Johne's Disease
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| Also known as: | Paratuberculosis |
Description
Johne's Disease is a contagious and chronic disease of ruminants caused by the bacteria Mycobacterium avium subsp. paratuberculosis. Affected species include cattle, sheep, goats and camelids. The disease is present worldwide with the highest prevalence occurring in dairy cattle. In these animals the disease is responsbile for severe economic losses due to reduced reproductive performance, increased susceptibility to disease, reduced milk production and culling losses.
Pathogenesis
Mycobacterium paratuberculosis is spread through herds via faecal-oral transmission, contaminated water and in utero. Young animals less than a year of age are most susceptible to infection and this usually occurs via 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 and uptake by the Peyer's Patches, infection begins in the ileum. M. paratuberculosis infects macrophages in the gastrointestinal tract and lymph nodes leading to a granulomatous inflammatory response. This leads to malabsorption and a protein-losing enteropathy.
Clinical Signs
Cattle become infected by the bacteria as calves but do not display the clinical signs of disease until between two and five years of age. In addition 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 clinical signs become more severe. The characteristic clinical signs are of watery 'pipe-stem' diarrhoea and severe wasting (despite maintenance of 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.
Gross Pathology
Lesions are usually localised to the terminal portion of the ileum. The mucosal surface is diffusely thickened with transverse, corrugated rugae which do not disappear when the intestinal wall is stretched. In sheep and goats however, the lesions usually described are nodule formation with necrosis and calcification .
Infected animals may also have enlarged mesenteric lymph nodes.
Diagnosis
Diagnosis is difficult, particularly in the case of subclinical disease as there is no single test that will detect all stages of the disease. Bacterial culture is the most definitive test but is time-consuming, requiring up to twelve weeks for incubation.
The most commonly used diagnostic test is the ELISA which detects antibodies to M. paratuberculosis in clinically affected animals. This is typically used in combination with post mortem and identification of the classic pathological lesions of the disease. Other commercially available tests include baceterial culture, PCR and agar gel immunodiffusion. Test sensitivity may be increased by using different tests in combination.
Diagnosis in sheep may also be complicated due to the disease occurring in two different types in these animals. Multibacillary disease is associated with large numbers of bacteria in the gut and a high antibody response. Paucibacillary disease is characterised be few or no bacteria within the alimentary tract and is associated with a low antibody response. Due to the different type of immune response occurring with each type of disease, there is no single test available to detect the presence of disease. Definitive diagnosis is provided following post mortem exam, histopathology and culture in order to determine the type of disease present.
Treatment
Several studies have reported an improvement in clinical signs following antimicrobial treatment of affected animals. However the same studies have shown that faecal shedding of the bacterium still occurs following treatment, therefore such treatment does not appear to be currently viable. Control measures are aimed at providing good hygiene and husbandry, maintaining adequate stocking densities and minimising exposure of young animals to the organism. Current recommendations include providing clean, manure-free areas for cows to calve and only administering pasteurised or test-negative colostrum to new born calves.
References
- Jones, T. C., Hunt, R. D., King, N. W. (1997) Veterinary Pathology Wiley-Blackwell'
- Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial