Salt, Tadpoles, and Unravelling Assumptions
By Vishva Nalamalapu, Office of the Chancellor
“It’s a bit of a cautionary tale,” says Jesse Brunner, a disease ecology professor at WSU.
The tale begins with a long-held assumption: stress, any prolonged challenging condition, makes humans and many other animals more susceptible to infection. Researchers often extend that to disease and epidemics at the population-level. The effects of stress on disease and epidemics could, however, be much more complicated. “If you start picking at that sort of scaling, some of it starts to unravel,” says Brunner.
Brunner, Erica Crespi, an ecological developmental biology professor at WSU, and their collaborators at the University of Connecticut recently published a paper in Ecology and Evolution testing that assumption using wood frogs, salt, and ranavirus. Their findings are a step forward in understanding how stress impacts disease and epidemics, which will help predict and prevent them.
Road salts and agricultural practices have made freshwater saltier, a growing cause of stress for amphibians. And one type of viruses, ranaviruses, commonly causes wood frog mass deaths in the wild. The team had previously found that salt made tadpoles more susceptible to infection and that tadpoles in saltier ponds were more likely to die from ranavirus. They hadn’t, however, tested how salt affects ranavirus at the population-level.
In a field surrounded by farmland in Connecticut, the team set up 128 cattle tanks. They filled each with water and adjusted the temperatures and saltiness. Then, they added tadpoles, including a couple with ranavirus infections.
“It all seems very simple on paper, and then you go out into the field and any small problem all of a sudden becomes a very big problem,” says Brunner. Some water heaters broke. Some tadpoles got stuck in filters. By the time they were able to perfect the setup, the tadpoles were too old for the experiment to work.
The next summer, the team tried again. Every day for 50 days, they checked the tanks for tadpoles that had died. Checking often was important because, otherwise, the bodies may have disappeared. “Everyone thinks that tadpoles are vegetarians, but they’re actually quite cannibalistic and will eat carcasses,” says Brunner. They collected the tadpoles that had died and verified they were infected with ranavirus.
The team was surprised by what they found. Salinity didn’t have much of an effect. Regardless of salinity, most of the tadpoles died from ranavirus, and quickly.
A few things could be at play. Saltiness could be causing more infections, but infected tadpoles could be dying so quickly that they don’t have as much time to transmit the virus. Those two effects could cancel each other out, resulting in similar deaths in more and less salty conditions.
Or, the virus could have spread too fast and been too deadly to show differences between more and less salty conditions. Like all experiments, the cattle tanks were smaller and simpler than the wood frogs’ natural environments. In their natural environment, individuals have more separation from each other, which could reduce transmission, slow the spread, and make it less deadly.
The team will now turn to understanding why stress may affect disease and epidemics differently at the population-level. “With each experiment, I learn what I didn’t know before. Then I run with it,” says Crespi. They learn from each cautionary tale, until they create another. In the process, a truer understanding of the world comes to be.