U.S. Fish & Wildlife Service

Sacramento Fish & Wildlife OfficeServing the people, conserving the fish, wildlife, and plants of California

A Unit Of The Pacific Southwest Region

Service Working to Combat Killer Chytrid in California Frog Populations

In some cases it’s good to have a thin skin, especially if you’re a frog. Yet millions of amphibians globally are facing extinction because their skin is too thick; victims of Chytridiomycosis (Chytrid). This virulent, highly transmissible, often fatal infectious disease attacks the host throughout its life-cycle, and has caused the extinction or critical decline of over 200 species of frogs and salamanders.

Photo of Mountain Yellow-Legged Frog

According to Dr. Lee Skerratt, a senior fellow at James Cook University, Australia, "The impact of Chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history." Photo Credit: Rick Kuyper / USFWS

The U.S. Fish and Wildlife Service (Service) and our partners are working hard to understand and combat this terrible disease.

As tadpoles, frogs breathe much like fish; they have gills and stay completely submerged in water. But, during metamorphosis into froglets, they lose their gills, develop rudimentary lungs, and start breathing through their nose and mouth (which have membranes that transfers oxygen much like the skin). In all stages, frogs absorb oxygen and exchange water through their skin; which means they need moist, thin skin in order to live.

Chytrid is caused by the aquatic fungal pathogen Batrachochytrium dendrobatidis (Bd). Bd feeds on keratin, which is a protein found in hair, skin, scales, feathers, fur, among other places, like your fingernails or a rhino’s horn. This isn’t a significant problem for tadpoles, because keratin is only found around their mouths and Bd usually doesn’t interfere with breathing or the ability to forage for food.

However, when a tadpole becomes a frog, keratin begins to be produced in other parts of their body. Bd creates cysts in the keratinized areas on an adult frog’s skin, resulting in thickening and sloughing of the skin, which stops the frog from breathing and absorbing water. The infected frog becomes lethargic and slow to react, making it an easy meal for predators or, if the disease progresses long enough, the frog dies from a heart attack, brought on by electrolyte imbalance, oxygen starvation, and nervous system collapse.

In April, 2014, the Service finalized listing three Sierra amphibians (the Sierra Nevada yellow-legged frog, the northern Distinct Population Segment (DPS) of the mountain yellow-legged frog, and the Yosemite Toad) under the Endangered Species Act (Act), bringing the total listed amphibian species under the Act in the U.S. to 35. As a part of the listing and recovery efforts for these various species, the Service has been actively involved in researching and fighting this disease; funding myriad studies and treatment efforts in the U.S. and globally; and collaborating with various partners in an effort to prevent the decline or extirpation of all at risk amphibian species.

In 2010, the Service started the Amphibians in Decline Program through Wildlife without Borders and has since awarded over $3,500,000 to fund 40 projects in 23 countries.

Photo of Mountain Yellow_Legged Frogs killed by chytrid

The Sacramento Fish and Wildlife Office has funded two grants,totaling $175,000, which will help pay for (B. dendrobatidis) monitoring and treatment efforts for five years. Photo Credit: Rick Kuyper / USFWS

Closer to home, in the Service’s Sacramento Fish & Wildlife Office (SFWO), there are approximately 22 Sierra Nevada yellow-legged frog and Northern DPS mountain yellow-legged frog communities in the Sierra Nevada that remain Bd free. Thirteen of these occur within Sequoia and Kings Canyon National Parks (SEKI) and the remainder occur in the adjacent Sierra, Sequoia, and Inyo National Forests.

These populations comprise the largest remaining groupings of both species, with sizes commonly exceeding 1000 adults. As such, these communities are critically important for their role as donor populations for current and future frog conservation efforts. Unfortunately, based on the current rates of Bd spread, all are expected to become Bd-positive and suffer serious declines or extinctions within the next 10 years.

It's possible that the severity of these declines can be mitigated by using anti-Bd treatments applied at the beginning of an outbreak. This can increase community survival by allowing time for treated frogs to develop an effective immune response which subsequently renders them much less susceptible to Bd. To allow effective treatments to be conducted, intensive monitoring is necessary to detect Bd infections at an early stage and quickly implement a treatment effort.

SFWO has allocated funding for two grants, totaling $175,000, which will pay for these monitoring and treatment efforts for five years. Partners in this and other efforts to help the species the event of an outbreak include: the University of California-Santa Barbara, Sierra Nevada Aquatic Research Laboratory (SNARL) and the Sequoia and Kings Canyon National Parks (SEKI).

All sites will be visited at least twice each summer for the first three years, with SNARL personnel visiting two-thirds of the sites (14-16 of the 22) and SEKI personnel visiting the remaining third, in order to assess the disease state of the incumbent population. Ten to twenty swabs will be taken at each site and analyzed within 2 weeks (~1,000 swabs per year).

If an outbreak is identified at a site, 500 frogs will be collected, penned, and separated into two groups (treatment and control). All will be tagged, swabbed, and held on-site. The control group will only be monitored, while the treatment group with receive daily Itraconazole exposure for seven days; after which all frogs will be released. To accurately determine the effectiveness of treatment, funding will be sought to continue monitoring the affected site(s) for an additional 2-3 years.

Finally, if conditions warrant significant intervention, early life cycle (eggs or tadpoles) may be collected and brought to a captive rearing location (i.e. zoo or aquarium) and the frogs will be raised to adulthood, infected with Bd, treated until disease free, and then reintroduced into the site from which they were taken.

Chytridiomycosis is a significant hurdle to recovery efforts and threatens amphibians globally. With ongoing research, varied treatment plans and captive rearing efforts currently underway, it is hoped that through the efforts of the Service and our partners, we will be able to meet the challenge it presents.


Feature story by John Garn, on detail from Carlsbad FWO.

Last updated: November 8, 2017