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