In some parts of the world, amphibian numbers are in decline. Activists are quick to blame everything from fracking to pesticides for reduced numbers of some frogs, but scientists have linked it to an emerging fungal disease called chytridiomycosis.

New research from the National Institute for Mathematical and Biological Synthesis (NIMBioS) findss 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 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 eastern United States and laboratory data on the effects of ranavirus. It determined that the life stage during which a frog was exposed to ranavirus was one of the most important factors in determining extinction and declines.

Extinction was most likely to occur when the tadpole or metamorph was 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.

Ranavirus infection could contribute to extinction of wood frogs in populations that are demographically isolated. Credit: Thomas Brown

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 anti-viral 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 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 across 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 they were associated with large-scale mortality and disease.

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 when tadpoles scavenge other dead and infected frogs. There is no cure or treatment for the disease.