Health authorities found in July that an 11-year-old beef cow in Alabama had bovine spongiform encephalopathy, a degenerative prion disease.
"This animal never entered slaughter channels and at no time presented a risk to the food supply, or to human health in the United States," Department of Agriculture officials said in an announcement.
The cow had an atypical—or spontaneously developing—form of the disease, which differs from the classical form that is contracted through infected feed ingredients, according to the announcement from the USDA Animal and Plant Health Inspection Service. The cow is the fifth known to be infected in the U.S., the first one having been discovered in 2003. That cow had a classical form of BSE, and the rest had atypical forms.
BSE spreads among cattle and to some other animals through consumption of transmissible prion proteins. Consumption of BSE-contaminated materials has been linked with a variant of Creutzfeldt-Jakob disease, a prion disease in humans. But APHIS officials have noted that animal tissues that could contain the BSE agent are prohibited from use in human and animal foods.
Atypical forms of BSE have been identified as L-type—the form found in the Alabama cow—or H-type.
Ryan Maddox, PhD, an epidemiologist in the Prion and Public Health Office of the Centers for Disease Control and Prevention, said L-type atypical BSE has been shown in laboratory conditions to infect both primates and humanized transgenic mice more efficiently than H-type atypical BSE or classical BSE. But he noted that there is no evidence of direct transmission of the L-type form to humans.
Dr. Maddox also said that BSE infections since the 1980s have been associated with disease in about 230 people worldwide, so he would not consider the risk to be extremely high. He also noted that the U.S. has regulations to reduce the risk to humans and animals.
Regulations from the Food and Drug Administration and the USDA prohibit inclusion of mammalian protein in ruminant feed or inclusion of high-risk materials in any animal feed. Examples of materials presenting the highest risk of transmission are the brains and spinal cords of cattle ages 2.5 years and older.
The USDA announcement states that atypical BSE occurs at a low rate in all cattle populations, usually in cattle age 8 years and older. At press time, the World Animal Health Organisation (OIE) had received three other reports of atypical BSE in cattle during 2017, two of them in Spain and one in Ireland.
The OIE lists the United States and Spain among countries considered to have a negligible risk of BSE, and Ireland is considered to have a controlled risk.