EXAMPLES OF EMERGING AUSTRALIAN WILDLIFE DISEASES
The following discussion, addressing only some diseases of Australian mammals, focuses primarily on drivers of emergence. For a more detailed discussion of these diseases, see Chapter 16 and the relevant taxonomic or disease chapters in this volume and in Vogelnest and Woods (2008) and Ladds (2009).
Although several identified emerging pathogens have likely been present in Australia for millenia, a variety of pathogens responsible for significant disease in Australian mammals appear to have spilled over more recently from domestic animal hosts and at least one (DFTD) has emerged spontaneously in the past few decades.Some wildlife pathogens emerging in Australia are known to have global significance. Toxoplasmosis and sarcoptic mange are considered emerging wildlife diseases within and outside Australia, although the factors driving emergence may differ (Tompkins et al. 2015). Others, recognised as emerging in other regions of the world, have only recently been detected in Australian wildlife (e.g. tularaemia and cetacean morbilliviruses). In many cases, the significance of the disease on wildlife populations has yet to be determined.
5.1 Leishmaniasis
Australian leishmaniasis is a recently recognised protozoal disease of macropods, reported only from a limited geographical area of northern Australia (Rose et al. 2004; Dougall et al. 2009). Infection causes skin lesions on the extremities of susceptible individuals and appears to be transmitted by a novel vector of Leishma- nia, a day-feeding midge (Dougall et al. 2011). Until recently, Australia was considered free of leishmaniasis and not at risk of incursion, because of the absence of known suitable vectors (phlebotomine sand flies). The novel Australian strain of Leishmania is distinct from strains found elsewhere in the world and has likely been present in Australian for millennia.
The disease first became apparent following spillover into managed macropod species held outside their normal geographical range and environmental conditions. The mammalian reservoir host is unknown. Factors relevant to emergence include the aberrant presence of a naive or susceptible host, compounded by limited surveillance for wildlife disease.5.2 Tularaemia
Francisella tularensis ssp. holarctica, the bacterial agent responsible for tularaemia, was believed to be absent from Australia until two human cases occurred in Tas. in 2011. Exposure to eastern ring-tailed possums (Pseu- docheirus peregrinus) had occurred in both cases (Jackson et al. 2012). Surveillance of Tasmanian wildlife found no evidence of the pathogen. Recently, retrospective studies using molecular techniques demonstrated the organism in archived eastern ring-tailed possum samples from the Australian mainland (Eden et al. 2017). It is likely that the pathogen has been present in Australian wildlife populations for millennia; factors influencing the emergence of this zoonosis in Australia remain unknown. Increasingly sophisticated diagnostic tools enabled detection of the pathogen in possums and will no doubt aid ongoing investigations.
5.3 Macropod orbiviruses
Orbiviruses have emerged as a cause of disease in macropod species. A sudden death syndrome appeared in managed tammar wallabies (Notamacropus eugenii) in the 1990s, caused by an orbivirus of the Eubenangee serogroup (Rose et al. 2012). The likely factors for emergence of this syndrome include seasonally abundant vectors (biting insects) and the holding of tammar wallabies in managed care facilities well outside their natural geographical range, presumably introducing naive hosts to this virus. It is not known if other wildlife species act as reservoirs (presumably asymptomatic) for these orbiviruses between outbreaks. Orbiviruses from the Wallal serogroup are responsible for epidemics of choroid blindness in kangaroos in Australia, first reported in the 1990s.
Archived tissues suggest the disease was present as early as 1975 (Vogelnest and Portas 2008). The factors driving the apparent emergence of this virus are not well understood, but are at least in part linked to a surge in prevalence of the vector after rainfall events (Hooper et al. 1999).5.4 Hendra virus
Hendra virus has probably been present in flying-fox populations for millennia (Halpin et al. 2007) but was only identified following extensive investigation prompted by an outbreak of a novel fatal disease in horses and humans in Qld (Black et al. 2015). The drivers of HeV emergence have now been extensively studied; spillover events are likely in part due to increased interface between flying-foxes and horses, as a result of anthropogenic (and perhaps natural) changes in the ecosystem. The complex factors influencing shedding of the virus from the bat host are under continued investigation. This pathogen was unknown before its dramatic emergence in 1994, probably because of the absence of clinical disease in the reservoir host and the limited study of health in Australian wildlife. Although many questions remain to be answered, the collaborative investigation process has greatly advanced the understanding of HeV disease dynamics, as well as identifying numerous ‘new’ zoonotic viruses in bats, including ABLV. Our current knowledge of HeV (including remaining gaps) demonstrates the complexity of causality in emerging infectious disease and the challenges in unravelling the story of disease emergence in wildlife species (see Chapter 43).
5.5 Sarcoptic mange
Sarcoptes scabiei causes mange in a wide range of mammalian hosts globally (Escobar et al. 2022). Host range in Australia has recently expanded to include koalas, wallabies, possums and bandicoots (Obendorf 1983; Skerratt et al. 1998; McLelland and Youl 2005; Wicks et al. 2007; Holz et al. 2011; Speight et al. 2017; Botten et al. 2022). Prevalence appears to be increasing in free-ranging barenosed and southern hairy-nosed wombat populations (Ruykys et al.
2009; Fraser et al. 2016; Martin et al. 2017; Old et al. 2017). The origin of sarcoptic mange in Australian mammals is still under debate; however, it most likely arrived in Australia with European settlers and their domestic dogs and from there spilled over into wombats and other native mammals (Fraser et al. 2019). Australian mammals are assumed to have little innate resistance to this pathogen, because of a lack of evolutionary exposure. The behavioural ecology of wombats likely predisposes them to infection, but it is not yet known if other inherent immunological factors are contributory. Habitat disruption, host demographics, inadequate nutrition and stress are also likely factors influencing the emergence of the disease (Fraser et al. 2016).5.6 Toxoplasmosis
It is assumed that T. gondii arrived in Australia with cats accompanying European settlers. Many species of Australian marsupials appear to be extremely susceptible to infection with T. gondii, again inferring lack of prior exposure to this pathogen. Although the population level impacts of toxoplasmosis on free-ranging Australian mammals are largely unquantified, this pathogen is considered emerging, with new reports in a widening range of species, including marine mammals (see Chapter 21).
5.7 Chlamydiosis
Both Chlamydia pecorum and to a lesser extent C. pneumoniae have a significant impact on free-living koala populations and although clinical disease appears rare outside this host, there is growing evidence that a wide range of marsupials are susceptible to infection with Chlamydia spp. (Burnard et al. 2017). The origins of chlamydiosis in Australian wildlife remain unclear, although there is growing evidence that the pathogen in koalas originated from introduced livestock (Polking- horne et al. 2013; Burnard and Polkinghorne 2016). It is not yet known why koalas experience such severe disease, compared with other marsupials; however, investigations continue to explore potential differences in the virulence of pathogen strains and the role of host immunological response and concurrent infection (see Chapter 35).
5.8 Tasmanian devil facial tumour disease
In recent decades, two genetically distinct cancers have arisen in Tasmanian devils (Pye et al. 2016). DFTD has spread across most of the geographical range of the free- ranging population, causing a dramatic decline in species numbers (Cunningham et al. 2021). Transmissible cancers are rare and can only emerge when conditions allow their spread. It appears that the ability of DFTD to evade the host’s immune response, coupled with the host’s behavioural ecology (allowing tumour transmission through biting), are significant factors in the emergence of this disease. Although studies have not shown any compromise in the devil’s immune system, it is possible that a reduction in genetic diversity, caused by previous population bottlenecks, contributes to the emergence of this disease (Siddle 2017) (see Chapter 40).
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