New research suggests runoff from road salt makes frogs more susceptible to a lethal virus.

Scientists at Washington State University surveyed 18 ponds with similar populations of wood frogs tadpoles during summer months when Ranavirus, a disease that kills amphibians and reptiles in North America and Europe, was at its yearly peak. 

Ranavirus infections were found in every pond. Tadpoles in ponds close to major roads died faster and had more severe infections than tadpoles in ponds that were farther from roads. 

The researchers also raised frogs in a lab. They found that frogs that were put in saltier water developed fewer immune cells. When exposed to the virus, the frogs in salty water had higher stress hormone levels and were more likely to die.

The study was published this month in Royal Society Open Science.

Emily Hall, the lead author on the paper, said Ranavirus can cause mass moralities where over 95% of amphibians in an infected pond die. Road runoff might explain why certain ponds see these die-offs.

“We study these things in isolation in the lab,” she said. “You expose animals to different concentrations of road salt and check what level is lethal, but what that doesn’t consider is in nature, tadpoles are not only exposed to salt, but also exposed to Ranavirus, or other runoff toxins like heavy metals.”

Those different stressors can interact in nature and make a level of salt that is otherwise safe turn deadly, said Hall.

It is not clear exactly why salt might be weakening frogs against the virus. 

Previous research Hall conducted suggested that tadpoles exposed to higher concentrations of salt swam less and grew more slowly. This might be because the tadpoles have to expend energy to pump excess salt out of their bodies — energy that should be used for growing. The additional stress of pumping out the salt might similarly divert energy from frogs’ immune systems.

She said the increased stress hormones could be suppressing the frogs’ immune systems. It is also possible the salt is directly harming the frogs’ immune cells.

Rick Relyea, a professor at Rensselaer Polytechnic Institute who has studied the impact of salt on a number of freshwater animals, said road salt flew under the radar of scientists for a long time. He said roads have been salted for the past 60 years, but the effect of the salt on ecosystems has only really been studied in the past decade.

“I don’t know of another study that has found such a strong interactive effect between pollution from road salt and disease killing wildlife,” he said of the new paper. “We now know a fair bit more about what salt does to animals directly, but know so little about how it might make animals or plants susceptible to natural diseases.”

Relyea said there is no wildlife-safe alternative to salt, but municipalities can keep drivers and animals safe by applying salt more efficiently.

“It’s not safety versus protecting the environment — we can do both,” he said.

Relyea said some towns in New York have been coating roads in brine before snowfalls. That makes it harder for the snow to stay on the pavement, leading to less salt use later. Better plows can also remove more snow so that less salt is needed.

One of those towns is Lake George, which has been implementing better salting techniques over the past few years to reduce the amount of salt flowing into its namesake lake. Relyea said Lake George is setting an example for the entire county of how salt should be applied to protect the environment.  He said the new study underscores the importance of using better salting techniques.