Deadly frog fungus strikes NZ species

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A fungus responsible for deci­mating amphibian populations around the world has struck one of New Zealand’s rare native frogs. Researchers working in the Coromandel Range have found a dead Archey’s frog with skin lesions, suggesting that it died from a chytrid fungal infection caused by Batrachochytrium dendrobatidis.

Chytrid fungi are found mainly in fresh water, but the group has estuarine and soil-dwelling representatives as well. Some species live in the guts of herbivores and some are parasites and saprophytes of algae, plants, nematodes, insects and even other chytrids

B. dendrobatidis is the only species known to parasitise vertebrates. It infects an animal’s epidermal cells, causing a thickening of the outer layer of the skin, but it is not known how that change leads to death.

Globally, two amphibian orders, the Anura and Caudata, comprising 14 families and 93 species, are known to be infected by this fungus. Diseased species have been found in Africa, South America, Central America, North America, Europe, Australia, and now Oceania. The first record was from North America in 1974. Australia, where the fungus arrived in 1978, has been the hardest hit, with 46 species affected, including eight listed as endangered and five as vulnerable.

The fungus was first found in New Zealand in 1999 in two well-established Australian species, the southern bell frog and the green and golden bell frog.

Archey’s frog is found only in the Moehau and Colville Ranges on the Coromandel Peninsula and at Whareorino Forest west of Te Kuiti. The species was in decline even before this current fungal problem was discovered.

“Frog populations have been decreasing worldwide since the 1980s, and New Zealand is unfortunately following the global trend,” says Department of Conservation officer Andrew Harrison. “Chytrid fungus has the potential to decimate New Zealand frogs if it spreads.”

He said that more information was required on the fungus, particularly its method of transfer from introduced frogs to native species. Other research is centred on detecting the fungus in water and on vegetation.

Harrison says New Zealand’s leiopelmid frogs are “of huge scientific interest interna­tionally, as they are the most ancient frogs left on the planet, survivors of the Jurassic period some 200 million years ago.”

Because the fungus is a possible biosecurity issue, a technical advisory group to the Ministry of Agriculture has been set up. Victoria University researcher Ben Bell, a member of the group, says, “The next step is to survey the extent of popula­tion decline and to find out if the survivors have been infected.”

The group is anxious to stop the fungus spreading to New Zealand’s other three native species, particularly Hochstetter’s frog, which shares native forest habitat with Archey’s frog.

Bell says the group is considering moving some frogs to a safe facility where they may be bred in captiv­ity. Meanwhile, Department of Conservation workers are taking strict precautions when visiting frog habitats to try to minimise the spread of the fungus.

Harrison said an infected frog would appear emaciated and lethargic, often with abnormalities of the skin or eyes. “The fungus infects the skin of frogs. We’re not yet sure whether it suffocates them or kills them as they absorb the toxins released by the fungus.”

While some funding has been directed at chytrid research in the United States and Australia, many ques­tions remain unanswered. For instance, no one knows why the fungus, which belongs to a group that has been in existence for 550 million years, has suddenly come to be toxic to frogs and tadpoles. One theory speculates that the pathogen is a mutated form. Other researchers suggest that climate change, such as drier conditions, are causing amphibians to crowd together at water sources, helping the fungus to spread.

Other factors have been blamed for some frog die-offs, such as uncontrolled chemical use. But nearly half the dead frogs handed in during a three-year Austral­ian survey were diagnosed with chytridiomycosis, showing no other disease and no evidence of depressed immunity.

Chytridiomycosis has recently been reported in amphibians collected for the global pet trade. Given that 180,000 animals of at least 21 endangered European species were imported into the UK alone between 1981 and 1990, there is huge potential for chytrid infec­tion to spread still further.

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