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==Description==

==Description==

Johne's Disease is a contagious and chronic disease of ruminants caused by ''[[Mycobacterium avium subsp. paratuberculosis]]''. Affected species include cattle, sheep, goats and camelids.  The pathogen primarily affects the small intestine and is responsbile for severe economic losses due to culling losses and reduced reproductive performance and milk production.

Johne's Disease is a contagious and chronic disease of ruminants caused by the bacteria ''[[Mycobacterium avium subsp. paratuberculosis]]'' (MAP). Affected species include cattle, sheep, goats and camelids.  The disease is responsbile for severe economic losses due to reduced reproductive performance, increased susceptibility, reduced milk production and losses due to culling of affected animals.  

Calves are most commonly affected by this condition, and can be infected via a number of routes.

==Pathogenesis==

==Pathogenesis==

MAP is primarily spread through herds via faecal-oral transmission, contaminated water, in utero and through infected milk and colostrum. Following ingestion of M. paratuberculosis, infection begins in the ileum. Following uptake by the Peyer's Patches, M. paratuberculosis infects macrophages in the gastrointestinal tract and lymph nodes leading to a granulomatous inflammatory response.

MAP is primarily spread through herds via faecal-oral transmission, contaminated water and in utero. Young animals are most susceptible to infection and usually become infected through ingestion of contaminated milk or colostrum. Environmental factors such as overcrowding and dirty pens may increase the risk of infection. Following ingestion of M. paratuberculosis, infection begins in the ileum. Following uptake by the Peyer's Patches, M. paratuberculosis infects macrophages in the gastrointestinal tract and lymph nodes leading to a granulomatous inflammatory response.

There are '''three''' stages of infection in Johne's disease.

There are '''three''' stages of infection in Johne's disease.

==Clinical Signs==

==Clinical Signs==

There is a long incubation period between infection and the development of clinical signs; this may range from a year to several years. Animals may be affected subclinically before overt clinical signs are displayed, displaying reduced milk production, reduced reproductive performance and increased susceptibility to infection or disease.

There is a long incubation period between infection and the development of clinical signs ranging from a year to several years. Animals may be affected subclinically before overt clinical signs are displayed. In the early stages, the disease is characterised by reduced milk production, reduced reproductive performance and increased susceptibility to infection or disease.

In the later phases of infection, the disease is characterised by profuse, watery 'pipe-stem' diarrhoea and weight loss despite maintaining a good appetite. The disease is progressive and advanced cases may develop submandibular or ventral oedema due to a protein losing enteropathy.

In the later phases of infection, the clinical signs become more severe. The characteristic clinical signs are of watery 'pipe-stem' diarrhoea and severe wasting (despite maintaining a good appetite). The disease is progressive and advanced cases may develop submandibular or ventral oedema due to a protein losing enteropathy.  

The disease is similar in goats and sheep except diarrhoea is not a feature.

The disease is similar in goats and sheep except diarrhoea is not a feature.

==Diagnosis==

==Diagnosis==

Diagnosis is by; Histology, Serological tests, [[ELISA]] and AGID.

Diagnosis is by; Histology, Serological tests, [[ELISA]] and AGID.