Johne's disease in Canada Part I: clinical symptoms, pathophysiology, diagnosis, and prevalence in dairy herds.

Recent international developments in the area of infectious disease control and nontariff trade barriers, along with possible zoonotic concerns, have provoked a revival of interest in Johne's disease in Canada and elsewhere. The bacterium causing Johne's disease, Mycobacterium avium subspecies paratuberculosis, is distributed worldwide and causes chronic granulomatous enteritis, also known as paratuberculosis, in domestic and exotic ruminants, including cattle. The subclinical form of this disease results in progressive weight loss, reduced milk production, lower slaughter value, and premature culling, with possible impacts on fertility and udder health. Eventually, infection can lead to the clinical form that manifests as chronic diarrhea, emaciation, debilitation, and eventual death. Currently, available tests to detect infected animals produce many false-negative results and some false-positives, particularly in subclinically infected animals, thus making their interpretation and utilization challenging in control programs. The objective of this 2-part review is to critically review the literature about Johne's disease in dairy cattle for bovine practitioners in Canada. Part I covers the clinical stages, pathophysiology, diagnosis, and prevalence of infection in Canada, while Part II discusses impacts, risk factors, and control programs relevant to Canadian dairy farms. By reviewing the scientific literature about Johne's disease, control of the disease could be pursued through informed implementation of rational biosecurity efforts and the strategic use of testing and culling.