Spotlight on Zoo Science
August 22, 2003
Solving the Mystery of Frog Fatalities
National Zoo scientists track down the cause of—and a cure for—a new, fatal amphibian disease.
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| Arroyo toad (U. S. Geological Survey/photo by Chris Brown) |
In November 1991, Don Nichols, veterinary pathologist at the Smithsonian’s National Zoo, received three dead, formalin-fixed California arroyo toads from the University of California at Santa Barbara. The three had been part of the University’s 120-animal colony, which had been devastated by a skin disease of unknown case. Sixty percent of the university’s toads had died in two months before Nichols took on the case.
Nichols, who has a special interest in the diseases of reptiles and amphibians, was initially frustrated in his attempts to identify the disease organism. But, after searching the National Zoo’s pathology files, he found three reports of skin disease in frogs with symptoms similar to those of the California toads. Then Nichols instructed the pathology residents at the Zoo to be especially vigilant in collecting skin samples from any frogs or toads sent in for post-mortem examination.
GBPsArFrogJC.jpg) |
| Green -and black poison dart frogs suffered from an unknown skin disease. |
In September 1996, an outbreak of skin disease among several species of frogs in the National Zoo’s animal collection gave Nichols the information he needed. This outbreak resulted in the deaths of a number of blue poison dart frogs, green and black poison dart frogs, and White’s tree frogs. Pathology staff members were able to collect many skin samples.
The disease organism appeared to most resemble species identified as belonging the fungi phylum Chytridiomycota. One of the few chytrid experts in the world, Joyce Longcore of the University of Maine, was first sent photographs and reports, and, later, fresh skin samples. Longcore’s studies confirmed that not only was this organism a new chytrid species, but it represented a novel genus as well. Longcore and Nichols eventually named the organism Batrachochytrium dendrobatidis in 1999.
The news circulated quickly. The newly emergent, fatal skin disease (cutaneous chytridiomycosis) in frogs, toads, and salamanders that was—and still is—devastating populations of captive and wild amphibians, contributing to global decline of amphibian populations, had been identified.
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Don Nichols and Elaine
Lamiand examine a poison arrow frog. |
The pioneering studies performed in the Zoo’s Pathology Department continued into 2001 and 2002 to prove beyond doubt that B. dendrobatidis can be lethal to poison dart frogs. Tadpoles harbor the disease for many weeks without showing any symptoms. Ultimately they die of chytridiomycosis near the end of metamorphosis or shortly thereafter.
Of perhaps equal importance to the discovery and identification of the disease is that National Zoo scientists have developed methods to successfully treat infected frogs and tadpoles in captivity. So, Nichols’ work has come full circle, from successful research on the cause to successful research on the cure for this once unknown amphibian killer.
—Robert Hoage
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