Amphibian declines and extinctions around the world have been linked to an emerging fungal disease called chytridiomycosis, but new research from the National Institute for Mathematical and Biological Synthesis (NIMBioS) and the Center for Wildlife Health at UT shows that another pathogen, ranavirus, may also contribute.
In a series of mathematical models, researchers showed that ranavirus, which causes severe hemorrhage of internal organs in frogs, could cause extinction of isolated populations of wood frogs if they are exposed to the virus every few years, a scenario that has been documented in wild populations.
The most widely distributed amphibian species in North America, wood frogs have been shown to be highly susceptible to ranavirus infection, particularly as tadpoles. But little research has been done into how ranavirus affects frogs at all stages of their life cycle–from egg to hatchling to tadpole to metamorph, the stage when they emerge as frogs. Little is also known about how the infection could hasten extinction in entire populations.
The study, published in the journal EcoHealth, investigates the effect of ranavirus on the entire life cycle of wood frogs in demographically isolated populations, where there is no movement of frogs into the population from surrounding areas.
The study used mathematical simulations based on long-term data sets from wild populations of wood frogs in the eastern United States and laboratory data on the effects of ranavirus. It determined that the life stage during which frogs were exposed to ranavirus was one of the most important factors in predicting extinction and declines.
Extinction was most likely to occur when tadpoles or metamorphs were exposed to ranavirus at frequent intervals in small populations. Under the worst-case scenario, extinction could occur in as quickly as five years with exposure every year and 25-44 years with exposure every two years.
The egg stage had a 57 percent survival rate when exposed to ranavirus, which was high enough to prevent extinction. Scientists speculate that eggs have a greater survival rate than other stages because they are protected by a thick gelatinous membrane that may serve as a structural barrier or contain antiviral properties.
“Just as the chytrid fungus has decimated frog populations, the results of our study suggest that ranavirus infection, too, could contribute to extinction of amphibian populations that are demographically isolated,” said lead author and NIMBioS postdoctoral fellow Julia Earl.
Amphibians are already considered the most imperiled of vertebrates, and a third of amphibians are threatened or endangered, according to the International Union for Conservation of Nature, the main international body that assesses the conservation status of species.
Disease also may be playing a role in amphibians’ extinction. Since the 1990s, chytridiomycosis, which has been called the worst disease affecting vertebrate animals in recorded history, has caused massive die-offs and species extinctions around the world, particularly in Australia; the Caribbean; and North, Central, and South America.
“Ranavirus infections in amphibians have been known since the 1960s, but it wasn’t until the 1980s when we started to see large-scale die-offs,” said collaborating UT scientist and director of the Global Ranavirus Consortium, Matthew Gray, “Hypothesized mechanisms for emergence include global transport of novel virus strains during the commercial trade of cold-blooded animals and an increase in anthropogenic stressors, such as pesticides, on the landscape.”
Once exposed to ranavirus, in susceptible species like wood frogs, mortality can be as quick as three days. Transmission can occur through water, direct contact, and tadpoles scavenging other dead and infected frogs. There is no cure or treatment for the disease.
NIMBioS is an NSF-supported center that brings together researchers from around the world to collaborate across disciplinary boundaries to investigate solutions to basic and applied problems in the life sciences.
To learn more about this research, visit the NIMBioS website.
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