Cattle Health · Dairy Disease

Mastitis in Cattle: Symptoms, Treatment & Prevention (Complete Guide)

Updated: Feb 2027 · 12-minute read · Reviewed by a licensed dairy veterinarian

What Is Mastitis?

Mastitis is inflammation of the mammary gland (udder), almost always caused by bacterial infection. Bacteria enter the udder through the teat canal — either during milking, in the barn between milkings, or during the dry period — and multiply in the milk-producing tissue. The cow's immune response causes swelling, milk changes, and in severe cases fever and systemic illness.

Mastitis affects up to one in three cows in poorly managed herds and costs the U.S. dairy industry roughly $2 billion annually through discarded milk, treatment costs, reduced production, culling losses, and bulk tank premium losses when herd SCC rises. Even well-managed herds see 15–25 clinical cases per 100 cows per year — the question isn't whether mastitis will occur, but how to keep it below thresholds that damage productivity.

Clinical vs Subclinical Mastitis

Subclinical mastitis is the hidden version — the cow shows no visible signs, milk looks normal, but somatic cell count (SCC) is elevated because white blood cells are fighting an infection you can't see. Subclinical outnumbers clinical mastitis by 15–40 to 1 in typical herds and causes the majority of production and bulk tank premium losses. You detect it only through SCC monitoring or the California Mastitis Test (CMT).

Causes: Contagious vs Environmental

The bacteria that cause mastitis fall into two clear groups based on how they spread. Understanding which group is dominant in your herd shapes both treatment and prevention.

Diagnosis: CMT & SCC

The California Mastitis Test (CMT)

The CMT is a cheap, cowside test that detects subclinical mastitis in seconds. A few squirts of milk from each quarter are mixed with a purple reagent on a paddle. Healthy milk stays fluid; infected milk with high somatic cell counts thickens and gels. CMT paddles and reagent cost under $50 and last for hundreds of tests — this is the single most valuable tool a small dairy can own.

Somatic cell count (SCC)

Individual cow SCC is measured by DHIA testing (monthly) or from bulk tank samples (every load). Interpretation:

Treatment Protocol

Grade 1 (mild) — abnormal milk only

Culture the affected quarter. If Gram-positive, treat with an intramammary antibiotic tube (Today, ToDAY, Pirsue, Spectramast LC) after each milking for the labeled duration (typically 2–8 days). If Gram-negative or no growth, most cases self-cure without antibiotics — monitor and re-culture.

Grade 2 (moderate) — abnormal milk plus swelling

Intramammary antibiotic tubes plus systemic anti-inflammatory (flunixin meglumine at label dose) to reduce inflammation and pain. Strip the affected quarter frequently (4–6 times daily) to remove infected milk and bacterial toxins.

Grade 3 (severe / toxic) — systemic illness

This is a veterinary emergency. Treatment includes IV fluids to combat dehydration and endotoxic shock, systemic antibiotics (ceftiofur, oxytetracycline), intramammary treatment, aggressive anti-inflammatory therapy, frequent stripping, and supportive nursing care. Even with treatment, toxic mastitis mortality is 20–40%.

Dry-Cow Therapy

The dry period (60 days between lactations) is both the most vulnerable time for new infections and the best opportunity to cure existing ones. Dry-cow antibiotic therapy at dry-off cures roughly 70–90% of existing subclinical infections and dramatically reduces new infections during the dry period.

Blanket vs selective dry-cow therapy

Blanket dry-cow therapy treats every quarter of every cow with a long-acting intramammary antibiotic (Tomorrow, Spectramast DC, Quartermaster) at dry-off. Selective dry-cow therapy (SDCT) treats only cows or quarters with a history of infection or elevated SCC. SDCT reduces antibiotic use by 30–60% while maintaining udder health, and it's now standard in most European countries and increasingly common in North America. Both approaches should be combined with an internal teat sealant (Orbeseal) to prevent new infections during the dry period.

Prevention Program

The five-point plan developed by the National Mastitis Council is the foundation of modern mastitis prevention. Herds that follow it consistently keep bulk tank SCC below 150,000 and clinical cases below 20 per 100 cows per year.

1. Post-milking teat dip — every teat, every milking, no exceptions
2. Prompt treatment of clinical cases — culture-guided, follow written protocol
3. Dry-cow therapy — antibiotic + teat sealant at dry-off
4. Cull chronic cases — cows with 3+ clinical cases or persistent high SCC
5. Regular milking equipment maintenance — vacuum, pulsators, liners

Milking-time hygiene

• Pre-dip every teat with an approved germicidal dip; contact time 30 seconds minimum
• Forestrip 3 squirts from each quarter into a strip cup — checks for clinical cases
• Dry teats with individual paper towels or single-use cloth wipes
• Attach the unit within 90 seconds of pre-stimulation for proper milk letdown
• Post-dip immediately after unit removal, covering 100% of the teat surface

See our Dairy Hygiene Protocol and Milking Equipment Maintenance guides.

Economic Cost

Each clinical mastitis case costs $250–$450 on average, broken down roughly as:

• Discarded milk during and after treatment: $100–$200
• Reduced production for the rest of lactation: $60–$100
• Antibiotic drugs and veterinary time: $30–$60
• Increased culling risk and involuntary replacement: $30–$60
• Labor for treatment, monitoring, and record-keeping: $20–$40

Subclinical cases cost less per case but far more in aggregate — $150–$200 per infected cow per lactation, with 30–50% of a typical herd carrying subclinical infection at any given time. When bulk tank SCC crosses processor premium thresholds (typically 200,000 or 250,000 cells/ml), the entire tank loses its quality premium — often $0.50–$1.50/cwt, or $10,000–$30,000 per year on a 100-cow herd.

Frequently Asked Questions

This article is educational and does not replace veterinary care. All antibiotic use requires a prescription from a licensed veterinarian under a valid VCPR. Follow all milk and meat withhold times to protect food safety and your operation.