AMPHIBIAN CHYTRID FUNGUS:
An unfortunate overview

This article was written by Craig Cleeland and originally published in "In the Spotlight" (Vol. 6, No. 1) in the first half of 2001. Craig presents his personal experience with the Chytrid fungus alongside a brief history of the disease and the research being conducted for the future.

DISCOVERY

It was with some remorse that I retrieved the last bloated and lifeless body of a Leaf Green Tree Frog (Litoria phyllochroa) from out of the enclosure in which they lived for almost three years now. This was the last of five, and with in the space of a week all had died. The symptoms always the same: unusual daytime behaviour; unusual lack of response to being disturbed; exaggerated reddening of the inner thighs and lower belly; and then the morning when I find the bodies lifeless floating in the water or sitting on rocks and logs.

Akin to, but at nothing like the scale of, the experience that frog biologists had to contend with in the late 1980s and early 1990s when whole populations and whole species appeared to disappear almost over night. In Queensland, at least seven rainforest frog species have disappeared over the past twenty years. The first extinctions occurred in the D'Aguilar and Conondale mountains near Brisbane in the late seventies and early eighties. The amazing Southern Gastric Brooding Frog (Rheobatrachus silus) was last seen in 1979. This was an incredible species whose tadpoles developed in the stomach of the female and the newly metamorphosed froglets emerged through the mouth. In the mid eighties frog populations in central eastern Queensland declined and the Northern Gastric Breeding Frog (Rheobatrachus vitellinus), the only other gastric brooding frog in the world, has not been seen since. By then it was clear that our frogs were in trouble, and so remaining high altitude populations were intensively monitored. In the early nineties, populations in north Queensland suffered similar sudden declines but this time zoologists were present to witness ill, dying, and dead frogs as mass mortalities occurred.

Interestingly, in many of these dramatic declines tadpoles still swam and fed in the very same streams that had frogs dying on the banks but their time was limited. Within months of leaving the stream and emerging as young frogs these animals would also join the growing list of frog mortalities.

It is now over twenty years since the disappearance of the Southern Gastric Brooding Frog. Since that time many other species have disappeared or have shown signs of significant population declines. Sick and dying frogs have been collected for examination and found to be infected with a chytrid fungi of the skin. Chytrids are small spherical fungi that produce mobile infective stages called zoospores. Some species are commonly found free living in soil and water where they degrade organic matter such as chitin or keratin, and others are parasites of algae, plants, nematodes or insects. Before the discovery of the amphibian chytrid, none had been found to cause disease in vertebrates.

AMPHIBIAN DISEASES GROUP

In an effort to investigate these declines and the role disease has played, a group of scientists and interested members of the general public from right around Australia have got together to form the Amphibian Diseases Group. Members include veterinarians with expertise in amphibian diseases, specialists in pathogenic micro-organisms, frog ecologists, and members of frog groups.

Lee Berger is a key person in routine and experimental work on amphibian diseases in Australia. Most amphibian specimens for testing are sent to the Australian Animal Health Laboratory at Geelong for Lee to do comprehensive post-mortems.

Other specialists involved include:

• Alex Hyatt who works at the Australian Animal Health Laboratory and is a specialist in electron microscopy and Rana viruses. He is a world expert in the identification of unknown viral agents.
• Rick Speare who works at James Cook University at the School of Public Health and Tropical Medicine with the frog disease work done through the Co-operative Research Centre for Tropical Rainforest Ecology and Management. Rick specialises in infectious diseases of wildlife and of humans.

Over the last few years the Amphibian Disease Group has been slowly unravelling the mystery surrounding the role of Chytrid fungus in frog declines.

What frog biologists were witnessing in the wild and what I had been seeing in the frog enclosure was of course was chytridmycosis - or put another way acute Amphibian Chytrid Fungus infection.

INFECTION

While it is still uncertain how chytrids arrived in Australia (most scientists are confident this is a novel pathogen for Australian frogs) the culprit for me was undoubtedly four Whistling Tree Frogs (Litoria verreauxi) that I had placed into the enclosure four weeks prior. Yet the Whistling Tree Frogs are themselves long-term captive animals and both species originally came from the same breeder.

How is this possible?

It is now becoming increasingly apparent that this disease and its effects on frog populations are much more complicated than many people first thought. Some, if not all, frog species can carry chytrid infections over prolonged periods of time without any clinical symptoms.

This was probably the case with my new Whistling Tree Frogs. Once added to the enclosure the fungus quickly spread to the other frogs that had no prior exposure and along with the stress of having new arrivals these frogs quickly succumbed to the infection. We also know tadpoles, although usually unaffected by the fungus, carry the infection in their keratin mouth-parts from soon after hatching until metamorphic climax when the tadpole beaks are shed and chytrid sporangia are rapidly redistributed to the skin. The fungal sporangia grow within cells in the superficial layers of the skin and that results in the thickening of the epidermis and the keratin layer. The exact cause of frog mortality is as yet not known.

RESEARCH CONTINUES

Late in 2000 an international conference on amphibian diseases was held in Cairns, Queensland. Interested scientists from around the world gathered in an effort to co-ordinate resources and priorities in addressing the issue of amphibian diseases. Three members of the VFG attended the conference: myself [Craig Cleeland]; Raelene Hobbs (who presented a paper on the Lost Frogs' Home and its role in preventing disease spread); and Gerry Marantelli. Gerry presented and co-authored a staggering 6 papers for the conference and was part of the steering committee. While a few other community frog groups were represented at the conference, the VFG had a major contribution to make so well done and thankyou to all those members who have contributed.

Further information about the conference and the draft recommendations can be obtained via the Amphibian Diseases Web Page.

Since the conference priorities for chytrid work have been set and research continues. This includes:

• DNA comparison of the cultured chytrid strains.
• culturing the fungus from different species and areas.
• transmission experiments to investigate pathogenicity of the fungus and how this varies with life stage, temperature, dose and species.
• treatment trials to enable tadpoles and frogs to be collected from infected, declining populations and raised in captivity.

Author: Craig Cleeland

FURTHER NOTES

Currently, there is no wholly successful treatment for the Amphibian Chytrid Fungus. Trials at the Amphibian Research Centre (ARC) have shown Fluconazole (a human antifungal) to be somewhat effective in the treatment of tadpoles but not frogs. Work at Washington Zoo has shown another drug Itraconazole (used for the treatment of oral candida in HIV+ patients) has been successful in treatment of poison arrow frogs. Trials at the ARC proved Itraconazole to be toxic to tadpoles, while preliminary testing of Itraconazole on Australian frogs has been promising.