Environmental survivability of Mycobacterium avium subsp. paratuberculosis (Mptb) on northern grazing properties

Summary

This study primarily examined the environmental survival of the Bison and Bovine strains of Mptb provided by Queensland Department of Agriculture and Fisheries from identified cases in Queensland cattle.

The study was conducted over a fifteen month period that allowed samples to be evaluated over a full summer and winter period to determine the survival attributes for the Mptb strain isolated in Queensland at four distinct sites using site specific soils.

The identification of two Queensland beef properties in 2012 and 2013 infected with Mptb not related to introduction to the properties of Australian cattle infected with known strains occurring in southern States, altered the epidemiological paradigm for Mptb in Queensland. It was necessary to establish environmental survival parameters of the new strain to provide guidance for control programs.

In addition, the potential for atypical mycobacteria, known to be present in the Queensland environment and previously isolated from cattle lymph nodes, to confound results of the recently approved HTJ PCR for diagnosis of presence of Mptb in faeces was investigated.

The final aspect of the research related to the development of techniques to demonstrate viability of bacteria in faecal and environmental samples positive to PCR testing for bacterial DNA.

Previous work (Whittington 2001) has indicated that Mptb ovine strain is capable of surviving in cool moist environments for periods of time exceeding 12 months. Temperature flux was thought to be a critical factor in survival times for these projects. This study has established that under certain conditions the Bovine and Bison strains can survive in northern Australia for periods exceeding 12 months. The effects of temperature flux were investigated but not clearly established. The previously proposed concept of dormancy for Mptb appears to be confirmed by this project.

The project also investigated the use of supplementary tests to PCR for determining viability of Mptb isolated from the environment and faecal samples. The use of propidium monoazide techniques to confirm viability was found to be unsatisfactory. Using commercial RNA extraction test kits proved to be more reliable than bacteriological culture to demonstrate the potential for recovery of viable organisms from faeces and environmental samples.

Key findings

The study confirmed that the PCR used in diagnostic tests was specific for Mptb isolates found in cattle in Australia. Environmental mycobacterial isolates were for the most part not a factor in confounding the results and any that were replicated were able to be easily distinguished by the product size.

The study provides evidence that survival of viable Mptb organisms is possible in northern regions of Queensland for a period exceeding 12 months. Survival is enhanced in intact faecal pats protected from moisture.

While the study provides evidence of survivability of the organism in faeces and the environment it does not confirm the potential for infection of stock grazing land that has previously been contaminated by Mptb in faeces. The potential for infectivity of the environment over time was not tested in this study.