Wobbly possum disease Fact sheet Introductory statement Wobbly possum disease is a condition of brushtail possums (Trichosurus vulpecula) that was first identified in a research facility in New Zealand in 1995 (Mackintosh et al 1995). Since then it has also been recognised in free-living possums (Perrott et al 2000b, Perrott et al 1999). A similar condition has been reported from brushtail possums in Australia, but it is thought that the Australian disease may be caused by a different, unidentified agent (Rose 1999). Aetiology Recent research indicates that wobbly possum disease is possibly caused by a novel nidovirus, most closely related to the family Arteriviridae (Dunowska et al 2012). Natural hosts The condition has only been described in brushtail possums (Ladds 2009). World distribution Wobbly possum disease has only been recognised in New Zealand. Occurrences in Australia A condition, clinically similar to wobbly possum disease, has been reported from brushtail possums in eastern Australia and Tasmania. Epidemiology Case fatality rate is greater than 95%. Incubation period is between five and 20 days when possums are inoculated intra-peritoneally with tissue suspensions (Mackintosh et al 1995).
In one study possums were administered tissue suspensions (by the intra-gastric, intra-tracheal and intradermal routes), urine (by the intra-peritoneal route), blood (by the intra-dermal and intra-peritoneal routes) and homogenised mites (Trichosurolaelaps crassipes) (by the intradermal route) from infected possums. All possums given these inoculates developed wobbly possum disease. Non-inoculated possums, both adults and joeys that had direct contact with the inoculated possums also developed the disease. Individually caged control possums did not, indicating that close contact is required for disease transmission (Perrott et al 2000a). The agent remains infective after freezing at -75 0 C and can pass through a 0.22 micron filter (O Keefe et al 1997). Clinical signs Clinical signs in New Zealand possums include docility and dullness progressing over one to six weeks to gait abnormalities (including incoordination, loss of balance, difficulty climbing, stumbling and lameness), aimless wandering, daytime feeding, wasting and blindness. Anaemia and hyperglobulinaemia also occur (Perrott 1998). The disease in Australian possum s progresses over weeks to months and includes depression, ataxia, blindness and persistently dilated pupils (Hufschmid and Holz 2012, Ladds 2009, Rose 1999). Diagnosis Diagnosis is based on clinical signs presenting in conjunction with typical histological changes. Pathology There are no significant gross lesions apart from reduced fat reserves. Histologically wobbly possum disease presents as perivascular infiltration of a range of tissues with mononuclear leukocytes, especially lymphocytes and plasmacytes. There tends to be a mild to moderate nonsuppurative meningoencephalitis with more severe changes occurring in the liver and kidney, including periportal or diffuse mononuclear infiltrates, focal hepatic necrosis and non-suppurative interstitial nephritis. Similar lesions have also been reported from the spleen, lung, bladder, heart and lymph nodes. The Australian condition is characterised by moderate to marked non-suppurative inflammation in the meninges and perivascular infiltration within the brain parenchyma. In addition, non-suppurative inflammation and Wallerian degeneration in the optic tract commonly occurs along with atrophy of the cerebellar folia and retina. The eyes may also show foci of tapetal discolouration, a pale optic disc or an optic disc that lacks the normal vascular tuft (Hufschmid and Holz 2012, Ladds 2009, Rose 1999). Differential diagnoses Differential diagnoses include traumatic injury and toxoplasmosis. Though it has never been reported, consideration should be given for ruling out Australian bat lyssavirus in Australian possums with neurological disease and/or encephalitis. WHA Fact sheet: Wobbly possum disease Jan 2014 2
Laboratory diagnostic specimens A complete necropsy should be performed. Collect a range of tissues, including brain, liver and kidney, and submit them in formalin for histopathology. Frozen tissues should be held for possible further testing. Treatment None. Prevention and control Control and prevention of the disease in New Zealand is not desirable as it is being investigated as a possible biological control agent. There are no recommendations for control of the Australian disease because the aetiology is unknown and it appears to exist at a low level in the possum population. Surveillance and management Wildlife disease surveillance in Australia is coordinated by Wildlife Health Australia. The National Wildlife Health Information System (ewhis) captures information from a variety of sources including Australian government agencies, zoo and wildlife parks, wildlife carers, universities and members of the public. Coordinators in each of Australia's States and Territories report monthly on significant wildlife cases identified in their jurisdictions. NOTE: access to information contained within the National Wildlife Health Information System dataset is by application. Please contact admin@wildlifehealthaustralia.com.au. There is no targeted surveillance program, AUSVETPLAN, or Import Risk Analysis for wobbly possum disease. Wildlife Health Australia (WHA) is interested in receiving reports of this syndrome in Australian possums. Contact admin@wildlifehealthaustralia.com.au. WHA recommends consideration of exclusion of Australian bat lyssavirus in Australian possums with neurological disease and encephalitis. Statistics There is currently only one report in the National Wildlife Health Surveillance Database (ewhis). A wild, blind, ataxic brushtail possum from NSW was euthanased in 2005 and diagnosed with wobbly possum syndrome. Between 1985 and 1993 540 brushtail possums were submitted to the wildlife clinic at Taronga Zoo. Thirty of these had clinical signs of depression and blindness. Tissues from these animals, along with an additional 12 possums from NSW, Victoria and Tasmania were examined histologically. Twenty-three had chronic nonsuppurative meningoencephalitis (Rose 1999). A survey in 1999 of 887 possums in New Zealand found wobbly possum disease in 39 (4.4%) of the possums sampled (Thompson and McLeod 1999). A second survey in 2005 found one positive possum out of six that were necropsied (McLeod 2007). Research Further research is required in New Zealand to ascertain the potential for this disease to be used as a possum control agent. Confirmation is needed that the nidovirus that was isolated from affected possums is the WHA Fact sheet: Wobbly possum disease Jan 2014 3
causative agent of the disease. More information is also required on the viability of the virus, its rate of spread and distribution, mortality rates of possums in the wild, and the time between natural infection and death. Research on the Australian disease is necessary to determine the aetiological agent. To gain a more complete epidemiological picture as many cases as possible should be entered into ewhis. Human health implications None. Conclusions Wobbly possum disease, like tuberculosis, appears to be peculiar to New Zealand brushtail possums. As possum control costs New Zealand in excess of $100 million annually it would be prudent to continue research into this disease to determine its feasibility as a control agent. Surveys indicate that the disease is present in wild New Zealand possums but does not seem to be exerting a significant effect on numbers. Similarly, the Australian disease appears to exist at low levels in wild possums, also exerting minimal influence on populations. It will be interesting to see, once the aetiological agents have been identified, if we are in fact dealing with two different diseases or just variations of the same one. Acknowledgements This fact sheet was written by Peter Holz. References and other information Dunowska M, Biggs PJ, Zheng T, Perrott MR. Identification of a novel nidovirus associated with neurological disease of the Australian brushtail possum (Trichosurus vulpecula). Veterinary Microbiology 2012:156:418-424. Hufschmid J, Holz P. Dasyurids, numbats, possums and gliders. In: Diagnostic Pathology of the Diseases of Aquatic, Aerial and Terrestrial Wildlife. Wildlife Pathology Short Course, Sydney, 2012:180-206. Ladds P. Viral diseases in terrestrial mammals. In: Pathology of Australian Native Wildlife. CSIRO Publishing, Collingwood, 2009:9-24. Mackintosh CG, Crawford JL, Thompson EG, McLeod BJ, Gill Jm, O Keefe JS. A newly discovered disease of the brushtail possum: Wobbly possum syndrome. New Zealand Veterinary Journal 1995:43:126. McLeod BJ. Wobbly Possum disease in the Wilkin and Young Valleys, Mount Aspiring National Park. DOC Research and Development Series 268, Wellington, 2007. O Keefe JS, Stanislawek WL, Heath DD. Pathological studies of wobbly possum disease in New Zealand brushtail possums (Trichosurus vulpecula). Veterinary Record 1997:141:226-229. Perrott MRF. Viruses of the common brushtail possum (Trichosurus vulpecula). PhD Thesis, Massey University, 1998. WHA Fact sheet: Wobbly possum disease Jan 2014 4
Perrott MR, Meers J, Thomson DM, Wilks CR. Viruses of the brushtail possum (Trichosurus vulpecular) in New Zealand. In: Proceedings of the Wildlife Disease Association (Australasian Section) Annual Meeting, Jervis Bay, 1999:18-19. Perrott MRF, Wilks CR, Meers J. Routes of transmission of wobbly possum disease. New Zealand Veterinary Journal 2000a: 48:3-8. Perrott MRF, Meers J, Cooke MM, Wilks CR. A neurological syndrome in a free-living population of possums (Trichosurus vulpecula). New Zealand Veterinary Journal 2000b: 48:9-15. Rose K. Common diseases of urban wildlife. In: Wildlife in Australia. Proceedings 327 of the Post Graduate Foundation in Veterinary Science, Sydney, 1999:365-427. Thompson EG, McLeod BJ. The prevalence of Wobbly Possum disease in a bush/farmland environment. In: Proceedings of the New Zealand Society of Animal Production, 1999:59:233-235. To provide feedback on this fact sheet We are interested in hearing from anyone with information on this condition in Australia, including laboratory reports, historical datasets or survey results that could be added to the National Wildlife Health Information System. Negative data are also valuable. If you can help, please contact us at admin@wildlifehealthaustralia.com.au Wildlife Health Australia would be very grateful for any feedback on this fact sheet. Please provide detailed comments or suggestions to admin@wildlifehealthaustralia.com.au. We would also like to hear from you if you have a particular area of expertise and would like to produce a fact sheet (or sheets) for the network (or update current sheets). A small amount of funding is available to facilitate this. Disclaimer This fact sheet is managed by Wildlife Health Australia for information purposes only. Information contained in it is drawn from a variety of sources external to Wildlife Health Australia. Although reasonable care was taken in its preparation, Wildlife Health Australia does not guarantee or warrant the accuracy, reliability, completeness, or currency of the information or its usefulness in achieving any purpose. To the fullest extent permitted by law, Wildlife Health Australia will not be liable for any loss, damage, cost or expense incurred in or arising by reason of any person relying on information in this fact sheet. Persons should accordingly make and rely on their own assessments and enquiries to verify the accuracy of the information provided. WHA Fact sheet: Wobbly possum disease Jan 2014 5