Paratuberculosis is a gastrointestinal disease of cattle worldwide and has huge economic and zoonotic implications for the cattle industry. Characterised by chronic diarrhea, weight loss and debility, it is serious enough to warrant a listing by the OIE. This disease is found in over 68% of cattle herds in the United States.
M. paratuberculosis is excreted in the feces and milk and is highly resistant in the environment for up to one year. Infection is usually acquired through oral ingestion of contaminated grass or feed.
Early clinical signs are characterised by persistent diarrhoea and weight loss which is unresponsive to standard antimicrobial therapy. Up to 50% of herds may be infected, though death rates are not usually as high. In pregnant cows, infections may result in abortions.
Cattle are most often infected as young calves, before 6 month of age, but some studies have shown that infection may also occur in adult cattle. Fecal shedding of MAP generally starts after 2 years, and clinical symptoms appear after an incubation period of 2–10 years. However, most infected cattle show no clinical signs during the prolonged incubation stage of infection. In addition, the elimination of the agent through the stool is very variable.
Diagnosis is based on presenting clinical signs and supportive diagnostic tests, including isolation of M. paratuberculosis in culture as well as ELISA tests and PCR testing.
Cell-mediated immune response wanes with progression of the disease and when this occurs, a humoral immune response becomes measurable. Therefore, comparative studies of ELISAs with different antigens have shown discrepancies in the ability of these tests to identify all infected animals.
In-field tests such as intradermal testing may be of use but microscopic examination of Ziehl-Neelsen-stained fecal samples remains the most definitve testing method.
Poor response to antimicrobial therapy is a consistent finding. Supportive anti-diarrhoeal formulations and broad-spectrum antimicrobials may be of palliative use in valuable cattle.
Control measures must be aimed at having calves birthed in areas free of manure, removed from the dam immediately after birth in the case of dairy cattle, bottle-fed colostrum that has been pasteurized or obtained from dams that test negative, and then reared segregated as much as possible from adults and their manure until >1 yr old. Use of milk replacer is recommended instead of waste milk unless the milk has been pasteurized.
- Dept of Agriculture, Ireland
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