The ecology of Hendra virus and Australian bat lyssavirus Thesis submitted by Hume Field BVSc MSc MACVS in fulfilment of requirements for the degree of Doctor of Philosophy in the School of Veterinary Science, The University of Queensland, Brisbane, Australia. November, 2004 i Declaration of Originality The work presented in this thesis is, to the best of my knowledge and belief, original, and my own work except as acknowledged. The material has not been submitted, either whole or in part, for any other degree at this or any other university. Hume Field November 2004 ii Acknowledgements I would like to thank a number of colleagues in the Department of Primary Industries and Fisheries, Queensland for the opportunity to be involved in this research, and for their sustained support over the years; in particular, Kevin Dunn (Deputy Director General and Chief Veterinary Officer), Ron Glanville (General Manager, Animal Health), Chris Baldock (now a Director of Ausvet Animal Health Services) and Russell Rogers (Principal Scientist, Laboratories). Thanks are also due to Baden Pearse and Sandy Mckenzie for field assistance in the early days at Cannon Hill. The initial funding for the project followed a successful research proposal developed by Russell Rogers and funded through a Queensland Government New Initiatives program. This funding employed myself as a research epidemiologist, and a technical officer (initially Natasha Smith, and subsequently Craig Smith) for three years from 1996, during which time the bulk of the fieldwork was done. Additional funding from DPI&F, the Commonwealth Department of Agriculture, Fisheries and Forestry’s Wildlife and Exotic Diseases Preparedness Program, and The University of Queensland broadened the scope of the research by supporting two additional PhD students - Kim Halpin and Janine Barrett. This expanded research team was capably lead by Peter Young (Queensland Agricultural Biotechnology Centre) and strongly supported of Professor John Mackenzie, then Head of the UQ Department of Microbiology and Parasitology. I sincerely thank all these people for their commitment and assistance. Very special thanks are due to Craig Smith for his remarkable abilities in the field, the lab and the office, and for his unfailing good-humour regardless of the hour, the temperature or the location. Acknowledgement and thanks are due to wildlife carers in Queensland and New South Wales, with special thanks to Helen Luckhoff and Helen Gormley from ONARR, and Marjorie Beck from the Kuringai Bat Conservation group for their open minds and their genuine interest in the research. Thanks also to Norm Mckenzie (Conservation and Land Management, Western Australia) without whose knowledge and expertise the Western Australian surveillance would not have been possible, and to Len Martin (ex-UQ Department of Physiology and Pharmacology) for access to his captive flying fox colony and archived sera. Particular thanks to Les Hall (ex-UQ School of Veterinary Science) for freely sharing his remarkable knowledge of flying fox biology and thus greatly facilitating my ascent of a very steep learning curve. iii I would also like to acknowledge the support of DPI&F laboratory colleagues in Townsville, Rockhampton and Yeerongpilly for lab access, advice and diagnostic support, particularly to Barry Rodwell and Mark Kelly at YVL. Likewise colleagues at AAHL, especially to Peter Daniels, Chris Morrissey and Ross Lunt for generous diagnostic support. Thanks are also due to Micheal Ward (ex-DPI&F) and John Morton (UQ School of Veterinary Science) for assistance with the logistic regression, and to Evan Sergeant (Ausvet Animal Health Services) for assistance with the modelling. Special thanks to my PhD supervisors Peter Young, Joanne Meers (UQ School of Veterinary Science) and particularly Simon More (now at the University of Dublin) for advice, guidance, support and time, to Peter Black (DAFF), Simon, and Chris Baldock for mentoring me towards epidemiology chapter membership of the Australian College of Veterinary Science, and to Chris and Simon for support and encouragement to finally complete my thesis. Above all, thanks Michelle for your patience, understanding, love and friendship. And finally, thanks to my beautiful boys Isaac and Daniel, to whom I owe a large playtime debt! iv Abstract Chapter one introduces the concept of disease emergence and factors associated with emergence. The role of wildlife as reservoirs of emerging diseases and specifically the history of bats as reservoirs of zoonotic diseases is previewed. Finally, the aims and structure of the thesis are outlined. In Chapter two, the literature relating to the emergence of Hendra virus, Nipah virus, and Australian bat lyssavirus, the biology of flying foxes, methodologies for investigating wildlife reservoirs of disease, and the modelling of disease in wildlife populations is reviewed. Chapter three describes the search for the origin of Hendra virus and investigations of the ecology of the virus. In a preliminary survey of wildlife, feral and pest species, 6/21 Pteropus alecto and 5/6 P. conspicillatus had neutralizing antibodies to Hendra virus. A subsequent survey found 548/1172 convenience-sampled flying foxes were seropositive. Analysis using logistic regression identified species, age, sample method, sample location and sample year, and the interaction terms age*species and age* sample method as significantly associated with HeV serostatus. Analysis of a subset of the data also identified a significant or near-significant association between time of year of sampling and HeV serostatus. In a retrospective survey, 16/68 flying fox sera collected between 1982 and 1984 were seropositive. Targeted surveillance of non-flying fox wildlife species found no evidence of Hendra virus. The findings indicate that flying foxes are a likely reservoir host of Hendra virus, and that the relationship between host and virus is mature. The transmission and maintenance of Hendra virus in a captive flying fox population is investigated in Chapter four. In study 1, neutralizing antibodies to HeV were found in 9/55 P. poliocephalus and 4/13 P. alecto. Titres ranged from 1:5 to 1:160, with a median of 1:10. In study 2, blood and throat and urogenital swabs from 17 flying foxes from study 1 were collected weekly for 14 weeks. Virus was isolated from the blood of a single aged non-pregnant female on one occasion. In study 3, a convenience sample of 19 seropositive and 35 seronegative flying foxes was serologically monitored monthly for all or part of a two-year period. Three individuals (all pups born during the study) seroconverted, and three individuals that were seropositive on entry became seronegative. Two of the latter were pups born during the study period. Dam serostatus and pup serostatus at second bleed were strongly associated when data from both years v were combined (p<0.001; RR=9, 95%CI 1.42 to 57.12). The serial titres of 19 flying foxes monitored for 12 months or longer showed a rising and falling pattern (10), a static pattern (1) or a falling pattern (8). The findings suggest latency and vertical transmission are features of HeV infection in flying foxes. Chapter five describes Australian bat lyssavirus surveillance in flying foxes, insectivorous bats and archived museum bat specimens. In a survey of 1477 flying foxes, 69/1477 were antigen-positive (all opportunistic specimens) and 12/280 were antibody-positive. Species (p<0.001), age (p=0.02), sample method (p<0.001) and sample location (p<0.001) were significantly associated with fluorescent antibody status. There was also a significant association between rapid focus fluorescent inhibition test status and species (p=0.01), sample method (p=0.002) and sample location (p=0.002). There was a near-significant association (p=0.067) between time of year of sampling and fluorescent antibody status. When the analysis was repeated on P. scapulatus alone, the association stronger (p=0.054). A total of 1234 insectivorous bats were surveyed, with 5/1162 antigen–positive (all opportunistic specimens) and 10/390 antibody-positive. A total of 137 archived bats from 10 species were tested for evidence of Australian bat lyssavirus infection by immunohistochemistry (66) or rapid focus fluorescent inhibition test (71). None was positive by either test but 2 (both S. flaviventris) showed round basophilic structures consistent with Negri bodies on histological examination. The findings indicate that Australian bat lyssavirus infection is endemic in Australian bats, that submitted sick and injured bats (opportunistic specimens) pose an increased public health risk, and that Australian bat lyssavirus infection may have been present in Australian bats 15 years prior to its first description. In Chapter six, deterministic state-transition models are developed to examine the dynamics of HeV infection in a hypothetical flying fox population. Model 1 outputs demonstrated that the rate of transmission and the rate of recovery are the key parameters determining the rate of spread of infection, and that population size is positively associated with outbreak size and duration. The Model 2 outputs indicated that that long-term maintenance of infection is inconsistent with lifelong immunity following infection and recovery. Chapter seven discusses alternative hypotheses on the emergence and maintenance of Hendra virus and Australian bat lyssavirus in Australia. The preferred hypothesis is that both Hendra virus and Australian bat lyssavirus are primarily maintained in P. scapulatus populations, and that change in the population dynamics of this species due to ecological changes has precipitated emergence. vi Future research recommendations include further observational, experimental and/or modeling studies to establish or clarify the route of HeV excretion and the mode of transmission in flying foxes, the roles of vertical transmission and latency in the transmission and maintenance of Hendra virus in flying foxes, and the dynamics of Hendra virus infection in flying foxes. vii Publications Refereed journal articles Halpin K, Young PL, and Field HE (1996) Identification of likely natural hosts for Equine Morbillivirus.