Chytridiomycosis: the lethal threat to amphibians

Chytridiomycosis is a disease caused by the fungus Batrachochytrium dendrobatidis (Bd), a fungus belonging to the class Chytridiomycetes that, although generally found in decomposing plants or animals under water, this species mainly attacks amphibians, especially anurans. It acts by attacking the skin of amphibians, a tissue of great importance in this animal group, producing hyperkeratinosis, necrosis, edema... which leads to the impossibility of proper respiratory exchange, leading in many cases to the death of the animal.

Ciclo quitridiomicosis en anfibios

This fungus has been the cause of the disappearance of at least 200 species of amphibians in the last three decades, defining it even as the major cause of biodiversity loss caused by a disease, with no other animal group facing such a population decline. This problem is of such magnitude that the World Organisation for Animal Health has included it in the list of notifiable diseases.

According to a study published this year in the journal "Science" led by Simon O'Hanlon of Imperial College, London, has managed to sequence the genome of the fungus Batrachochytrium dendrobatidis determining that its origin is in Korea. This study has also defined four genetically different lineages, three of which are distributed throughout the areas inhabited by amphibians and the fourth found only in anurans native to Korea. This Korean lineage is much more genetically diverse and similar to the ancestral lineage from which the other three remaining lineages originate.

On the European continent, the first outbreak of this fungus was found in Madrid, in the Peñalara Natural Park, affecting individuals of the common midwife toad (Alytes obstetricans).

Although still lethal, projects are being carried out to eradicate the fungus, or at least reduce its virulence on amphibians. In 2015, chytridiomycosis in a wild population was completely eliminated for the first time. This was achieved in Mallorca through antifungal treatments on tadpoles of Balearic midwife toad (Alytes muletensis) together with a decontamination of the natural environment where they live, allowing, according to MNCN researcher Jaime Bosch, the permanent eradication of the fungus in this region.

Subsequently, in the Sierra de Guadarrama National Park in 2017, the infection of the common midwife toad (Alytes obstetricanus) by SOS Amphibians has been reduced. This has been possible by means of actions such as the extraction of larvae during periods of time when the temperature was optimal for the development of the fungus, reducing contagion to the larvae of subsequent clutches. These removed larvae are treated in the Threatened Amphibian Captive Breeding Center of the Sierra de Guadarrama and later returned to their natural habitat. This process of attacking the fungus by heat is important, as it provides a way of action to treat infected animals without the use of chemical agents. The fungus is most active between 17ºC and 25ºC causing the death of 100% of the affected individuals, being Spain an area of high risk of contagion. As the temperature rises, the proportion of deaths drops to 30ºC where, from this temperature, the fungus seems to disappear completely.

These actions lead us to dream of a global solution to control the attack of Batrachochytrium dendrobatidis on anurans, although not all are good news, as urodelos (salamanders and newts) are also victims of a fungus very similar to the previous Batrachochytrium salamandrivorans (Bsal). This fungus has been recently discovered in Europe and North America, being of notable importance its threat in Holland and Belgium where it has finished with 5% of the population of urodelos. Finally, it has also been located in Asia and South America, which leads us to think of a global threat like the fungus that attacks anurans.

Salamander chytridiomycosis

Human beings play an important role in this process, since human action can promote the spread of the fungus in cases, for example, of the trade of wild amphibians captured in legal imports and the illegal trade, being important the establishment of measures of action and prevention that allow the decrease of the attack of these fungi in the populations of world amphibians, one of the most threatened animal groups.

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