Bramley

Mathematical and Molecular Epidemiology of Mastitis Control in Dairy Herds and Direct and Indirect Effects of Subclinical Treatment

Dr. John Bramley

NON-TECHNICAL SUMMARY: Mastitis is one of the most common and costly diseases affecting dairy cattle. A major limiting factor in US dairy production systems is insufficient knowledge concerning the effects of subclinical mastitis treatment programs. This knowledge gap limits producers' ability to prevent animal health problems and reduce economic losses from mastitis. This project examines the effects of diagnosis-based antibiotic treatment of subclinical mastitis in dairy herds. The impact of subclinical mastitis treatment on the spread of mastitis in dairy herds, and on the development of antimicrobial resistance among mastitis pathogens are being quantified in this research. Our research will deliver knowledge that allows dairy producers to make informed rational decisions regarding the implementation of mastitis treatment practices.

OBJECTIVES: Our goal is to quantify the direct and indirect effects of rational diagnosis-driven antibiotic treatment of subclinical mastitis of dairy cattle. Direct effects are the impact of treatment on the individual animal, indirect effects are the population level impact of treatment. The latter has been disregarded in mastitis treatment studies. The primary objectives are: 1) to develop a population dynamic model for evaluating the impact of antibiotic treatment of subclinical mastitis caused by major gram-positive mastitis pathogens in lactating dairy cattle, and 2) to evaluate changes in strain diversity and antimicrobial resistance of bacterial isolates obtained from populations of dairy cattle receiving an intensive antibiotic mastitis control program.

APPROACH: We will develop a deterministic state-transition mathematical model of subclinical mastitis transmission dynamics among lactating dairy cattle. Initial parameter estimates for this model will be obtained from a literature review of refereed publications. The model will be used to explore the effect of varying cure rate associated with antimicrobial treatment on output values describing the transmission rate for contagious mastitis pathogens, and the prevalence of infected and uninfected cows. The sensitivity of model behavior to parameter values (sensitivity analysis) will be explored. We are especially concerned with the sensitivity of model behavior to changes in the parameter duration of infection. Data obtained from a multi-herd longitudinal negative controlled clinical field trial will be used to in an assessment of goodness of fit of model output to observed data. Antimicrobial resistance testing (agar dilution methods, minimum inhibitory concentration profiles and PCR-based identification of resistance determinants) will be combined with results of molecular typing (Pulsed Field Gel Electrophoresis) to quantify clonal patterns of transmission among susceptible and resistant Staphylococcus aureus and Streptococcus uberis isolates. S. aureus and S. uberis isolates obtained from a multi-herd longitudinal negative controlled clinical field trial are available. This field trial evaluated the intensive use of the lincosamide antibiotic, pirlimycin hydrochloride, for treatment of subclinical mastitis in lactating dairy cattle. Molecular methods will focus on identification of genes that confer resistance to macrolide-lincosamide-streptogramin drug classes. Results of DNA fingerprinting and antimicrobial susceptibility testing will be combined to quantify the molecular epidemiology of antimicrobial resistance in populations receiving an intensive antimicrobial treatment intervention, and will be compared to control populations where this intervention is not implemented.