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===Johne's disease (paratuberculosis)===
===Johne's disease (paratuberculosis)===
*
*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