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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