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Amphibian Disease

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Photos of amphibian health evaluations. Photos: T. Barnes Amphibian health evaluation includes taking blood samples and oral swabs from boreal toads (endangered in the State of Colorado), collecting representative live specimens of other amphibian species and the opportunistic collection of dead animals. The adult animals in these photographs were unharmed. Photos: T. Barnes.
 National Elk Refuge, Jackson, Wyoming. Chytrid fungus was identified here on boreal toads in 2000. Photo: D. Patla. National Elk Refuge, Jackson, Wyoming. Chytrid fungus was identified here on boreal toads in 2000. Photos: D. Patla.
Batrachochytrium dendrobatidis photo. Photo: Allan Pessier. Batrachochytrium dendrobatidis. Two jar-shaped zoosporangium, or thalli (A), and zoospores inside are clearly visible. The discharge tubule is also visible where the flagellated zoospores are released from the thallus (B). Photograph courtesy of A. Pessier, University of Illinois Zoological Pathology Program, Loyola University Medical Center.

Disease is emerging as an important concern in the study of amphibian decline. While amphibian research at the U.S. Geological Survey (USGS) Northern Rocky Mountain Science Center and USGS Fort Collins Science Center does not focus entirely on disease, the issue plays an important role in our activities. There are several diseases of amphibians that warrant attention, Rana viruses, a perkinsus-like organism and the amphibian chytrid fungus are three that are of concern in the United States.

In the Rocky Mountain Region, we suspect that chytridiomycosis is affecting populations of boreal toads in Rocky Mountain National Park (RMNP). The amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd) has been identified in the park and some populations are exhibiting the same characteristics of large amphibian die-offs noted in Panama and Australia in 1998. We believe that Bd is the proximate cause of the recent decline of boreal toads in RMNP. Further north, the amphibian chytrid fungus was found on dying boreal toads at the National Elk Refuge, Wyoming, in 2000, increasing our concern that the disease may be widespread or may become widespread in the Rocky Mountains.

Because of the recent concern about disease and the role it appears to be playing in the decline of many amphibian populations, many of our current projects focus on disease. For example, In collaboration with National Wildlife Health Center (NWHC; Dr. David Green) and RMNP, we conducted an evaluation of amphibian health in and around RMNP. This study provides baseline data on amphibian health including incidence of chytrid and other fungal infections, the presence of viral infections, and overall health of amphibians.

Amphibian chytridiomycosis:

Amphibian chytridiomycosis is the infection of amphibian skin cells by the microscopic fungus Batrachochytrium dendrobatidis (Bd).

The amphibian chytrid fungus has been identified in a number of states from Maine to North Dakota to California. In general, chytrids are a group of fungi that are found ubiquitously in soil, water, and even in the rumens of cows. This particular chytrid, however, seems to be specific to amphibians. It has been documented in various frogs, toads, and salamanders both in captivity and in the wild. Infection by this fungus causes thickening of the skin, and infected toads and frogs tend to shed skin frequently. Infection can be lethal in some amphibian species, including boreal toads, but the mode of death is unknown. The thickening of the skin may impair gas exchange and affect the animal's ability to absorb water. An alternate hypothesis is that the fungus may produce a toxin.

There are other amphibian diseases as well. Ranaviruses belong to the Iridovirus family and infect insects, fish, and amphibians. This type of virus is not known to infect persons or other warm-blooded animals. In the western United States, most die-offs attributed to ranaviruses have occurred in tiger salamanders, although recent testing has detected the virus in Columbia spotted frogs in Montana and mountain yellow-legged frogs in California. Hundreds or thousands of sick and dead amphibians can be found at affected sites. Usually, the virus infection occurs in larvae (tadpoles and aquatic salamanders with gills) and frogs that have just completed metamorphosis. Sick and recently dead amphibians show small ulcers in the skin and extensive reddening of the skin along the ventrum (stomach) and base of the limbs. Internally, the virus affects and destroys many organs, including blood vessels, skin, stomach, liver, kidneys, and spleen. Studies are under way to determine where the virus originates each year and whether the ranaviruses of frogs and salamanders are different. So far, ranaviruses have not been detected in toads, but have caused disease outbreaks in true frogs and tiger salamanders. Die-offs of amphibians due to ranavirus have occurred in at least 15 states, including California, Arizona, Utah, Idaho, Montana, North Dakota, Wyoming and Colorado.

What we are doing about amphibian disease:

As we learn more about the amphibian chytrid fungus and other amphibian diseases, concern has grown that field scientists may be vectors for transmitting disease among study sites. We follow standard protocols developed by the Declining Amphibian Populations Task Force (DAPTF is now the Amphibian Specialist Group [ASG] of the IUCN and Species Survival commission) to reduce the risk of carrying disease accidentally between habitats. These protocols should be used by anyone conducting fieldwork in and around amphibian habitat including wetlands, breeding sites, or upland areas known to be used by amphibians. In general, these protocols recommend disinfection of all equipment – including, but not limited to, waders, nets, and calipers – in a >10% bleach solution between drainages. Chytrid fungus spreads through waterborne zoospores (see photo above). These spores, and the fungal thalli in which they are formed, can remain viable in mud or any damp place. For example, zoospores have been found in mud caked into the tread of a wader. Thorough scrubbing with a bleach solution is critical for proper disinfection of equipment. We disinfect equipment between all drainages and sometimes between discrete sites. These suggestions are endorsed by the Colorado Division of Wildlife (CDOW), who provide more information for field technicians working with amphibians in a presentation by Lauren Livo, Ph.D. and the CDOW.

In addition to following the protocols suggested by the ASG and the linked presentation, we are especially alert for apparent mass mortality or die-off events. We collect amphibians that are found dead and submit them to USGS-NWHC for pathological examination. We encourage partners and collaborators, and the general public, to report such events to NWHC or their local wildlife department.

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