Effects of farm management on ecology of virulent Rhodococcus equi

Muscatello, Dr Gary (2005) Effects of farm management on ecology of virulent Rhodococcus equi. PhD thesis, Veterinary Science, The University of Melbourne.

Abstract

Environmental samples (air and soil) were collected from Thoroughbred breeding farms with different prevalences of R. equi pneumonia to increase our understanding of the ecology of virulent R. equi on horse farms. The airborne population of virulent R. equi was a major focus of this research, as inhalation of the pathogen from the environment is considered the primary route of pulmonary infection. Air sampling was performed using an air monitoring system with selective media to facilitate the recovery of R. equi, allowing quantitative measurement of airborne virulent R. equi. Polymerase chain reaction and DNA hybridisation techniques were used to evaluate environmental samples to identify and differentiate R. equi. Associations were observed between the concentration of airborne virulent R. equi, the proportion of airborne R. equi that were virulent and the prevalence of disease due to R. equi on farms. No association was observed between the concentration of virulent R. equi in soil, the proportion of R. equi in soil that were virulent and the prevalence of disease due to R. equi. Environmental samples were taken at a variety of locations and at regular intervals to identify high risk areas. Elevated concentrations of virulent R. equi were detected in the middle and later periods of the breeding season, yet cases of R. equi pneumonia fell late in the season, illustrating the age specificity associated with the disease. The holding pens and lanes were associated with elevated airborne concentrations of virulent R. equi and these areas were typically dry and barren. Lower soil moisture and lower pasture heights were significantly associated with elevated airborne concentrations of virulent R. equi. Few environmental variables were associated with elevated concentrations of virulent R. equi in soil, although the data suggested that acidic soil conditions may lead to an elevated proportion of virulent R. equi in soil. The management characteristics of farms were compared with disease prevalence and severity, using questionnaires completed by managers and veterinarians. An association between high disease prevalence and mortality was found, along with an association between low foal numbers and low disease prevalence. There was no conclusive evidence that immunisation, parasite control or other health management practices in mares or foals reduced the prevalence or severity of R. equi pneumonia. Isolation of virulent R. equi from the breath of clinically affected and healthy foals demonstrated for the first time the possibility for foal-to-foal aerosol transmission. The concentration of virulent R. equi in exhaled air was significantly greater than that seen in environmental air. These results implicate breath from infected foals as a source of spread of the pathogen within the herd. The ecological links established between factors influencing air contamination and the epidemiology of the disease may be used to implement effective environmental and foal management strategies to reduce the level of virulent R. equi to which susceptible foals are exposed, thus reducing the prevalence and severity of R. equi pneumonia in the horse breeding industry.