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      Batrachochytrium dendrobatidis  visible as transparent spherical bodies growing in lake water on (a) freshwater arthropod and (b) algae (Photo: Johnson ML, Speare R. via Wikimedia Commons ) - Click for full size
    Taxonomic name: Batrachochytrium dendrobatidis gen. et sp. nov.
    Synonyms:
    Common names: chytrid frog fungi, chytridiomycosis (English), Chytrid-Pilz (German), frog chytrid fungus (English)
    Organism type: fungus
    Batrachochytrium dendrobatidis is a non-hyphal parasitic chytrid fungus that has been associated with population declines in endemic amphibian species in upland montane rain forests in Australia and Panama. It causes cutaneous mycosis (fungal infection of the skin), or more specifically chytridiomycosis, in wild and captive amphibians. First described in 1998, the fungus is the only chytrid known to parasitise vertebrates. B. dendrobatidis can remain viable in the environment (especially aquatic environments) for weeks on its own, and may persist in latent infections.
    Description
    Fungal Morphology: Batrachochytrium dendrobatidis is a zoosporic chytrid fungus that causes chytridiomycosis (a fungal infection of the skin) in amphibians and grows solely within keratinised cells. Diagnosis is by identification of characteristic intracellular flask-shaped sporangia (spore containing bodies) and septate thalli. The fungus grows in the superficial keratinised layers of the epidermis (known as the stratum corneum and stratum granulosum). The normal thickness of the stratum corneum is between 2µm to 5µm, but a heavy infection by the chytrid parasite may cause it to thicken to up to 60 µm. The fungus also infects the mouthparts of tadpoles (which are keratinised) but does not infect the epidermis of tadpoles (which lacks keratin).
    The fungus produces inoperculate, smooth-walled zoosporangia (zoospore containing bodies), which are spherical to subspherical in shape. Each zoosporangium (10µm to 40µm in diameter) produces a single discharge tube, which penetrates (and protrudes out of) the skin. Eventually the plug that blocks the release of immature zoospores is shed and the mature zoospores are released. The zoospores (0.7µm to 6µm in diameter) are elongate to ovoid in shape. Each possesses a single posterior flagellum, rendering it motile in water (Mazzoni et al. 2003; Daszak et al. 1999; Berger, et al. 1998; Berger et al. 1998, Berger, Speare and Hyatt, 2000, in Daszak et al. 1999; Speare et al. 2001; Weldon et al. 2003).
    To view a scanning electron micrograph of infected skin of a wild frog (Litoria lesueuri) please see: Daszak et al. 1999. Emerging Infectious Diseases and Amphibian Population Declines.
    To view histological sections of infected skin of Bufo haematiticus and Atelopus varius (showing the sporangia and discharge tubes of the fungus) please see: Daszak et al. 1999. Emerging Infectious Diseases and Amphibian Population Declines.
    To view a histological section of severely infected skin of a wild frog (Litoria caerulea) please see:
    Berger et al. 1998. Chytridiomycosis causes amphibian mortality .

    Click here to see information about Symptoms of the disease caused by Batrachochytrium dendrobatidis.

    Pathogenesis of chytridiomycosis: Authors of a recent study, Voyles et al. (2009) have found that B. dendrobatidis, causes such severe electrolyte imbalances that the frog’s heart stops. The skin of amphibians maintain proper osmotic balance inside the animal and regulate respiration. The authors found that the skin of infected frogs was less adept at transporting sodium and chloride ions. Sodium and potassium concentrations in the blood of infected frogs dropped, more so as the infection intensified and the animals’hearts began to beat irregularly and ultimately stopped.

    Occurs in:
    host, lakes, natural forests, riparian zones, water courses
    Habitat description
    Chytridiomycosis has now been reported from 38 amphibian species in 12 families, including ranid and hylid frogs, bufonid toads, and plethodontid salamanders. Although chytridiomycosis is found in a range of species and habitats (including African frogs in lowland regions in Africa) it has caused population declines of amphibians species confined to montane rain forests (Weldon et al. 2004; Daszak et al. 1999). The fungus prefers lower temperatures which may explain the high precedence of the fungus in high elevations in the tropics. In culture conditions optimum growth occurred at 23°C, with slower growth occuring at 28°C and (reversible) cessation of growth occuring at 29°C (Longcore, Pessier, Nichols, 1999, in Daszak et al. 1999).
    General impacts
    Batrachochytrium dendrobatidis has been found to affect at least 93 amphibian species from the orders Anura (frogs and toads) and Caudata (salamanders) in all the continents except Asia. It is thought to be one of the main causes of the global decline in frog populations since the 1960s, and the dramatic population crashes from the 1970s onwards (Parris and Beaudoin, 2004). The chytrid fungus kills frogs within 10 to 18 days (Michigan Frog Survey, 2003), although it is not known how. It may be physical, affecting respiration by altering the frog’s skin, or the fungus may give off a toxin (Michigan Frog Survey, 2003). Tadpoles are not affected, although the fungus may infect the keratinised mouthparts (Berger et al. 1999).
    For a summary on the impacts of B. dendrobatidis please follow this link impacts.

    Key findings of the The Global Amphibian Assessment has revealed that one-third (32%) of the world’s amphibian species are threatened, representing 1,896 species. Threats include viral diseases, habitat loss, drought, pollution, and hunting for food. The biggest single threat appears to be B. dendrobatidis.
    A search on the database using "diseases" as a keyword in "all" habitat types, biogeographic realm and countries results in a list of 547 species impacted by diseases (IUCN, Conservation International, and NatureServe. 2006).

    Notes
    Salamanders can act as host reservoirs of chytrid infection in frogs, and vice versa (Davidson et al. 2003).
    Geographical range
    Native range: Africa may be the origin of Batrachochytrium dendrobatidis.
    Evidence supporting this include the facts that: (i) the site of the earliest known occurence of the disease was in Africa, (ii) the major host (Xenopus laevis) shows no disease symptoms when infected by the fungus, and (iii) there is a plausible route (i.e. the international trade of X. laevis) to explain the movement of the disease from Africa to other parts of the world. A recent paper has outlined genetically-based evidence (including multilocus sequencing) that suggests that in at least two areas (Panama and Australia) where amphibian populations are declining the disease caused (chytridiomycosis) has emerged due to a recent introduction of B. dendrobatidis. This can be compared with competing hypotheses that suggest that climate and/or pollutant-level changes have altered previously existent host-pathogen relationships (Weldon et al. 2004; Morehouse et al. 2003).
    Known introduced range: The fungus has been found on every continent that has amphibians except Asia: Africa, Australasia-Pacific, North America, and South America (Weldon et al. 2004; Michigan Frog Survey, 2003; Garthwaite, Undated; USGS, 2000; Lips et al. 2003).

    For a detailed list of species reported to have been infected with B. dendrobatidis (including the country, location, date and source of the report) please see: Speare and Berger. 2004. Global Distribution of Chytridiomycosis in Amphibians.

    Introduction pathways to new locations
    For ornamental purposes:
    Live food trade: If X. laevis was responsible for carrying the fungus out of Africa, other amphibian species could subsequently have distributed it between and within countries. The American bullfrog (Rana catesbeiana) may be an important vector, mainly through international trade as a food item but also within countries as populations established for the food trade escape and spread (Weldon et al. 2004).
    Other: Amphibians carrying Batrachochytrium dendrobatidis have been detected in frogs for scientific purposes, particularly X. laevis and X. tropicalis (Parker et al. 2002, Reed et al. 2000, Speare and Berger, 2000, in Johnson and Speare, 2003).
    Other: It has been suggested that Africa was the origin of the fungus and the pathogen was subsequently disseminated around the globe via the international trade of Xenopus laevis (and to a lesser extent X. tropicalis). In 1934 a biological assay to indicate pregnancy in humans was developed based on X. laevis, which lead to their export. In 1970 approximately 5,000 frogs were shipped abroad (Weldon et al. 2004; Prov. Admin. Of the Cape of Good Hope, 1970, in Weldon et al. 2004).
    Pet/aquarium trade: Amphibians carrying Batrachochytrium dendrobatidis have been detected in the pet trade in the United States, Europe and Australia (Groff et al. 1991, Mutschmannet al. 2000, Speare, 2000, in Johnson and Speare, 2003)
    Taken to botanical garden/zoo: A captive blue poison dart frog (see Dendrobates azureus IUCN Red List of Threatened Species) diagnosed with amphibian chytridiomycosis died at the National Zoological Park in Washington D.C. (Berger et al. 1999). Infected amphibians have also been identified in European zoos (Daszak 2000, in USDI, 2003).
    Transportation of habitat material: Batrachochytrium dendrobatidis may have been introduced into new areas by movement of infected amphibians or in contaminated water or soil containing zoospores. In Australia, chytridiomycosis-infected cane toads (Bufo marinus), a recently introduced species, have been found. In North America bullfrogs (R. catesbeiana) and other species of amphibians have been translocated or introduced widely (Cunningham et al. 1993, Mahony, 1996, Richards, McDonald and Alford, 1993, in Johnson and Speare, 2003; Berger, Speare and Hyatt, 2000, in Daszak et al. 1999).


    Local dispersal methods
    For ornamental purposes (local): Dwarf African clawed frogs ( Hymenochirus curtipes IUCN Red List of Threatened Species) bred in captivity in California were found infected with B. dendrobatidis (Groff, 1991, in Daszak et al. 1999). Their introduction into ornamental garden ponds throughout the United States in the late 1980s may have facilitated the spead of the fungus (Daszak et al. 1999).
    On animals: The fungus can survive in wet or muddy environments, and could conceivably be spread by livestock carrying mud on their hooves and moving among frog habitats (USDI, 2003). It also may be spread by waterfowl (Waldman et al. 2001).
    On clothing/footwear: The disease could also be spread by contaminated footwear and equipment (USDI, 2003).
    Other (local): "The infective stage of Batrachochytrium dendrobatidis is the zoospore and transmission of the disease requires water as the zoospore is not tolerant to dehydration. B. dendrobatidis remains viable in tap water for up to 3 weeks, in deionized water for up to 4 weeks and in lake water for even longer. Infection with an extremely small inocula (100 zoospores) can prove fatal (Berger et al. in Speare et al., 2001; Johnson and Speare, 2003; Berger, Speare and Hyatt, 2000, in Daszak et al. 1999).
    The fungus can exist in water or mud and could be spread by wet or muddy boots, vehicles, cattle, and other animals moving among aquatic sites, or during scientific sampling of fish, amphibians, or other aquatic organisms (USDI, 2003). "
    Road vehicles: Chytrids could be carried on mud clinging to wheel wells or tires, or on shovels or other equipment (USDI, 2003).
    Management information
    Preventative measures: Knowledge of the infectiveness and spread of Batrachochytrium dendrobatidis is relevant to all control strategies, particularly in the development of preventative measures. The infective unit of the fungus is the zoospore. Infection by the fungus (and thus spread of the disease) requires water because the zoospore does not tolerate dehydration. B. dendrobatidis remains viable for up to 3 weeks in tap water, up to 4 weeks in deionised water and even longer in lake water. Infection by an extremely small inoculum (100 zoospores) is sufficient to cause a fatal infection (Berger et al. in Speare et al. 2001; Johnson and Speare, 2003; Berger, Speare and Hyatt, 2000, in Daszak et al. 1999).

    Please see main preventative management strategies for a summary under the following headings: improving diagnostics and knowledge of epidemiology, developing trade and quarantine regulations, raising awareness and control options.

    The Amphibian Conservation Action Plan (ACAP) is designed to provide guidance for implementing amphibian conservation and research initiatives at all scales from global down to local. Chapter 4 outlines action steps relating to the detection and control of chytridiomycosis.

    Nutrition
    Its occurrence solely in keratinised tissues suggests that it uses amphibian keratin as a nutrient. Batrachochytrium dendrobatidis will grow for at least one generation on cleaned epidermal keratin or on amphibians that have died of the infection. The fungus may also be cultured in vitro on tryptone agar without the addition of keratin or its derivatives (Daszak et al. 1999; Longcore, Pessier and Nichols, 1999, Pessier et al. 1999, in Daszak et al. 1999).
    Reproduction
    Batrachochytrium dendrobatidis is diploid and primarily reproduces asexually (and clonally) by producing aquatic uniflagellated zoospores in a zoosporangium (Johnson and Speare, 2003).
    Lifecycle stages
    Batrachochytrium dendrobatidis has two life stages: a spherical reproductive sessile zoosporangium and a motile zoospore. The motile zoospore directs itself and attaches to the keratinised outer layers of its host. It then absorbs its tail and buries itself below the surface of the skin. It matures into a zoosporangia with rhizoids within about four days and produces and releases up to 300 zoospores into the external environment (via a discharge tube). The cycle is initiated again once a suitable substrate (in the same or a different host) is found. The presence of the fungus in the keratinised mouthparts of frog tadpoles (without actually killing them) supports the role of larvae as reservoirs for the pathogen. (The larvae of amphibian species may survive for as long as 3 years before metamorphosing.) Syntopic salamanders and frogs may also act as reciprocal pathogen reservoirs for chytrid infections. It has been suggested that B. dendrobatidis may not be an obligate amphibian parasite, possibly living in other non-amphibian hosts or even sapropytically (off dead tissue) (Michigan Frog Survey, 2003; Speare et al. 2001; Daszak et al. 1999; Davidson et al. 2003).
    As of yet, no resting structures (either asexual or sexual) have been identified for B. dendrobatidis. The fact that sexual reproduction in chytrid fungi has been associated with the production of resistant, thick-walled resting spores has lead to the hypothesis that the production of airborne spores explains the widespread distribution of B. dendrobatidis in relatively pristine areas. However recent research has found evidence that shows that the population structure of B. dendrobatidis is largely clonal, supporting the hypothesis that the fungus lacks a sexual stage (as is the case for many chytrid fungi). This suggests that dispersal by human (or perhaps other long distance travellers, such as birds), rather than natural causes, are more likely to be the cause of the pathogen's entry into pristine areas (Morehouse et al. 2003; Berger et al. 1999, Daszak et al. 1999, in Morehouse et al. 2003)."
    This species has been nominated as among 100 of the "World's Worst" invaders
    Reviewed by: Matthew J. Parris Assistant Professor, Department of Biology University of Memphis USA
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG) with support from the Terrestrial and Freshwater Biodiversity Information System (TFBIS) Programme (Copyright statement)
    Last Modified: Monday, 14 August 2006


ISSG Landcare Research NBII IUCN University of Auckland