In vitro and in vivo studies on treatment and prevention of bovine mastitis : a thesis presented in partial fulfilment of the requirements for the degree of Philosophy Doctor in Veterinary Science at Massey University, Palmerston North, New Zealand

Abstract

Mastitis prevalence on dairy farms depends on the number of infected cows and the duration of each intramammary infection. Strategies aiming to influence these factors are the subject of research presented in this thesis. Decreasing the duration of infection can be achieved by successfully treating infected quarters. Treatment of mastitis can occur during lactation or in the dry period. Treatment success is influenced by the concentration of antimicrobial achieved at the site of infection and the length of time it is present. The concentration of antimicrobial should exceed the relevant minimal inhibitory concentration. The susceptibility of mastitis-causing organisms varies among geographical areas and over time. New Zealand’s susceptibility data demonstrated a high susceptibility to penicillin. A formulation containing this antimicrobial was administered to healthy lactating cows milked once or twice daily. The concentrations of penicillin in milk were above the minimal inhibitory concentrations for the entire inter-dosing interval. Doubling the number of treatments or milking once-a-day resulted in a significantly increased time above the minimal inhibitory concentrations. The number of new infections is greatest during the early dry period in mature cows and in the pre-calving period in both heifers and mature cows. Pre-partum administration of delayed release antimicrobial formulations in heifers decreased the incidence of clinical mastitis and resulted in better reproductive performance, but not in increased milk production, when compared to control heifers. More effective prevention of new infections within the dry period was achieved by administering a novel teat sealant to mature cows when compared to a commercial teat sealant and untreated controls. Strategies for shortening the duration of intramammary infections and decreasing the number of affected cows at the start of lactation investigated in this thesis should reduce the prevalence of mastitis on dairy farms in New Zealand.