Mastitis in Cows: Infectious Causes, Symptoms, and Treatment
- Mastitis is inflammation of the udder, most often caused by bacteria entering the teat canal. Common infectious causes include Staphylococcus aureus, Streptococcus agalactiae, environmental streptococci, coliforms such as E. coli and Klebsiella, and sometimes Mycoplasma.
- Signs can range from mild clots or watery milk in one quarter to a hot, swollen, painful udder with fever, poor appetite, dehydration, or shock in severe cases.
- Subclinical mastitis may not change the udder or milk visibly. It is often found through a high somatic cell count or a positive California Mastitis Test.
- Prompt veterinary guidance matters because treatment depends on severity, likely pathogen, milk culture results, lactation stage, and milk-withdrawal rules for any medications used.
- Typical 2025-2026 US direct treatment cost range is about $40-$120 for a mild on-farm case, $120-$350 for a standard treated case including drugs and discarded milk, and $300-$1,000+ for severe cases needing intensive care, IV fluids, repeated exams, or hospitalization.
What Is Mastitis in Cows?
Mastitis is inflammation of the mammary gland and udder tissue. In cows, it is usually linked to infection, although trauma and teat-end damage can also contribute. The infection may affect one quarter or multiple quarters, and it can be clinical or subclinical.
Clinical mastitis means you can see changes, such as abnormal milk, swelling, heat, pain, or redness in the udder. Subclinical mastitis is quieter. Milk may look normal, but the quarter is still inflamed, milk production drops, and somatic cell count rises. That makes subclinical disease easy to miss without routine monitoring.
Infectious mastitis matters for both cow health and herd performance. It can reduce milk yield, worsen milk quality, increase culling risk, and in severe gram-negative cases can make a cow systemically ill very quickly. Early recognition and a treatment plan from your vet can improve comfort, protect the rest of the herd, and support more careful antibiotic use.
Symptoms of Mastitis in Cows
- Clots, flakes, or stringy material in milk
- Watery, thin, or discolored milk
- Hot, swollen, firm, or painful quarter
- Redness of the udder skin or teat
- Drop in milk production from one quarter or the whole cow
- High somatic cell count or positive California Mastitis Test with normal-looking milk
- Fever, poor appetite, depression, or dehydration
- Weakness, cold ears, sunken eyes, diarrhea, or signs of shock
See your vet immediately if a cow with suspected mastitis also has fever, weakness, dehydration, reduced rumen fill, or signs of shock. Severe coliform mastitis can become life-threatening fast. Even milder cases deserve timely attention because the best treatment choice depends on whether the problem is likely contagious, environmental, culture-negative, or caused by an organism that responds poorly to antibiotics.
What Causes Mastitis in Cows?
Most infectious mastitis starts when bacteria enter through the teat canal. Some pathogens spread mainly during milking from cow to cow. These contagious causes include Staphylococcus aureus, Streptococcus agalactiae, and Mycoplasma bovis. They often persist in the udder and can be harder to clear, especially S. aureus and Mycoplasma.
Other cases come from the cow's environment. These environmental pathogens include coliforms such as E. coli and Klebsiella, plus environmental streptococci. Bedding is a major source, but mud, manure, contaminated water, poor udder prep, teat-end injury, flies, and contaminated intramammary products or equipment can also play a role.
Risk rises when teat ends are damaged, housing is wet or dirty, milking hygiene slips, liners are not functioning well, or dry cows and fresh cows are not kept clean and dry. Some unusual causes, including yeast, Prototheca, and Trueperella pyogenes, can also occur. Those cases may respond poorly to routine treatment, which is one reason milk culture and herd-level investigation are so useful.
How Is Mastitis in Cows Diagnosed?
Diagnosis starts with history and a careful udder exam. Your vet will look at which quarter is affected, how abnormal the milk is, whether the cow has local signs only or whole-body illness, and whether this is a first case, repeat case, or herd-wide pattern. Severity matters because mild, moderate, and severe mastitis are managed differently.
Milk testing is central. Common tools include foremilk examination, the California Mastitis Test, somatic cell count data, and milk culture. In cows, a somatic cell count of 200,000 cells/mL or higher strongly suggests intramammary infection, while herd goals are usually lower than that. For clinical cases, milk should ideally be sampled before treatment so your vet can decide whether antibiotics are likely to help and whether the herd problem looks contagious or environmental.
In more serious cases, your vet may also recommend bloodwork, hydration assessment, and herd-level review of records, bulk tank culture, and recent case patterns. That bigger picture helps identify whether the farm is dealing with fresh-cow disease, dry-period infections, milking-time spread, or chronic high-SCC cows that may need segregation or culling.
Treatment Options for Mastitis in Cows
Spectrum of Care means you have options. Here are treatment tiers at different price points.
Budget-Conscious Care
- Prompt exam or herd-protocol review with your vet
- Milk sample collected before treatment when possible
- Frequent stripping-out or milking of the affected quarter if your vet advises it
- Anti-inflammatory care when appropriate and legal for the animal's class and stage
- Targeted intramammary treatment only when signs, culture, and herd protocol support it
- Clear milk-withdrawal instructions and monitoring plan
Recommended Standard Treatment
- Veterinary exam and severity scoring
- Milk culture or submission to a diagnostic lab
- Intramammary antibiotics when indicated by your vet and label use
- Systemic anti-inflammatory treatment and supportive care as needed
- Recheck of milk appearance, udder comfort, appetite, hydration, and temperature
- Review of milking routine, bedding, and fresh-cow or dry-cow risk factors
Advanced / Critical Care
- Urgent veterinary assessment for toxic or down cows
- IV or oral fluids, aggressive supportive care, and close monitoring
- Systemic medications based on severity, lactation status, and food-animal regulations
- Repeated milk and herd diagnostics, including culture for unusual pathogens
- Segregation, milk-last protocols, or culling discussions for chronic contagious cases
- Hospitalization or intensive on-farm care when needed
Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.
Questions to Ask Your Vet About Mastitis in Cows
Bring these questions to your vet appointment to get the most out of your visit.
- Does this look like mild, moderate, or severe mastitis, and what signs would make it an emergency?
- Should we collect a milk sample for culture before starting treatment?
- Is this case more likely contagious or environmental based on our herd history and this cow's stage of lactation?
- Which treatment options fit this cow, and what are the expected tradeoffs for each?
- What milk-withdrawal and meat-withdrawal times apply to the medications you recommend?
- If this quarter does not respond, when should we recheck, change plans, dry off the quarter, or consider culling?
- Should we culture recent cases or bulk tank milk to look for Staph aureus, Strep agalactiae, or Mycoplasma?
- What prevention steps on our farm would likely lower new mastitis cases the most over the next month?
How to Prevent Mastitis in Cows
Prevention works best when it focuses on both the cow and the environment. Keep stalls, bedding, and loafing areas clean and dry. Pay close attention to dry cows, close-up cows, and fresh cows, because many new infections begin during the dry period or around calving. Good teat-end condition also matters, so milking equipment should be functioning correctly and liner slips should be minimized.
Milking routine is a major control point. Clean, dry teat prep before unit attachment and complete post-milking teat dip coverage help reduce spread. Gloves during milking, milking infected cows last when needed, and screening purchased animals can be especially important when contagious pathogens are a concern.
Routine monitoring helps catch problems before they become herd-wide. Work with your vet to track clinical case rate, somatic cell counts, repeat offenders, and fresh-cow patterns. Cows with SCCs over 200,000 cells/mL for multiple months, chronic clinical cases, or poor response to treatment may need culture, segregation, dry-off planning, or culling discussions. Some herds also use internal teat sealants at dry off and coliform vaccination programs to reduce the severity of environmental mastitis. The best prevention plan is the one that fits your herd's facilities, labor, and disease pattern.