B059 - JOHNE’S DISEASE

IDevice Icon

B059 - JOHNE’S DISEASE

Nature of the disease
Johne's disease (JD) is a chronic, progressive, wasting disease of ruminants caused by Mycobacterium avium paratuberculosis. The disease is characterised by a long incubation period, with clinical disease usually only occurring in older animals.
Classification
OIE List B disease
Susceptible species
Cattle are the main species affected, with goats, sheep, deer and alpacas also susceptible. Susceptibility to infection is greatest in young animals, especially those less than 3 months old. Clinical disease is usually seen in older animals.
Distribution
JD occurs in many countries, especially those with cool climates, and is most prevalent in intensive dairying areas.
Clinical signs 
Clinical signs are rarely seen in cattle younger than 2 years and are usually observed between 2 and 6 years of age. Clinical disease may occur at a younger age in sheep and alpacas. Appetite is usually unaffected. The main features are:

Cattle and deer 

  • Diarrhoea
  • Wasting
  • Dehydration

Sheep and goats 

  • Emaciation
  • Weakness
  • Depression
  • Diarrhoea, is not usually as severe as in cattle

Sub-clinical infections are common in all species and sub-clinically infected animals are a source of transmission of the disease.

Post-mortem findings 
Thickening and corrugation of the intestinal mucosa, particularly in the terminal ileum and ileo-caecal area and associated lymph nodes are swollen and oedematous
Differential diagnosis 
Other conditions with signs that could be confused with Johne’s disease include:
  • Salmonellosis
  • Coccidiosis
  • Parasitism
  • Chronic molybdenum poisoning
  • Wasting due to cobalt deficiency
Specimens required for diagnosis 
The detection of sub-clinical diseases can be done by delayed-type hypersensitivity (DTH), consisting in intradermal inoculation of avian tuberculin and the observation 72 hours later of swelling at the inoculation point. However this test as a poor specificity and should be only used in herds to give an idea of the number of sensitised animals.

Common serological tests include complement fixation and ELISA but those are of unknown sensitivity and therefore are better used to confirm a case rather than to identify sub-clinical infections.

Faecal samples can be submitted for identification of the Mycoplasma by bacteriology, the technique permits to diagnose the disease up to 6 months prior to the apparition of symptoms. Faecal samples can also be submitted for DNA probes detection associated with PCR, commercial kits have been recently developed.

At post-mortem, samples of small intestine especially terminal ileum and ileo-caecal valve region for histopathology, samples are stained with Ziehl-Neelsen's method. Pathognomonic lesions include infiltration of the lamina propria, Peyer's patches and cortex of the mesenteric lymph nodes with Langhan's giant cells and epitheloid cells.

Transmission   
Infection of young animals occurs through the ingestion of infected faecal material on contaminated teats of dams, or contaminated pasture or water. 

The causative bacterium is excreted in large numbers by diseased animals and to a lesser extent by asymptomatic carriers, some of which may have extensive intestinal lesions.

The bacteria are stable in the environment and may survive 12 months or more in sheltered situations.

M. paratuberculosis has been isolated from male and female reproductive tracts, from semen and from unborn calves of infected cows. It is excreted in the milk of infected cows, especially if clinically affected. However, the importance of transmission by means other than ingestion has not been determined.

Risk of introduction   
JD is most likely to be introduced through the importation of JD infected live animals. 

Asymptomatic carriers can be a source of infection. Testing of animals may be unreliable because of the low sensitivity of serological tests for JD.

Importation of susceptible animal from an infected country to a non-infected country should require that the animals are coming from a herd free from the disease for more than 5 years and that diagnostic tests have been done prior to shipment.

Control / vaccines  
All available tests for JD in the live animal have significant drawbacks, contributing significantly to the difficulties experienced in controlling and eradicating the disease.

Control of spread of infection has been constrained by the lack of an efficient serological test for the detection of sub-clinically infected animals. Control programs involve preventing exposure of young animals to contaminated faeces or milk and the prompt removal of known infected animals and any close relatives (dam/offspring) from the herd or flock.

Vaccination against Johne's disease reduces, but does not eliminate, bacterial excretion. Vaccination may interfere with bovine tuberculosis programs because vaccinated animals are sensitised to tuberculin, and the vaccine antibodies interfere with serological testing for JD to detect infected animals.

References
  • Office International des Epizooties, 2002
  • Paratuberculosis, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 537-539
  • Paratuberculosis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 841-850