Johne's Disease

Also known as:	Midges
The most important veterinary species	Culicoides

Paratuberculosis/ Johne's Disease is caused by Mycobacterium avium subsp. paratuberculosis, commonly seen in cattle, but known to affect all ruminants. Common signs include enteritis and diarrhoea.

• • May be seen in zoo ruminants and goat herds.
  • Particularly prevalent in Channel Island breeds.
  • Is now also becoming a problem in Limousin breeds.
• Produces a chronic proliferative enteritis.
• Is usually fatal, since the disease cannot be got rid of.
• Animals may sometimes be carriers without showing clinical signs.
  • Once disease is present in a herd, it is very difficult to get rid of it.
• Mycobacterium is excreted in urine and milk as well as in the faeces.

Clinical

• Clinical signs develop in older cows after calving i.e. 3 to 4 years of age.
• BUT animals are infected as calves less than 6 months old
  • The disease develops very slowly.
• Clinical signs include:
  • Ongoing, chronic profuse diarrhoea.
    • Paint like consistency.
  • Hindquarters and tail-caked with faeces
  • Faeces also splattered on walls.
  • Animal gradually fades away and dies over the course of months.

Pathogenesis

• Organisms get in through the M-cells of Peyer's patches.
• Mycobacteria invade macrophages and cause a granulomatous inflammatory response.
• Death results from:
  • Damage to the mucosa.
    • Nutrients cannot be absorbed.
  • Inflammatory loss of protein
    • I.e. a protein losing enteropathy (hypoalbuminaemia).

Pathology

Gross

Johnes Disease (Courtesy of Bristol BioMed Image Archive)

• Quite typical
• Cows appear very emaciated.
  • Depends on how long the disease has been there.
  • Not very much to see!
    • Fat is pale and oedematous, and there is not much of it.
• Signs are confined to the terminal small intestine (especially the ileum) but are characteristic.
  • Diffusely thickened mucosa
    • Transverse, corrugated ruggae with reddened crests.
      • Cannot extend the gut to remove these (i.e. they are permanent ruggae).
    • Velvety mucosal surface.
    • Mucosa may take on a 'corn-on-the-cob' appearance in advanced cases.
      

      Proliferative ileitis in Johnes disease (Courtesy of Bristol BioMed Image Archive)
  • Serosal oedema.
  • Distended lymphatics.
• Enlarged mesenteric lymph nodes.
• Changes are milder in sheep and goats.
  • Often missed.
  • May produce small areas of necrosis not usually seen in cattle.
  • Sheep may get a pigmented form.

Histologically

Histological appearance of Johnes disease (Courtesy of Bristol BioMed Image Archive)

• Many large macrophages (epithelioid macrophages) in mucosa, submucosa and lymph nodes.
  • Mesenteric lymph nodes are pale and enlarged (though not necrotic).
• The lamina propria is infiltrated by sheets of macrophages with some lymphocytes.
• Acid-fast bacteria are found in the macrophages and giant cells.
  • Detected by Ziehl-Neelson stain.
  • Bacteria act like foreign body producing a type IV hypersensitivity reaction.
• Sheep have two different forms.
  1. Paucibacillary
     • Many T cells
     • Few bacilli
  2. Multibacillary
     • Many macrophages
     • Many bacilli in macrophages
     • Few lymphocytes

Diagnosis

• Diagnosis is by
  • Histology
  • Serological tests
    • ELISA & AGID
  • Culture of organisms
  • 60% of cases have lesions in colon and rectum and can be diagnosed by rectal biopsy.