Genetic variation and spread of Staphylococcus aureus, Streptococcus dysgalactiae and Streptococcus uberis in mastitis in dairy cows

Mastitis, inflammation of the udder, is a common disease among dairy cows worldwide. It is often associated with bacterial intramammary infections (IMI) and is subdivided into clinical mastitis (CM) and subclinical mastitis (SCM). Both CM and SCM influence milk quality and yield negatively, and mastitis is therefore of major economic concern for the farmer. Clinical mastitis is also of potential concern from an animal welfare perspective. The dairy sector in Sweden is undergoing major changes. The number of farms is continuously decreasing, and the average number of cows per farm continues to increase. In addition, a switch from tie stalls to free-stall housing is in progress. As management systems influence spread of udder pathogens, the ongoing changes in the dairy sector result in new challenges for farmers, as well as for veterinarians, other advisers, and researchers. Therefore, research providing up-to-date knowledge about infection patterns within and between herds of the most common udder pathogens is warranted.

The general aim of the project was to gather knowledge about three common udder pathogens, Staphylococcus (Staph.) aureus, Streptococcus (Strep.) dysgalactiae, and Strep. uberis, causing mastitis in Swedish dairy cows, with focus on genotype variation, spread, and short- and long-term outcomes of these infections. With increased knowledge, improved prevention strategies can be designed in the future. More specific aims were: 
- To investigate the genetic variation of Staph. aureus, Strep. dysgalactiae, and Strep. uberis isolates collected from cases of CM within Sweden, and to investigate if genotype within species differ regarding spread between herds and disease outcome. 
- To investigate the occurrence of Staph. aureus, Strep. dysgalactiae, and Strep. uberis IMI at and just after calving in large free-stall herds with mastitis problems, and to investigate if the infection patterns differed between bacterial species, herds, seasons, and parities. 
- To investigate associations between Staph. aureus, Strep. dysgalactiae, or Strep. uberis IMI in early lactation, and udder health, production, and culling during the following lactation, and to investigate if the outcome varied depending on when IMI occurred in relation to calving. 
- To investigate potential sources of Staph. aureus and Strep. dysgalactiae in body sites and the close environment of late-gestation heifers, dry cows, and in calving premises, in large free-stall herds with mastitis problems.

In the first study of the project, bacterial isolates collected in a previous national study of veterinary-treated CM (VTCM) were used to study between-herd genotype variation in epidemiologically independent isolates and differences between genotypes and streptococcal species in outcome (milk somatic cell count (SCC), new cases of VTCM, milk yield, and culling) up to 120 days after registration. Genotyping was performed using pulsed-field gel electrophoresis (PFGE). In the second study of the project, quarter milk samples were collected during a 12-month period from cows in selected free-stall herds with mastitis problems on the day of calving and four days later to evaluate occurrence of IMI. A selection of isolates were genotyped using spa-typing (Staph. aureus) or PFGE (streptococci). The importance of these IMI for short-term (first month after calving) and long-term (whole lactation) effects on udder health (SCC, VTCM), milk yield, and culling was also investigated. In the third study, four of the herds were revisited one year after the end of the second part, for additional milk sampling, and sampling of body sites and environments of animals close to calving. Occurrence of Staph. aureus and Strep. dysgalactiae was evaluated and isolates genotyped.

The results of the first study showed that the two most common Staph. aureus genotypes among the 185 VTCM isolates studied 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 (n=132) varied moderately compared to the ones of Staph. aureus and Strep. uberis. The common genotypes of Staph. aureus were associated with a lower 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, IMI were common at or just after calving. Staphylococcus aureus was the most common pathogen found, followed by Strep. dysgalactiae, and Strep. uberis. Infections with Staph. aureus and Strep. dysgalactiae, but not Strep. uberis, were common in primiparous cows on the day of calving, suggesting transmission of those pathogensbefore the start of first lactation. 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 herd variation in occurrence of IMI at or close to calving, indicating that the predisposing factors for udder infections at calving differed between herds. Intramammary infections just after calving were associated with increased SCC during the first month of lactation, as well as throughout the lactation, but associations with other outcomes were variable, depending on pathogen, on at which sampling or samplings IMI was found in relation to calving, and on breed and parity. In the third study, Staph. aureus was common in body site and environment samples, and the isolates were mostly of the same genotype as those found in IMI at the same herd. In contrast, Strep. dysgalactiae was not found in the environment and only in two body site samples. These isolates were of a genotype found in IMI.

Overall, the results showed that genotype variation of Staph. aureus, Strep. dysgalactiae, and Strep. uberis is pathogen-dependent. The variation and infection patterns of Staph. aureus suggest that infected udders are the main reservoirs of infection for this pathogen, and that contagious spread both within and between herds is common. However, environmental sources were found within some herds. The patterns of Strep. dysgalactiae suggested contagious spread between and within some herds, but environmental spread was suggested in other herds. Isolates of Strep. uberis showed high diversity, suggesting that the environment is the main source of this pathogen. Altogether, this project contributes new knowledge about Staph. aureus, Strep. dysgalactiae, and Strep. uberis that can be used in preventive work against these IMI.