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The F-Files: The frog fungus and worldwide amphibian declines A most dreadful fungus is amongst us! In 1985 an exotic fungus hit the southwest around Albany. The fungus had spread to Perth by 1989 This is a brief summary of the chytrid fungus which is infecting our local frogs. It is designed to answer some of the most commonly asked questions. How does the disease affect frogs? The frog or "chytrid" (pronounced ki-trid) fungus (scientific name Batrachochytrium dendrobatidis) is from an ancient group of fungi and is unusual for having a vertebrate host. Most members of this group are free-living but can sometimes be parasitic on algae and aquatic invertebrates. The fungus attacks keratin which is embedded within the sensitive skin of frogs. The fungus gets within the skin of frogs and eats away at the keratin. The fungus creates a "discharge tube" from which it releases new fungus spores to the outside world that may infect new frog victims. Tadpoles are relatively immune from the fungus as they do not have keratin in their skin. However, they do have keratin in their mouthparts and the fungus can cause some damage to them. The main problem is at metamorphosis. As soon as the tadpole metamorphoses from a fish-like creature in to a small hopping frog-shaped creature, the fungus spreads to the rest of the body and can kill the young frogs before they even get a taste of life out of water. How has it affected frogs across the world? The frog fungus has caused the extinctions of many species of frogs world-wide. Areas especially hard-hit were Central and South America and Australia. How has it affected Western Australia's frogs? Most of our information for the prevalence of the frog fungus in WA comes from a study conducted by Alcoa Frog Watch's Ken Aplin and Murdoch University student Peter Kirkpatrick. Is it a fungus or a virus? The chytrid fungus is a parasitic fungus. It is a member of a large group of soil and aquatic fungi that do not normally invade animal tissues. Where does it originally come from? The Australian frog fungus appears to be identical with that infecting Central and North American frogs. The fungus appears to be a recent invader to both of these continents. It may have come out of Africa, perhaps with frogs destined for zoos or on the African Clawed Toad, Xenopus, which is commonly used in laboratories around the world. How did it get to W.A.? One possible route to WA was through frogs shipped accidentally with boxes of fruit. It is not uncommon for a tree frog to emerge from these boxes, and many of the shipments are coming from areas where the fungus is endemic. Anyone coming across one of the imported animals should report it to the W.A. Museum, CALM's Wildlife Branch or the Australian Quarantine Inspection Service. How does it infect frogs? The fungus spreads via a free-swimming spore which enters the skin of its victim. Once inside the skin cell, the spore matures and reproduces to produce a new batch of spores, which are released back onto the skin of the frog and then into the environment. This cycle takes about 4 days. Frogs probably catch the fungus through direct contact with another infected animal, or from infected water. Does it affect tadpoles as well as frogs? Yes. But in tadpoles only affects the mouthparts which are made of keratin. Other than damage to the mouthparts, overall health of tadpoles does not appear to be affected by the fungus. However, as tadpoles change into froglets, the skin becomes keratinised and the fungus spreads across the skin. Froglets often die soon after metamorphosis. What actually kills the frogs? Most appear to die as a result of toxic damage to internal organs. This is probably caused by enzymes produced by the fungus when it reproduces. Some may die because of disruption to the physiology of the skin, which plays a major role in respiration for amphibians. The time from infection to death has been found to be 1-2 weeks in experimental tests. Are all frog species killed by the fungus? Possibly not. Some species are known to be infected but have not shown any decline in population numbers in the wild. This may be because they are less susceptible, or it may reflect differences in their behaviour or reproductive biology. The southwest has a disproportionate number of burrowing species compared to the rest of Australia, and this may contribute to the lack of extinction of southwest frog species. Can it spread in water, or on plants or soil? Almost certainly in water, and possibly in moist soil or on wet plants. The fungus cannot survive drying out, so probably can't be spread in dry soil or by wind dispersal. Can it infect humans or other animals? Humans, almost certainly not. Trials have been conducted on fish, without any sign of infection. It is possible that the frog fungus can infect some kinds of invertebrates, but so far it has not been found anywhere except on frogs themselves. Resources - good links to fungus sites/Gerry's ARC site Further information about amphibian diseases, including chytrid fungus: http://www.jcu.edu.au/school/phtm/PHTM/frogs/ampdis.htm CSIRO's Research Project http://www.csiro.au/index.asp?type=faq&id=Frog%20fungus Further information about amphibian declines is available at: Decling Frog Research at James Cook University www.jcu.edu.au/school/tbiol/zoology/herp/decline/jcu.shtml Declining Amphibian Task Force (UK) http://www.open.ac.uk/daptf/index.htm The Draft Threat Abatement Plan is available at www.deh.gov.au/biodiversity/threatened/publications/tap/amphibians/index.html Orange and white-bellied frog recovery plan link - http://www.deh.gov.au/biodiversity/threatened/publications/recovery/frogs/ |
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