Prevalence of Johne’s disease in rabbits and kangaroos

Summary

Following reports from Scotland that rabbits on JD-infected farms in the Tayside region were infected with M avium subsp paratuberculosis, a study of rabbits and kangaroos on OJD-infected farms in NSW was commenced. Three hundred rabbits and 300 eastern grey kangaroos from 10 farms grazing OJD-affected sheep flocks were killed and examined for evidence of JD between late 1996 and late 2000.

Two hundred and fifty three rabbits were tested by radiometric culture of their faeces, while 47 were examined by smear and ZN stain of tissues combined with histopathology of the lower small intestine and regional lymph nodes. No evidence of JD or the causative organism was detected in any rabbit.

For kangaroos, 206 were examined primarily by faecal culture and 94 by smear and histopathology. Some animals were examined by faecal culture and histopathology. One kangaroo specimen produced evidence of low numbers of M avium subsp paratuberculosis in faeces but histopathological examination revealed no evidence of active infection, which might cause multiplication of the bacteria within the kangaroo. It was concluded that the bacteria identified in this animal were bacteria which had been ingested from pasture contaminated by OJD-infected sheep and which had survived passage through the gut.

Considering information available from studies of wildlife in Kangaroo Island and of rabbits in Scotland, we conclude that the prevalence of JD in rabbits and kangaroos on OJD-affected farms in Australia is very low (less than 1% of the adult population) or zero. Nevertheless, we recognise that there is a risk that adaptation of the organism to wildlife hosts could occur in future. There is evidence that the grazing pressures exerted by rabbits and kangaroos on sheep pastures in Australia is similar to that exerted by rabbits on beef cattle pastures in Scotland. We hypothesise that the S strains responsible for all or most of the OJD infection endemic in NSW sheep flocks are more host specific than C strains, particularly the C strains identified in cattle and rabbits in the Tayside region of Scotland.

The positive finding of M avium subsp paratuberculosis in the faeces of a kangaroo implies that there is a risk of physical transfer of organisms from one farm to another which may lead to transfer of infection from an infected flock to a neighbouring uninfected flock by kangaroos.

These results, together with the overseas findings, suggest that further research activities should be conducted into the host specificity of strains of M avium subsp paratuberculosis and the species studied should include the common domestic animals farmed in Australia and the rabbit and kangaroo. Further action on this recommendation should be postponed until the current study on KI is completed.