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A fungus is threatening midwife toads in Majorca because of captive breeding

An international team of scientists, with Spanish participation, has confirmed that the fungus that causes chytridiomycosis reached Majorca through a reintroduction programme for threatened amphibians bred in captivity in England, and which were “apparently” healthy. Four populations are currently affected on the island, but only one has developed the disease, causing the death so far of one tenth of Balearic midwife toads.

The research study, published this week in Current Biology, shows that midwife toads (Alytes muletensis), also known as “Ferreret” in Majorca, were infected by the fungus Betrachochytrium dendrobatidis coming from South African amphibians (Xenopus gilli) on a captive breeding programme carried out in England.

“The two species (Xenopus gilli and Alytes muletensis) were mutually infected in captivity by contaminated water, since the fungus is waterborne and highly infectious, and we know that there are high levels of infections from chytridiomycosis in the African species”, explained the main researcher, Susan Walker, from Imperial College London, to SINC.

The fungus, which lives in the water and on the skin off host amphibians such as frogs, toads, salamanders and newts, has led to the extinction of amphibian populations in Europe. In general, the disease is found in 87 countries and has resulted in the rapid decline of populations in areas such as Australia and Central America, pushing some species to extinction.

“The fungus creates sacs which spread their spores when reproducing. It is not known too clearly whether amphibians die from some type of toxic substance or the tearing of their skin which dries as it loses water due to piercing of the skin caused by the fungus”, informed Juan Antonio Oliver Valls, from the Palma Department of the Environment which, together with the National Science Museum (CSIC), was also involved in the study.

The disease has affected four Balearic midwife toad populations of the more than 30 which exist in Majorca. Only one of them has developed the infection, but has already led to the reduction in a tenth of the population. “Other populations are positive, although no changes in number of animals has been observed”, Oliver pointed out.

The research suggests that there are unknown factors preventing the extinction of these populations, although, according to Walker, “it is difficult to quantify the survival rate of affected toads”.

Temperature, a possible factor

Researchers point out that the disease is linked to temperatures. They indicate that above 27°C or 28°C the fungus dies and animals are cleaned naturally. “The fungus grows with low temperatures, below 20°C. Majorcan pools can reach a higher temperature though, which is why the growth of the pathogen slows down”, the English researcher commented.

Other factors that can protect amphibians from the fungus is their effective immunological response which is currently being studied in the laboratory. Susan Walker also believes that “the variety of Betrachochytrium dendrobatidis introduced on the island is not virulent, or has evolved with less virulence due to the islands’ unique environmental conditions”.

Despite the fungus’s presence, Balearic midwife toad populations have improved their situation as the number of animals has tripled over the last 12 years, with almost half of the total number recently introduced. “Last year record population numbers were recorded”, reports the Palma Department of the Environment.

Reintroduction of species, a risk

The study showed the risk of reintroducing species into the wild and highlighted the need to ensure that animals bred in captivity do not infect other species with pathogens.

“To prevent future infections, toads and pools already infected need to be treated with antifungal drugs. We have developed a treatment that cures infections 100%, which is why a treatment in captivity is now possible, even if the cure for their habitat is more complex”, added Walker.

In the affected areas in Majorca there are regional regulations prohibiting entry to excursionists to prevent other areas from becoming infected. Furthermore, worldwide control of the disease has been included in the conservation plans for amphibians. The fungus has also been added to a list of diseases “which need to be in quarantine”, according to the World Organisation for Animal Health.

SINC Team | alfa
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