Written by Ken Kingery
MOSCOW, Idaho – Frogs around the world are dying from a fungal pathogen perhaps because they don’t realize they are sick.
In a study conducted at the University of Idaho, scientists found that the immune system of the study’s frog species failed to respond to the chytrid fungus known as Batrachochytrium dendrobatidis (Bd). In fact, it appears the fungal infection actually may suppress its victim’s immune system.
The findings are reported in a paper recently published in the Public Library of Science online edition (PLoS ONE), an interactive open-access journal for the communication of all peer-reviewed scientific and medical research.
“The biggest thing we found was a surprising lack of response by the frogs,” said Erica Bree Rosenblum, assistant professor of biological sciences and lead author of the study. “If you are hit with a deadly disease, you would expect that your body would do something about it. But we found that these frogs are not turning on immune function genes the way you would expect them to.”
Bd is an ancient fungus that only recently began killing frogs around the world. It lives inside of a frog’s skin, wreaking havoc by some unknown mechanism. Previous studies have shown that once Bd is introduced to a habitat, up to 50 percent of amphibian species and 80 percent of individuals can die within one year.
Scientists do not know how or why Bd kills its host, so Rosenblum is attacking the problem through genetics.
The study examined gene expression in the skin, liver and spleen of infected frogs of the species Silurana (Xenopus) tropicalis – a species highly susceptible to Bd – both three days after exposure and shortly before death. The skin was studied to determine what mechanisms cause frogs to die, while the liver and spleen were chosen to study the immune system’s response to the fungus.
Results found that not only are the genes related to immune response in infected frogs not turning on, but those genes actually are being suppressed.
“And that is the exact opposite of what one would expect to find,” said Rosenblum.
Besides the immune system, the study also looked at the frog’s skin to determine what processes, if any, are responsible for what amounts to a skin infection killing a vertebrate. This is a rare occurrence because, after all, humans don’t die from fungal skin conditions such as ringworm. So why should a fungal skin infection cause frogs to croak?
Though the study did not find any smoking guns, it did point to some disrupted genes in the frog’s skin – an organ that is much more important to an aquatic animal’s health than a land lover’s.
Other disrupted genes seem to affect cellular detoxification, which could make the frogs susceptible to toxins created by the fungus, the natural environment, or both.
According to Rosenblum, though this study is not good news for frogs, and only encompasses one species in a controlled environment, it is an important piece of the puzzle.
“This study is not the answer, but it is a necessary first step to help us find the answer,” said Rosenblum. “The next step in finding this answer involves further research on the generality of these findings in other species and other conditions.”
The research paper can be found online at: www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0006494.