Coagulase-Negative Staphylococci in Bovine Sub-Clinical Mastitis

Mastitis is a common disease in dairy cows. The majority of the cases are subclinical, and many of those are due to infection with coagulase-negative staphylococci (CNS). CNS is a heterogeneous group of bacteria consisting of a large number of different species, but limited information is available on the epidemiology of mastitis due to these species. The overall aim of the thesis was to improve the knowledge on prevalence and significance of different CNS species in connection with sub-clinical mastitis in dairy cows. In the first study, the epidemiology of CNS species, i.e. ability to induce persistent intra-mammary infections (IMI), and association with milk production, SCC, parity and month of lactation, was studied in dairy herds with problems due to sub-clinical CNS mastitis. The most commonly isolated CNS species were S. epidermidis, S. simulans, S. chomogenes, S. xylosus and S. haemolyticus. Persistent IMI were common in quarters infected with S. chromogenes, S. epidermidis and S. simulans. The results did not indicate differences between these CNS species in their association with daily milk production, cow SCC, and month of lactation. S. epidermidis was mainly found in multiparous cows, and S. chromogenes in primiparous cows. The second study concentrated on S. epidermidis by investigating possible transmission of S. epidermidis from milkers to cows, the discriminatory capacity of the sub-typing methods used and the clonal diversity within unrelated bovine S. epidermidis strains. Pulsed-field gel electrophoresis had high discriminatory power and showed that many different S. epidermidis types exist in bovine milk samples. Identical isolates were found in samples from the milker’s skin and in milk samples. As dairy cows are not a natural host for S. epidermidis the results suggest a human source of these udder infections.