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Research on Threatened Species


(1) Museo Nacional de Ciencias Naturales (CSIC), (2) Imperial College London, (3) Institute of Zoology, Zoological Society of London

The relentless fight against chytridiomycosis: the big threat to the world’s amphibians

The fungus Batrachochytrium dendrobatidis is decimating the world’s amphibians. The authors of this article are studying the disease from the epidemiological angle to build models that will one day be able to predict how it evolves and spreads.

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 Amphibians around the world are in dire straits. These creatures, which have lived on the planet for millions of years, and even coexisted with the dinosaurs, are today facing the threat of an infectious disease that is decimating their populations at an unprecedented rate. More than a third of the almost 7,000 known species of amphibian are at risk of disappearing. And HIGHLIGHTSProfile: Andrés Fernández Loras and Jaime Bosch Pérez
as if that were not bad enough, over 250 species are now already considered extinct in nature. These shocking figures are extremely worrying, and in the animal kingdom, no other class of vertebrate facing such a severe decline as the Amphibia. This has led scientists worldwide to get to work to find what is at the root of the frequent episodes of mortality witnessed on all continents in recent years. And, at the same time, efforts are underway to find solutions to this extremely serious problem, which affects the planet’s biodiversity as a whole.

Despite the fact the new disease that is killing amphib­ians was only discovered recently, some of the many studies conducted have already borne fruit. The culprit of the epidemic has been identified as a fungus in the Chytridiomycota division, with the scientific name Batrachochytrium dendrobatidis (Bd). The problem is so ser­ious, and has such a severe impact, that the World Organisation for Animal Health (the former Inter­national Epyzootes Office, 01E) has included the disease this fungus causes in amphib­ians, known as chytridiomycosis, on its list of notifiable diseases. This makes it the first disease to be included on the list on account of the threat to biodiversity it represents.

Along with the disease’s causes, new discoveries have been made about its characteristics. Research teams around the world are working against the clock, and have, among other things, managed to decipher how this pathogen kills vulnerable amphibian species. The fungus invades their skin, which is highly sensitive and vital in these animals, destabilising their ion balance and triggering cardiac arrest, for example. Thanks to this research we also know much more about the distribution of the chytrid fungus, and the factors encouraging its growth and incidence. In parallel, the fungus’s genome has been sequenced, and it has been found to have three genetically distinct lineages with different degrees of virulence.

Specimen of a Majorcan midwife toad killed by chytridiomycosis  in a water course in Majorca. / Photo J. Bosch.

Spain is among the leaders in contributing to our understanding of this disease and the pathogen causing it, and Spanish scientists are playing an active role in a variety of research projects with institutions around the world. One of these is the so-called “Proyecto Cero,” financed and backed by the Fundación General CSIC, in which prestigious organisations such as Imperial CoIlege London, the Zoological Society of London (ZSL) and the Museo Nacional de Ciencias Naturales (MNCN-CSIC) are taking part. As part of this project, the disease is being studied at the epi­demiological level in order to build models to predict its future evolution and spread. These models will also help identify which species of amphibians are worst affected. And knowing what species are at greatest risk, will allow us to devote our limited resources to trying to save them.

Supporting all these studies, which aim to broaden our knowledge of the disease, over the last few years conservation and captive breeding programmes have been set up for the most threatened populations or species. However, the speed with which the fungus has spread around the world, and the number of species at risk of extinction, mean time is not on the amphibians’ side. The scientific community has recently realised that there is a need to address the problem from a new perspective that aims to combine captive breeding programmes with a variety of strategies to mitigate the disease in nature. The aim of this new approach would be to start to combat the effects of the disease directly in amphibian populations in the wild, although we are not yet able to eradicate the pathogen from the envir­onment.

This Proyecto Cero combating chytridiomycosis has taken a step forward in this regard, making it a pioneering project worldwide, in that it seeks to mitigate the devastating effect of the disease on amphibians in the natural environment.
Along these lines, this Proyecto Cero is running mitigation trials at various points in Spain, focusing its efforts on midwife toads in the Ayltes genus, the European species most vulner­able to the disease. One of these species the common midwife toad, Alytes obstetricans, was precisely the unfortunate victim of the first outbreak of chytridiomycosis discovered in Europe, at the Peñalara natural park in Madrid. To achieve this goal of mitigating the disease, we have started to combine fungicide treatment of the animals (while they are in their larval state), with various interventions on their (aquatic) environment. For these activities, various points of Spain have been selected, at locations as different as Majorca, Zamora, Málaga and Teruel, in an effort to eradicate the fungus from them, or at least significantly reduce the fungal burden on infected populations that have not been wiped out. In Majorca, for example, two years ago one of the world’s first attempts was made to treat the disease directly in nature. This involved collecting and treating all the tadpoles of the Majorcan midwife toad, Alytes muletensis, at a specific site, and returning them to their pond after it had been dried out. We observed that although the chytrid fungus was not completely eliminated from the population, its burden was greatly reduced. In Málaga, one of the other areas in which the project worked, in close collaboration with David García from Fuengirola Zoo, we are investigating various different fungicide concentrations to treat the tadpoles of the Betic midwife toad, Alytes dickhilleni, and we are also evaluating raising the water temperature as an alternative way of eliminating the pathogen. Increasing the temperature improves these cold-blooded (poikilotherms) animal’s immune response, and at the same time, creates a somewhat unfavourable environment for the fungus, which dies at high temperatures. In Teruel, on the other hand, we have teamed up with the regional government’s local environment department (Servicio Provincial de Teruel de Medioambiente del Gobierno de Aragón) to use riskier mitigation strategies in the field such as temporarily drying out water holes harbouring infected populations. The good news is that the pathogenic fungus appears to have no resistance to the absence of water so is un­able to survive in a completely dry environment. Nevertheless, when all the water has been drained from their environment, adult amphibians can survive underground in contact with the traces of environmental moisture, en­abling the pathogenic fungi that have colonised their skin to survive with them. Therefore, at some sites, as well as treating infected larvae found in the water, we are fencing off bodies of water to avoid adult specimens coming into contact with it.

The Proyecto Cero on chytridiomycosis mitigation is in full swing. To back these mitigation strategies, research is also being done to look at the various protein components of the amphibians’ skin, which help them keep undesirable pathogens at bay. Thus, we have embarked on studies to characterise amphibians’ immune response to the disease, with a view to achieving some form of resistance to it. We are also trying out possible immunisation pro­cesses, exposing the animals to less virulent strains of the fungus under experimental conditions. This enables exposed animals to develop less severe infection when subsequently exposed to more virulent strains of the fungus, demonstrating evidence of an acquired immune response. Unfortunately our research has also shown the chytrid fungus’s capacity for genetic recombin­ation, which means that these mitigation strategies need to be managed carefully.

A multitude of efforts are being made by numerous researchers around the world. The success of this difficult undertaking will depend on their collaboration. We hope for a promising final outcome and that we are able to safeguard the existence of these creatures, which are among the planet’s oldest but still least understood.

Profile: Andrés Fernández Loras and Jaime Bosch Pérez

Andrés Fernández Loras
Andrés Fernández Loras is a contract veterinarian at the Spanish National Research Council (CSIC), where he is working on the Proyecto Cero project on chytridiomycosis mitigation. He has worked previously on infectious diseases in African bats, mitigating chytridiomycosis on the island of Montserrat in the Caribbean, receiving training at the London Zoological Society and Jersey Zoo. He is currently preparing a doctoral thesis on immune response and mitigation of chytridiomycosis.

Jaime Bosch Pérez
Jaime Bosch Pérez is a CSIC staff scientist and vice president of the Asociación Herpetológica Española (Spanish Herpetological Association). He has written over 90 scientific publications, and began his career with a study of sexual selection and acoustic communication in amphibians. He has focused on the study of emerging diseases specific to amphibians for many of years. He is the lead researcher on various research projects in various locations around the world on amphibian population conservation and monitoring programmes. For over ten years he has been in charge of the Peñalara Natural Park Amphibian Monitoring Programme, where he runs the Sierra de Guadarrama Endangered Amphibians Breeding Centre.

Matthew Fisher
Matthew Fisher is a researcher at Imperial College London, and an expert on infectious diseases caused by fungi, in plants and humans, as well as in wild animals. He is interested in the environmental factors triggering fungal diseases, and the role of humans in their dispersal and perturbation of natural systems. He is currently heading the most ambitious European project on chytridiomycosis and has been working with Dr. Jaime Bosch since the disease was discovered in Spain in 1999.

Trenton WJ Garner
Trenton WJ Garner is a researcher at the Zoological Society of London. His PhD was on aspects of sexual selection and the genetics of amphibian and reptile populations. For many years his research work has centred on experimental evaluation of the costs of parasitism in hosts, fundamentally in the case of chytridiomycosis and viral diseases of amphibians. He works with researchers from around the world, participating in research projects in a multitude of countries, and is one of Europe’s most active herpetologists.

Published in No. 09

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  • Lychnos. ISSN: 2171-6463 (Spanish print edition),
    2172-0207 (English print edition), 2174-5102 (online edition)
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