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Frogs Get Their Shots: Vaccination May Curb Lethal Fungus

Image: Pacific chorus frog

A Pacific chorus frog is seen in a meadow located in Point Reyes National Seashore, Calif. Scientists have known for years that pesticides drift from California’s agricultural heartland and accumulate in frogs at the highest elevations of the Sierra Nevada mountains. Now they have a better understanding of exactly which ones collect in tissue, the first step toward stopping the sharp decline of threatened amphibians across the environmentally sensitive mountain range. U.S. Geological Survey via AP file

Around the world, the rapid spread of a pathogenic fungus has sent frogs and other amphibians hopping toward extinction. Hope for their survival may come in the form of vaccination programs similar to those that protect humans from contagious diseases, according to a new study.

No, this doesn't mean that newborn tadpoles will be paying a visit to the veterinarian for a round of shots, or that conservationists with mini-syringes will be mucking through rain forests in search of frogs. But the reality isn't all that different.

Hundreds of amphibians already have been removed from habitats contaminated with the fungus Batrachochytrium dendrobatidis, which causes chytridiomycosis — the fungal disease implicated in the global amphibian decline. One route is to vaccinate these captive-bred amphibians, explained Jason Rohr, an ecologist at the University of South Florida and the study's senior author.

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Once immunization is induced, these hoppers could be reintroduced to the wild and potentially reduce the spread of the fungus to the point that it goes locally extinct. "This is the whole idea behind vaccinating a portion of the human population and being able to protect everyone," Rohr explained.

Another possibility is to dump enough vaccine into ponds to induce an acquired immune response in the field, he added.

Caution urged

The research brings up some interesting avenues to explore, but "there're a lot of technological hurdles still to come," Allan Pessier, an amphibian disease expert at the San Diego Zoo who was not involved with the study, said. In other words, no one yet is jumping for joy.

For starters, Rohr and colleagues found that the immune response can be acquired via repeated exposure to dead fungus through a series of laboratory experiments with Cuban tree frogs, a species that is not generally considered susceptible to the disease. Replicating the results with a species of conservation concern, such as Panamanian golden frogs, is a vital next step, Pessier said.

What's more, the immune response appears to involve ramping up production of white blood cells that are targeted at the pathogen. This reduces the amount of fungus on an amphibian, but does not eliminate it altogether. In susceptible amphibians, any amount of fungus may be lethal.

Extinct frog didn't actually croak

Then come questions about the "practical aspects" of implementing a vaccination program outside the controlled conditions of a laboratory, Pessier said. Frogs that are immunized in captivity and then released to the wild, for example, are unlikely to pass their immunity on to offspring.

And any vaccine released into ponds "is going to get dispersed really quickly — the water is going to move it around," he said. That raises questions about of how to implement such a program effectively.

Rohr agreed that these are early days in the vaccination effort. He said the paper published in this week's issue of the journal Nature presents "some of the basic proof-of-concept research that is necessary to push forward."

Research continues

In addition to vaccines, scientists are developing probiotics that appear to fight the chytrid fungus. Other research teams are racing to figure out why some amphibian populations are naturally able to live with the fungus, in hopes of using the information to protect other populations.

Meanwhile, conservationists continue to breed and maintain populations in captivity, said Anne Baker, the executive director of Amphibian Ark, a non-profit organization that oversees global efforts to protect populations of amphibians that cannot be safeguarded in their natural environments.

"It doesn't make any sense to put them back [into the wild] until we know that we can put them back with some level of assurance that at least some of them will survive," Baker said.