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).\r\n
To view a scanning electron micrograph of infected skin of a wild frog (Litoria lesueuri) please see: \r\n Daszak et al. 1999. Emerging Infectious Diseases and Amphibian Population Declines.\r\n
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.\r\n
To view a histological section of severely infected skin of a wild frog (Litoria caerulea) please see:\r\n
Berger et al. 1998. Chytridiomycosis causes amphibian mortality .

\r\nClick 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.

Salamanders can act as host reservoirs of chytrid infection in frogs, and vice versa (Davidson et al. 2003).

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).\n
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).\"

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).

Batrachochytrium dendrobatidis is diploid and primarily reproduces asexually (and clonally) by producing aquatic uniflagellated zoospores in a zoosporangium (Johnson and Speare, 2003).

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).

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).

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.