WHAT ARE EMERGING INFECTIOUS DISEASES?

The World Organisation for Animal Health (WOAH) defines an emerging (or re-emerging) disease as:

• a new infection resulting from the evolution or change of an existing pathogen or parasite, resulting in a change of host range, vector, pathogenicity or strain, or

• the occurrence of a previously unrecognised infection or disease, or

• an already known disease that either shifts its geo­graphical setting or expands its host range, or signifi­cantly increases its prevalence.

There is a tendency to classify many currently impor­tant infectious diseases as emerging. In many cases, and particularly with wildlife EIDs, it may be difficult to accurately determine if a disease fits the necessary crite­ria to support a definition as ‘emerging’, in part because of the lack of existing or baseline information (Tompkins et al. 2015). The following simplified criteria may help to better assess the emergent status of a disease:

• previously unrecognised disease

• known disease in a new host

• changed geographic distribution of a disease

• increased incidence of a disease within a population

• re-emergence of previously controlled disease (C Sangsterpers. comm.).

Wildlife EIDs may be broadly divided into three groups:

1. Pathogens that have a wildlife species as a reservoir host; infection in the wildlife reservoir does not

Those in bold are discussed further in this chapter; also see taxon or disease specific chapters for further information.

20 - Emerging infectious diseases 333

Table 20.1. (continued)

*Likely reservoir host.

MC = managed care; FR = free-ranging.

334 CurrentTherapyin MedicineofAustraIian Mammals

cause clinical disease, but infection in humans or domestic animals results in disease (e.g. Hendra virus [HeV] in flying-foxes [Pteropus spp.]).

2. Pathogens that result in clinical disease in wildlife species, as well as in domestic species and/or humans (e.g. Australian bat lyssavirus (ABLV) and Toxoplasma gondii).

3. Pathogens and syndromes where the only known disease occurs in wildlife (e.g. devil facial tumour disease (DFTD), orbiviruses in macropods and Australian leishmaniasis).

Wildlife hosts may be a necessary part of the epidemi­ology of the pathogen (as with HeV and ABLV) or they may be one of several host species (e.g. T. gondii is reliant on the presence of the definitive host, the cat (Felis catus), and may infect a broad range of intermediate hosts).

Emerging disease investigation has primarily focused on pathogens affecting either humans or production ani­mals. Globally, the majority of EIDs are zoonoses and an increasing proportion of emergent zoonoses have been shown to originate from wildlife (Jones et al. 2008; McFarlane et al. 2012). Consequently, the role that wild­life plays in EIDs has been under increasing scrutiny. Arguably, the greatest concerns around wildlife EIDs have focused on impact of wildlife-sourced zoonoses on human populations. Recent global examples of emerging zoonoses of wildlife origin include COVID-19 (SARS- CoV-2), Hendra, Nipah and Ebola viruses (Wong et al. 2007; Zhu et al. 2020). In Australia, wildlife are confirmed or suspected reservoirs for emerging zoonoses such as ABLV, HeV, Buruli (Bairnsdale) ulcer and tularaemia (Prowse et al. 2009; Eden et al. 2017) (Table 20.1).

Wildlife populations may be the source of pathogens associated with emerging diseases that have impact on domestic animals. Global examples include bovine tuber­culosis, transmitted from common brush-tailed possums (Trichosurus vulpecula) in New Zealand or badgers (Meles meles) to cattle (Nolan and Wilesmith 1994; O’Neil and Pharo 1995); free-ranging birds acting as reservoirs for avian influenza (Alexander 2007); and African buffalo (Syncerus caffer) as subclinical carriers of foot-and-mouth disease virus (Hedger and Condy 1985). Examples of emerging diseases moving from Australian mammals to domestic species are limited, but include Menangle and Hendra viruses (Selvey et al. 1995; Philbey et al. 2008).

Emerging diseases that only affect wildlife species have until recently received relatively little attention, particularly in the Australian context. This situation is now changing, with the growing awareness of the impacts (both potential and realised) of infectious disease on wildlife populations. Infectious disease may impact indi­viduals directly (through increased mortality or reduced fertility) or indirectly, through reduced immune func­tion, fitness and behaviour change.

Some emerging diseases have had demonstrable cata­strophic impacts on wildlife populations. White-nose syndrome (in North American bats), chytrid fungus (in frogs) and DFTD in Tasmanian devils (Sarcophilus har- risii) have caused significant population declines and continue to pose a major threat to species survival (Sker- ratt et al. 2007; Alves et al. 2014; Cunningham et al. 2021). Other emerging diseases recognised as having a current (or potential) impact on populations or species of Aus­tralian mammals include chlamydiosis in koalas (Phasco- larctos cinereus) and sarcoptic mange in bare-nosed (Vombatus ursinus) and southern hairy-nosed (Lasiorhi- nus latifrons) wombats (Table 20.1).

An emerging disease may have a significant impact on a wildlife population even when the host is a clinically unaffected reservoir. For example, the public concern over the potential health risks posed by flying-foxes has resulted in attempts to relocate colonies in close proxim­ity to human settlements. In North America, attempts were made to reduce the risk of Lyme disease by culling white-tail deer (Odocoileus virginianus) populations, despite evidence to suggest that deer density is not a key determinant in the emergence of this zoonosis (Levi et al. 2012).

2.