Mad cow disease, or bovine spongiform encephalopathy (BSE), is a type of transmissible spongiform encephalopathy (TSE) that causes brain deterioration and only affects cattle. When food from infected cattle is consumed by humans, it is believed to cause another type of TSE that only affects humans, called variant Creutzfeldt-Jakob disease (vCJD). Like other types of TSEs, BSE and vCJD are characterized by long incubation periods and progressive nerve and brain damage and are always fatal. BSE and vCJD are most common in the United Kingdom but have also occurred in several other countries, including the United States. While BSE in cattle has reached crisis levels in some areas, vCJD as a human infection has remained relatively rare, even where BSE cattle infection is more common.

Scientists believe that BSE arose from the practice of supplementing cattle feed with meat and byproducts from animals infected with BSE or another type of TSE. All types of TSEs are related to infectious prions, which are otherwise healthy proteins that have folded into abnormal shapes and become infectious. It is believed that when a person consumes contaminated beef, the infectious prions are absorbed into the person’s body, where they may create more infectious prions. This process ultimately causes symptoms of vCJD to appear.

Although the prions may be transmitted from cattle to humans, the transmission may not produce infection. Cattle and humans are two different species and infectious prions from one species are less likely to create infectious prions in another species. In addition, not all parts of BSE-infected cattle are equally risky to consume. High-risk tissues include the intestines and areas that contact nervous tissue (e.g., brain, spinal cord, eyes, tonsils). Infectious prions do not appear to be present in milk or muscle meat. However, certain cuts of meat (e.g., t-bone steaks) may have a greater risk of being contaminated by nervous tissue.

When vCJD develops in a person, the first symptoms are psychiatric problems (e.g., depression, anxiety, insomnia). Eventually, neurologic signs (e.g., unpleasant sensations, problems walking, lack of coordination, vision problems) develop. Patients also begin to feel confused, forgetful, and have difficulty thinking and speaking. In later courses of the diseases, patients become unable to speak or move and enter a coma. Like all TSEs, vCJD is ultimately fatal.

The only ways to positively diagnose vCJD is by means of a brain biopsy (an invasive and risky procedure) while a patient is alive or examination of brain tissue during an autopsy. However, a probable diagnosis can be made from the patient’s medical history, signs and symptoms, magnetic resonance imaging (MRI) of the brain and an electroencephalogram (EEG).

There are no known methods to cure BSE or vCJD. Treatment of vCJD is aimed at making the patient feel as comfortable as possible. No methods have been developed to detect the infectious agents of BSE in food. Because contaminated food cannot be made safe, prevention relies on avoiding contaminated meat and meat byproducts. This includes halting the spread of BSE among cattle. The United States, United Kingdom and many other countries have put several strict regulations in place for the purpose of stopping the spread of BSE in cattle and preventing vCJD in humans.