Dairy cow characteristics related to Staphylococcus aureus isolation from quarter samples

A comprehensive description of the health status of 2406 cows in 28 herds was used to study complex interrelationships between Staphylococcus aureus occurrence and potential risk factors at the cow level over a broad spectrum of production conditions. The objective of the study was to characterize those cows shedding Staph. aureus in milk and to provide indications of possible causal relations that should be further explored. The estimates are expected to aid in providing valid and precise evaluations of results from cross sectional microbiological examination of bovine (quarter) milk samples. Staph. aureus were isolated at microbiological examination of quarter milk from a 5.2, 13.5, 17.9 and 22.0% of 1552, 490, 296 and 277 cows in lactations 1, 2, 3 and 4-9 respectively. Simultaneous isolation of other mastitis pathogens was not significantly associated with occurrence of Staph. aureus. Of the positive samples, 95% had a positive California mastitis test reaction. Among the clinical measures of udder health, only visibly abnormal milk combined with normal udder tissue was associated with increased occurrence of Staph. aureus, although not consistently. Prior antibiotic treatment for udder disorders was not significantly associated with Staph. aureus isolation. The occurrence of sole ulcers in multiple digits was consistently associated with Staph. aureus in the first lactation. Milk yield was involved in interactions with other pathogens isolated, prior veterinary disease treatment, body weight and season of calving. Strong herd-year effects were revealed. This, combined with the interactions, indicated that some herd-specific factors were major determinants of Staph. aureus occurrence. The study indicated which cow characteristics should be taken into account in epidemiological analyses and causal interpretations of data from cross sectional microbiological examinations of dairy herds.