A Great Leap Forward

Frogs and toads throughout the world are being killed by a fungus that is new to science. The fungus, which coats their undersides and legs, is thought to be suffocating the animals, which breathe through their skins. It could be a major factor underlying the decline in amphibian populations reported worldwide.

The fungus has been found independently by teams in the US and Australia. It belongs to a new genus of chytrid, a group thought to be related to the earliest fungi. Although other chytrids parasitise a range of organisms, from microscopic algae to insects, they have never before been found to cause disease in vertebrates.

The fungus, which has yet to be given a scientific name, is known to have struck down 10 species of frogs and toads from 10 locations in Australia, seven species from two locations in Panama, a toad from southern California, and six species of frogs in four American zoos and aquariums. "There's little doubt that this is a worldwide phenomenon," says Allan Pessier, a veterinary pathologist at the National Zoological Park in Washington DC.

The scientists don't yet know if the fungus is the primary cause of death, or is killing animals weakened by other factors, such as ultraviolet radiation penetrating the atmosphere due to the thinned ozone layer or agricultural chemicals. "Many factors could be at work, but the fungus is probably right up there," says Pessier.

Nobody knows where the fungus came from, or how it is spread (see below). It was first noticed by Don Nichols, a colleague of Pessier's at the zoo in Washington DC, in arroyo toads, Bufo microscaphus californicus, from a captive colony in California.

At first Nichols didn't recognize it as a fungus. Because infected skin contained a proliferation of round cells, rather than fungal filaments, Nichols thought it was a protozoan. Earl Green of the Maryland Animal Health Laboratory in College Park made the same mistake when he examined specimens collected in 1996 and 1997 from western Panama by Karen Lips of St Lawrence University in Canton, New York.

By 1996, Nichols and Pessier had begun to doubt the protozoan diagnosis. Then an outbreak of the infection began to ravage the National Zoological Park's population of blue poison-dart frogs, Dentrobates azureus, which come from Surinam. Nichols and Pessier teamed up with Joyce Longcore, a chytrid taxonomist at the University of Maine in Orono, who described her findings to the Mycological Society of America in Puerto Rico last week.

Author: Ian Anderson, Melbourne

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