Continuing education — Voortgesette opleiding Bovine mastitis therapy and why it fails

INTRODUCTION Mastitis can occur at any stage of a cow’s productive life. Microbiological investigations of clinical bovine mastitis reveal a causative agent in 75–95% of cases. There is good evidence that microorganisms are involved in almost all cases. At least 137 infectious causes of bovine mastitis are known, but the majority of cases are caused by only a few common bacterial pathogens, namely staphylococci, streptococci, coliforms and Arcanobacterium pyogenes. For antibacterial mastitis therapy to be successful, the active drug must attain and maintain concentrations exceeding the minimum inhibitory concentration (MIC) at the focus of infection for long enough to break the production and toxin-producing cycle of the causative pathogen. This may be prevented by a number of factors that include udder pathology and poor selection of antimicrobials, and is also influenced by the route of administration. Therapy may have poor results owing to tissue damage and introduction of new infections during treatment, and/or failure to eliminate the management factors that predispose to mastitis. Although there is an underlying assumption that the primary goal of antibacterial mastitis therapy is to kill bacteria and that the normal udder is sterile, usually the best that can be achieved is temporary reduction or suppression of the bacterial population to allow the host to overcome the infection. The mechanisms to cleanse the udder are evidently poorly developed in some cows, as relapses and re-infections commonly follow antimicrobial therapy. Udder infections tend to be dynamic, and stress may contribute to udder infections becoming clinically apparent. The purpose of this article is to provide an update on bovine mastitis therapy and reasons for its failure.