Johne's Disease

Also known as:

Paratuberculosis

Description

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.

Pathogenesis

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.

Stage 1: This often goes unnoticed, as it is subclinical. It typically affects animals less than two years of age, and will advance to stage II, only a few months later.

Stage 2: This is again a subclinical infection, usually affecting older heifers, or young adults. Infected animals are apparently healthy, but are shedding infection within their manure, infecting the environment.

Stage 3: Advanced infection. Animals will show clear clinical signs, including weight loss, decreased milk production, and a severe reduction in appetite.

Animals with advanced stage 3 will appear gaunt, and the meat is no longer deemed fit for human consumption. Once the disease is present in the herd, it is ver difficult to get rid of it.

Clinical Signs

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 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.

Gross Pathology

The infected host appears emaciated. Fat is pale and oedematous, and only present in small amounts. Signs are confined to the terminal small intestine (especially the ileum) but are characteristic. The mucosal surface is diffusely thickened with transverse, corrugated rugae. Infected animals may also have enlarged mesenteric lymph nodes.

Proliferative ileitis in Johnes disease (Courtesy of Bristol BioMed Image Archive)

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

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. The most commonly used diagnostic test is the ELISA which detects antibodies to M. paratuberculosis in clinically affected animals.

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

60% of cases have lesions in colon and rectum and can be diagnosed by rectal biopsy.