Epidemiology of Mycoplasma bovis in Scottish dairy herds

Abstract

The bacterium Mycoplasma bovis (M. bovis) causes major economic losses to dairy herds resulting from increased mortality and morbidity, treatment costs, and reduced growth of young stock. There was limited knowledge on the prevalence of M. bovis in Scotland and no national monitoring scheme. Two studies were conducted; a longitudinal bulk tank milk (BTM) prevalence study and a cross-sectional seroprevalence study on dairy calves. In the longitudinal BTM prevalence study, one hundred and eighty-one dairy herds across Scotland participated in the study which required them to submit four BTM samples roughly three months apart that were tested for the presence of active M. bovis infection and for recent exposure. A short questionnaire on general herd management practices were issued to farmers to identify potential risk factors associated with seropositivity. At each of the four sampling points, the proportion of antibody positive herds were 76%, 71%, 83%, and 79%, and overall, 86% of herds tested seropositive in at least one of their four samples. Multivariable logistic regression identified herd history of M. bovis as a potential risk factor for the presence of M. bovis antibodies. The questionnaire results also provide an updated overview of the common structures and practices on Scottish dairy farms. Herds were then classified based on the antibody results of their four BTM samples using various methods. Sixty-one percent of herds tested consistently positive for all four samples, 15% consistently negative, and 24% transitional. When classified by k-means clustering of the optical density (OD) trend, the majority of herds had a stable trajectory (44%). A cross-sectional seroprevalence study was then carried out on a subset of herds from the BTM study (n=36) to determine if there was evidence of exposure to M. bovis in youngstock and if there was an association between the BTM and calf seroprevalence. Twenty calves were sampled on each farm (10 animals 4-8 months old and 10 animals 10-14 months old) and a BTM sample collected. There was evidence of youngstock exposure in most herds (58%), and this was associated with the BTM prevalence. The results of this thesis have demonstrated that M. bovis is likely endemic in Scottish dairy herds and has raised further questions on risk factors and within-herd prevalence estimates of M. bovis