Johne’s disease, Biology

Johne’s disease

The disease, discovered by Heinrich A. Johne, a German bacteriologist and veterinarian, in 1905, is caused by a bacterium named Mycobacterium avium subspecies paratuberculosis, an acid-fast bacillus often abbreviated MAP. Johne’s disease, also called as paratuberculosis, is a specific chronic contagious enteritis of cattle, sheep, goats, camels, buffaloes and occasionally of pigs. The disease is characterized by progressive emaciation, and in cattle and buffaloes by chronic diarrhoea and thickening of the intestine.Mycobacterium avium spp. paratuberculosis, the causative acid-fast organism is short, thick, rod like and found in clumps. It is slow growing organism and can be cultivated on artificial media (Herrolds medium containing mycobactin J). The organism survives for a long time in bovine faeces and urine in cattle sheds and in pastures. The organism is resistant to chemical disinfectants and heat.


Transmission:
The disease spreads by ingestion of feed and water contaminated by the faeces of infected animals. The infection occurs mostly in the early months of life. The incubation period extends from 12 months to several years. The animals aged 3 to 6 years mostly suffer from the disease. Affected animals may not show clinical symptoms and continue to discharge organisms in the dung. The organisms persist in pastures for about 1 year. The organisms are susceptible to sunlight, drying and high pH of soil; continuous contact of urine with faeces reduces the life of bacteria.In cattle, clinical signs appear mainly during 2-6 years of age. The infected animals which are apparently healthy, often show clinical signs after parturition. Emaciation is the only abnormality in early stages, accompanied by oedema of submandibular region. The oedema disappears when diarrhoea sets in. The faeces are soft and thin with mucus, and are without offensive odour. Diarrhoea may be continuous or intermittent with a tendency to improve in late pregnancy and appears again after parturition. The course of the disease varies from weeks to months and finally ends in death.In sheep and goats, the disease is manifested by emaciation. Diarrhoea is less pronounced. In sheep, there may be shedding of wool.


Lesions: The carcasses of chronically infected cattle are emaciated. The lesions occur most frequently in the vicinity of terminal portion of small intestine and the ileocaecal valve. In early stages this area may be oedematous, and in more chronic cases the intestinal mucosa may become wrinkled and corrugated. The mesenteric and ileocaecal lymphnodes may be enlarged and oedematous. In advanced cases of the disease there is high incidence of arteriosclerosis.The lesions in sheep are similar to those in cattle. Infection with pigmented variety gives rise to orange discolouration of mucus surface, although corrugation of mucosa may not be present and the intestinal wall may be thickened. The mesenteric and ileocaecal lymphnodes may be enlarged and oedematous, there may also be caseation and calcification.


Diagnosis: Clinical symptoms like intermittent diarrhoea and gradual loss of condition are indications of Johne’s disease. Microscopical examination of a number of samples of faeces from an individual should be undertaken because a single sample from each case results in diagnosis of about 30% cases. The presence of acid-fast organisms confirms the diagnosis. The bacteriological examination of faeces is a cumbersome diagnostic procedure. Johnin is inoculated intradermally which results into a hypersensitivity reaction. Johinin is manufactured and supplied by the Division of Biological Products, Indian Veterinary Research Institute, Izatnagar (Uttar Pradesh).


Treatment: The organisms is more resistant to chemotherapeutic agents in vitro and therefore the practical utility of treatment in clinical cases is poor.


Control:
The affected animals should be segregated and their faeces properly disposed off. A live vaccine has been developed. It reduces the incidence of clinical disease. It consists of a non-pathogenic strain of Johne’s bacillus with an adjuvant. The calves and lambs soon after birth are inoculated with the vaccine subcutaneously. The vaccinated animals become reactors to Johnin. Vaccination is generally done in heavily infected herds. In India, vaccination is not practiced.

Posted Date: 9/17/2012 6:42:26 AM | Location : United States







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