be effectively tested for BSE because the infectious agent in milk cannot be concentrated enough for the experiment to be read. Buying organic milk bears no advantage in terms of avoiding exposure to BSE.
This government website offers more information about BSE: The Centers for Disease Control and Prevention at www.cdc.gov
