Australia has over 90 native bat species and of these, 13 species are threatened, with 3 species listed as critically endangered under the Environmental Protection and Biodiversity Conservation Act 1999. The exotic emerging disease white-nose syndrome has been identified as a potential threat to Australian insectivorous bat populations, which are already under pressure from a variety of other anthropogenic processes.
White-nose syndrome is a disease of insectivorous, hibernating bats, associated with infection by Pseudogymnoascus destructans, a psychrophilic (‘cold-loving’) fungal pathogen. It is endemic to the Palearctic regions and has been identified in bat populations throughout Europe, Russia, and north-east China. It is believed to have spread to eastern North America by anthropogenic means and was first documented in New York State in 2006. Since then, P. destructans has spread rapidly throughout the eastern USA and into eastern Canada. Though apparently innocuous in endemic environments, P. destructans infection in some North American bat species is associated with mortality rates of up to 100% in infected cave populations and substantial declines in these species. It has been estimated that over 5.7 million bats have died in North America as a result of the infection.
In Australia, limited free-ranging bat population sampling, environmental sampling and diagnostic investigations of clinically ill bats have not identified P. destructans or white-nose syndrome.
There is no evidence that P. destructans are capable of infecting humans or domestic animals. Flying-foxes are not considered susceptible to white-nose syndrome.
Clinical features and diagnosis
Pseudogymnoascus destructans infects the skin and infection may be visible as a white to grey powdery fungal growth on the bat’s muzzle, ears and wing membranes. As wing membranes are involved in the gas exchange and fluid balance, infection-related damage can cause a metabolic disturbance, resulting in emaciation, dehydration, altered behaviour (such as flying during the day), more frequent arousal from torpor and death. Correspondingly, differences in the mortality rates of infected bats in the USA and Europe have reflected in the degree of P. destructans dermal invasion.
Laboratory tests to detect P. destructans infection include histopathology of wing tissue, quantitative PCR of skin swabs or tissues samples, and fungal culture.
Epidemiological features
Pseudogymnoascus destructans is transmitted primarily by direct contact and indirectly by contamination of the cave environment. It may also be transmitted indirectly by the batwing mite, Spinturnix myoti, and contaminated fomites such as equipment, clothing, and footwear. Though S. myoti is not present in Australia, other Spinturnix spp. are present and may be potential vectors of infection if the fungus was introduced.
Pseudogymnoascus destructans growth on bats and subsequent transmission between bats is prompted by the sustained decrease in bat body temperature and reduced immune function that occurs during hibernation.
In Palearctic bat populations, P. destructans infection is seasonally common, but the disease is not. In European bat colonies, infected animals start appearing in late winter, with numbers peaking in early spring. Bats are usually not infected between late summer and midwinter.
In contrast, in North America, seasonal P. destructans infection is associated with death and has caused substantial declines in bat populations. The seasonal pattern of disease differs to that in Europe; in North American bat populations, white-nose syndrome lesions appear in early autumn, with deaths starting midwinter and peaking in early spring. Infected bats that survive the hibernation period rapidly clear the infection in spring. Factors that are thought to be associated with the higher mortality rates in North American bat populations include differences in species susceptibility to infection or immune response; species torpor physiology; roosting microclimates (particularly humidity and temperature) and other environmental conditions; and colony size and density during hibernation, with larger colony sizes in North American bat populations.
As P. destructans does not grow in temperatures above 20°C, it is not likely to be capable of growing and spreading efficiently in bat populations in consistently warm climates, such as northern Australia. Conversely, in favourable cave environments (P. destructans grows optimally at 12.5–15.8°C), the fungus can persist between hibernation seasons and survive in the absence of bats for at least 5 years. As a result, infected caves remain an infection risk for a long time.
There is no evidence that white-nose syndrome has substantially affected populations of bats that do not roost in caves or mineshafts, although P. destructans infection has been detected in tree-roosting bats.
The potential impact of white-nose syndrome on Australian bat species
The susceptibility of Australian bat species to P. destructans infection is unknown, as is the likely effect of infection. Some Australian bats are of the same genera as species in North America that have suffered substantial population declines.
A recent qualitative risk analysis1 predicted that without risk mitigation measures, it is highly likely that a bat population in Australia will be exposed to P. destructans at some time in the next 10 years and that the most likely pathway for entry is a fomite such as clothing or equipment that has been used in an infected cave in another country. The Australian bat species considered most likely to be affected by white-nose syndrome are cave-dwelling bat species in southern Australia. These include: southern bent-winged bats (Miniopterus schreibersii bassanii); eastern bent-winged bats (M. schreibersii oceanensis); eastern horseshoe bats (Rhinolophus megaphyllus); chocolate wattled bats (Chalinolobus morio); large-eared pied bats (C.dwyeri); southern myotis (Myotis Macropus); and Finlayson’s cave bats (Vespadelus Finlayson). Of these, the two bent-winged bat species are believed to be at greatest risk of an adverse population affects. Australia’s relatively milder climate may limit the deaths associated with P. destructans infection, compared with North American bat populations. However, as the southern bent-winged bat is already critically endangered, this species is particularly vulnerable to the effect of a disease epidemic.
Minimising the risk of introduction and spread of white-nose syndrome in Australia
Risk mitigation measures such as education campaigns targeting cave users and Australian border interventions are important to minimise the risk of P. destructans entering Australia and coming into contact with local bat populations. The 17th International Congress of Speleology (the study or exploration of caves) is being held in Sydney in July and includes field trips to a number of Australian caves that will be attended by cavers from Australia and other countries. This represents a particular risk period for entry of P. destructans, but also an opportunity to raise awareness of white-nose syndrome among the Australian and international caving communities and alert them to Australia’s incoming requirements for decontamination of clothing and equipment that has been used in caves in other countries. In collaboration with conference organisers and Wildlife Health Australia, the Department of Agriculture and Water Resources is promoting awareness of white-nose syndrome among congress delegates and discouraging the use of caving equipment that has been used in other countries. Meanwhile, Biosecurity Officers in Australia’s airports have been equipped with information about this disease and how to effectively decontaminate any clothing or equipment being brought into the country. Information about required treatments for these items will shortly be available through BICON, the department’s database for biosecurity import conditions.2 These requirements will apply not only to cavers but also to other tourists and those with an occupational reason to enter caves.
The department has also provided funding for the preparation of response guidelines that can be used in the event of an incursion of white-nose syndrome in Australia.3 Management options in an outbreak could include disease surveillance, containment actions such as closing infected caves to human activity, public education activities, and conservation activities.
Reporting potential cases of white-nose syndrome
Suspected cases of white-nose syndrome in Australia should be reported to the Emergency Animal Disease Watch Hotline (free call 1800 675 888, 24 hours a day) or to the government biosecurity agency in the state or territory in which the disease has been seen. Contact details for the Wildlife Coordinators in each of the states and territories are available on the Wildlife Health Australia website.
Further information on white-nose syndrome is available through the Department4 and Wildlife Health Australia.5
References
- Holz P, Hufschmid J, Boardman W et al. Qualitative risk assessment: white-nose syndrome in bats in Australia. wildlifehealthaustralia.com.au/Portals/0/ Documents/ProgramProjects/WNS%20Disease%20Risk%20Analysis%20 Australia.pdf Accessed April 2017.
- Department of Agriculture and Water Resources. BICON Home. https://bicon.agriculture.gov.au/BiconWeb4.0 Accessed May 2017.
- Wildlife Health Australia. White-nose syndrome response guidelines. 2017. wildlifehealthaustralia.com.au/Portals/0/Documents/ProgramProjects/ WNS%20response%20guidelines%20-%201.0%20-%20May%202017.pdf Accessed May 2017.
- Department of Agriculture and Water Resources. White-nose syndrome in cave-hibernating bats. 2016. agriculture.gov.au/pests-diseases-weeds/ animal/white-nose-syndrome Accessed May 2017.
- Wildlife Health Australia. Bat Health Focus Group. wildlifehealthaustralia.com.au/ProgramsProjects/BatHealthFocusGroup.aspx#WNS Accessed May 2017.
This article appeared in the July 2017 issue of the Australian Veterinary Journal