#Climate Change Accelerates Amphibian Disease Spread

In May 2011, herpetologist Robert Zappalorti stood at the edge of a pond in Ocean County, New Jersey, watching green frog tadpoles die by the hundreds. They were lethargic and swollen, their skin erupting in angry red lesions. Within a week, 99 percent of them would be dead. This was New Jersey's first documented encounter with ranavirus, a pathogen that kills so quickly and brutally that researchers call it "the Ebola of the frog world."

Fifteen years later, scientists still can't predict where or when it will strike next.

The Five-Day Killer

Ranavirus doesn't waste time. Once an infected amphibian shows clinical signs, it has roughly five days to live. The virus attacks multiple organ systems simultaneously, causing hemorrhaging, tissue necrosis, and systemic failure. In a pond outbreak, the pathogen can wipe out nearly an entire population within 7 to 10 days.

Lisa Hazard and Kirsten Monsen-Collar, both associate professors at Montclair State University, documented that first New Jersey outbreak and have been tracking the virus ever since. What they've observed is deeply unsettling: ranavirus doesn't discriminate. It jumps between species with ease. At that initial Ocean County site, Fowler's toad tadpoles were seen eating the corpses of dead green frog tadpoles. A week later, the toads began dying in droves.

This cross-species transmission matters because New Jersey hosts over 30 amphibian species, nearly 40 reptile species, and 90 freshwater fish species. All of them are vulnerable. The virus has already caused mass die-offs in both captive and wild fish populations, raising what Monsen-Collar calls "profound risk of really serious economic impact" for fisheries and the communities that depend on them.

The Temperature Connection

The relationship between ranavirus and climate change isn't speculative—it's thermal. The virus "tends to be more infectious at higher temperatures," according to Monsen-Collar, at least up to a certain threshold. Trent Garner at the Zoological Society of London found a "relatively linear" relationship between rising temperatures and growth rates for the ranaviruses responsible for most North American amphibian deaths.

A 2019 study of common frogs in the UK projected that severe ranavirus outbreaks would occur "over wider areas and an extended season" under future climate scenarios. Warmer water doesn't just speed up viral replication—it can weaken amphibian immune systems while simultaneously creating ideal conditions for pathogen spread.

But the picture gets murkier when you zoom in. Different ranavirus species flourish at different temperatures. Some might be suppressed if water gets too hot, while others thrive. The relationship between temperature and virulence, as researchers have discovered, is "not uniform across the board."

The Unpredictable Pattern

In the late 2010s, researchers surveyed 17 sites across New Jersey, expecting to find patterns that would help predict outbreaks. They measured pond size, depth, canopy cover, tree species composition, and proximity to human development. None of it mattered. Some sites tested positive for ranavirus every single year. Others alternated. Some showed the virus three years out of four with no apparent logic.

A larger 2016 study surveyed 122 ponds across five Northeastern states over two years. Of the 30 ponds sampled both times, just under half tested positive for ranavirus in both years. The virus persists in the environment, lurking even when no die-offs are visible.

William Pitts, a senior zoologist with the New Jersey Department of Environmental Protection, describes an "ebb and flow cycle" to ranavirus outbreaks. But scientists don't know what drives it. Rainfall? Temperature fluctuations? Population density? Immune resistance in surviving populations? All of the above?

"We're all kind of waving our hands, predicting, 'Well, we think that this could happen,'" Monsen-Collar admits, "and we're kind of just holding on to our seats, waiting to see what is going to happen."

A Crisis Within a Crisis

Ranavirus is just one threat in a much larger catastrophe. Amphibians are the most threatened vertebrate class on Earth. The 2023 Global Amphibian Assessment found that 40.7 percent of the 8,011 species evaluated—2,873 species total—face extinction. That percentage has climbed steadily from 37.9 percent in 1980 and 39.4 percent in 2004.

What changed between 2004 and 2023 is the primary driver. Climate change now accounts for 39 percent of status deteriorations, essentially tied with habitat loss at 37 percent. In earlier decades, disease and habitat destruction dominated. The fungal pathogen Batrachochytrium dendrobatidis, which causes chytridiomycosis, devastated populations worldwide and drove many of the "enigmatic declines" that puzzled scientists in the 1980s and 1990s.

Now climate change is accelerating both old threats and new ones. Garner believes it's having "direct and catastrophic effects" on many amphibian species and is "proceeding too fast for many of these to keep up through adaptation."

Ecosystems Without Tadpoles

Ranavirus doesn't threaten humans directly, but its downstream effects ripple outward in ways we're only beginning to understand. A 2022 study linked amphibian declines from fungal disease to increased human malaria incidence in Central America—fewer frogs meant more mosquitoes, which meant more disease transmission.

Amphibians occupy a critical middle position in food webs. As tadpoles, they graze on algae and detritus. As adults, they consume vast quantities of insects. Lose them, and ponds choke with algae while insect populations explode or crash unpredictably. Fish that feed on tadpoles lose a food source. Snakes and birds that hunt adult frogs go hungry.

New Jersey, the most densely populated state in the U.S. outside Washington D.C., offers a preview of what happens when disease, climate change, and human pressure converge. Fifteen years after that first outbreak, state officials continue surveillance. "We're going to keep surveilling [ranavirus] and documenting it," Pitts says.

But documentation isn't prevention. And as temperatures continue to rise, the windows for outbreaks will keep expanding, the pathogen will keep evolving, and scientists will keep watching ponds turn into graveyards, still unable to predict which one will be next.