Johne's disease
It is also known as paratuberculosis characterized by chronic enteritis and progressive weakness in dairy animals.
Etiology: Mycobacterium paratuberculosis, the causative organism for this chronic disease, is an acid fast and comparatively resistant bacillus. The principal mode of spread of disease is through ingestion of contaminated feed or water or through intrauterine route. Morbidity rates are high but mortality is only 1-10% and main losses result from reduced production or weight gain.
Pathogenesis: Organisms localize in the intestinal mucous membrane and adjoining lymph nodes and multiply there.
Clinical signs: In cows, there is reduced milk production, progressive weight loss and submandibular edema. There is diarrhoea and faeces resemble pea soup and are without offensive odour. Feed intake remains normal but water intake is increased. In sheep and goats, faeces are soft but diarrhoea is usually not seen. The animals show progressive weakness and alopecia. In goats depression and dyspnoea are additional signs.
Diagnosis: It is diagnosed by clinical signs and history of persistent diarrhoea and continuous but slow weight loss. It can be confirmed by performing intra-dermal Johnin test. Johnin PPD 0.2 ml is injected by intra-dermal route in the cervical region. In positive cases, skin thickness at injected site increases by 3 mm or more by 48 to 72 h. Faecal or rectal pinch examination also confirms the disease. Serological tests like indirect fluorescent antibody, ELISA and complement fixation tests are also confirmatory. The organisms can be grown from faecal samples. However, none of these tests taking one at time can detect all the cases of paratuberculosis, so 2 or 3 tests are performed for its diagnosis. The advanced clinical cases can be diagnosed easily by clinical symptoms, faecal examination and antibody response of the host while preclinical cases can be detected by lymphocyte transformation, antibody response and faecal examination. Resistant animals can only be diagnosed by lymphocyte transformation test.
Treatment: The treatment is difficult as organisms are mostly resistant to antibiotics and it requires prolonged treatment. There is some recovery, if animals are treated with streptomycin @ 50 mg/kg body weight for 20-30 days. Combination of 500 mg dihydrostreptomycin intramuscularly and 300 mg- each of rifampin and isoniazied given twice daily orally were found effective in goats.
Control: The long incubation period and lack of single diagnostic test makes the disease difficult to control. Eradication of infected animals and carriers and maintenance of strict hygienic conditions are of some value. An animal herd may be declared free of paratuberculosis only when they do not show symptoms up to 3 years, feacal culture of above 2-year-old animals is negative and Johnin testing gives negative results.