Mastitis in Dairy Cows: Causes, Signs and Prevention

Mastitis is inflammation of the udder, nearly always caused by bacteria entering through the teat. It is widely described by dairy scientists as one of the most costly diseases in dairy cattle, because it cuts milk production, changes milk quality, requires treatment and discarded milk and can end a cow's career. It also matters to consumers, because milk quality and safety begin at the udder.

This guide explains how mastitis happens, what to watch for, how it is detected and the routine practices that prevent it. Treatment decisions belong to you and your veterinarian, so use this as background and not as a treatment plan. If you want to find dairy health meetings near you, agriculture.directory can help you find agriculture events in your area.

What Mastitis Is

Bacteria enter the udder through the teat opening, multiply in the mammary tissue and trigger an immune response. The result may be obvious, with a swollen, hot udder and abnormal milk, or hidden, with no visible signs but elevated somatic cells and reduced milk. The National Mastitis Council publishes research-based guidance on mastitis control and milk quality.

Clinical and Subclinical

Clinical mastitis is visible. You may see clots, flakes or watery milk, a swollen or hard quarter, heat and pain, and in severe cases a sick cow with fever, reduced appetite and decreased milk. Subclinical mastitis shows no obvious signs. The milk looks normal, but the cow has an infection that lowers production and raises the somatic cell count. Because subclinical cases are more common than clinical cases in many herds, testing matters.

Two Main Types of Bacteria

Contagious pathogens spread from cow to cow, usually during milking, through contaminated hands, towels, liners or machines. Examples include Staphylococcus aureus and Streptococcus agalactiae. Environmental pathogens live in bedding, manure and mud, and infect cows between milkings. Examples include coliforms like E. coli and environmental streptococci. Knowing which type is causing your herd's problems tells you where to focus, which is why culturing milk samples is useful. Your veterinarian can help set up culture programs, and extension programs such as University of Minnesota Extension publish guides.

Detecting Mastitis

• Strip every cow at milking. Squirting a few streams of foremilk onto a dark cup or plate shows clots or abnormal color.
• Watch the udder. Look for swelling, redness or a cow that kicks or guards a quarter.
• Track production. A sudden drop can be an early sign.
• Test for subclinical cases. The California Mastitis Test is a simple cow-side test, and electronic conductivity and somatic cell counts add information.
• Culture milk. Culturing identifies the organism, which helps guide treatment and prevention.

Treatment: Work With Your Vet

Treatment depends on the organism, the severity and the cow's history. Some mild cases may resolve with little or no antibiotic treatment, and some require specific treatment, and your veterinarian can help you decide which. All antibiotic treatments have withdrawal periods during which milk must be discarded, and you must follow label directions and your vet's instructions closely. The FDA's food pages and your state regulators describe the rules that keep drug residues out of milk. Never ship milk from treated cows before the withdrawal period ends.

Prevention: The Milking Routine

A consistent, clean milking routine is the heart of mastitis control. Common recommendations include:

• Wear gloves and keep hands and equipment clean.
• Pre-dip teats with an approved germicidal dip and allow contact time, then wipe dry with a clean single-use towel.
• Attach units promptly, and avoid overmilking.
• Post-dip teats after milking with an approved teat dip to protect the open teat canal.
• Milk infected cows last, or use separate equipment, to reduce spread.
• Keep the milking system maintained and tested regularly.

Prevention: Environment and Management

Clean, dry bedding is one of the strongest defenses against environmental mastitis. Keep freestalls clean, groom stalls often, avoid overcrowding and make sure cows can lie down comfortably. Keep cows cool in summer, since heat stress weakens immunity. Dry cow management and a good transition program also reduce infections around calving. Nutrition matters too, as balanced rations with appropriate vitamins and minerals support immune function.

Dry Period Strategies

The dry period is a critical time for udder health. Many herds use dry cow therapy or teat sealants, based on culture data and veterinary advice, and the trend is toward targeted use instead of blanket treatment. Discuss options with your vet, since antibiotic stewardship matters. The FDA's pages on antimicrobial resistance explain why using antibiotics carefully protects both animal and human health.

Know Your Numbers

Track your herd's clinical mastitis rate, somatic cell count and bulk tank culture results. Trends tell you whether your program works. If numbers climb, review the milking routine, equipment, bedding and cow comfort. Dairy plant field staff, extension specialists and veterinarians can help troubleshoot.

Common Mistakes

• Skipping foremilk stripping. You miss early cases.
• Inconsistent teat dipping. Coverage and contact time matter.
• Using the same towel on several cows. This spreads bacteria.
• Treating without a diagnosis. Culture first when possible.
• Ignoring withdrawal times. This risks drug residues.
• Dirty bedding. Environmental pathogens thrive.

Why Consumers Should Care

Milk is pasteurized, which kills harmful bacteria, and regulated for safety. Good udder health helps farms produce high-quality milk and reduces antibiotic use. If you buy from a local dairy, it is fair to ask how the farm manages udder health and milk quality. For a lighter look at dairy life, farm.fm has a humor piece about dairy farm regulations multiplying faster than the herd, which any dairy producer who has filled out a milk quality report will feel in their bones.