BSE, which is not contagious, exists in two forms: classical and atypical. The incubation period for classical BSE from time of infection, thought to occur early in life, until the onset of clinical signs averages three to six years. The primary source of infection for classical BSE is feed contaminated with the infectious prion agent, such as meat-and-bone meal containing protein derived from rendered infected cattle. Regulations from the Food and Drug Administration (FDA) have prohibited the inclusion of mammalian protein in feed for cattle and other ruminants since 1997 and have also prohibited high risk tissue materials in all animal feed since 2009.
Atypical BSE occurs in older cattle, usually 8 years of age or greater, and does not appear to be associated with contaminated feed. Like classic or sporadic CJD in humans, it seems to arise rarely and spontaneously.
BSE was first diagnosed in 1986 in the United Kingdom, which has had the vast majority of cases worldwide. However, the disease has been detected in many other countries, including four cases in the United States from 2003 to 2012. Of the four U.S. cases, the first was a case of classical BSE that was imported from Canada; the rest have been atypical BSE.
BSE presents a public health concern because occurrences of variant CJD in humans have been linked to the consumption of food containing ingredients derived from BSE-infected cattle. BSE has also caused substantial losses in international trade for the U.S. beef industry.
The World Organization for Animal Health (OIE) assigns each country a risk status for BSE based on the country’s history with the disease, the implementation and enforcement of their feed bans and their BSE surveillance. In 2013, the U.S. status for BSE was upgraded to negligible risk, the highest status available. In 2015, the OIE determined that atypical BSE occurred spontaneously at a low rate in all cattle populations and would be excluded for BSE risk.