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Pilot Study: Tracer Weaner Trial for Ovine Johne's Disease
Eradication of ovine Johne’s disease requires destocking sheep and spelling land until Mycobacterium avium subsp. paratuberculoisis has died out. At the conclusion of a decontamination period, currently deemed to be 2 consecutive summers or 15 months, there is no simple way to assess either the residual level of contamination of the environment or the risk residual contamination poses to livestock.
This study was undertaken to determine whether it is possible to detect infection in young sheep exposed to low levels of contamination. Previous experiments in the United Kingdom suggested that this might be possible, but the trials had been done with cattle strains of M. avium subsp. paratuberculosis. Consequently in this project small groups of Merino weaners were orally dosed on repeated occasions with graded numbers of an Australian sheep strain of M. avium subsp. paratuberculosis. Samples were collected from each weaner at regular intervals and evaluated using several tests.
Within 7 to14 weeks the organism could be cultured from various locations in the gastrointestinal tract and associated lymph nodes of each sheep that was given a total dose of 1.3 to 6.9 x 107 organisms. Infection was not established in weaners given lower doses. The next lower dose was 8.2 x 103 . Therefore the infectious dose of this strain of M. avium subsp. paratuberculosis is somewhere between about 103 and 107 organisms, a value higher than that found in the UK experiments. Reevaluation of the methods used in the UK experiments suggest that the doses given may have been underestimated by a wide margin. Thus the present results suggest that the infectious dose of M. avium subsp. paratuberculosis may be higher than previously thought with the implication that it may be possible to reduce decontamination periods for land. In other words it might be easier to achieve decontamination of land than previously imagined.
Although infection was found by culture of tissues of these weaners, there would be a need to culture all sentinel sheep unless an indirect method could be found to identify sheep most likely to be culture positive. Fortunately the results of a skin test were positive in 66% of weaners in which the organism had established an infection and were negative in uninfected sheep, regardless of whether they had been dosed with the organism or were in a control group. The results of tests for gamma interferon were positive in some infected weaners but also in some of the controls so this test was not of value. An ELISA test also was not useful at this early stage of infection. Overall, the results suggest that weaner sheep could be used as sentinels in an infected environment, and that the skin test could be used to select individual sentinel sheep for culture. This approach requires validation in a controlled field trial.
The experimental infection method developed in this study can be used later for other purposes such as evaluating the pathogenesis of ovine Johne’s disease, the behaviour of new diagnostic tests and vaccine efficacy.
This page was last updated on 10/11/2014
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