Batrachochytrium dendrobatidis (Bd)

Other Names:

Amphibian chytridiomycosis, Infection with Batrachochytrium dendrobatidis, chytridiomycosis, amphibian chytrid fungus

Pathogen Type:

a waterborne chytrid fungus; infects keratinized skin, disrupting functions critical for respiration, hydration, and electrolyte balance

Population:

wild and captive amphibian populations

Global Distribution:

All continents with amphibians; more than 90 countries

Importance

Since its discovery in the late 1990s, Bd has been associated with population declines and extinctions in hundreds of amphibian species worldwide. Some species develop severe disease with high mortality, while others remain asymptomatic carriers that contribute to pathogen spread.

Disease Agent

  • Batrachochytrium dendrobatidis (Bd), a waterborne chytrid fungus
  • Optimal growth at 63-77 °F (17-25°C)
  • Motile zoospores infect hosts
  • Persists for weeks in the environment
  • Growth declines above 82°F (30°C); ~98.6°F (37°C) is lethal to Bd

Species and Ages Affected

  • All amphibians, especially frogs and toads
  • Species differ in susceptibility
  • All life stages, except eggs; disease is most severe in post-metamorphosis animals
  • Some species may be asymptomatic carriers

In the United States

If You Suspect the Disease

  • Notify aquatic animal health authorities
  • Do not move affected animals
  • Isolate affected animals
  • Increase biosecurity and disinfection

How the Disease Spreads

  • Water containing fungal zoospores
  • Direct contact between amphibians
  • Movement of infected animals, mud, water, or contaminated equipment or footwear

Contributing Factors for Outbreaks

  • Cool, moist environments
  • Cool-month seasons
  • Higher elevations
  • High host density and breeding habitats

Field Tip: Bd can persist in aquatic environments and on contaminated equipment. Avoid moving amphibians, water, or wet soil between locations.

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Technical Brief: Batrachochytrium dendrobatidis

Clinical Signs Seen with B. dendrobatidis Infection

Signs may appear 1-4 weeks after exposure.

AssessmentTypical findings
Population impactSudden death, up to 100% for some species and life stages; disease is most severe in post-metamorphosis animals
BehaviorLethargy, loss of appetite (anorexia); loss of righting reflex, abnormal posture (e.g., splayed hind legs); erratic or uncoordinated movement, abnormal swimming)
External signsExcessive or irregular skin shedding (hyperkeratosis), skin discoloration or erythema (redness) on ventral surface or limbs; thickened or rough skin; in tadpoles, loss of pigmentation or discoloration of mouthparts (jaw sheaths, oral discs)

Select each photo to enlarge:

Alpine tree frog (Litoria verreauxii alpina) with severe chytridiomycosis, exhibiting ventral erythema (skin reddening) and an impaired ability to maintain normal upright posture. Laura Grogan /PLoS Pathog, CC BY 4.0
Amphibian Chytridiomycosis abnormal posture
Adult Sierra Nevada yellow-legged frog (Rana sierrae) with severe chytridiomycosis, exhibiting abnormal posture characterized by rigid, abducted extension of the hind limbs I. Chellman/Wikipedia, CC BY-SA 4.0
Close-up of tadpole mouthpart, with depigmented jaw sheath (arrows). Vance Vredenburg/Amphibiaweb, CC BY-NC 4.0
A common midwife toad (Alytes obstetricans) with chytridiomycosis, exhibiting abnormal posture (hind leg abduction) and sloughing skin. Pascale Van Rooij/Vet Research, CC BY 4.0
A dead adult frog lying with rigid, splayed legs and ventral erythema (reddening) of the limbs. Forrest Brem and Virginia Gewin/PLoS Biol, CC BY 4.0
salamander hemorrhage

Prevention Best Practices

  • Do not move amphibians between locations.
  • Thoroughly clean and disinfect footwear, equipment, and boats between sites.
  • Avoid transporting water, mud, or debris.
  • Handle amphibians only when necessary.
  • Wear disposable gloves and change gloves between animals.
  • Never release captive amphibians into the wild.

Disinfection*

  • B. dendrobatidis is susceptible to most disinfectants.
  • Clean and rinse surfaces before disinfection.
  • Use EPA-approved disinfectants as directed.
  • Heat: 98.6°F (37°C) for 4 hr, 116°F (47°C) for 30 min, or 140°F (60°C) for 5 min
  • Drying: minimum of 3 hours

* Caution: Disinfectants can be toxic to amphibians and aquatic organisms. Prevent disinfectants from entering natural water bodies.

Important: Bd cannot be diagnosed by visual examination. Infected animals can appear healthy. Laboratory testing is required to confirm the disease.

Diagnostics

  • Quantitative PCR
  • Histopathology: skin biopsies or toe clips
  • Cytology: skin scrapings or fresh skin sheds
  • Environmental DNA (eDNA) for population monitoring

Sample Collection

Contact aquatic animal health authorities before collecting samples.

  • Samples should only be collected by trained personnel using strict biosecurity measures.
  • Sterile skin swabs from live animals:
  • Adult frogs, toads, and salamanders: ventral surfaces of abdomen, thighs, feet, and toe webbing
  • Tadpoles: jaw sheaths and oral cavity
  • Freshly dead animals, chilled
  • Water samples (for eDNA)

Treatment

  • There is no approved treatment or vaccine available for free-ranging wildlife.
  • Supportive care and antifungal treatment may be beneficial for infected captive animals.
  • Bd is sensitive to heat; warm temperatures may suppress or clear infection in some amphibian species.

Public Health

  • B. dendrobatidis does not infect humans and poses no zoonotic disease risk.
  • People can spread Bd via contaminated hands, footwear, equipment, or water.

For More Information

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