Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Animal, Nutrition and Food Science

First Advisor

Schukken, Ynte


Subclinical mastitis remains the dominant form of mastitis affecting dairy cattle, and is responsible for the greatest economic losses associated with mastitis in dairy herds in the major dairy producing countries. Mastitis control has relied on a series of well established management practices that have led to significant improvements in mastitis prevalence and milk quality measures over the past 3 decades. Changes in pathogen prevalence, including the shift in absolute and relative importance of pathogens such as Escherichia coli, Klebsiella spp. and Streptococcus uberis combined with the ongoing importance of Staphylococcus aureus, highlight the need for continued research to evaluate mastitis control practices. This dissertation examines targeted antimicrobial treatment of chronic subclinical mastitis during lactation as a control measure implemented in herds that have applied standard mastitis control practices. Treatment of subclinical mastitis caused by the major gram-positive mastitis pathogens Staphylococcus aureus, Streptococcus uberis, and Streptococcus dysgalactiae in dairy herds is examined. Methods include predictive modeling of the effects of lactation therapy using a deterministic state-transition model of pathogen transmission dynamics, and evaluation of lactation treatment in a negative controlled clinical field trial. Concepts put forward include the distinction between direct and indirect effects of mastitis control practices and the value of molecular diagnostics to improve our understanding of mastitis epidemiology and the impact of control programs. Results obtained from predictive modeling indicate that overall positive population level effects of lactation therapy would be realized for herds that have successfully implemented practices that reduce pathogen transmission. A novel finding was the prediction that under management scenarios with high pathogen transmission rates treatment of subclinical mastitis will have little impact on the proportion of infected quarters and no positive population level effect in reducing new infection rates. In a field trial, positive direct and indirect effects of treatment of S. aureus mastitis were observed suggesting benefits of lactation therapy targeting this pathogen. Potential benefits were off-set by the frequent finding of post treatment infections which resulted in no improvement in somatic cell count of treated cows compared to untreated controls. Lactation therapy of S. uberis and S. dysgalactiae mastitis resulted in cure proportions and duration of infection did not differ from spontaneous cure of untreated controls and there was limited evidence of an effect of treatment on rates of new infection. A unique finding was the identification of an association between Coxiella burnetii shedding and subclinical mastitis in dairy cattle. It is concluded from this research that treatment of subclinical mastitis during lactation may be justified under specific conditions, and it is recommended that dairy farm managers and their advisors should not routinely implement lactation therapy of subclinical mastitis without careful consideration of the potential benefits and risks.