Yes, you can drink milk from a cow with mastitis, but only after it has been pasteurized and only if antibiotic withdrawal periods have elapsed. Raw milk from any cow is unsafe. Milk from a treated cow may contain antibiotic residues that regulators test for. Understanding the biology, the drug rules, and the pasteurization evidence will tell you when that milk is safe and when it is not.
What Mastitis Does Inside the Udder
Mastitis is inflammation of the mammary gland, usually triggered by bacteria entering through the teat canal. The immune response floods the gland with white blood cells called somatic cells. Those cells release enzymes and oxidants that damage milk-secreting tissue. The result is a shift in milk composition that matters for both safety and quality.
Inflammation opens the blood-milk barrier. Proteins leak from blood into milk, including immunoglobulins and the iron-binding protein lactoferrin. Sodium and chloride rise while potassium and lactose fall. The milk tastes salty and its electrical conductivity climbs, which is why inline sensors can flag it. Milk yield drops as secretory cells are destroyed or shut down.
Bacterial species shape severity. Streptococcus uberis and Escherichia coli are common environmental culprits. Staphylococcus aureus often causes chronic, fibrotic infections that resist clearance. Mycoplasma bovis causes contagious outbreaks with little visible swelling. Each organism changes the residue and shedding picture differently.
How Much Somatic Cell Count Rises, and What It Means
A healthy quarter typically sits below 100,000 somatic cells per milliliter. The US regulatory limit for bulk tank milk is 750,000 cells/mL, a threshold that has tightened over decades. An infected quarter can spike past 1,000,000 cells/mL within 24 to 48 hours of infection. Clinical cases may exceed 5,000,000 cells/mL.
Cell count is a proxy, not a verdict. It measures immune cells, not pathogens, and not drugs. High counts correlate with infection risk but cannot confirm which organism is present. Only culture and sensitivity testing answer that question. A cow with 400,000 cells/mL may be infected or merely resolving an old case.
Pasteurization destroys the somatic cells themselves. What survives is the composition change, not the cell count. The saltiness and reduced yield persist after heat treatment. A plain-language version: heat kills the cells, but the altered chemistry stays.
Antibiotic Residues Are the Real Safety Issue
Treating mastitis usually means intramammary antibiotics, often combined with systemic drugs. Those drugs partition into milk and clear at different rates depending on the compound, dose, and route. Regulators set withdrawal periods to keep residues below tolerances such as the FDA's safe levels.
Residue risk is not hypothetical. Bulk tank testing programs exist precisely because treated milk occasionally reaches the pool. Beta-lactam drugs cause allergic reactions in sensitive people, and some carry concerns about antimicrobial resistance. The FDA's Grade A Pasteurized Milk Ordinance sets the testing framework. Pasteurization does not reliably degrade most antibiotic residues.
The counterintuitive detail: a cow can look completely recovered while her milk still fails a residue test. Clinical signs resolve in two to five days, but drug clearance can lag by days or weeks. Visual inspection is worthless here.
Withdrawal Period Comparison by Drug Class
| Drug class | Typical milk withdrawal | Residue test method | Notes |
|---|---|---|---|
| Beta-lactams (penicillin, ceftiofur) | 3 to 8 days | Beta-lactam receptor assays | Most common residue finding |
| Tetracyclines | 4 to 7 days | Charm II or LC-MS | Tissue residues linger longer |
| Macrolides (erythromycin class) | 5 to 10 days | LC-MS | Tissue-bound fraction |
| Sulfonamides | 3 to 5 days | Charm II | Carryover risk from feed |
| Aminoglycosides | 5 to 10 days | LC-MS | Kidney accumulation |
| Untreated clinical case | None (no drug) | Not applicable | Pasteurize before drinking |
Withdrawal periods are label and jurisdiction specific. Consult the product label rather than this table.
Timeline: From Infection to Safe Milk
| Day | Event | Somatic cells | Drug status |
|---|---|---|---|
| 0 | Bacteria enter teat canal | Normal or rising | No drug yet |
| 1 to 2 | Clinical signs, yield drop | 1,000,000 to 5,000,000/mL | Treatment may start |
| 2 to 5 | Clinical resolution | Peak then falling | Within withdrawal window |
| 5 to 14 | Subclinical persistence possible | 200,000 to 1,000,000/mL | Withdrawal ends per label |
| 14 to 30 | Tissue repair, yield recovery | Falling toward baseline | Residue negative |
| 30+ | Chronic cases remain elevated | Above 200,000/mL | Culture needed |
The timeline is illustrative, not a protocol. Chronic S. aureus cases can stay elevated for months. Culling decisions rest on culture results and economics.
Edge Cases: Raw Milk, Chronic Cows, and Discard Milk
Raw milk from a mastitic cow is a genuine hazard. Mastitis pathogens like E. coli O157:H7, Salmonella, and Listeria can be present. The CDC advises against raw milk for anyone, and mastitis raises the odds of pathogen shedding. The raw milk aesthetic argument collapses here.
Discard milk is a separate question. When a farmer pulls milk from a treated cow, is that milk salable? No. It must be withheld from the bulk tank per the label. Drinking it at home is a personal decision, but it bypasses the testing chain.
Chronic infected cows are the hardest case. They may have normal-looking milk, intermittent high cell counts, and bacterial shedding that fluctuates. Culture three times is the usual diagnostic rule to separate chronic from transient. Meanwhile, their milk is still legal to sell if pooled counts stay under thresholds.
Subclinical cases are the quiet majority. Estimates put subclinical mastitis at 15 to 40 percent of quarters in some herds. These cows show no visible signs. Their milk enters the bulk tank every milking.
Common Misconceptions, Corrected
Misconception one: the milk is inherently poisonous. It is not. Hazard comes from residues and pathogens, not mastitis inflammation by itself. Properly withdrawn and pasteurized milk from a recovering cow is not a poison.
Misconception two: pasteurized cow's milk kills the bacteria anyway. Pasteurization reduces pathogens, but it is not sterilization. Spore formers survive. Highly contaminated milk carries residual risk. The industry relies on both a healthy bulk supply and pasteurization.
Misconception three: high somatic cell count means antibiotic in the milk. Wrong. Cells are immune cells. Residue is chemical. The two are measured by completely different tests.
Misconception four: mastitis milk is unsafe for cheese or yogurt. Heat treatment makes the milk safe for processing. The altered composition does affect texture and flavor, but safety is a function of pasteurization and residue status.
Practical Actions for Anyone With a Sick Cow's Milk
For the consumer: visit consumer-facing dairies with clean tank reports. Drink only pasteurized milk. Check any farm you buy from for bulk tank somatic cell count and residue test records.
For the farmer or homesteader, follow this sequence.
First, culture the quarter before treating, so you know the organism. Second, read the label's milk withdrawal period and record the exact date. Third, test milk with a beta-lactam residue strip before any household use. Fourth, pasteurize at 72°C (161°F) for 15 seconds or the equivalent, and refrigerate promptly. Fifth, keep discard milk out of the bulk tank regardless of appearance. Sixth, recheck somatic cell count 30 days after clinical cure.
Red-flag warnings:
Any milk from a systemically ill cow with fever or loss of appetite. Uneven, bloody, or watery quarters during the acute phase. Milk from cows treated off-label or with extra-label drugs. Untested milk entering a household raw-milk supply. Any commercial milk sale during an active withdrawal period.
Threshold reconciliation: the FDA bulk tank limit of 750,000 cells/mL is a herd-level sale standard, not a drinkability measure. A household limit of 200,000 cells/mL for a single quarter is stricter and not regulatory. Withdrawal periods are label law and override any general advice here. When these thresholds conflict, follow the label and your vet's written guidance.
Limitations and the Honest Caveat
Evidence on direct human consumption of mastitic milk is thin. Most data come from commercial processing and residue surveillance, not from controlled drinking studies. The safest course is the one the regulatory system already enforces: withhold during treatment, test before use, and pasteurize.
One professional caveat: veterinarian guidance is farm-specific. An organic operation may have different withdrawal constraints than a conventional one. Detecting residue in a bulk tank is a regulatory event with economic consequences.
The bottom line: mastitis alone does not make milk undrinkable. Drugs and pathogens do. Pasteurize, verify withdrawal, and test.
Sources
US Food and Drug Administration, Grade "A" Pasteurized Milk Ordinance (bulk tank standards and testing). Centers for Disease Control and Prevention, raw milk safety guidance. World Health Organization, food safety and antimicrobial residue information. National Mastitis Council, somatic cell count interpretation resources. USDA Animal and Plant Health Inspection Service, dairy health monitoring.