Heat therapy for sick frogs lowers sperm levels

Bad news for frogs getting sauna therapy: The heat treatment for their fatal fungal infection may come at a cost.

Exposing infected green and golden bell frogs (Ranoidea aurea) to thermal therapy — a commonly used treatment technique — can compromise their sperm levels. While the frogs became disease free, their sperm concentrations substantially dropped and remained that way months later, researchers report August 4 in the Journal of Experimental Biology.

“We’ve looked at the survival of frogs after these treatments… but it’s less common that we’ve taken into account effects on reproduction,” says Cori Richards-Zawacki, an amphibian biologist at the University of Pittsburg who wasn’t involved in the work. “That seems important,” she says, if the idea is to revive populations “by treating frogs and then putting them back into the wild.”

Over the past half century or so, at least 500 amphibian species — including R. aurea — have suffered population declines driven in part by a chytrid fungus called Bd (Batrachochytrium dendrobatidis). Chytrid colonizes the animals’ skin and disrupts the uptake of electrolytes, which on depletion can ultimately cause cardiac arrest.

Reproductive and conservation biologist Rose Upton witnessed a chytrid outbreak firsthand in July 2024 at a captive breeding facility housing bell frogs in Australia’s University of Newcastle, where she works.

To treat infected frogs, scientists often opt for strategies including antifungal baths and heat therapy. Temperatures above 29° Celsius (84° Fahrenheit) can kill the fungus in days, or even within a few hours at 37° C (99° F). Green and golden bell frogs naturally prefer temperatures around 29° C.

At the Newcastle facility, Upton and colleagues raised the temperature of the cabinets that housed frogs by 2° C every day from 25° C to 37° C, before holding the temperature for six hours and ramping it down in similar increments. While treating the frogs, Upton’s team wanted to understand whether the therapy posed a reproductive cost for bell frogs, given heat stress can impair fertility in many animals, including humans.

The researchers collected sperm from 13 mature male frogs — four Bd negative and nine Bd positive — before and five weeks after the heat treatment. They found that irrespective of the frogs’ infection status, their sperm concentrations nearly halved after the thermal therapy. Six months later, these levels remained substantially reduced in four infected individuals that were tested again, which came as the “biggest surprise,” Upton says. The team also found that, post-treatment, frogs had a higher proportion of motile sperm, though these levels declined slightly months later.

“I would be a little bit cautious in interpreting these results,” Richards-Zawacki says, given that “we don’t have a really good handle on what causes fluctuations in sperm production in amphibians.” It’s especially hard to say whether the longer-term findings can be attributed to the heat treatment, the lingering effects of the infection, or something else, she adds.

Upton acknowledges that the study is preliminary. It lacks a comparative frog population that didn’t undergo heat treatment; it didn’t include female frogs; and it didn’t track whether heat-treated males produce fewer offspring. It’s also unclear how sperm concentrations and motility may change if infected frogs were exposed to lower temperatures that can also kill the fungus.

“I’m certainly not suggesting that we should stop heat treatments or other mitigative actions,” Upton says. “I think it’s just good to be monitoring reproduction so that we know ahead of time if there is a dip in that fertility.”