Amphibian Diseases: Chytrid Fungus (Chytridiomycosis) and Ranaviruses
Causative Agents
Chytrid fungus:
  • Chytridiomycosis is a skin disease of amphibians that is caused by the chytridiomycete fungus, Batrachochytrium dendrobatidis, which seems to be specific to amphibians.
  • In general, chytrids are a group of fungi that are found ubiquitously in soil, water, and even in the rumen of cattle.
  • It has been documented in various species of frogs, toads, and salamanders both in captivity and the wild.
  • Infection can be lethal in some amphibian species, including boreal toads, but the mode of death is unknown, but it has been speculated that this fungus may produce a toxin, although this idea has not been confirmed.
  • Ranavirus is a genus within the Iridoviridae family of viruses.
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Chytrid Fungus
Mountain yellow-legged frogs killed by chytrid fungus (August 2008 - California, USA). Photo © by Vance Vredenburg
Chytrid fungus
  • Worldwide.
  • Opportunistic surveys for chytrid fungus in 2008-2009 indicated that it is widely distributed in all regions of British Columbia and in all of the frogs and toads tested.
  • Prevalence of Chytridiomycosis has been reported to be seasonal in temperate areas, with a higher incidence during cooler months and a lower incidence during warmer months.
  • Americas, Asia and Pacific, Europe.
  • As mortality events due to ranaviruses mostly affect larval amphibians, die-offs associated with ranaviruses occur in spring and summer amphibian when larvae are present.
Hosts, Transmission and Life Cycle
Chytrid fungus
  • Amphibians.
     Transmission and Life Cycle:
  • Chytrid spores are motile (zoospores) but have limited swimming ability (distances often less than 2 cm) and the fungus appears to depend on water flow or host movement for long-distance dispersal.
  • The fungus grows best between 17 and 25°C and cannot grow at air temperatures higher than 28°C.
  • The USGS National Wildlife Health Center has isolated amphibian ranaviruses from 16 species of frogs, one species of toad and six species of salamanders. Mortality events due to ranaviruses occur most commonly in larval amphibians such as mole salamanders (Ambystoma spp.), true frogs (Lithobates spp. and Rana spp.) and chorus frogs (Pseudacris spp.).
     Transmission and Life Cycle:
  • Transmission is via direct contact, ingestion of virus or infected animals and water exposure.
Signs and Symptoms
Chytrid fungus
  • The waterborne spores of this fungus have been found to affect a variety of tissues, including the skin of post-metamorphic individuals and the mouthparts of tadpoles.
  • In post-metamorphic individuals, chytridiomycosis causes a marked thickening of the skin and excessive skin sloughing (shedding), which can impair respiration through the skin and osmoregulation (water balance), resulting in death.
  • Adult amphibians infected with chytrid fungi also have exhibited symptoms such as extended back legs, lethargy, and loss of righting reflex.
  • In larvae, jaw sheaths and tooth rows of tadpoles lack pigment or appear deformed, which may impede feeding activity.
  • Overall, it appears that chytrid infection disrupts the ability of amphibians to maintain fluid and electrolyte balance, impacting proper muscle and nerve function.
  • Field signs of a ranaviral epizootic event include sudden or explosive onset of illness in amphibians in a wetland, often with hundreds or thousands of sick and dead amphibians found in a 1 to 5 day period.
  • Overall mortality rates in juvenile frogs and salamanders in a wetland can exceed 90%.
  • Affected individuals usually present with subtle to severe hemorrhages in the ventral (belly) skin, especially at the base of the hind limbs and around the vent opening. Hemorrhages may be present from tip of chin to tip of tail ventrally and may be pinpoint or irregular patches.
  • Other clinical signs include lethargy, swimming erratically, weakly, or on their sides, and mild to severe fluid accumulation under the skin (in lymphatic sacs) of the abdomen and proximal hind limbs.
  • Internally, there may be fluid accumulation (clear or red-tinged) in the body cavity (called hydrocoelom), and hemorrhages on the surfaces the heart, stomach and liver.
  • Occasionally, white, pinpoint areas of dead tissue are evident in the liver or spleen. Ulcers of the skin and palate tend to be randomly scattered.
Meat Edible?
  • Meat from amphibians is generally not consumed.
Human Health Concerns and Risk Reduction
  • No risk of human zoonoses has been reported in relation to infection of amphibians with chytrid fungus or ranaviruses.
Samples for Diagnosis
  • Fresh, intact carcasses. Photos of affected animals.
Further Reading
Return to Manual Home Page Disease List - Body Region Affected Disease List - Causative Agent or Risk Factor Disease Surveillance Form Glossary Contact Information
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