Mad Cow Disease in the US
Apparently I misspoke the other day in class in our discussion of transmissible prion based disease, when I said that beef herds in the United States were free of cases of bovine spongiform encephalopathy (BSE), or Mad Cow Disease. A report that is hitting the national news media has indicated that a dairy cow from a farm in California has been confirmed with having the disease.
Officials stress that this appears to be an isolated case, but are continuing to monitor for the presence of other animals to gauge the scope of the situation. Reports like this have a huge impact on both consumer confidence in this country, as well as on international relations. As noted in the Washington Post article in the above link, reports that BSE had been detected in Canadian herds in 2003 resulted in an import ban by the US; at present, Canada is not reciprocating with the current situation with US beef.
Historically, BSE levels in cattle worldwide have been diminishing from a high of over 37,000 cases in 1993, down to only 29 cases last year. Much of this decrease is directly attributable to heightened awareness of the infectious nature of spongiform encephalopathies, and how they can be transmitted into susceptible herds via rendered animal proteins in feedstock.
What epidemiologists may be seeing now is a baseline or sporadic incidence of the disease in domesticated herds. A similar disease of deer called chronic wasting disease is also transmitted via infectious prion particles when they are ingested. As noted on this blog last year, the number of deer exhibiting chronic wasting disease is increasing, however susceptible animals appear to be acquiring the disease during forage. Infectious prion proteins are extremely durable, and when an animal dies of CWD and decomposes, prions may get into the soil at the site, and may be ingested months or years later when another animal forages at the same spot. The durability of bovine prion particles may also explain the sporadic outbreak of disease in that animal.