Europe's largest and best known salamander species, the fire salamander, is falling victim to a deadly fungus, and new research is making scientists more pessimistic about its future. A 2-year study of a population in Belgium, now entirely wiped out, has revealed that these amphibians can't develop immunity to the fungus, as was hoped. To make matters worse, it turns out the fungus creates a hardy spore that can survive in water for months and also stick to birds' feet, offering a way for it to spread rapidly across the continent. Two other kinds of amphibians, both resistant to the disease, also act as carriers for the highly infectious spores.
"This is terrible news," says geneticist Matthew Fisher of Imperial College London, who studies the fungus but was not involved in the new research. "This isn't a problem that's going to go away. It's a problem that's going to get worse."
The pathogen, Batrachochytrium salamandrivorans (Bsal), is a chytrid fungus, a type that lives in damp or wet environments and typically consumes dead organic matter. Bsal infects and eats the skin of salamanders, causing lesions, apathy, loss of appetite, and eventually death. Over the past few decades, a related fungus, B. dendrobatidis (Bd), has struck hard at amphibian populations around the world, particularly in the Americas, Australia, Spain, and Portugal. More than 200 species of frogs and toads are thought to have gone extinct, including many kinds of Costa Rica's striking stream-breeding toads.
Bsal was identified in a nature reserve in the Netherlands in 2013 after fire salamanders started dying with ulcers and sores similar to those caused by Bd. Fire salamanders (Salamandra salamandra) grow up to 35 centimeters long, can live more than 40 years, and hunt insects and other small prey in forest streams. Their bright yellow spots warn predators of poison around their head and back. In the Dutch nature reserve, the population plummeted 99.9%. The fungus is thought to have arrived in Europe via salamanders or newts imported from Asia for the pet trade. Bsal has since been found in Belgium and Germany in both fire salamanders and alpine newts.
As soon as Bsal was spotted in Belgium in April 2014, veterinarian An Martel of Ghent University in Merelbeke, Belgium, and her colleagues began visiting every month to track the population. About 90% of the fire salamanders died within 6 months, and after 2 years all were gone. The fieldwork revealed that adult animals were more likely to get infected, which makes sense because they are in closer contact with each other—through fighting for mating and breeding, for example—than are juveniles. But the death of these adults means that the population likely won't recover.
There was no immune response detected in any of the sick animals in the lab, suggesting that it will be impossible to develop a vaccine, the team reports today in Nature. "We really wanted to find solutions to mitigate disease, to save the salamanders, but everything turned out bad," Martel says. The team had also hoped that the fungus would become less virulent—as often occurs when a pathogen reaches a new host that lacks any immunity—but that hasn't happened: Fungal spores taken from the last fire salamanders in the Belgian forest, when dripped onto the backs of healthy salamanders in the lab, were just as lethal as those collected early in the outbreak. "When they come in contact with a single spore, they will die."
This is a very important piece of work, and it's terrifying. … If [this] gets loose in the United States, it's going to be bad.
The paper has more bad news. Researchers knew that Bsal makes spores with a tiny tail called a flagellum, which propels them toward amphibians. If spores dry out, they die. Otherwise, they typically survive for a few days before being eaten by protozoa. But Martel's group discovered that Bsal makes a second type of spore that looks much hardier and is rarely eaten by protozoa. "This will make it almost impossible to eradicate the fungus from the environment," says Martel, who adds that the spores can survive in pond water for more than 2 months.
Another experiment showed that soil remained infectious for 48 hours after it was walked on by a sick salamander. In a separate lab test, the spores adhered to goose feet, suggesting they could hitchhike long distances on birds.
The group also showed that two species that share the same habitat as the fire salamander are likely carriers of the disease. Midwife toads (Alytes obstetricans) could be infected with the fungus and shed spores for a few weeks, but they didn't get sick. A high dose of the fungus killed alpine newts (Ichthyosaura alpestris), but low doses made them infectious for months without killing them. As has happened with Bd in the Americas, Bsal will lurk in these reservoirs of disease even after local populations of fire salamanders vanish. Any fire salamanders that arrive from elsewhere will likely get infected by newts or toads.
According to results from previous infection trials, most salamander species in Europe are likely just as vulnerable to Bsal. The fire salamander has a range that extends across Europe, and the fear is that the fungus will reach endangered salamanders. With small populations, these species could more easily be driven extinct, Fisher says. "The assumption is that they are all at risk," he says, and the findings in the new paper "have really upped their risk status." Martel and European colleagues recently started monitoring for Bsal in seven countries.
It is possible to cure amphibians in the lab. For animals that can take the heat, like fire salamanders, 10 days at 25°C will kill the fungus. Other species can be cured with a combination of two drugs. But there is no practical solution for animals in the wild, especially when their habitat is contaminated with fungal spores.
Herpetologist Jaime Bosch of the National Museum of Natural History in Madrid had a rare success in eliminating a chytrid fungus from the wild. A few years ago, he and colleagues got rid of Bd on the Spanish island of Mallorca by temporarily removing some 2000 tadpoles of the Mallorcan midwife toad (Alytes muletensis) and disinfecting their ponds with powerful chemicals. But this success would be hard to replicate in less isolated locations, he says. "Right now, we are very far away from having any solution." The only hope in the meantime, Bosch and others say, is to slow the spread of the disease by ending the importation of amphibians.
The United States, a hot spot of amphibian diversity, has already taken steps in that direction. Last year, the U.S. Fish and Wildlife Service banned the import of 201 species of salamanders on the grounds that they might introduce the fungus. Joe Mendelson, a herpetologist at Zoo Atlanta, says the new research suggests the list should be expanded to include other carriers such as the toad and newt studied in the new paper. "This is a very important piece of work, and it's terrifying," he says. "If Bsal gets loose in the United States," he says, "it's going to be bad."