Climate Change Could Make This Horrific New Jersey Wildlife Disease Worse

It was May 2011 when herpetologist Robert Zappalorti called scientists about the tadpoles.

Researchers at Montclair State University went to see for themselves. At the site in Ocean County, New Jersey, they found green frog tadpoles that looked like something out of a medical drama or a frog horror film—lethargic and swollen, with red lesions on their skin.

In the same area, they found Fowler’s toad tadpoles eating the corpses of dead green frog tadpoles.

“Approximately one week after the initial [green frog] die off, there was mass [Fowler’s toad] tadpole mortality,” the researchers wrote in a paper published in 2013. In it, they identified the culprit: the first recorded occurrence of ranavirus in New Jersey.

Documented around the world, ranaviruses can affect frogs, salamanders, fish and reptiles, of which New Jersey has plenty. The state with more humans per square mile than anywhere else in the country in 2025 (excepting the District of Columbia) is also home to over 30 species of amphibians, nearly 40 species of reptiles and 90 species of freshwater fish. 

Fifteen years after ranavirus was first in New Jersey, scientists still don’t know the broad, long-term impacts the disease is having on the state’s ecosystems. There’s also a chance climate change could exacerbate ranavirus in New Jersey and elsewhere, threatening both the state’s environmental and economic health.

“Ranavirus is here, and we don’t fully understand what it’s going to do, but it’s happening,” said Lisa Hazard, a physiological ecologist and associate professor at Montclair State University in New Jersey. “We’re living it.”

For amphibians, ranavirus can cause swelling, hemorrhaging, ulcers and abnormal spine curvature; multiple organ failure usually kills animals displaying clinical signs within five days. Hazard refers to it as the Ebola of the frog world. All of the die-offs in New Jersey she’s aware of have been amphibians, but that’s the group of animals being tested and paid attention to, she said.

Hazard and Kirsten Monsen-Collar, a molecular ecologist and associate professor at Montclair State, were the ones who documented New Jersey’s first recorded ranavirus outbreak in 2011. The pair had spoken at a meeting of the state chapter of the Wildlife Society that same year and invited attendees to report back if they encountered any signs of ranavirus, Hazard said. Not long after, they got Zappalorti’s phone call.

Hazard and Monsen-Collar have seen outbreaks start with one or two symptomatic animals with ranavirus DNA inside them, said Monsen-Collar, “and within seven to 10 days, 99 percent of the population’s dead.”

Part of what makes the pathogen frightening is its ability to survive in the environment for a long time, Monsen-Collar added. She contributed to a 2016 study of ranavirus that surveyed 122 ponds over two years across five Northeastern states, including New Jersey. Of the 30 ponds sampled in both years, just under half tested positive both times.

It’s proven difficult to pin down what environmental factors are triggering outbreaks, Hazard said. She and Montclair State undergraduate student Adriana Messyasz surveyed a total of 17 sites in New Jersey for the disease in the late 2010s, resampling sites from the 2016 regional study as well as adding their own. 

Combining their results with those of the larger study gave them four years of data for multiple locations, but they still struggled to see any consistent pattern, Hazard said: Some sites had ranavirus every year, while others alternated or had the disease three out of four years. They looked at variables like pond size and depth, canopy cover, tree species and proximity to human impact, but none of these were clearly associated with presence or absence of the disease.

The pair concluded ranavirus is “everywhere,” Hazard said, and might hit one site but not another in any given year, with seemingly no rhyme or reason.

“I think it would take a lot more data—a lot more ponds, a lot more years—to actually pick up on any kind of actual environmental factors that are influencing it,” she said.

William Pitts, a senior zoologist with the endangered and nongame species program at the state Department of Environmental Protection’s Fish & Wildlife arm, voiced similar unknowns. Ranavirus in the state seems to have an “ebb and flow cycle” right now, he said, but scientists aren’t sure precisely what’s driving that cycle.

“We’re going to keep surveilling [ranavirus] and documenting it,” he said, “and seeing if we do have more evidence to say this is a longer-term problem, or this probably is just a thing that [animals have] always been dealing with.”

Ranavirus does not directly endanger humans, but wildlife diseases like it can upset ecosystem functioning and have serious impacts on humanity without ever infecting people. A 2022 paper linked amphibian declines from a fungal disease to increased incidence of human malaria in Central America. Malaria isn’t a concern in New Jersey, but Hazard said there are always “downstream effects” that people don’t initially think about.

“People think, ‘Oh, it’s a frog. Who cares about a frog?’” she said.

Ranavirus has caused mass die-offs in both captive and wild fish populations, Monsen-Collar said, and the disease could spell financial trouble for those whose livelihoods involve fish.

“In my opinion, there is a profound risk of really serious economic impact because of these kinds of pathogens,” she said.

Climate change could throw an extra wrench into the gears. Ranavirus has a temperature range where it’s optimally active and infectious, and it tends to be more infectious at higher temperatures, Monsen-Collar said—to a point. A small temperature increase could make it more likely that pathogens will infect and spread faster within organisms, but going above their limit could actually prevent infection.

“We don’t know. We’re all kind of waving our hands, predicting, ‘Well, we think that this could happen,’” Monsen-Collar said. “And we’re kind of just holding on to our seats, waiting to see what is going to happen.”

A 2019 study of United Kingdom common frogs predicted severe ranavirus outbreaks would occur “over wider areas and an extended season” under certain climate projections. Trent Garner, a professor at the Zoological Society of London’s Institute of Zoology and a co-author on the paper, wrote in an email that there is a “relatively linear” relationship between increasing temperature and growth rate for a specific group of ranaviruses—a group very similar to the ranaviruses responsible for the majority of amphibian deaths in North America.

The relationship between temperature and virulence is not uniform across the board, Garner said—different ranaviruses appear to flourish at different temperatures. But he nonetheless thinks climate change is having “direct and catastrophic effects” on many species of amphibians and is proceeding too fast for many of these to keep up through adaptation. 

Even if warming isn’t directly affecting the ways amphibians and ranavirus interact, “amphibians in rough shape due to climate change will undoubtedly be less able to cope with any infectious disease,” he wrote.

“While I do think that in some cases warming may mean some amphibians that are susceptible to disease caused by ranaviruses are less exposed to or better able to cope with infections, the majority of outcomes involving climate change and ranaviruses will not be to the benefit of amphibians,” he added.

Like many fields, studying wildlife disease is restricted by time and financial limitations that force researchers into decisions on what’s worthwhile. Hazard and Messyasz’s study didn’t lead to any concrete findings, despite the pair visiting their sites multiple times a year over a two-year stretch (sometimes hiking an hour just to reach a site) and using a “gold standard” DNA testing method that was also time-consuming and expensive, Hazard explained.

“How much effort do we put in for what feels like not much reward?” she said.

Instead of further sampling studies, Hazard and Monsen-Collar are collaborating with Nina Goodey and Ulrich Gubler in the Montclair State chemistry and biochemistry department to develop a method to test for ranavirus more cheaply and efficiently in the field. Hazard withheld details since the project is still in progress; the concept is strong, she said, but implementing it has turned out to be trickier than expected.

“We’re making progress, but it’s slow progress,” she said. “And part of the reason it’s slow progress is that we’re MSU, and we’ve got little pots of money, and we’ve got faculty and some undergrad and master’s level research students [in our lab group]. We are not a big, high-powered lab.”

Once this new testing method is complete, however, it could potentially serve as a building block to develop similar tests for other wildlife diseases. These applications come with questions—whether a disease is naturally occurring or human-influenced, for example, could change our imperative to do something about it, Hazard said.

“You could argue about the ethics of it,” she said. “But when I see a problem, I want to solve the problem.”