Mastitis
Also called Udder infection
Inflammation of the udder, nearly always from bacteria entering through the teat. It lowers milk yield and quality, and can be hidden (subclinical) with normal-looking milk.
Cause and spread
- Cause
- Many bacteria, mainly streptococci, staphylococci and gram-negative bacteria such as coliforms, entering through the teat canal.
- How it spreads
- Contagious bacteria such as Staphylococcus aureus and Streptococcus agalactiae spread from cow to cow mainly at milking. Environmental bacteria come from dirty bedding and wet or muddy ground.
Signs to watch for
- Clots, flakes or change of colour in the milk
- Swollen, hot, painful or red udder quarter
- Fever, loss of appetite or shock in severe cases
- Falling milk yield
- Subclinical mastitis: no visible change, only a raised cell count in the milk
Many diseases look alike. Only a laboratory test can tell them apart, so please see a veterinarian for any sick animal.
How it is diagnosed
A somatic cell count (SCC) finds hidden infection: about 200,000 cells per mL or more in a cow indicates a high likelihood of infection. Culture of milk from the affected quarter is the only reliable way to find the bacteria in clinical cases, and sensitivity testing shows which antibiotics work.
- Usually the first testMilk Somatic Cell Count (SCC)Finds hidden mastitis from the number of cells in the milk.
- Confirms the diseaseMastitis Panel (Cell Count + Culture and Sensitivity) (panel)Cell count plus culture and antibiotic sensitivity of the milk.
- Confirms the diseaseBacterial Culture and Sensitivity (panel)Grows the bacteria in the milk and shows which antibiotics work.
Prevention and control
- Keep bedding clean and dry, and the udder clean and dry at milking
- Dip teats in an effective germicide straight after milking
- Use individual towels to dry teats, and gloves for milkers
- Do not use water during milking except to keep milking units clean, and keep teat ends healthy
Where this information comes from
- Mastitis in Cattle · Merck Veterinary Manual
General information for animal owners and veterinarians, last checked October 2026. It does not replace the advice of your veterinarian, who should decide which tests and treatment your animal needs.