No, you should not drink milk from a cow with mastitis. The milk can contain dangerous bacteria, bacterial toxins that survive boiling, and inflammatory byproducts that pose both short-term and long-term health risks. Commercial dairy operations are required to discard milk from cows with mastitis, and it never legally enters the supply chain if somatic cell counts exceed regulatory limits. The real risk falls on people drinking raw milk from a home or small-farm cow with an active or undetected udder infection.
What Mastitis Does to Milk
Mastitis is an infection of the cow’s udder, usually caused by bacteria like Staphylococcus aureus, E. coli, or Streptococcus species, though fungi can also be responsible. The infection floods the milk with white blood cells (somatic cells), bacteria, and inflammatory compounds. It also changes the milk’s basic composition: fat, protein, lactose, and casein levels all drop significantly. In one study, healthy cows produced milk with 4.30% fat and 3.39% protein, while cows with mastitis dropped to 4.08% fat and 3.35% protein, with even steeper losses in key nutritional proteins like casein.
The milk often looks and tastes different, too. Clinical mastitis can produce visible clots, watery consistency, or a yellowish color. Even in milder cases, the milk becomes noticeably saltier due to increased sodium content, and it develops a stronger savory (umami) taste. These changes reflect the breakdown happening inside the udder as the cow’s immune system fights the infection.
Bacteria and Toxins in Mastitis Milk
The most concerning pathogen in mastitis milk is Staphylococcus aureus, one of the most common causes of subclinical mastitis and a major foodborne pathogen. In Europe, milk and dairy products account for 5% of all food-poisoning outbreaks linked to staph bacteria. But S. aureus isn’t the only threat. Mastitis milk can also harbor Salmonella, Listeria monocytogenes, E. coli, and Campylobacter, all of which cause serious gastrointestinal illness and can be life-threatening for young children, elderly people, and anyone with a weakened immune system.
What makes this particularly dangerous is that some of these risks don’t go away with heat. S. aureus produces enterotoxins, essentially bacterial poisons, that are extraordinarily heat-stable. The bacteria themselves die during pasteurization, but enterotoxin A can remain active even after being heated to 121°C (250°F) for 28 minutes. That’s well above the temperature of boiling water and far beyond what standard pasteurization achieves. So if the bacteria have had time to produce toxins in the milk before it’s heated, boiling won’t make it safe. Spores from other pathogens like Bacillus cereus and Clostridium species also survive pasteurization temperatures.
Longer-Term Health Concerns
Beyond acute food poisoning, mastitis milk contains compounds linked to more serious long-term effects. The inflammatory response in the udder increases levels of nitrates, nitrites, and nitrosamines in the milk. Nitrosamines are classified as probable human carcinogens by the International Agency for Research on Cancer.
There’s also an emerging concern about amyloid proteins. Mastitis alters casein and related proteins in ways that may promote their transformation into amyloid fibrils, structures similar to those found in Alzheimer’s disease and a condition called AA amyloidosis. Research has shown these amyloid fibrils can be transmitted orally, cross the gut barrier, and trigger disease. This area of research is still developing, but it adds another reason to avoid consuming milk from infected animals over time.
Why Pasteurization Isn’t a Full Safety Net
Many people assume that if milk is pasteurized, any contamination from mastitis becomes irrelevant. Standard pasteurization (72°C for 15 seconds) kills most live bacteria, but it does not destroy heat-stable enterotoxins already present in the milk. It also doesn’t eliminate spores from certain dangerous bacteria. And there’s ongoing debate about whether pasteurization fully eliminates Mycobacterium avium subspecies paratuberculosis, a pathogen sometimes found in dairy herds.
This is why commercial dairies don’t simply pasteurize mastitis milk and sell it. They discard it entirely. The European Union deems milk unfit for human consumption when somatic cell counts exceed 400,000 cells per milliliter, a threshold that flags subclinical mastitis even when the cow shows no visible symptoms.
Raw Milk From a Home Cow: The Highest Risk
If you’re milking your own cow or buying raw milk from a small farm, the stakes are much higher. There’s no pasteurization step at all, so every bacterium in the milk reaches you alive. Subclinical mastitis is particularly risky here because the cow may look perfectly healthy and the milk may appear normal. The infection can be present for weeks without obvious signs, silently shedding bacteria into every milking.
If you suspect mastitis, visible signs in clinical cases include swelling, heat, or hardness in one or more quarters of the udder, along with milk that looks watery, clumpy, or discolored. But subclinical cases require testing. A simple cowside test called the California Mastitis Test can detect elevated somatic cells in minutes and costs very little. Regular testing is the only reliable way to catch infections before they become a food safety problem.
What Happens to Milk After Mastitis Treatment
Cows with mastitis are typically treated with antibiotics, which creates a second food safety issue: drug residues in milk. After treatment, there’s a mandatory withdrawal period before the milk can be used again. Research on two of the most common antibiotics shows that one clears to safe levels within about 72 hours (3 days), while the other may take closer to 5 days. In the United States, official withdrawal periods for some formulations are set as short as 24 to 60 hours, but studies have found that certain antibiotics may still be present at or near regulatory limits at those timeframes.
For home dairy producers, this means you should not drink milk from a treated cow until the full withdrawal period for the specific antibiotic has passed. Your veterinarian can tell you exactly how long to wait based on the drug used. Some farms use test strips to confirm the milk is free of residues before returning it to consumption.
Mastitis Milk and Cheese Making
Even if you’re not drinking the milk straight, using mastitis milk for cheese is a bad idea for both safety and quality reasons. The altered protein profile, specifically the lower casein-to-protein ratio, makes the milk curdle poorly. It takes longer to form a curd, the curd is weaker, and it doesn’t drain whey effectively. The resulting cheese has higher moisture content, lower fat and protein, and a shorter shelf life. High somatic cell counts in bulk tank milk are one of the strongest predictors of poor cheese quality, which is why cheesemakers monitor cell counts closely.
The safety concerns carry over as well. Bacterial toxins concentrate rather than dilute during cheesemaking, and the aging process does not destroy heat-stable enterotoxins.