Infection of frogs by amphibian chytrid causing the disease chytridiomycosis - key threatening process listing

NSW Scientific Committee - final determination

The Scientific Committee has found that:

1. Chytridiomycosis is a fatal disease of amphibians and is caused by the chytrid Batrachochytrium dendrobatidis (Longcore et al 1999). The chytrid sporangia grow in the keratinised epidermis of amphibians and the waterborne zoospores remain viable for over 24 hours.

2. The chytrid is virulent only to adult amphibians, however it has been found, and therefore may be carried, on the keratinised mouthparts of tadpoles (Berger et al 1999). The chytrid does not appear to rely on susceptible (stressed) hosts for survival and persists in populations where the density of adults has been reduced (Daszak et al 1999). Further, other poikilotherms with keratinised surfaces could harbour and help to spread the disease, although this has not yet been observed.

3. Two explanations have been proposed as to how the organism kills frogs (Berger et al 1998, Pessier et al 1999). The sporangia may release fatal toxins that are absorbed by the semi-permeable skin or alternatively, the disease damages the epidermis which affects water and/or electrolyte balance.

4. Chytridiomycosis is a global epidemic. It is now known to occur in Australia, New Zealand, Europe, North America, Central America, South America and Africa (Speare and Berger 2000a). In Australia, chytridiomycosis has been observed in frog populations from four geographic areas: a large eastern zone extending from Big Tableland (near Cooktown) south to Melbourne; an Adelaide zone; a zone in south-west Western Australia to just north of Perth; and a zone in the central Kimberleys.

5. The first known occurrence of the disease in Australia (determined via retrospective examination of museum specimens) was in south-east Queensland from a specimen collected in December 1978 (Speare and Berger 2000b). The subsequent spread of the disease may in part be attributed to both the international and domestic trade inamphibians as pets. For example, three of six exotic Axolotyls, Ambystoma mexicanum, purchased in Perth and Townsville were infected with the chytrid (Speare 2000). Similarly, frogs are often accidentally transported with agricultural produce, as demonstrated by infected frogs that were rescued from produce in Melbourne (Marantelli and Hobbs 2000). Collection and handling of frogs and inadvertent transport of infected material between frog habitats may also promote the disease's spread.

6. The chytrid appears to be a novel pathogen of Australian frogs, given its apparent sudden appearance and rapid spread through previously unexposed and thus highly susceptible populations. Chytridiomycosis is potentially fatal to all native species of amphibian. As such, all frog species that are listed under the schedules of the Act may be affected by the disease. Chytridiomycosis has been reported from the following:

Endangered frog species and populations:

Vulnerable frog species:

*Species where chytridiomycosis is a known cause of death.

7. High altitude (>400m) populations are more severely affected by chytridiomycosis. This may be explained by the influence of temperature on Batrachochytrium dendrobatidis. An experimental study demonstrated a 100% mortality of infected frogs at temperatures of 17°C and 22°C while at 27°C, 50% of individuals survived as asymptomatic carriers (Speare 2001). Further, an increased prevalance of infection during winter has been observed in frog populations from Western Australia and Queensland (Aplin and Kirkpatrick 2000, Retallick and Dwyer 2000). Such population declines have been reported from the NSW uplands (Gillespie and Hines 1999, Hines et al 1999), the Victorian alps (Gillespie and Marantelli 2000) and the Queensland wet tropics (Laurance et al 1996, McDonald and Alford 1999).

8. Stream-associated frog species are more likely to be infected because the pathogen is waterborne. The following are stream-breeding species of the NSW coast and ranges and may be threatened by chytridiomycosis (Gillespie and Hines 1999):

Endangered stream-breeding frog species and populations:

Vulnerable stream-breeding frog species:

9. Species of stream-breeding frog which are not threatened but may become threatened include:

10. The appearance of chytridiomycosis in a frog population may make it less able to withstand other threatening processes. The death of adult frogs from the disease - in concert with such threats as predation of tadpoles by exotic fish (e.g. plague minnow, Gambusia holbrooki), loss and degradation of habitat, and isolation of sub-populations would increase a local population's vulnerability to extinction.

11. In view of the above the Scientific Committee is of the opinion that Infection of frogs by amphibian chytrid causing the disease chytridiomycosis adversely affects two or more threatened species, populations or ecological communities or could cause species, populations or ecological communities that are not threatened to become threatened.

Associate Professor Paul Adam
Chairperson
Scientific Committee

Proposed gazettal date: 22/08/03
Exhibition period: 22/8/03 - 3/10/03

References

Aplin K and Kirkpatrick P 2000, 'Chytridiomycosis in southwest Australia: historical sampling documents the date of introduction, rates of spread and seasonal epidemiology, and sheds new light on chytrid ecology', Proceedings of Getting the Jump on Amphibian Disease, Cairns, Australia, 26-29 August 2000, p. 24.

Berger L, Speare R, Daszak PI, Green DE, Cunningham AA, Goggin CL, Slocombe R, Ragan MA, Hyatt AD, McDonald KR, Hines HB, Lips KR, Marantelli G and Parkes H 1998, 'Chytridiomycosis causes amphibian mortality associated with population declines in the rainforests of Australia and Central America', Proceedings of the National Academy of Science 95, pp. 9031-6.

Berger L, Speare R and Hyatt A 1999, 'Chytrid fungi and amphibian declines: overview, implications and future directions', in Declines and disappearances of Australian frogs (ed. A. Campbell), pp. 23-33, Environment Australia, Canberra.

Daszak PI, Berger L, Cunningham AA, Hyatt AD, Green DE and Speare R 1999, 'Role of emerging infectious diseases in amphibian population declines and global implications', Emerging Infectious Diseases 5, pp. 735-48.

Gillespie G and Hines H 1999, 'Status of temperate riverine frogs in south-eastern Australia', in Declines and disappearances of Australian frogs (ed. A. Campbell), pp. 109-130, Environment Australia, Canberra.

Gillespie G and Marantelli G 2000, 'The role of the amphibian chytridiomycete fungus in population dynamics of the spotted tree frog (Litoria spenceri), a declining riverine species in south-eastern Australia', Proceedings of Getting the Jump on Amphibian Disease, Cairns, Australia, 26-29 August 2000, p. 45.

Hines H, Mahony M and McDonald KR 1999, 'An assessment of frog declines in wet subtropical Australia', in Declines and disappearances of Australian frogs (ed. A. Campbell), pp. 46-63, Environment Australia, Canberra.

Laurance WF, McDonald KR and Speare R 1996, 'Epidemic disease and the catastrophic decline of Australian rainforest frogs', Conservation Biology 77, pp. 203-12.

Longcore JE, Pessier AP and Nichols DK 1999, 'Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians', Mycologia 91, pp. 219-27.

Marantelli G and Hobbs R 2000, 'Disease vectors and the community: from lost frogs to frog friendly gardens - how do we help frogs while containing disease spread?', Proceedings of Getting the Jump on Amphibian Disease, Cairns, Australia, 26-29 August 2000, p. 49.

McDonald KR and Alford RA 1999, 'A review of declining frogs in northern Queensland', in Declines and disappearances of Australian frogs (ed. A. Campbell), pp. 14-22, Environment Australia, Canberra.

Pessier AP, Nichols DK, Longcore JE and Fuller MS 1999, 'Cutaneous chytridiomycosis in poison dart frogs (Dendrobates spp.) and White's tree frogs (Litoria caerulea)', Journal of Veterinary Diagnosis and Investigation 11, pp. 194-9.

Retallick R and Dwyer L 2000, 'Using translocations to learn about frog declines and disease', Proceedings of Getting the Jump on Amphibian Disease, Cairns, Australia, 26-29 August 2000, p. 41.

Speare R 2000, 'Understanding the pieces of the amphibian disease puzzle', Proceedings of Getting the Jump on Amphibian Disease, Cairns, Australia, 26-29 August 2000, p. 19.

Speare R 2001, 'Attachment 7: Nomination of amphibian chytridiomycosis as a key threatening process' in ' Developing Management Strategies to Control Amphibian Diseases: Decreasing The Risks Due To Communicable Diseases, pp. 185-208. Report to Environment Australia: Canberra. Accessed: January 30.

Speare Rand Berger L 2000a, 'Global distribution of chytridiomycosis in amphibians'. Accessed: November 11.

Speare R and Berger L 2000b, 'Chytridiomycosis in amphibians in Australia'. Accessed: November 11.