Image Credit: This image has been modified.

Mad Cow California: Is the Milk Supply Safe?

In announcing the detection of a new case of mad cow disease in California, the USDA emphasized that her carcass never made it into the food supply. The fact that the infected animal was a dairy cow, though, raises the question about the disposition of her milk. There’s currently no direct evidence that cow’s milk poses any mad cow risk, but when it comes to this mysterious class of diseases, we’ve learned time and time again that absence of evidence of risk, does not equal evidence of absence of risk.

The USDA flatly declared that milk doesn’t transmit mad cow disease, but that’s what we used to think about scrapie, the parallel disease in sheep, before research in the last 5 years unequivocally demonstrated that milk could indeed become contaminated with prions, the infectious proteins that cause these transmissible sponge-like (spongiform) brain diseases (encephalopathies). Similar experiments have not detected infectivity in mad cow milk, but this may be a function of the limited sensitivity of testing methods. And we have no such data on the atypical strain of mad cow disease found in California, which research suggests is more “lymphotrophic,” meaning more likely to invade the lymph system, which could mean mean milk is more likely to become infected.

If prions are present at low levels in bovine milk, then this could present a particular problem in the United States, since inflammation has been shown to dramatically boost the infectivity of prion-infected milk and there is an epidemic of mastitis in the U.S. dairy herd. Last year researchers demonstrated that the ingestion of as little as 1 to 2 quarts of milk from scrapie-infected sheep stricken with mastitis could cause prion infection in lambs at an attack rate of 86%. This is thought to be because the prions may concentrate in the blood and pus cells sloughed into milk from inflamed udders.

Today’s dairy cows endure annual cycles of artificial insemination, pregnancy and birth, and mechanized milking for 10 out of 12 months (including 7 months of their 9-month pregnancies). This excessive metabolic drain overburdens the cows, who are considered “productive” for only two years and are slaughtered for hamburger when their profitability drops, typically around their fourth birthday, a small fraction of their natural lifespan. Turning dairy cows into milk machines has led to epidemics of so-called “production-related diseases,” such as lameness and mastitis, the two leading causes of dairy cow pre-slaughter mortality in the United States.

Because 1 in 6 dairy cows in the United States suffers from mastitis, the dairy industry continues to demand that American milk retain among the highest allowable “somatic cell” concentration in the world. When a cow’s udder is inflamed, greater than 90% of the somatic cells in her milk are neutrophils, the inflammatory immune cells that form pus. As the dairy industry points out, the accumulation of pus is a natural part of an animal’s defense system. So pus itself isn’t a bad thing, we just may not want to drink it, especially if it can increase the risk of transmission of cellular pathogens that can survive pasteurization such as paratuberculosis and, in theory at least, prions.

-Michael Greger, M.D.

See also:


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous "meat defamation" trial.

6 responses to “Mad Cow California: Is the Milk Supply Safe?

Comment Etiquette

On, you'll find a vibrant community of nutrition enthusiasts, health professionals, and many knowledgeable users seeking to discover the healthiest diet to eat for themselves and their families. As always, our goal is to foster conversations that are insightful, engaging, and most of all, helpful – from the nutrition beginners to the experts in our community.

To do this we need your help, so here are some basic guidelines to get you started.

The Short List

To help maintain and foster a welcoming atmosphere in our comments, please refrain from rude comments, name-calling, and responding to posts that break the rules (see our full Community Guidelines for more details). We will remove any posts in violation of our rules when we see it, which will, unfortunately, include any nicer comments that may have been made in response.

Be respectful and help out our staff and volunteer health supporters by actively not replying to comments that are breaking the rules. Instead, please flag or report them by submitting a ticket to our help desk. is made up of an incredible staff and many dedicated volunteers that work hard to ensure that the comments section runs smoothly and we spend a great deal of time reading comments from our community members.

Have a correction or suggestion for video or blog? Please contact us to let us know. Submitting a correction this way will result in a quicker fix than commenting on a thread with a suggestion or correction.

View the Full Community Guidelines

  1. It sickens me to read a post like this. I hate knowing what we do to cows. And as this post points out, even if someone happened to have a pathological desire to see cows suffer, there is also self-interest to point out. What is bad for the cows is also bad for the humans. (sigh)

    Thank you for putting these paragraphs together. I love having the little videos put into perspective like this. It not only helps me, but it is something I can share with others.

  2. I am going to link this Blog in tomorrows Video to get the word out!
    People should have to see and read this info weekly so they don’t forget and make better food choices that not only help themselves but help the abused animals and expose the dairy industry practices.

Leave a Reply

Your email address will not be published. Required fields are marked *

Pin It on Pinterest

Share This