Effect of lactation therapy on Staphylococcus aureus transmission dynamics in two commercial dairy herds

The direct effect of 8？days intramammary lactation therapy with pirlimycin hydrochloride was demonstrated by an increased proportion of cure and a reduction in duration of infection in quarters receiving treatment compared to untreated controls. The indirect effect of lactation therapy was demonstrated by reduction of new S. aureus intramammary infections (IMI) caused by the dominant strain type in both herds. Strain typing of representative isolates taken over the duration of all IMI, including pre- and post-treatment isolates, provided more precise estimates of new infection, cure, and re-infection rates. New S. aureus infections in recovered susceptible quarters and the emergence of a new strain type in one herd influenced incidence measures.In addition to demonstrating positive direct effects of lactation therapy, this study provides evidence that treatment of subclinical S. aureus mastitis during lactation can have indirect effects including preventing new IMI and reducing incidence of clinical mastitis within dairy herds. Strain specific transmission parameter estimates for S. aureus MLST clonal complexes 5, 97 and 705 in 2 commercial dairy herds are also reported.Mastitis continues to be recognized as one of the most economically important health problems of dairy cattle [1,2]. Subclinical mastitis, which can be characterized by an elevated milk somatic cell count (SCC), is the dominant form affecting cows. A number of authors have suggested that dairy producers frequently leave subclinical mastitis undetected or untreated for extended periods during lactation [3-5].Mastitis control interventions are intended to reduce the duration of infection and prevent new infections. Examples of effective mastitis control practices include the use of long acting antibiotics at the end of lactation (dry-cow therapy), application of a post-milking teat disinfectant following each milking, and segregation or culling of infected individuals [6,7]. Treatment of mastitis durin