Field evaluation of the tracer weaner model : early detection of sheep with Mycobacterium avium subsp. paratuberculosis.

Abstract

Experiments were conducted on two separate farms in the endemic area for OJD to determine whether tracer animals could be used to detect infective levels of S strain M. a. paratuberculosis on pasture. Culture from tissues was shown to be the most sensitive method for the detection of early infection in sheep after natural exposure to S strain M. a. paratuberculosis. The organism was detected in at least one naive introduced sheep from every potentially exposed group, 6 to 12 months post-exposure. Antemortem diagnostic tests (skin testing, IFN-γ and faecal culture) were shown to have low sensitivity at this early stage of naturally acquired disease. The prevalence of infection early after exposure was similar in sheep first exposed as neonates, as weaners or as adults. Lambs born from an infected flock became tissue culture-positive sooner than naive lambs suckling uninfected ewes introduced to the same infected environment. These findings suggested that groups of naive sheep, used as tracer animals and tested by tissue culture at slaughter after 6 months exposure, might be useful to assess pasture infectivity in disease control programs.