Mycobacterium avium subsp. paratuberculosis: pathogen, pathogenesis and diagnosis

Johne's disease, or paratuberculosis, is a chronic intestinal infection caused by Mycobacterium avium subsp. paratuberculosis. The usually fatal disease is characterised by cachexia, and in some species diarrhoea, after a long pre-clinical phase. Treatment is ineffective and economically impracticable. The infection primarily affects domestic and free-ranging ruminants, but has also been reported in primates, rabbits, stoats and foxes. Since paratuberculosis is often subclinical, under-reporting is suspected, even though the disease is notifiable in numerous countries. Herd prevalence of bovine paratuberculosis in Europe ranges from 7% to 55%. In the United States of America, herd prevalence is strongly associated with herd size; 40% of herds of more than 300 head were found to be infected. In Australia, reported dairy herd infection rates range between 9% and 22%. Paratuberculosis in domestic livestock entails significant economic losses due to several factors (e.g. reduced production, premature culling and increased veterinary costs). Free-ranging and captive wildlife are also at risk from paratuberculosis.