Johne's Disease

Also known as:

Paratuberculosis

Description

Johne's Disease is a contagious and chronic disease of ruminants caused by Mycobacterium avium subsp. paratuberculosis. Affected species include cattle, sheep, goats and camelids. The pathogen primarily affects the small intestine and is responsbile for severe economic losses due to culling losses and reduced reproductive performance and milk production. Calves are most commonly affected by this condition, and can be infected via a number of routes.

Pathogenesis

MAP is primarily spread through herds via faecal-oral transmission, contaminated water, in utero and through infected milk and colostrum.

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

The infection primarily affects the intestine, and causes thickening of the mucosal wall. This causes impaired function, affecting its absorptive ability. This may cause, severe emaciation, and chronic profuse diarrhoea.

Clinical signs may also develop in older cows during times of stress; around calving for example.

In advances cases the infected animal gradually fades away and dies over the course of months.

Pathogenesis

Organisms penetrate the M-cells of the Peyer's patches. Mycobacteria invade macrophages and cause a granulomatous inflammatory response. Death may result from: damage to the mucosa, not absorbing nutrients, or inflammatory loss of protein.

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. It has diffusely thickened, velvety mucosa surface and transverse, corrugated ruggae with reddened crests. Infected animals may also have enlarged mesenteric lymph nodes

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

Changes are milder in sheep and goats, than seen in cows, and is symptoms are often missed.

Histologically

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.

Bacteria act like foreign bodies producing a type IV hypersensitivity reaction. Sheep have two different forms: 1. Paucibacillary; many T cells, and few bacilli.

2. Multibacillary: Many macrophages, many bacilli in macrophages, and few lymphocytes.

Diagnosis

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.