U.S. team studies what drives amphibian extinctions

Emerging infectious diseases, in particular chytridiomycosis, are responsible for drastic declines in amphibian diversity around the world, and determining the elements that influence these pathogens and how they spread can help scientists understand the dynamics involved.

Chytridiomycosis is lethal to many species of amphibians. It is caused by fungal pathogens, Batrachochytrium dendrobatidis (Bd) or Batrachochytrium salamandrivorans (Bsal). Bd was discovered in the 1990s, and is a priority research subject because of its rapid worldwide spread and the amphibian mortality it has caused.

A group of U.S. scientists, some associated with the University of Nevada, set out to study the "emergence, spread, pathogenicity, and ecology" of the Bd pathogen. They focused on how temperature across different latitudes might influence the growth rates and viability of different strains of Bd. Their study appears in Frontiers in Veterinary Science, June 22.

"No other pathogen is known to have had such a ubiquitous effect on such a broad range of host species and in so many different environments," the researchers said. "As a result, Bd-related declines have been called, `the most spectacular loss of biodiversity due to disease in recorded history.'”

A focus on Bd viability 

The Bd pathogen's life cycle is in two stages. First it is a zoospore, an asexual spore that moves around aquatically and attaches itself to the amphibian skin or larval mouthparts. Then it matures into a spore case (zoosporangium) to reproduce more zoospores. Lethality depends on the intensity of the infection, so the researchers were interested in determining what regulates Bd growth and reproductive rates.

The researchers collected five isolates (strains) of Bd from amphibians in different U.S. states (Ohio, Vermont, Louisiana, New Mexico and Texas) and cultured them in the laboratory. They then measured the population growth of each strain and its genetic variants under different temperature conditions.

Previous studies had not found any environmental factors, such as temperature or precipitation, linked to Bd growth. But this study found what the researchers called "intriguing patterns" in differences among the isolates.

Although the isolates were similar at the intermediate temperature of 21 °C (70 degrees F), "within each temperature, the isolates frequently differed from each other in their maximal viability, zoospore densities, fecundity, growth rates, and carrying capacities. These differences were pronounced at either end of the thermal spectrum, at low (4 °C) and high (26 and 27 °C) temperatures." The researchers also found that all the isolates had highest fecundity in low temperatures.

The researchers write that the mean temperatures across latitudes "may influence the maximal performance of Bd." There was some evidence that the Vermont and Ohio isolates, states in northern latitudes with generally lower mean temperatures, "exhibited lower viability and carrying capacities across temperatures" than the isolates from the more southern latitudes.

However, some Bd reproductive traits that the study looked at did not follow this pattern.

More research needed

The researchers conclude that variations in abiotic factors among Bd genetic variants are important in determining amphibian risk of disease.

"To intervene in the continued population declines of amphibians, we must understand how pathogen biology is mediated across different environments, and within and among genetic lineages," they wrote. "We must also determine what environmental factors are driving the disease dynamics responsible for the disease-induced losses of amphibian biodiversity."