Ewe-Lamb Transmission Of Ovine Johne's Disease

Summary

Knowledge of the likelihood of intrauterine or transmammary transmission of M. a. paratuberculosis infection in sheep is important in the design of control programs. Little has been published on this aspect of ovine Johne's disease, although studies in cattle have found up to 25% of foetuses from clinically affected cows to be infected.

In this study 145 ewes from a heavily infected flock on Farm A, and their late term foetuses were examined. A sub-sample of 125 of the ewes had been screened for OJD between August and November 2000. At that time, blood samples were collected for AGID and IFN-γ testing, faecal samples were collected for OJD culture, and skin testing for delayed-type hypersensitivity (DTH) was performed. In August 2001, in the weeks preceding necropsy, blood samples were collected for AGID and IFN-γ, and faecal samples for OJD culture and direct PCR. At necropsy, tissue samples were collected from the ewes and their foetuses for OJD culture and histopathological examination. During other studies on Farm B, a further six pregnant sheep were necropsied after showing clinical signs suggestive of OJD, and results from these were included in the current study.

Five of six ewes with clinical OJD had infected foetuses. These results suggest that 53 – 99% of ewes with clinical and confirmed OJD could be expected to have an infected foetus. These five ewes were all in the advanced stages of OJD. In two for which full pathological results were available, severe diffuse pathology was present (one multibacillary and one with fewer organisms).

Only one of 54 subclinically affected ewes had an infected foetus. This ewe was culture positive in the uterus only, but did have clinical signs suggestive of OJD. These results suggest that 0.3 –12.5% of ewes with subclinical OJD could be expected to have an infected foetus.

None of 16 uninfected control ewes had an infected foetus.

Only two of 48 ewes (43 infected and five apparently uninfected) had M. a. paratuberculosis in milk or mammary tissue. These results suggest that 0.5 – 17%. of infected ewes could be expected to excrete culturally detectable M. a. paratuberculosis in milk.

The findings from this study are unlikely to significantly alter the current approaches taken by industry for control of OJD. Even on farms with a high prevalence of infection, the risk of intrauterine and transmammary transmission is likely to be small if suspected clinical cases of OJD are promptly culled. Such sheep are of greatest risk to the rest of the flock by their potential excretion of enormous numbers of M. a. paratuberculosis. The congenital risk to their own offspring is thus a minor risk for OJD transmission at the flock level. The low level of demonstrated foetal infection in subclinically affected ewes and in uninfected ewes is of more importance. Considering these two groups together, the upper 95% confidence limit for infected foetuses was just 7.5%. Remembering that only 16 uninfected ewes were included and that this is a worst case scenario in heavily infected flocks, these finding provide some assurance that congenital infection is unlikely to be a significant barrier to existing control or stud recovery programs, especially if antemortem testing is undertaken.

This project was also designed to provide samples from adult sheep with defined M. a. paratuberculosis infection for gamma interferon test validation, to enable CSIRO to carry out its obligations under the project OJD.025. Samples were successfully collected for testing, and culture and pathology results provided, from 145 ewes.