Mad Cow Risk and Reasonable Precautions

Reading time: 3 – 4 minutes

A case of so-called “mad cow disease” was found in a California dairy cow last week. The disease, known to veterinary scientists as bovine spongiform encephalopathy (BSE), is fatal, and can be transmitted from cow-to-cow or cow-to-human through the ingestion of contaminated tissue. There is no evidence BSE spreads to humans — the human form of the disorder is called Creutzfeldt-Jakob disease — through drinking milk. As such, authorities claim that the infected animal (which is being held and will be destroyed) posed no risk to humans.

Dairy cows


While a BSE-infected dairy cow is somewhat less distressing than a BSE-infected meat cow, there are nevertheless serious issues raised by this discovery. BSE isn’t caused by a virus or by bacteria. Instead, it’s a prion disease (a prion is a misfolded protein). Proteins are major components of all cells. They’re made up of long chains of smaller molecules, and are folded into specific shapes. The folded shape of a protein determines its function; a misfolded protein acts differently than a properly folded protein. Strangely enough, prions — misfolded proteins — possess the ability to cause normal proteins that they encounter to misfold; this is how they propagate themselves. They are not living, however, and they can’t be “killed.” This is one of the reasons that prions in the food supply are so concerning; unlike bacterial contamination, the threat of which is mitigated by cooking, prions are heat-resistant. A person who eats thoroughly cooked, BSE-contaminated meat can still get Creutzfeldt-Jakob disease.

Prion diseases like BSE can’t be transmitted thorough the air or through animal-to-animal or animal-to-human contact. They are transmitted only through consuming infected animal material. One might reasonably assume, therefore, that a case of BSE in a cow would be an isolated incident, as cows are not normally cannibals. Unfortunately, common cattle husbandry practices in the U.S. do, in fact, produce cow cannibalism. It used to be common for waste products left over from cattle slaughter (the brain and spinal cord, for instance) to be ground up and added back into cattle feed. This practice created optimal conditions for transmission of prion disease. The practice was banned by the FDA in 1997, but it’s currently legal to use cattle products in chicken feed, as it’s not possible to transmit BSE to a chicken. Conversely, it’s currently legal to use chicken litter (the swept-up scraps off the floor of a chicken coop that include feces, feathers, and so forth) in cattle feed. This creates a problem, however; chickens scatter (cow part-containing) chicken feed on the floor, which is swept up and fed back to cows. The vehicle for cow-to-cow BSE transmission is still in place in conventional cattle operations in the United States.

Symptoms of Creutzfeldt-Jakob disease include loss of muscle control, hallucinations, and dementia. Infected individuals do not develop symptoms for many years, but once symptoms occur, death inevitably follows within about 13 months. There is no known treatment for prion disease. Because cooking does not render prion-infected meat safe, the safest route for avoiding infection is to avoid cattle raised on chicken litter-containing feed. Such cattle include those that are exclusively grass-fed and those labeled “certified organic.”

About the Author

Kirstin Hendrickson, Ph.D., is a science journalist and faculty in the Department of Chemistry and Biochemistry at Arizona State University. She has a PhD in Chemistry, and studied mechanisms of damage to DNA during her graduate career. Kirstin also holds degrees in Zoology and Psychology. Currently, both in her teaching and in her writing, she’s interested in methods of communicating about science, and in the reciprocal relationship between science and society. She has written a textbook called Chemistry In The World, which focuses on the ways in which chemistry affects everyday life, and the ways in which humans affect each other and the environment through chemistry.

Comments

  1. Kirstin – can you provide documentation that meat from an infected animal contains the prion? From my understand, the prion is only found in the brain and spinal cord. Also, an atypical case is not linked to feed, therefore it could occur in grass-fed or organic. There’s more information at bseinfo.org.

  2. Slaughtering practices contaminate all of a cow’s meat with contaminants in the brain. With regard to BSE, the organ meat is considered most dangerous, followed by bone meal, followed by muscle cuts. Of course, a dairy cow isn’t slaughtered for human food…but it can be rendered into chicken feed, which is then fed back to cows (including meat beeves).

    You are right that an atypical case (as this was, meaning that the cow got BSE from a random mutation rather than from feed) isn’t feed-linked. However, the practice of feeding cows to chickens and chicken litter to cows allows even the atypical cases to propagate. Had this case not been caught (a great likelihood, given the rarity of testing), the cow’s infected tissue could have made its way into animal feed.

    Bottom line, by eating organic or grass-fed cattle, one avoids the possibility of eating a cow that has eaten cow…and one avoids the vast majority of the risk associated with BSE.