Aliases: mad cow disease bovine spongiform encephalopathy, BSE
Agriculture isn’t just cornfields; sometimes it’s what that corn gets fed to. In the West, we love meat and dairy, and that means we need cows. Lots of cows. In an effort to maximize production and minimize costs, while keeping the livestock healthy, humans have done some pretty wild things. One of mankind’s more eccentric ideas was to provide protein to cows in the form of meat-and-bone meal made from, among other things, cows. In the 1980s, this dubious practice led to the first recorded outbreak of bovine spongiform encephalopathy (BSE), better known as mad cow disease1.
BSE is a transmissible spongiform encephalopathy or prion disease that results in a neurological disorder in cattle. It likely originated with a different prion disease, scrapie, which afflicts sheep. In the UK sheep were also used in the meat-and-bone meal fed to cows, and it is believed that scrapie-infected feed transmitted the illness to their bovine brethren, where it evolved into mad cow disease. It was then spread through UK herds via contaminated meal. The first cases were detected in 1986 and the epidemic peaked in 1993, with almost 1,000 new cases per week.
There is a (very thin) silver lining to this story. BSE is transmitted exclusively through feed, not cow-to-cow, so once the cause of the outbreak was identified and preventative measures were put in place, the number of cases plummeted. In 1995 there were 14,562 cases in the UK, which dropped to 1,443 by 2000 and 11 in 2010. Now, back to the bad news: BSE took a serious toll. As of 2010, there have been 184,000 cases in the UK altogether (in more than 35,000 herds)1.
To add injury to injury, BSE may have given rise to another prion disease, this time in humans. Variant Creutzfeldt-Jakob disease (vCJD) was discovered in the UK in 1994, within the incubation period of the beginning of the BSE epidemic1. By the end of 1996, there had been 15 cases of vCJD2, with symptoms similar to BSE2. If BSE did cause vCJD, then humans are not entirely innocent in its emergence, but the punishment may exceed the crime. vCJD is a brutal disease; the median duration of illness is 13-14 months, and the median age of death is 281.
Cause: The pathology of mad cow disease is not well understood, but the disease is thought to be caused by a misfolded protein, called a prion. There are 3 strains: typical (the strain that the caused the outbreak in the UK, and is linked with vCJD), and two atypical strains, H and L. The atypical strains are very rare, and are thought to arise spontaneously in cow populations, and not be transmitted1. Speaking of transmission, it appears that BSE, like some other prion diseases is not transmitted cow-to-cow, but through environmental exposure; in this case, eating contaminated meat-and-bone meal2.
Consequence: BSE damages the central nervous system of cattle1, causing lesions in brain that result in behavioral changes (fear or aggression), ataxia (uncoordinated gait, falling, tremors), and dysesthesia (abnormal response to touch and sound)2.
Cure: Because there is no cure for mad cow disease, the best protection against it is prevention: not feeding BSE-contaminated meal to livestock. Fortunately, legislation has been enacted to eliminate contaminated meal from the animal feed system. In the UK, no cow over 30 months of age can be used for human food or animal feed, and as of 2009, most proteins, including tissue potentially infected with BSE (opaquely referred to as “specified risk materials” or SRMs) have been banned from the entire animal feed system, including fertilizers and pet feeds, in the US and Canada. Aside from prevention strategies, herds are kept under surveillance, and sick animals are culled1.
- “BSE”. Centers for Disease Control and Prevention. 21 February 2013. Web. 16 November 2014. http://www.cdc.gov/ncidod/dvrd/bse/
- Nathanson, N, J Wilesmith, & C Griot. (1997). Bovine Spongiform Encephalopathy (BSE): causes and consequences of a common source epidemic. American Journal of Epidemiology, 145(11):959-969.
Image source: Creative Commons, https://www.flickr.com/photos/usdagov/13250283675/