Fighting Chytrid: How do biologists fight pandemics in the animal kingdom?

A mountain yellow-legged frog quietly swims in a mountain lake in California. The frogs are highly susceptible to a chytrid fungus commonly known as “Bd” that is sickening millions of frogs around the world. Photo courtesy of Isaac Chellman

While this is the first pandemic many of us humans have experienced, pandemics are not uncommon in the animal kingdom. As biologists learn about these diseases, they also identify ways to treat the infected individuals and boost immunity among the larger population. Right now, biologists in California are helping mountain yellow-legged frogs fight a deadly skin fungus, and the strategies they’re using aren’t too different from the strategies being used to fight the current pandemic in the human world.

1. Understand the threat

When an outbreak is detected, biologists collect skin samples from frogs to confirm the finding. Photo courtesy of Isaac Chellman

Biologists and researchers from multiple agencies, including the U.S. Fish and Wildlife Service, are working to combat Batrachochytrium dendrobatidis, a form of chytrid fungus commonly known as Bd. Infections from the fungus have wiped out dozens of frog species around the world. The fungus attacks the frogs’ permeable skin, and their breathing and water balance become compromised. Bd can be transmitted through water or skin-to-skin contact. While some frog species can live through infection or even be asymptomatic carriers, others are vulnerable.

“Mountain yellow-legged frogs are highly susceptible to the fungus,” said Jill Seymour, senior biologist with the Service’s Sacramento field office. Seymour has been working on recovery actions for the frog since 2017.

Mountain yellow-legged frogs, which include two species, are listed as endangered and live in California’s Sierra Nevada and Transverse ranges. While non-native fish have been the biggest threat to the frogs historically, Bd is moving through the southern Sierra Nevada like an explosion, outpacing the frogs’ ability to build an immune response.

2. Build a cadre of experts

Isaac Chellman (rear), and Tom Smith (front) get boots on the ground to treat sick frogs when an outbreak is detected. Photo courtesy of Erica Rosenblum

Having a team of experts to make decisions and determine next steps is an important part of fighting a pandemic. In 2014, amphibian and disease experts from federal and state agencies, zoos and universities in California formed the Mountain Yellow-legged Frog Working Group to devise a plan to put these species on the road to recovery. The group meets annually to discuss recovery actions, and in 2019, the group created a rapid response team to tackle Bd outbreaks.

“With a team in place to quickly respond when an outbreak is detected, we have a better chance of saving mountain yellow-legged frog populations that have not previously been exposed to the fungus,” said Isaac Chellman, an environmental scientist at the California Department of Fish and Wildlife who monitors frog populations in the northern Sierra Nevada.

Jill Seymour, senior biologist at the Service, is part of the Bd Rapid Response Team. Photo by USFWS

3. Have a coordinated plan

Biologists working to treat sick frogs in a remote lake have an on-site meeting to discuss the outcome of the day’s activities and next steps. Photo courtesy of Isaac Chellman

Knowing who’s doing what speeds response times. For the response team, each team member has a specific role that helps biologists get to the outbreak site as quickly as possible. Some prepare and pack the equipment while others secure permits and recruit qualified biologists to go into the field. The response team can get boots on the ground within five to 10 days of learning of a potential outbreak.

“It is no small effort to get the people and equipment to do this,” said Seymour, who leads coordination of the recovery and outbreak response efforts for the Service. “The more you can coordinate and collaborate, the more effective you can be in managing outbreaks.”

The response team also developed a protocol for treating infected frogs in the wild. The protocol includes an eight-day, step-by-step process for treating the frogs with an anti-fungal drug to reduce the amount of fungus on their bodies.

4. Know the level of infection

Large swabs like these are used to collect skin samples from the frogs. The samples are taken to labs and analyzed for the presence of Bd. Biologists review the results to determine if an outbreak is underway. Photo courtesy of David Burkart

Monitoring and early testing are essential to understanding the extent of the infection.

“Timing helps flatten the curve,” said Tom Smith, a research biologist with the University of California Santa Barbara Earth Research Institute. Smith leads the on-the-ground response efforts and has completed multiple backcountry frog fungus treatments. “A delay of even a week can make the difference between having hundreds of frogs to treat and having very few frogs still alive.”

Currently, about 15 populations of mountain yellow-legged frogs have not been exposed to the fungus and are the most vulnerable to an outbreak. Biologists from the various member agencies trek into the mountains from mid-June through September to look for signs of Bd infection among these populations. They swab 20 to 30 frogs at each site at least once each summer to determine if Bd is present in the population. And if they see an unusual number of dead frogs in a single location, they swab the frogs and take skin samples back to a lab to confirm the presence of Bd. Biologists review the results to determine the degree of the infection and if an outbreak is underway.

“We cannot properly respond to an outbreak if we don’t know where that outbreak is occurring and how infected the population may be,” said Chellman.

5. Activate first responders to mitigate the impact

Mountain yellow-legged frogs soak in an antifungal treatment solution for approximately 10 minutes once a day for eight days. This treatment reduces the amount of fungus on the frogs’ skin, giving their immune systems an opportunity to catch-up and begin to fight the fungus naturally. Photo courtesy of Isaac Chellman

While the response team and in-field protocol were formalized in 2019, the protocol was based on methods developed in several laboratory experiments and used during a successful in-field treatment of more than 400 frogs in Kings Canyon National Park in 2015. That effort was the first to enlist coordinated support from several agencies to get biologists in the field for treatment activities.

In 2015, Chellman was working for the National Park Service, and during routine frog monitoring, he noticed a die-off in progress at a lake in Kings Canyon National Park.

“I quickly contacted my supervisor, who contacted other members of the working group in the hope of getting rapid approval for a response,” said Chellman. The Service and Department issued the research permits to the National Park Service while the Sierra Nevada Aquatic Research Laboratory secured qualified biologists to go into the field for the treatment.

“Within a week, we had the permits, the gear and a team ready to treat the frogs,” said Smith.

Approximately 10 biologists hiked 17 miles to the site of the outbreak, carrying packs loaded with 10-days’ worth of food, nets, large plastic bins, the antifungal medicine, and other equipment. Once there, they built large, 6-foot by 6-foot net pens to hold the frogs in the lake and started catching as many frogs as possible.

Captured frogs were placed in square, plastic bins filled with a highly diluted solution of itraconazole (the anti-fungal medicine) and water. The frogs were lightly sloshed around for 10 minutes until their bodies were completely covered with the solution. Each frog received a small identification tag called a PIT tag and was returned to the net pen. The frogs received one treatment a day for about eight days.

“The goal was to use the drug to decrease the fungal load and allow their immune systems to kick in and improve their chance of survival,” said Smith. “We bet that if we could do that to a sufficient number of frogs in the population, those individuals would survive long enough to prevent local extinction.”

Only adult frogs are treated under the protocol. While tadpoles can carry the disease, by treating adult frogs, reproduction can continue beyond the outbreak and the population has a better chance to persist.

“Although the logistics can be challenging given the terrain, remoteness and difficulty in detecting an outbreak in time, this kind of effort is relatively straightforward and inexpensive,” said Chellman. “It could make the difference between a population persisting or disappearing.”

6. Prevent cross-contamination

Dr. Roland Knapp wears gloves while releasing frogs after their treatment cycle. Photo courtesy of Isaac Chellman

Hand sanitizer is a necessity in the field.

“You always have to assume you could be carrying the fungus,” said Seymour. While Bd is not airborne, it can be spread by reusing contaminated equipment or protective gear like boots and frog nets. “You have to be hyper-conscious of what you’ve touched and the potential for cross contamination.”

In addition to keeping their hands clean, biologists follow strict protocols for decontaminating all equipment, including nets and water shoes, in a disinfectant solution such as diluted bleach to kill the fungus.

7. Monitor and retest

Tom Smith captures frogs at a Sierra Nevada lake. Returning to sites where frogs received treatment to perform health checks helps biologists and researchers understand the disease and effectiveness of the treatment. Photo courtesy of Jeff McFarland

While the anti-fungal treatment is a heavy lift, monitoring and retesting are just as important.

The response team uses capture-mark-recapture surveys, which require the biologists to return to the sites, capture frogs again and perform a health check. The frogs are tracked using the unique identifier information stored in the PIT tag.

“When we returned to the Kings Canyon site in 2016 and 2017, we saw a high survival rate among frogs that were treated,” commented Smith.

8. Have hope

A healthy mountain yellow-legged frog rests in its mountain lake home. With efforts like those taken by the Bd Response Team, the mountain yellow-legged frogs have a better chance of recovering from recent population declines. Photo courtesy of Isaac Chellman

Success lies within the ongoing commitment of the members of the working group to sustain the monitoring, rapid response and long-term actions.

“There are many people who are working hard to help this species recover,” said Chellman. “The efforts of all these dedicated people will hopefully help the species turn the corner and eventually recover to the point where it can be successfully delisted.”

Hope, collaboration and good science are essential elements to ending any pandemic — no matter human or frog.

Written by Meghan Snow, public affairs specialist, Sacramento Fish and Wildlife Office

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