Got Milk Microbes?

A Guide to Milk-borne Infectious Diseases

Milk being poured into glass, white background. Aaron Graubart/Getty Images

It’s the first food we taste. It’s in pasta sauces, candies, pastries, custards, cheeses, yogurts, and ice cream. Milk is arguably one of the most versatile ingredients among cooks and a staple in most households. However, as an animal product that is full of nutrients, there are several infectious diseases associated with microbe-contaminated milk and milk products. The good news is that most of these microbes are killed by pasteurization, and so in reality, infections due to milk and cheese are pretty rare -- but still possible.

This is why we pasteurize milk.

How Does Cow’s Milk Get Contaminated?

Just like all people carry microbes, all animals do as well. Sometimes the microbes that cows carry can be a problem.

Some dairy cows spend much of their time grazing in pastures, where they come in contact with a variety of environmental microbes. In other cases, cows are confined to buildings, where in more crowded conditions bacteria can grow and spread from cow to cow. In addition, many microbes that are “commensal” organisms (that co-exist with cows without causing disease) may be considered human pathogens.

Dairy processing facilities have many routes for the entry of contaminating microbes. First, as a nutrient-rich liquid, milk provides an ideal environment for microbial growth. Second, dairy processing plants are full of areas where “foot traffic” from employees can be accompanied by microbes.

Infectious Microbes Found in Cow’s Milk

  • Bacillus cereus: These bacteria produce a toxin that can cause diarrhea and another that causes vomiting. Bacillus cereus spores are heat-resistant and may survive pasteurization. There have even been very rare cases linked to dried milk and dried infant formula.
  • Brucella: Brucella is a bacterial microbe that is found in unpasteurized dairy products. Brucella infection, or Brucellosis, has also been called “Undulant Fever” because of the regular recurrence of fever associated with the disease.
  • Campylobacter jejuni: Campylobacter jejuni is the most common bacteria to cause diarrheal disease in the U.S. and is found in raw milk and poultry. It has an increased chance of causing disease when consumed in milk, because the basic pH of milk neutralizes the acidity of the stomach.
  • Coxiella burnetii: Coxiella infects a variety of animals, including livestock and pets. The microbe can be found in cow’s milk and is resistant to heat and drying. Infection by Coxiella results in Q fever, a high fever that may last up to 2 weeks.
  • E. coli O157:H7: This particular strain of E. coli has been associated with a number of food-borne outbreaks and is the cause of bloody diarrhea. Frequently associated with dairy cattle, microbial contamination of raw milk and soft cheeses can result in disease.
  • Listeria monocytogenes: Listeria is a common bacterial pathogen that is found in soft cheeses and unpasteurized milk. It can even survive below freezing temperatures and can therefore withstand refrigeration. It is particularly dangerous to individuals who have weakened immune systems, including pregnant women, AIDS patients, and the very young and very old.
  • Mycobacterium avium subspecies paratuberculosis: This strain of mycobacteria can withstand pasteurization and has been associated with the development of Crohn’s disease, also known as inflammatory bowel syndrome. However, whether or not these bacteria can actually infect humans remains controversial.
  • Mycobacterium: The cause of “consumption,” a horrific wasting disease that first affects the lungs, Mycobacterium bovis is associated with consumption of raw milk and was one of the most common contaminants prior to the practice of pasteurization. It's like the tuberculosis (or TB) we have now, but a different bacteria but related bacteria causes it. This type of TB is something we don't see much of any more because of work to make sure cows don't carry or spread it to us. M. bovis causes tuberculosis in cows and can be passed to humans via unpasteurized cow's milk, causing a disease that is very similar to M. tuberculosis.
  • Salmonella: Salmonella contamination of raw milk and milk products has been the source of several outbreaks in recent years. Symptoms include diarrhea and high fever.
  • Staphylococcus aureus: Staph aureus produces a toxin that causes explosive vomiting. The disease may be considered a consequence of actual food poisoning from simply consuming the toxin, rather than from an actual infection.
  • Yersinia enterocolitis: Associated with raw milk and ice cream, among other foods, contamination is believed to be a consequence of a breakdown in sanitization and sterilization techniques at dairy processing facilities.

    What about Mad Cow Disease?

    Mad Cow Disease, also known as bovine spongiform encephalitis (BSE), is a disease that affects the nervous system and is caused by an infectious protein called a “prion.” Consumption of meat from cattle with BSE can result in transmission of the disease. In humans, the disease is called “transmissible spongiform encephalopathy” or “variant Creutzfeldt-Jakob disease.”

    Fortunately for the dairy industry and milk consumers, the infectious prion has not been found in milk from infected cows, nor has transmission through drinking of cow’s milk been reported. In short, you can’t get Mad Cow Disease from milk.

    How to Prevent Milk-borne Infectious Diseases:

    1. Don’t drink raw milk. Drink only pasteurized milk.
    2. Keep dairy products refrigerated within the expiration date marked on the package.
    3. Be careful when you travel to developing nations, follow the recommended sanitary precautions for the country you are in and do not eat raw dairy products.


    Boor, KJ. Fluid Dairy Product Quality and Safety: Looking to the Future. J. Dairy Sci. 84:1-11 (2001)

    Foodborne Illness. US Food and Drug Administration. Center for Food Safety and Applied Nutrition

    Rowan NJ et al. Putative virulence factor expression by clinical and food isolates of Bacillus spp. after growth in reconstituted infant milk formulae. Appl Environ Microbiol. 2001 Sep;67(9):3873-81.

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