Johne's disease (paratuberculosis), caused by Mycobacterium avium subspecies paratuberculosis (MAP), is a chronic progressive intestinal disease affecting sheep and other ruminants. It causes significant suffering and major economic losses in affected flocks.
MAP infection typically occurs in young lambs through the faecal-oral route, but clinical disease usually manifests in adult sheep aged 2-5 years. The disease progresses through silent infection, subclinical disease with declining condition and production, and finally clinical disease with severe weight loss, bottle jaw (submandibular oedema), and eventual death. The protracted course means animals may suffer for months before detection.
Clinical Johne's disease causes chronic malabsorptive enteropathy leading to progressive wasting, protein loss, and weakness. Affected sheep cannot maintain bodyweight despite normal or increased appetite in early stages. As disease advances, weakness, recumbency, and inability to compete for food cause additional suffering. The condition is invariably fatal in clinically affected animals.
MAP serology (ELISA) identifies infected animals but misses early subclinical cases, with sensitivity improving as disease progresses. Faecal PCR testing detects MAP shedding but is most useful in moderate-to-high shedders. Definitive diagnosis requires post-mortem examination. The "tip of the iceberg" nature of Johne's disease means that for every clinical case, many more subclinically infected animals exist in the flock.
No effective treatment exists; control focuses on preventing spread. Management strategies include: removing and culling clinical cases promptly; identifying high-risk animals through test-and-cull programmes; hygiene protocols that reduce lamb exposure to MAP in the early postnatal period; colostrum management ensuring lambs receive clean colostrum; and pasture management to reduce environmental contamination.
A Johne's vaccine is available in some countries for sheep and can reduce the proportion of animals developing clinical disease and shedding MAP, though it does not eliminate infection. Vaccination programmes should be implemented alongside management improvements rather than as standalone measures.
Johne's disease control requires a flock health plan developed with a veterinarian. Purchasing replacement stock from accredited low-risk flocks, maintaining closed flocks where possible, and participating in national paratuberculosis control programmes reduces incidence over time. Whole-flock approaches over 5-10 years are needed to achieve meaningful reduction in prevalence.