Mad cow disease: What you should know about BSE
What is BSE?
Bovine spongiform encephalopathy (BSE), commonly known as mad cow disease, is a fatal brain condition in cattle. It causes the brain to deteriorate and develop a spongy texture when examined under a microscope.
BSE is not caused by bacteria or viruses, but by prions—abnormal proteins that cause other proteins in the brain to misfold. Over time, this damages the nervous system, leading to behavior changes, coordination issues, and eventually death. There is no treatment or cure.
How cows get BSE
There are two forms of BSE:
- Classical BSE: This form was historically spread when cattle were fed meat and bone meal made from other infected cattle. The U.S. banned this practice in 1997 and introduced even stricter rules in 2008.
- Atypical BSE: This extremely rare form appears to develop spontaneously in older cattle. It is not linked to feed and does not spread between animals.
Both forms affect the brain, but only classical BSE has been linked to feed-based transmission. Atypical cases are considered naturally occurring and isolated.
Who and what can get BSE
BSE affects older cattle, typically over 30 months of age. Because the disease develops slowly, younger animals do not show symptoms.
BSE is not contagious. It does not spread through contact between cows or from the environment.
Humans can contract a related illness called variant Creutzfeldt-Jakob disease (vCJD) by eating beef products contaminated with the BSE prion. This occurred in a small number of cases in the United Kingdom in the 1990s. In the U.S., food safety practices have successfully prevented any such cases.
What the signs look like
Signs of BSE may include:
- Nervousness or aggression
- Difficulty walking or standing
- Shaky or jerky movements
- Increased sensitivity to sound or touch
- Weight loss despite normal appetite
- Odd behaviors, such as repeated nose licking
Symptoms usually take 3 to 6 years to appear after infection. Once they begin, the animal’s health rapidly worsens and death typically occurs within 2 weeks to 6 months.
There is no test for BSE in live animals. Diagnosis requires brain tissue testing after death. The USDA operates a national surveillance program to test older or high-risk cattle for the disease.
Where BSE has been found
BSE was first identified in the United Kingdom in 1986, where it caused a major outbreak. Since then, cases have been reported in Canada, France, Germany, Japan, Brazil, and other countries.
In the United States, there have been seven confirmed cases since 2003:
- One classical case in Washington state in 2003, involving a Canadian-born cow
- Six atypical cases in Texas, Alabama, California, and South Carolina
The most recent case, an atypical one, occurred in South Carolina in 2023. All U.S. cases have been handled quickly and effectively, with no impact on the beef supply.
Worldwide, atypical BSE remains very rare, with fewer than 150 cases recorded between 2001 and 2023. The most recent classical case globally was found in Scotland in 2024.
How BSE is prevented
BSE is extremely rare in the U.S. because of strong preventive measures, including:
- Feed bans: Since 1997, it has been illegal to feed cattle any material that might carry the BSE prion. In 2008, additional restrictions were introduced.
- Tissue removal: High-risk tissues, such as the brain and spinal cord, are removed from slaughtered cattle to keep them out of the food supply.
- "No downer" rule: Cattle that cannot walk are not allowed to enter the food chain.
- Ongoing testing: The USDA routinely tests cattle, especially older or sick animals, to detect and prevent BSE from spreading.
The U.S. also bans the use of specified risk materials (such as brain and spinal cord tissue from animals over 30 months of age) in both animal feed and food products. These are key measures to prevent transmission.
Atypical BSE is not associated with feed and is believed to occur spontaneously in all cattle populations at a very low rate.
Is U.S. beef safe?
Yes. The beef supply in the U.S. is safe. Strong federal regulations, careful monitoring, and fast response to any potential threat help ensure that the risk of BSE is extremely low.
What the public should know
- BSE is not currently a threat to the U.S. beef supply.
- Decades of regulation and surveillance keep cattle and consumers safe.
- American beef remains safe to eat.
What vets and producers should know
- Veterinarians should report any older cattle showing unexplained neurological signs.
- Producers must comply with feed bans and maintain strong biosecurity practices.
- Preventing BSE protects public health and supports international trade in U.S. beef.
Final takeaway
BSE is a serious but rare and well-controlled disease. Thanks to decades of research, surveillance, and regulation, the U.S. continues to produce safe, high-quality beef. Staying informed and following science-based practices is essential to keeping it that way.