Chytrid fungi are primitive organisms that live in wet soils or in fresh water and most of them are saprophytes, although others are algae, plants or invertebrate animals parasites. Within this group, Batrachochytrium dendrobatidis, the fungus that produces the chytridiomycosis in amphibians, and the recently discovered Batrachochytrium salamandrivorans, which produces the disease in salamanders and newts exclusively, are the only ones that parasitize vertebrates.
B. dendrobatidis was described for science in 1998. Some years before, massive mortalities due to the pathogen had already been produced in different parts of the planet, such as Australia, Central America and Europe, where we detected the disease for the first time in 1997 in Peñalara. The specific origin of this fungus is still a mystery, although some studies point out that it could be originated in Africa and, due to international trade of the African clawed frog (Xenopus laevis), it could have been dispersed to the rest of the world. Nowadays, the fungus can be found in almost every place of the world where there are amphibians. Furthermore, another species of the genus, B. salamandrivorans, has been recently described. It can be found outside its area of origin in the Netherlands, Belgium and Germany. In this case, the origin of this new species is probably South-East Asia, from where it could have been introduced through trade of the amphibians for terrarium keeping.
The cycle of these organisms is simple: in the sessile stage, that is, in which the fungus is adhered to the substrate, it develops the zoosporangia that are in charge of releasing thousands of zoospores into the aquatic environment. These flagellate zoospores, which are the mobile stage, are active for less than 24 hours, in which they cover no more than two centimetres. Once they reach the host, the zoospores form a cyst under the skin and develop new zoosporangia. If the host's surface of the skin is densely populated by the fungus, the vital functions of the skin, such as the osmoregulation, the breathing or the water balance, are affected. A cardiac arrest is usually triggered, which finally produces the death of the animal.
B. dendrobatidis can grow within a wide range of temperatures, but its optimum temperature for growing in laboratory is between 17 and 25 degrees C (63 and 77 degrees F). It can withstand really low temperatures (up to 4 degrees C [40 degrees F]), so it survives in the amphibians larvae during the winter under the ice layer. On the other hand, it stops growing at 25 degrees C (77 degrees F) and dies beyond 28 degrees C (82 degrees F). The optimum temperature for growth of B. salamandrivorans is lower (between 10 and 15 degrees C [50 and 59 degrees F]), while temperatures beyond 25 degrees C (77 degrees F) are lethal.
The two species of fungi need humidity and don't bear the drying. For this reason, the adult stages of species such as the midwife toad, which live in dry areas and are very terrestrial, are not often infected, so all the burden of the pathogen is gathered in the larval stages that are in the water. This is very different in other more aquatic species or in tropical regions, where the environmental humidity makes the amphibians' terrestrial stages to be perfect for the chytrid fungi.
With this in mind, eliminating these fungi from the material that we use in the field in order to avoid their transmission is relatively easy through the complete sun-drying or through high temperatures. Furthermore, the disinfectants that are used in veterinary or in livestock facilities are very effective. For the amphibians' treatment, there are very effective antifungals used by veterinarians, such as the itraconazole.
Knowing these fungi’s biology is crucial to develop strategies to fight them and to prevent its expansion. Furthermore, knowing how they have spread around the world, highlights our responsibility in the amphibians' widespread decline, and it should encourage us to fight against them with all the means we have within our grasp.