Early Detection of natural infection of sheep with Mycobacterium avium subsp. paratuberculosis

Summary

Eradication of ovine Johne’s disease (OJD) requires de-stocking sheep and spelling land until M. a. paratuberculosis has died out. The use of tracer animals (sentinels) can provide an early warning that infectious organisms are still present. However, Johne’s disease differs from most other infectious diseases of farm livestock because of the long incubation period, and a method for early detection of infection in tracer animals was needed. In a pilot study (Project TR.073), 100% of weaner sheep experimentally infected with >107 organisms of S strain M. a. paratuberculosis were detected by culture of tissues two to four months later, and skin-testing with Johnin detected 66% of infected sheep with 100% specificity. If such early detection were also possible in naturally exposed sheep, and particularly if skin testing allowed targeting of likely-infected sheep for necropsy, then the use of tracer weaner sheep to determine the possible infectivity of pasture could be a practical option in the control of OJD.

Experiments were conducted on two separate farms in the endemic area for OJD. Despite significant uncontrolled differences, a general ranking of likely levels of exposure to M. a. paratuberculosis from very high to low was possible. The examination of the tracer concept in several different environments was considered important, if the technique was to be used in the future to examine pasture infectivity in a wide range of field situations.

In the most heavily contaminated environment, 43 to 75% of sheep sampled ≥8 months post-exposure were culture-positive. Infection in introduced naive sheep was first detected by culture of tissues 5 - 6 months after potential exposure. In environments with lower levels of contamination, infection with M. a. paratuberculosis was detected by tissue culture in at least one naive introduced sheep from every potentially exposed group, 6 to 12 months post-exposure. In the lowest contamination scenario, where pasture had been grazed by infected ewes for two months only, then left ungrazed for a month before the introduction of tracers, a single infected sheep was detected 12 months after first exposure.

Antemortem testing for specific CMI responses was not useful in the field application of the tracer concept. Skin-tests and IFN-γ tests had very low sensitivities in samplings <12 months post-exposure.​>