The skin fungus, Batrachochytrium dendrobatidis, also known as amphibian chytrid, first made its presence felt in 1993 when dead and dying frogs began turning up in Queensland, Australia. Since then it has sickened and killed frogs, toads, salamanders and other amphibians worldwide, driving hundreds of species to extinction.
The fungus is blamed for major declines in the Costa Rican amphibian population, too.
As a postdoctoral researcher Kevin Smith studied the fungus in South Africa, home to the African clawed frog, a suspected vector for the fungus. When he took a position at Washington University in St. Louis, Missouri, at the Tyson Research Center, he worked on other problems.
But whenever he visited a pond, he collected tadpoles and checked their mouth parts, often a fungal hot spot, under the microscope, just out of curiosity.
He found the fungus in about a third of the ponds whose tadpoles he checked. The obvious questions were: Why only a third? Why didn’t it occur in all amphibian populations in a region where it is found?
The amphibians and the fungus have reached an evolutionary truce in Missouri, where the chytrid is endemic rather than epidemic. Because there was no pressure to rescue an amphibian population, Smith had the time and the opportunity to look more broadly and to study the entire pond ecosystem.
Together with an undergraduate student Smith collected physical and chemical data and surveyed the species living in 29 ponds in east central Missouri. The results of this study are published in PLOS ONE.
Somewhat to Smith’s surprise, it was statistically possible to distinguish infected from non-infected ponds, a finding he likens to being able to predict that influenza will circulate in some cities but not others.
“We don’t know exactly what the key factors are but just knowing that not every pond appears to be suitable for chytrid in a given year is a very big step,” he said.
The study also suggested that patterns of fungus infection might be an indirect effect of variations in invertebrate communities. What this meant was unclear, since chytrid was thought to be an amphibian specialist.
But while the pond study was underway, other researchers announced that crayfish and nematodes can be infected with chytrid, raising the possibility that invertebrates act as alternative hosts or biological reservoirs for the fungus.
“Alternative hosts and reservoirs have been a key missing piece in our understanding of chytrid epidemiology,” Smith said. The fungus, like any pathogen, cannot be effectively controlled unless all its hiding places are known.
Chytrid, or more properly amphibian chytrid, since there are about 1,000 species of fungus in the class Chytridiomycetes, specializes on keratin, a structural protein found in the skin, hair, nails and similar tissues of vertebrates.
In amphibians, chytrid infects and damages the skin, which amphibians use to breathe and absorb water. Once the fungus takes hold, it causes a disease called chytridiomycosis, which is usually fatal.
“You can sometimes tell when a frog is infected,” Smith said, “by the way it walks. It is slow and spraddles its legs, as though its skin is painful or chafed. When we grabbed frogs like those in South Africa and took samples, they were always heavily infected with the fungus,” he said.
Unlike more familiar fungi such as mushrooms, which release spores that drift through the air, chytrids, among the earliest fungi to evolve, are aquatic and release flagellated zoospores that swim through the water.
“Laboratory studies suggest the zoospores can live independently only about a day or so. They’re considered to be very fragile,” Smith said. “They get expunged from the fungal cell inside the amphibian skin, they swim around for about a day, and if they don’t infect something with keratin, they’re no longer viable. That’s what’s generally thought.”
“That’s why we focused on the aquatic habitat,” Smith said.