Batrachochytrium dendrobatidis Longcore, Pessier & D. K. Nichols 1999
Batrachochytrium dendrobatidis (Bd) is a chytrid fungus that was first reported as the cause of chytridiomycosis (an often deadly and highly infectious disease of amphibians) in wild and captive frogs collected in North and Central America and Australia. It was formally described as a species from captive South American frogs in 1999 and virtually everything known about it has been discovered in the context of its role in global amphibian declines. This fungus has now been found on all continents except Antarctica and has been detected in frog specimens collected as long ago as 1938. Chytrid fungi are typically aquatic and differ from other fungi in that they have a motile, flagellated zoospore. Many chytrid species have been described from aquatic habitats and soils as free-living or commensal organisms and as parasites of algae, invertebrates, fungi, and plants. Bd is one of only two chytrids known to parasitize vertebrates and it is the only one known to infect and develop within the keratinized epidermal cells of living amphibian skin. Bd infects an extraordinarily broad diversity of host species--in fact, it has the widest known host range of any pathogen. It is known to infect over 350 species of amphibians and has been been implicated in driving the decline or extinction of over 200 of these species. (Fisher et al. 2009; Kilpatrick et al. 2009)
The origin of the BD chytridiomycosis pandemic remains controversial. According to the "novel pathogen hypothesis", Bd has been recently introduced to areas where it is causing population declines. An alternative scenario, the "endemic pathogen hypothesis", suggests that Bd has been present in recently affected populations for a long time, with its newly devastating impact resulting from changes in host susceptibility, pathogen virulence, environmental changes, or some combination of these factors. Molecular data from sites broadly dispersed around the world seem to suggest a single origin of Bd with evidence of local and global spread; these findings lend strong support to the novel pathogen hypothesis. However, in some populations Bd appears to be present without causing subsequent declines and in other locations, it appears to have been present before widespread declines. Similarly, in some species that declined regionally but persisted in small populations, Bd is still present and infecting frogs without driving these populations to extinction. (Fisher et al. 2009 and references therein; Kilpatrick et al. 2009 and references therein; Lam et al. 2010) The possible role of natural or anthropogenic global climate cycles, or long-term changes, in driving outbreaks of amphibian chytridiomycosis is controversial (for discussion, see, e.g., Lips et al. 2008; Rohr et al. 2008; Rohr and Raffel 2010).
Chytridiomycosis is notable for an overall lack of obvious disease pathologies. Metamorphosed amphibians infected with Bd typically exhibit epidermal hyperplasia and hyperkeratosis, and possibly upregulated skin shedding, but only rarely do they exhibit any lesions visible to the naked eye. Larval anuran amphibians may exhibit visible deformities of keratinized mouthparts. Bd is hypothesized to produce lethal toxins either before or after infection, to interfere with water uptake and lead to death due to dehydration, or to cause sharp osmotic imbalances that may interfere with water regulation and/or neurological function. (Fisher et al. 2009) Voyles et al. (2009) studied Bd infection in the treefrog Litoria caerulea. They found that in diseased individuals electrolyte transport across the epidermis was inhibited by >50%, plasma sodium and potassium concentrations were reduced by ~20% and ~50%, respectively, and asystolic cardiac arrest resulted in death. Given the critical importance of the skin in maintaining amphibian homeostasis, disruption of cutaneous function may be the mechanism by which Bd produces morbidity and mortality across a wide range of phylogenetically distant amphibian taxa.
Fungicidal protocols have been developed that are generally very effective for captive frogs, making captive breeding an important component of frog conservation efforts. These approaches include the use of elevated temperature, formalin/malachite green, and standard veterinary antifungal drugs. Good results for clearing infection in captive colonies have suggested the possibility of clearing infection in natural populations using catch, treat, and release methods using inexpensive, mobile biocontainment laboratories. An important new development in research on mitigation is the recognition that some bacteria that occur naturally on amphibian skin produce antifungal metabolites. This finding suggests the possibility of a probiotic approach to Bd mitigation, i.e., intentionally spreading these protective bacteria. (Fisher et al. 2009 and references therein; Lam et al. 2010)
It remains unclear how Bd has dispersed to and persisted in remote pristine environments where anthropogenic introduction is unlikely. If Bd can survive independently of amphibian hosts, it must use non-amphibian organic materials as nutrient resources. Although Bd DNA has been detected in water bodies and on rocks, conclusive evidence of Bd persistence in the environment is lacking. Species, populations, and individuals vary widely in susceptibility to chytridiomycosis. Mortality rates in laboratory infection experiments can range from 0% to 100%, depending on the species, age of animals, and temperature regime. In the wild, some species and populations are extirpated. while others, those that survive initial declines, persist with various levels of infection. While the disease dynamics are undoubtedly influenced by local environmental conditions, particularly temperature, inherent differences in host susceptibility and behavior are also important. Colonization by Bd and subsequent disease development may be influenced by host defense mechanisms, such as secretions of antimicrobial peptides or bacterial commensals with anti-fungal properties. Some species-specific behavioral characteristics such as microhabitat selection, basking, aggregating in retreat sites, or association with water bodies may also affect the likelihood of infection and disease. (Rosenbaum et al 2010 and references therein)