As you may have heard, there was a dairy cow in California that was diagnosed with mad cow disease last week. This was the only case of mad cow disease to be diagnosed in the United States in the last six years. Just the name of this disease makes it sound pretty scary, so I wanted to talk about it for a minute.
Before we get into the nitty gritty of the disease, I know the biggest question on everyone’s mind is about food safety. Can I still eat beef? Can I drink milk? Is it safe?
Because this particular case was found in a dairy cow, let’s start with that. Mad cow disease is not transmitted in milk, or in other dairy products.
The abnormal prion that causes mad cow disease is in the brain and spinal cord tissue. This disease has been linked with a disease in people, variant Creutzfeldt-Jakob disease (vCJD). People can get vCJD from eating the brain or spinal cord from cattle that have mad cow disease, or from eating beef that has been contaminated with the abnormal prion that causes mad cow disease. Ground beef is more likely to be contaminated than steaks or roasts, simply due to the type of handling and processing that goes into making ground beef.
As we’ll talk about later in this article, there are measures in place to help significantly decrease the risk of exposure and transmission of mad cow disease to animals and people.
And now for the quick-and-dirty information about mad cow disease…
Bovine Spongiform Encephalopathy
The scientific name for this disease is bovine spongiform encephalopathy (BSE). It is caused by an abnormal type of protein called a prion (pronounced PREE-on, it stands for proteinaceous infectious particle). Everybody has prions, but occasionally a prion will get a little crazy and become abnormal.
Normal prions live on the surface of cells in the brain and spinal cord. Scientists think that prions are necessary for normal nerve transmission, but they’re still trying to work that out for sure.
No one seems to know why or how the prion changes in the first place. When good prions go bad, whatever the reason, there is a change in their conformation and they look different than the normal prions. The abnormal prions can induce the same conformation change in other normal prions. No one is exactly sure how this change happens, but scientists think it is either due to direct contact between abnormal and normal prions, or the activation of an enzyme that causes the conformation change.
This video gives a great overview of how abnormal prion proteins may accumulate in a nerve cell.
The abnormal prions accumulate inside a nerve cell and cause the cell to die. When the nerve cell dies, it leaves a sort of “hole” in the brain and releases more abnormal prions into the brain. As more cells die, there are more “holes” left in the brain tissue, which is why the disease is called “spongiform encephalopathy.” (Because under a microscope, the brain tissue of an infected cow looks kind of like a sponge.)
Cows with BSE show signs that reflect brain and spinal cord damage. They may have abnormal behavior, have trouble walking, or even not be able to stand up. Cows with BSE may be very aggressive towards people, which led to the nickname “mad cow disease.”
The prion that causes BSE is only in the brain and spinal cord tissue. A cow can get BSE by eating the brain or spinal cord tissue of a cow that was infected with the disease. Farmers used to use cattle and sheep meat and bone meal products to feed to cattle and sheep. (By-products from butchering, sometimes including brain and spinal cord tissue, were processed into a high-protein feed for cattle and sheep.) Once it was learned that this was how BSE was transmitted to other cattle and sheep, this feeding practice was banned.
Originally, in the United States, the feed ban only included ruminants. Now, mammalian proteins can not be fed to ruminants at all, and tissues that may possible contaminated with abnormal proteins can not be included in any livestock feed, pet food, or fertilizers. The feed ban has significantly reduced the number of cases of BSE, which has dramatically reduced the risk of human exposure.
The cow that was diagnosed with BSE in California last week had atypical BSE. This means the abnormal prions in her brain were different from the abnormal prions in typical BSE (the one that is transmitted by eating infected brain or spinal cord tissue). The prions in atypical BSE can be either H-type prions or L-type prions, compared to the C-type prions in typical BSE. Atypical BSE prions have a higher (H-type) or lower (L-type) molecular mass (weight) than the prions in typical BSE.
What is important about the distinction between typical and atypical BSE is that it appears that while typical BSE can be transmitted to other animals, atypical BSE is a sporadic event that is not likely to be transmitted.
For more information about BSE, please check out the Centers for Disease Control and Prevention website.