Diagnostic, predictive and preventative tools for Johne’s disease in sheep and cattle

Summary

Johne's disease. Johne's disease (JD) is a chronic infection of the intestinal tract and associated lymph nodes caused by Mycobacterium avium subsp. paratuberculosis (Mptb). Most infections are acquired soon after birth but progress slowly, and signs of disease are usually not seen for 1 to 3 years in sheep and for 3 to 10 years in cattle. The hidden phase is called subclinical infection. During this long incubation period tests for JD are inaccurate. Mptb is passed in faeces (termed faecal shedding) long before there are overt signs of disease, and this perpetuates the infection cycle in a herd or flock. Subclinically infected animals can be moved to other farms, thereby spreading the infection. Eventually the signs appear: weight loss, and in cattle also diarrhoea, followed by death. Initially only a few animals are affected each year, but the numbers inevitably increase. While upfront costs are less than those of other endemic diseases, internationally JD is regarded as an undesirable problem, and some countries impose control measures that can impact trade. The distribution of JD in flocks and herds in Australia is very uneven, which creates an opportunity to reduce spread, provided better tools are developed to detect, control and prevent the disease.
Scope of Project P.PSH.0576. This is a large program of basic and applied research, commissioned to provide a foundation for better tests and future control options, while ensuring that current diagnostic tools remain available and are as accurate as possible.

The project was divided into three research subprograms and an animal trial program, and it contained an explicit objective to build animal health research capacity for the livestock industries.

1. Predictive and diagnostic tools - to enable producers to identify exposed and infected herds/flocks/individuals and to predict those likely to spread the disease.
2. Animal genomics - to identify resistant animals, and to determine the susceptibility of different breeds of sheep.
3. Improving vaccine technology – to conduct research towards a safer, more effective vaccine for sheep and cattle.
4. Research capacity for livestock health - to maintain and develop capacity in livestock health for the benefit of producers.

We concentrated on the disease in the period soon after exposure to Mptb (< 12 months), while animals had early subclinical infection, because we were most interested in determining early predictors of future disease outcome. This work required long term experimental infection trials in both sheep and cattle. There were remarkable discoveries and practical achievements.