How to save the frogs

11/04/2009

A great barred frog with severe chytridiomycosis, a fungal disease killing many amphibians around the world/Copyright (c) 2009 Lee Berger

An invasive and virulent fungus has devastated frog and other amphibian populations around the world over the past decade.  Scientists identified the chytrid fungus Batrachochytrium dendrobatidis (Bd) on the skin of dying amphibians, which leads to a disease called chytridiomycosis, but they didn’t know the mechanism for how the disease killed, since the fungus seemed to affect only the skin.

Amphibians have a unique ability to breathe and absorb electrolytes – charged ions – through their porous skin, and James Cook University disease ecologist Jamie Voyles and her colleagues put this idea to the test: Do Bd infections cause frogs' skin somehow disrupt their ability to maintain the necessary electrolyte balance? "We measured skin functioning in frogs with severe chytridiomycosis," says Voyles. "We also tracked physiological changes in blood and urine, and monitored heart function."

Using Australian green tree frogs (Litoria caerulea), Voyles’ research, published in the journal Science, demonstrated that frogs infected with chytrid fungus lost 50 percent more sodium and potassium ions into water compared to healthy frogs. That reduced sodium and potassium ion concentration in the frogs' blood by 20 and 50 percent, respectively, which in turn led to cardiac arrest. "Water is very dilute and draws out the frogs' ions when their skin is not functioning," explains James Cook University Research Fellow Lee Berger, a co-author on the study.  They compare this phenomenon to hyponatraemia, a condition where people, particularly athletes, have died of heart attacks from drinking too much water too fast because it drastically dilutes the ion concentration in the blood.

Voyles developed an electrolyte-replacement solution that helped diseased frogs better absorb essential potassium and sodium ions. They studied the solution, which is sort of like “froggie Gatorade,” on diseased frogs and it delayed their deaths and helped restore the frogs’ electrolyte balance, but the frogs tested still died. The scientists believe that they could not save the animals because those individuals had too severe of infections. They plan to continue research in hopes that they may someday use this or a similar solution to turn the tide on the global amphibian decline. 

Another group of scientists from James Madison University in Harrisonburg, Virginia found another promising solution – a probiotic bacteria called Janthinobacterium lividum. Reid Harris and colleagues gave critically endangered mountain yellow-legged frogs (Rana muscosa) a "bacterial bath" and found it helped them maintain weight, and also prevented them from dying.

The chytrid fungus Bd most likely came from exotic African clawed frogs (Xenopus laevis) which have been introduced around the world and have immunity to the fungus. In the 1930s and 1940s doctors would inject female urine under their skin to test for pregnancy, since the hormones in a pregnant female would cause the frogs to ovulate. Then the clawed frogs ended up discarded in waterways around the world and the fungus has spread through water or amphibian to amphibian contact. In cool moist environments which the fungus invades, it will cause 50 percent of species and 80 percent of individuals to disappear within a year, according to a study by Karen Lips.