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Investigation of potential diseases associated with NorthernTerritory mammal declines - Final report

Reiss, Andrea, Jackson, Bethany, Gillespie, Graeme, Stokeld, Danielle and Warren, Kris (2015). Investigation of potential diseases associated with NorthernTerritory mammal declines - Final report<br />. Darwin, NT: Charles Darwin University.

Document type: Research Report
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Author Reiss, Andrea
Jackson, Bethany
Gillespie, Graeme
Stokeld, Danielle
Warren, Kris
Title of Report Investigation of potential diseases associated with NorthernTerritory mammal declines - Final report
Publication Date 2015
ISBN 978-1-925167-32-0   (check CDU catalogue open catalogue search in new window)
Publisher Charles Darwin University
Place of Publication Darwin, NT
Total Pages 144
Field of Research 300800 Environmental Sciences
Abstract Executive summary

There is compelling evidence of broad-scale declines in populations of small terrestrial native mammals in northern Australia, including the Top End of the Northern Territory (NT) over the past 20 years. Causes under consideration include changed fire regimes, introduced fauna (including predators) and disease. To date information on health and disease in northern Australian mammals has been limited.

Disease is increasingly recognised as a primary driver of some wildlife population declines and extinctions e.g., Tasmanian devil facial tumour disease, white nose syndrome in bats and chytrid fungus in amphibians.

Disease has been identified as a risk factor for extinction in declining and fragmented wildlife populations globally, particularly in situations of increased environmental stressors, changing ecosystems, arrival of new vertebrate threats or climate change. Unless wild populations are studied in detail over long periods of time, the effects of disease are easily overlooked and may be difficult to determine.

This study is the largest and most comprehensive study of health and disease in small mammals in northern Australia and is one of a small number of studies worldwide to have approached investigation of wildlife populations in this comprehensive manner.

A total of 281 individuals from four target species were examined and sampled under anaesthesia across five main sites in the Top End of the NT, from June 2013 to Nov 2014. Non-invasive samples (ticks and faeces) were collected from a further 113 animals.

Nine prioritised pathogen groups were investigated by diagnostic testing:

• encephalomyocarditis virus (EMCV)
• mammalian herpesvirus
• Coxiella burnetii (disease agent causing Q fever)
• Leptospira spp.
• enteric Salmonella spp.
• enteric protozoa (Cryptosporidium spp. and Giardia spp.)
• protozoal haemoparasites (trypanosomes, Babesia spp. and Hepatozoon spp.)
• Toxoplasma gondii
• gastrointestinal helminths (worms)

Additional investigation was undertaken under collaborative agreements for pathogens of significance to human health: Ross River virus and Barmah Forest virus.

Results were analysed for associations with locations, species, seasons, body condition, sex, blood parameters and other potential health indicators such as level of ectoparasite burden.

The majority of individuals examined were assessed to be in good health and body condition.

The presence of several pathogens which are known to be associated with disease in wildlife populations was identified, including mammalian herpesvirus, enteric Salmonella spp., protozoal haemoparasites (trypanosomes, Babesia spp., Hepatozoon spp.), enteric protozoa (Cryptosporidium spp. and Giardia spp.), microfilaria and Toxoplasma. Of these, several were previously unreported in target species in the NT.

De novo molecular pathogen discovery studies on the northern brown bandicoot (Isoodon macrourus) used cutting-edge metagenomic techniques to look for unrecognised pathogens. Analysis is still underway; however there is evidence of several potentially significant pathogens including viruses from the Retroviridae family.

A number of ectoparasite taxa that may act as vectors for infectious disease were identified.

The study found evidence that several pathogens, capable of impacting population health, are circulating in Top End small mammal populations, but did not find compelling evidence that a single pathogen is responsible for, or a risk factor in, the decline of small mammals in the Top End of the NT.

The study found no serological evidence of infection with encephalomyocarditis virus, Leptospira spp. or Coxiella burnetii; however it is possible that these pathogens are present in populations (at levels below detection limits due to sample sizes), with resultant morbidity or mortality.

Limitations of this study include a short temporal span, a lack of longitudinal and survivorship studies, difficulties in collecting specimens from small species and an inability to study populations in the absence of feral cats and other predators. In combination these factors limit the ability to investigate the potentially complex interactions between disease and other pressures on mammal populations.

Top End mammal populations are assessed to be at risk of increased levels of environmental and host stresses and are vulnerable to the likely future impacts of infectious disease. The current situation in the Top End fulfils many of the criteria necessary for an emerging infectious disease and novel disease agents should be considered as risk factors for populations. It is recommended that disease investigation continues in mammal populations of concern in the Top End in the medium to long term (five to 20 years) to increase knowledge; improve data sets; maintain and build capacity; and maximise opportunities for detection and response to new disease threats.

It is recommended that future studies include, as priorities:

• longitudinal and survivorship studies to determine the impact of nominated pathogens on host
  survival and fitness (e.g., using cortisol and anti-oxidant capacity), and detect pathogen trends in
  individuals (e.g. whether certain pathogens are shed intermittently and whether infections persist or
  resolve).
• a particular focus on Toxoplasma gondii (due to its strong epidemiological link to presence of cats
  in the environment and as the only identified pathogen likely to impact a broad taxonomic range
  of mammals) and any potentially significant pathogens emerging from de novo molecular work.
  This should include a focus on sites where feral cats have been excluded, to investigate potential
  interactions between pathogen presence and predation as well as differences in prevalence of T.
  gondii in areas where the definitive host (the cat) is present/absent
• extension of serological studies to a wider host species range (including macropods) to determine
  the presence and prevalence of priority pathogens such as T. gondii in the landscape.

Ongoing collaborative efforts between NT Department of Land Resource Management and the Conservation Medicine Program (School of Veterinary and Life Sciences), Murdoch University may facilitate research and enable continued opportunities for work funded through competitive grants or industry sponsorship.
Additional Notes This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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