Mycobacterium avium subsp. paratuberculosis
Johne's Disease (paratuberculosis)
- Johne's Disease is a chronic, contagious enteritis of ruminants
- Caused by M avium subsp. paratuberculosis
- Epidemiology
- Transmitted to young calves by ingestion of mycobacteria in faeces of infected adults
- Organisms viable in environment for long periods
- Long incubation period with clinical signs appearing in cattle over 2 years of age
- Subclinical carriers can occur, shedding organisms in their faeces
- Pathogenesis and pathogenicity
- M avium subsp. paratuberculosis is an intracellular pathogen
- Mycobacteria are ingested by macrophages in the Peyer's patches
- Survival and replication of mycobacteria in macrophages initiate an immune-mediated granulomatous reaction
- Lymphocytes and macrophages accumulate in the lamina propria and submucosa, resulting in marked thickening and folding of the intestinal wall
- Mesenteric lymph nodes are enlarged
- A protein-losing enteropathy results, along with failure to absorb nutrients and water
- Clinical signs
- Diarrhoea, initially intermittent, and weight loss in cattle
- Weight loss in sheep and goats
- Rapidly fatal with weight loss and diarrhoea in some deer
- Diagnosis
- All diagnostic procedures have faults but include:
- Microscopy of rectal biopsies
- Faecal culture
- Serology of serum including complement fixation tests, agar-gel immunodiffusion test and an ELISA
- Histopathology of intestines and lymph nodes
- Isolation and identification of mycobacteria from faeces and tissues
- Ziehl-Neelson-positive smears
- Intradermal tuberculin test
- DNA probes for detection in faeces
- Control
- Slaughter of affected animals
- Detection and slaughter of subclinical shedders using faecal culture, DNA probes and ELISA
- Good hygiene to protect young calves
- Separation and isolation of calves from affected dams
- Inactivated adjuvanted vaccines are available and reduce shedding of mycobacteria but do not eliminate infection