What are the bacteria?

Bacteria that infect the udder and result in mastitis can be grouped into:

Contagious or cow-associated bacteria - those that spread from cow to cow during milking
Environmental bacteria - those that come from the environment in which the cow lives.

Knowing the likely source of the bacteria helps to prevent it from infecting the udder. To identify bacteria, samples of foremilk are collected aseptically and submitted for bacterial culture.

Bacterial culture involves smearing a small volume of milk onto an agar plate, incubating it for up to 48 hours and identifying the colonies that grow, as shown in the picture.

Contagious bacteria are spread mainly during milking by transfer of infected milk between cows, which can be via:

Contaminated liners
Milker's hands
Cloths and wipes

Staphylococus aureus (Staph. aureus) is the most important contagious bacteria, and is a common cause of mastitis in mid and late lactation in high SCC herds.

Streptococcus agalactiae (Strep. agalactiae) is a much less common contagious bacteria. Seek veterinary help if these bacteria are detected in your herd.

Environmental bacteria are found throughout the cow's environment, and can be isolated from:

Manure and soil
Muddy races and paddocks
Hair, legs and hooves
Cow's digestive and reproductive tracts

Streptococcus uberis (Strep. uberis) is the most important environmental pathogen in New Zealand and a common cause of clinical mastitis at calving. Other environmental bacteria also cause mastitis.

Bacteria that cause mastitis can also be grouped by the severity of mastitis they are likely to cause:

Major pathogens - these bacteria generally cause a significant rise in SCC, often resulting in clinical signs of mastitis
Minor pathogens - these bacteria generally cause only a mild rise in SCC, not usually associated with clinical signs
Major pathogens = Staph aureus, Strep. agalactiae, Strep uberis, E. coli, Klebsiella, etc
Minor pathogens = C. bovis, CNS