A central role in the recent catastrophic decline of amphibians is occupied by diseases, with one in particular – the chytrid fungus Batrachochytridium dendrobatidis – responsible for the largest disease driven species extinction event ever known to man. This fungus was described for the first time only in 1999 and since then hundreds of publications have detailed its mechanisms for infection in amphibians, the impact on different populations and species as well as numerous experiments designed to treat the infection in captivity or to stop it from spreading. As a result, we currently know a great deal about this pathogen although we are still unable to remove it from the environment or to efficiently treat it in the wild, making the future prospects of countless amphibian species extremely precarious. Several species, in particular habitat specialist species in high altitude Central America, a global biodiversity hotspot, are now so threatened that their survival might be completely dependent on captive breeding programs. However, as the fungus can’t be successfully removed from the environment and there is no vaccine or long term protection, it is unclear how any captive bread individuals could be released back in the wild. In addition, fighting this disease is complicated by the fact that not all species are affected and the fungus seems to be endemic in some countries but introduced and invasive in others, greatly facilitated by the global amphibian trade in spreading across all continents except Antarctica.
Interestingly, the chytrid is neither very resistant (it is vulnerable to desiccation and to temperatures above 29C) nor particularly good at active dispersal, yet, it can rapidly contaminate very large regions and moves very quickly between areas, including those without amphibians and with high enough temperatures to kill the fungus. This is even more surprising considering that direct transmission, animal to animal, is potentially the most important pathway of infection. Understanding more widely the mechanisms for the spread of the fungus and for infection could provide a much needed help for fighting this disease.
In a recent study Kolby et al. investigate rainfall events during tropical storms in Cusuco National Park in Honduras as a potential source of contamination with the fungus, away from contiguous bodies of water. They used single use large plastic sheets (16 square meters) in 5 areas with large openings in the forest canopy during 2011 and 2012 and additionally collected water from nearby rivers where high contamination of amphibians such as the critically endangered Plectrohyla dasypus (See image) had been detected. Very worryingly, the chytrid was detected in 3 of the 5 studied rivers but also in one of the rainfall samples, some 600m away from the nearest river, suggesting for the first time that aerial dispersal is possible for this pathogen. Even if the fungus spores were swept by wind from surrounding trees it still required aerial transportation over at least 50m. The implication is that the fungus could be moved rapidly between differ types of habitats and could potentially infect amphibian species in a terrestrial environment, away from the typical source of contamination which is rivers, pools of water and aquatic species. Cusuco National Park is an area of remarkable importance for amphibians and reptiles, with over 100 species recorded there and several single site endemics present. Hopefully more research such as this study can help us to understand how to protect these species before they are lost forever.
Kolby et al. 2015 Presence of amphibian chytrid fungus (Batrachochytrium dendrobatidis) in rainwater suggests aerial dispersal is possible. Aerobiologia DOI 10.1007/s10453-015-9374-6
Read more about the chytrid here: www.amphibianark.org/the-crisis/chytrid-fungus/