Refuge Notebook 3-30-2012


Mystery of the Frog-Killing Fungus

by John Morton

During a hot July day in 2002, Mari Reeves, a biologist with the Anchorage Field Office of the U.S. Fish and Wildlife Service, was poking around the Swanson River Road in the Kenai National Wildlife Refuge.  Reeves was just starting a multi-year investigation of structural abnormalities in our ubiquitous wood frog.  What she found was a dead subadult frog that was subsequently diagnosed with chytridiomycosis, a sometimes lethal disease caused by the fungus, Batrachochytrium dendrobatidis (Bd). 

Finding Bd in Alaska was (and is) a big deal.  This fungal pathogen was first described in the scientific literature in 1999, but was widely recognized within only a few years as the cause of recent global declines in amphibian species.   Bd appears to be capable of infecting most of the world’s more than 6,200 amphibian species, of which 30% are already threatened with extinction.  And Reeves’ frog was the first time that Bd was found in Alaska. 

Bd is a chytrid fungus, of which there are approximately 1,000 different species that live exclusively in water or moist environments.  The chytrids are among the most primitive types of fungi, known to be at least a half billion years old.  Bd is unusual because it is the only chytrid that parasitizes vertebrate animals, specifically amphibians.  Chytridiomycosis is the disease that occurs when an amphibian is infected with large numbers of the Bd fungus; it becomes lethal when zoospore count equivalents reach 10,000. 

Infection with Bd occurs inside the cells of the outer skin layers that contain large amounts of keratin, a protein that is also found in our own hair and nails.  With chytridiomycosis, the skin becomes very thick due to a microscopic change called “hyperplasia” and “hyperkeratosis”.  These changes in the skin are deadly to amphibians because— unlike most other animals— amphibians absorb water and electrolytes like sodium and potassium through the skin and not through the mouth.  Abnormal electrolyte levels as the result of Bd-damaged skin can cause the heart to stop beating and the death of the animal.  Common sub-lethal signs of chytridiomycosis are reddened or otherwise discolored skin, excessive shedding of skin, abnormal postures such as a preference for keeping the belly away from the ground, unnatural behaviors such as a nocturnal species that suddenly becomes active during the day, or seizures.

In southeast Alaska, boreal toads—Alaska’s only toad species—have gone from abundant to almost non-existent in less than ten years.  The cause remains uncertain, but biologists have since found boreal toads in Alaska’s coastal rain forest (and in other states) that died from the chytrid fungus.

In 2006, Reeves and her colleagues found Bd-infected wood frogs in the Swanson River canoe system as well as along the Swanson River Road.  In 2011, Bd-infected frogs were found at 17 sites, including 10 frogs carrying lethal loads of zoospores.  During that same year, Meg Perdue, who replaced Reeves as lead investigator, also employed a new genetic method for detecting Bd in water samples, which did not require the conventional method of swabbing the skin of captured frogs (see photo).  More than a third of 34 sites that she tested came back positive for Bd in the water.

Neither Reeves nor Perdue have yet to find Bd in wood frogs on Tetlin or Innoko National Wildlife Refuges, two other places their team has surveyed.  But other researchers have found Bd as far north as Denali National Park.  On the Kenai, many of the contaminated ponds and infected frogs were associated with gravel roads, and it’s possible that this may be a mechanism by which Bd is spreading.

Based on an examination of museum specimens going back to 1879, one school of thought believes Bd likely originated in South Africa and was spread by the international exportation of the African clawedfrog (Xenopus laevis), a commercial trade that started in 1934 with the development of a frog-based assay to determine pregnancy in humans! 

Bd outbreaks were initially thought to be triggered by recent global warming trends since they are temperature dependent.  However, a recent analysis suggests that the warming and cooling phases brought about by El Nino cycles may be driving episodes of fungal growth.     

Whatever the cause, declining boreal toad populations in southeast Alaska and other amphibians elsewhere in the world are especially alarming because amphibians are indicator species, an early warning system of environmental health because of their sensitivity to pollutants and changes in environmental conditions such as rapid climate change and increasing ultraviolet radiation.  The fact that so many frog species are going extinct worldwide suggests that something is ecologically out of whack.   

So when you next hear wood frogs chorusing this spring as the ice melts in your neighborhood wetland, really take the time to stop and listen.  Their calls, or perhaps the absence of them, may be trying to tell us something even as we try to understand this mystery.    

John Morton is the supervisory biologist at Kenai National Wildlife Refuge. You can find more information about the Refuge at