Experimental Staphylococcus aureus intramammary challenge in late lactation dairy cows: Quarter and cow effects determining the probability of infection

The purpose of this study was to identify factors at the quarter and cow le vel that determine whether a quarter remains infected after an intramammary challenge with Staphylococcus aureus Newbould 305. A total of 135 cows wer e studied. Information on animal characteristics, cow-conformation, cow som atic cell count (SCC), and bacteriology, blood vitamin E levels, serology f or retro-viral infections, bovine leukocyte adhesion deficiency-carrier sta tus, and the presence of bovine lymphocyte antigens class I alleles was col lected on each animal. All quarters of all cows were then challenged with S taphylococcus aureus Newbould 305. The challenge with S. aureus Newbould 30 5 resulted in 28 cows (20.7%) that did not establish infection in any of th e quarters, 21 (15.6%) cows had 1 quarter infected, 35 (25.9%) had 2 quarte rs infected, 24 (17.8%) had 3 quarters infected, and 27 (20.0%) had all qua rters infected. A higher prechallenge SCC decreased the risk of infection. An infection with Corynebacterium bovis prior to challenge decreased the ri sk of S. aureus infection. Of the bovine lymphocyte antigen alleles, the pr esence of the W20A allele proved to be significantly associated with a decr eased risk of infection. No other factors proved to be significant.