Changes

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 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.

==Pathogenesis==

==Pathogenesis==

==Gross Pathology==

==Gross Pathology==

The infected host appears emaciated.

The infected host appears emaciated. Fat is pale and oedematous, and only present in small amounts. 

Signs are confined to the terminal [[Small Intestine - Anatomy & Physiology|small intestine]] (especially the [[Ileum - Anatomy & Physiology|ileum]]) but are characteristic.  The mucosal surface is diffusely thickened with transverse, corrugated rugae.  Infected animals may also have enlarged mesenteric lymph nodes.

[[Image:johnes disease proliferative ileitis.jpg|thumb|right|150px|Proliferative ileitis in Johnes disease (Courtesy of Bristol BioMed Image Archive)]]

Signs are confined to the terminal [[Small Intestine - Anatomy & Physiology|small intestine]] (especially the [[Ileum - Anatomy & Physiology|ileum]]) but are characteristic.  It has diffusely thickened, velvety mucosa surface and transverse, corrugated ruggae with reddened crests.  Infected animals may also have enlarged mesenteric lymph nodes

==Histologically==

==Histologically==

There are many large macrophages (epithelioid macrophages) in mucosa, submucosa and lymph nodes.

There are many large macrophages (epithelioid macrophages) in mucosa, submucosa and lymph nodes.

The mesenteric lymph nodes are pale and enlarged (though not necrotic), and the lamina propria is infiltrated by sheets of macrophages with some lymphocytes.

The mesenteric lymph nodes are pale and enlarged (though not necrotic), and the lamina propria is infiltrated by sheets of macrophages with some lymphocytes. Acid-fast bacteria are found in the macrophages and giant cells, and this is detected by Ziehl-Neelson stain.  

Acid-fast bacteria are found in the macrophages and giant cells, and this is detected by Ziehl-Neelson stain.  

Bacteria act like foreign bodies producing a type IV hypersensitivity reaction.

Bacteria act like foreign bodies producing a type IV hypersensitivity reaction.