Mastitis in Dairy Cows Genotypes, Spread, and Infection Outcome of Three Important Udder Pathogens

Mastitis, inflammation of the udder, is a common disease among dairy cows worldwide. This thesis investigated the genotype variation and spread of three major udder pathogens: Staphylococcus aureus, Streptococcus dysgalactiae, and Streptococcus uberis. Isolates collected in a previous study of veterinary-treated clinical mastitis (VTCM) were used to study between-herd genotype variation in epidemiologically independent isolates and differences in outcome. Intramammary infections (IMI) were scrutinized for their occurrence on the day of calving and four days later by quarter milk sampling in selected herds with mastitis problems. The importance for long-term udder health and production of these IMI was also investigated. The two most common Staph. aureus genotypes among the VTCM isolates were detected in 64% of the herds. In contrast, none of almost 100 Strep. uberis isolates from different herds was of the same genotype. The Strep. dysgalactiae isolates varied moderately compared to the ones of Staph. aureus and Strep. uberis. The common genotypes of Staph. aureus were associated with a lower somatic cell count (SCC) during the follow-up period, compared to the less common genotypes. No differences were detected between genotypes of streptococci, but cows with Strep. dysgalactiae VTCM had a lower SCC during the follow-up period compared to those with Strep. uberis. In herds with mastitis problems, Staph. aureus was the most common pathogen found at and just after calving, followed by Strep. dysgalactiae, and Strep. uberis. Isolates of Staph. aureus showed the lowest within-herd genotype diversity, followed by an intermediate diversity of Strep. dysgalactiae and a high diversity of Strep. uberis. There was a marked variation in occurrence of IMI at or close to calving in herds with mastitis problems, indicating that the predisposing factors for udder infections at calving differed between herds. Most early lactation IMI were associated with an increase in lactation SCC, whereas associations with other outcome variables were more variable. Altogether, this thesis contributes knowledge about Staph. aureus, Strep. dysgalactiae, and Strep. uberis that can be used in preventive work against these IMI.