Staphylococcus aureus (S. aureus)

This bacterium is extremely difficult to control by treatment alone, because the response to antibiotic treatment is poor. Successful control is achieved by prevention of new infections and culling infected cows. S. aureus organisms colonize damaged teat ends or teat lesions. The infection is spread form cow to cow by a number of factors e.g. milkers’ hands, wash cloths, teat cup liners, and flies. The organisms probably penetrate the teat canal during milking. Irregular vacuum fluctuations impact milk droplets and bacteria against the teat end with sufficient force to cause teat canal penetration and possible development of new infection. Infected cows must either be culled, segregated from the milking herd and milked last or milked with separate milking units, or teat cup liners must be rinsed and sanitized after milking infected cows. The development of automatic dipping and flushing through the cluster has the potential to revolutionise the control of S. aureus mastitis.

Staphylococcus aureus causes chronic mastitis, often it is subclinical, where there is neither abnormal milk nor detectable change in the udder, but somatic cell count has increased. Some cows may flare-up with clinical mastitis, especially after calving. The bacteria persist in mammary glands, teat canals, and teat lesions of infected cows and are considered contagious. The infection is spread at milking time, when S. aureus contaminated milk from infected cows comes into contact with teats of uninfected cows, and the bacteria penetrate the teat canal. Once established, S. aureus often does not respond to antibiotic treatment, and infected cows eventually must be segregated or culled from the herd. Eradication of S. aureus, has proved to be almost impossible; however control is possible by practicing the highest level of hygiene and milking techniques.

Cows infected with S.aureus do not necessarily have high SCCs. Only 60% of infections are found in cows producing milk with more than 200,000 SCC. In several research trials, 3-8% of first lactation cows were found infected with S. aureus at calving. Many remain infected throughout the first lactation and are reservoirs for infecting other cows in the herd. Although as many as half of the cows with high SCC may be infected with S. aureus, somatic cell counts alone are not sensitive enough to positively diagnose S. aureus infections.

Control of contagious mastitis.