Close look at new fungus reveals origins, spread of salamander killer

A salamander-killing fungus first described in 2013 looks as if it originated in Asia and is hitchhiking around the world in the pet trade.

The fungus, nicknamed Bs, for Batrachochytrium salamandrivorans, came to the attention of science during baffling die-offs of rare fire salamanders in the Netherlands. Another Batrachochytrium fungus had ravaged amphibian populations in recent decades. But An Martel of Ghent University in Belgium and her colleagues ruled it out in the new die-offs and discovered that the culprit was an unknown relative, which researchers named last year (SN: 10/5/13, p. 18). Now, after more surveys and lab tests, Martel and collaborators start to answer questions on the spread and targets of Bs in the Oct. 31 Science.

“It is appropriate to be exceptionally concerned, if not alarmed,” says Jamie Voyles of New Mexico Tech in Socorro, who studies the killing mechanisms of a closely related chytrid fungus. So far, no evidence shows the newly recognized disease has reached North America, home to a quarter of the world’s known salamander species. But the continent is a market for amphibians from Asia, and as veterinary pathologist Allan Pessier of San Diego Zoo Global puts it, “we need to focus on disease screening and biosecurity for imported animals.”

This new salamander pathogen came as a shock to biologists already dismayed by more than 20 years of amphibian disease outbreaks. In 1999, Pessier and others identified the cause: a previously unknown species of chytrid fungus they named Batrachochytrium dendrobatidis, or Bd. Generally mild-mannered, chytrids usually break down dead stuff in the environment.

The second killer chytrid seems to target salamanders, especially newts, Martel and colleagues report. The team attempted to infect 35 species picked from different branches of the amphibian genealogical tree. The fungus didn’t seem able to attack frogs, toads or legless snake-shaped amphibians called caecilians. But it quickly killed 41 of the 44 individual salamanders tested from Western temperate-zone species.

“It’s not a pleasure to see,” Martel says. In a susceptible species, the fungus “literally eats the skin off an animal.”

In the infection tests, Bs generally didn’t kill Asian species, suggesting the fungus originated in Asia, the researchers say. Asia also looked like the probable origin after the researchers’ epic efforts to check for pathogen DNA on 5,391 wild amphibian specimens worldwide. Bs turned up on only two of the four continents surveyed. Animals tested positive in Thailand, Vietnam and Japan, where biologists have not seen salamander die-offs. These species may have evolved for millions of years with the pathogen. In contrast, Bs was detected in the Netherlands and Belgium, where salamanders have died.

The new study makes a good case for an Asian origin of Bs, says Timothy Y. James of the University of Michigan and a coauthor of a massive genetic study of the Bd fungus.

To see if an Asian disease could travel via the global trade in live animals, Martel and her colleagues checked 2,335 skin samples from captive animals in such places as European pet shops, a Hong Kong export business and London’s Heathrow Airport. Three salamanders, two of which were sent to Europe in 2010, carried the fungus. Lab tests with two other species show that one salamander touching another can transmit the disease, the researchers report.

No wild North American salamanders have tested positive for Bs, although surveys so far have big gaps, says study coauthor Karen Lips of the University of Maryland in College Park. She warns that if the disease does strike North America, the inner workings of ecosystems could change. Tiny salamanders darting through leaf litter or burrowing into soil often get overlooked, but “these things are superabundant,” she says. She fears a repeat of the nightmare she chronicled in Panama as the first amphibian-killer fungus swept through. Bd exterminated some 30 of 74 amphibian species at her study site and shook up the rest of the community, even reducing numbers of certain snakes that eat amphibians.

To prevent such loss and disruption, Lips calls on Congress to pass proposed legislation requiring health checks for imported amphibians. Regulations already address keeping rop plants and livestock from bringing new disease into the United States, she points out.