BSE progressively destroys the brains of infected cattle. Cows can catch the illness by eating feed made from the parts of infected cows or certain other animals, such as sheep. There is no treatment and there is a very long incubational period of up to eight years, during which time the infected animals appear healthy. There is no firm proof that mad cow disease can be passed on to humans as Creutzfeldt-Jacob disease (CJD), but other animals eating infected cattle brains develop the disease. New studies suggest BSE may also be hereditary.
BSE first appeared in 1984 in a cow in Britain thought to have eaten feed that included offal from sheep that harboured scrapie, a similar illness.
In Britain more than 120,000 cattle have probably died of BSE since the first case was reported in 1985. The highest incidence of confirmed cases was 3500 per year in the early 1990's. Since then, fewer cases have occurred, due to a 1988 government ban on offal as well as more careful monitoring, but the disease remains endemic in the British herd.
The names of drug manufacturers who produce products from bovine materials have been quashed by the UK inquiry into the handling of the BSE epidemic. Their identity is protected by confidentiality clauses in the Medicines Act. Three products could have been made using potentially dangerous material from infected cattle: a skin test for tuberculosis, a measles vaccine and a vaccine to prevent diphtheria, tetanus and whooping cough.
The livestock industry in the USA voluntarily banned sheep and certain other animal parts from US feed in 1996. The following year, FDA formally banned any proteins from cows, sheep, goats, deer or elk - animals that get similar brain-wasting diseases - from feed for cows, sheep or goats. Poultry or pigs can still eat those proteins, but feed must be labelled "do not feed to cows or other ruminants," and companies must have systems to prevent accidentally mixing up the feeds.