amphibians, disease, human impact

Ranaviruses: Ravaging Amphibian Populations

Over the past few years, amphibians have been hit hard on a global scale. (Amphibians include frogs, toads, salamanders, and the lesser-known caecilians, which are tropical, limbless, worm-like critters.) A few months ago, I wrote a post about the chytrid fungus, which may have already caused 125 to 500 amphibian species to become extinct. Another group of diseases, specifically ranaviruses, has also been recently ravaging amphibian populations (and other animal populations) all over the world. But while there are clearly multiple dieases responsible for these amphibian declines, people are likely the ultimate culprit here.

eastern box turtles ranaviruses
Among turtles, Eastern Box Turtles (Terrapene carolina carolina, shown here) are some of the ones that have been hit the hardest by the ranaviruses. (Image credit: Ken Thomas)

According to the USGS National Wildlife Health Center, it’s suspected that these ranaviruses may now be in every state in the U.S. (except perhaps Hawaii). In addition to the Americas, they’ve also been found in Asia, Australia, and Europe. And the consequences for this are devastating – the ranaviruses often kill more than 90% of an infected population of amphibians. (In the U.K. alone, ranaviruses are thought to kill tens of thousands of amphibians every year.) Because the ranaviruses are relatively easily transmitted – they can make their way from one individual to another through direct contact, or indirectly through contaminated water or ingesting contaminated materials – it is very difficult to control the spread of the disease. And, complicating matters more, local infections can involve multiple different species in the same area, with some species likely serving as reservoirs.

This video shows researchers discussing the appearance and spread of the ranaviruses in the U.S. (Video credit: Voice of America, U. S. government)

What exactly are the ranaviruses? As their name implies, they’re a type of virus. Specifically, they are large DNA viruses, and while we’ve focused on amphibians so far, these viruses actually infect reptiles (particularly turtles) and a variety of fish as well (causing significant concern for some fisheries). Signs of an epidemic usually manifest suddenly, with hundreds or thousands of animals getting sick or dying within a short period of 1-5 days. Usually juveniles are hit the hardest, which is why large die-offs are often seen in the spring or summer. Infected individuals become lethargic, have trouble swimming (making them easier targets for predators), accumulate fluid under their skin near their bellies and hind legs, have hemorrhages on their skin, and internally can also have unusual fluid accumulation and hemorrhaging.

Major die-offs were first reported in the mid-1990s in the U.S. and U.K., and they’ve spread since then. Why now – what’s caused these die-offs to happen only relatively recently? It’s thought that the viruses emerged due to introduced stressors in the animals’ environments (such as nitrogenous waste, climate change, agricultural land use, etc.), and/or from moving the ranaviruses to new amphibian populations, populations that weren’t previously exposed to the viruses. Both possible explanations are caused by human activities. The transportation of infected animals is also likely how the chytrid fungus came to be so destructive to amphibian populations. For example, until the 1960s, the African clawed frog (Xenopus laevis) was globally used as human pregnancy tests, and they’ve also been sold as pets and used in scientific research laboratories. These frogs are now known to be one of the few chytrid fungus-resistant amphibians that can serve as carriers – we’ve been spreading these potential carriers all over for decades.

To help prevent spread of the ranaviruses, it’s recommended to disinfect footwear and any equipment (such as for using fishing bait) that comes in contact with water where amphibians live.

The story of the ranaviruses is another example of how seemingly little actions we do – such as purchasing and releasing an exotic pet into an area it’s not native to – can have huge, unintended costs. But while it only took small actions to get started, it’ll take a lot more work to bring the devastating consequences to a hault.

(Special thanks to Amanda Duffus for suggesting writing a Byte on this topic!)

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