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 uncommon—but still possible.
Infections disease prevention is the reason why we pasteurize milk. You can learn about the methods and myths regarding pasteurization if you’re wondering if it’s really necessary after reading about the risks below.
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, wherein more crowded conditions the bacteria can grow and spread from cow to cow. In addition, many microbes that are “commensal” organisms (organisms that co-exist with cows without causing disease) may be considered human pathogens (they can cause infection in humans).
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
There are a wide variety of microbes that can be found in cow’s milk as well as milk products. The risk of many of these, but not all, is reduced by pasteurization. Some products can vary widely in their risk as well. For example, many soft imported cheeses (such as Brie) are not pasteurized and carry a much higher risk of infection (especially for pregnant women) than do hard and pasteurized cheeses. Let’s look at some of the specific infections that are associated with milk.
Bacillus Cereus Infections
Bacillus cereus is a bacteria which produce toxins. One type of toxin can cause diarrhea while another causes vomiting. Bacillus cereus spores are heat-resistant and may survive pasteurization. There have even been very rare cases linked to dried infant formula.
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. It is one of the possible causes of a prolonged fever of unknown origin in children.
Campylobacter jejuni Infections
Campylobacter jejuni is the most common bacteria to cause diarrheal disease in the U.S. infecting roughly 2.4 million people each year. The bacteria is found in raw milk and poultry and may cause bloody diarrhea along with cramping abdominal pain beginning two to five days after exposure. Campylobacter has an increased chance of causing disease when consumed in milk, because the basic pH of milk neutralizes the acidity of the stomach, allowing the bacteria to survive.
Coxiella Burnetii Infections
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 two weeks. Like Brucella, it may be a cause of an unknown prolonged fever in children.
E. Coli O157: H7 Infections
The E. coli O157: an H7 strain of E. coli has been associated with a number of food-borne outbreaks and is often a cause of bloody diarrhea (hemorrhagic colitis.) Frequently associated with dairy cattle, microbial contamination of raw milk and soft cheeses can result in disease. This bacteria may also cause hemolytic uremic syndrome (hamburger disease), which is marked by a low platelet count (thrombocytopenia), and may lead to bleeding and kidney failure.
Listeria monocytogenes is a common bacterial pathogen that is found in soft cheeses (especially imported 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, people with AIDS, and the very young and very old. Listeria is one of the infections known to cause miscarriage, and those who are pregnant are roughly 10 times more likely to acquire the infection.
Mycobacterium Avium Subspecies Paratuberculosis Infections
Mycobacterium avium subspecies paratuberculosis is a strain of mycobacteria that can withstand pasteurization and has been associated with the development of Crohn’s disease, a type of inflammatory bowel disease. It’s still not known if these bacteria can actually infect humans and the exact association of Mycobacterium avium paratuberculosis and Crohn’s disease remains controversial.
Mycobacterium Bovis Infections
Mycobacterium, the cause of “consumption,” is 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 is a different strain of the bacteria. Efforts to reduce the chance of cows carrying or spreading this type of TB is a reason we no longer see this disease often. M. bovis causes tuberculosis in cows and can be passed to humans via unpasteurized cow’s milk, resulting in a disease that is very similar to M. tuberculosis.
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 Infections
Staphylococcus aureus produces a toxin that causes explosive vomiting and is a common “potluck” cause of food poisoning. Food poisoning from Staphyloccous aureus is not caused by an infection with the bacteria, but rather the bacteria release toxins into food which is left out at room temperature. Upon heating, the bacteria are killed, but the toxin, being heat resistant, persists.
Yersinia Enterocolitis Infections
Yersinia enterocolitis infections are associated with eating 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
It can be frightening to learn of the different infections which can be transmitted with milk, but a few simple practices can greatly reduce your chance of contracting these infections:
- Don’t drink raw milk. Drink only pasteurized milk and other dairy products.
- Think twice and read labels when you shop “organic.” Many organic food stores sell unpasteurized dairy products.
- Beware of soft cheeses. Some of these, especially those which are imported, are unpasteurized. Since infections such as Listeria usually cause only mild illness in a mother, they often go unrecognized as a cause of miscarriage.
- Keep dairy products refrigerated within the expiration date marked on the package.
- Do not leave any foods, especially those which contain dairy products, outside of the refrigerator for more than two hours (and ideally, less.) Keep in the mind that bacterial toxins may persist despite reheating even if the bacteria themselves are killed.
- 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.
- Milk and unpasteurized dairy products are not the only sources of food poisoning. Food poisoning is likely much more common than most people think, considering most cases of “stomach flu” in adults are really food poisoning.