Toads In A Hole? Fungal Disease Threatens UK Toad Population

The deadly chytrid fungus Batrachochytrium dendrobatidis (Bd) is highly effective at causing extinctions among local populations of amphibians and it has already wiped out vast numbers of amphibians in areas including Australia and South America. In the UK, the chytrid is found only in Kent, but researchers fear that in time it may spread across the country.

The fungus lives in the water and on the skin of host amphibians and the new study shows that the length of time that is able to survive on its own in water is key to determining what its impact might be.

One reason suggested for its devastating impact is that it is able to survive in water for long periods of time, allowing it to infect large numbers of amphibians as they pass through. However, the amount of time that the chytrid is able to survive outside the host is not known.

The researchers at Imperial College London and the Institute of Zoology used mathematical models to see the effects of introducing the chytrid into a breeding population of common toads (Bufo bufo), varying the amount of time that it survived outside the host.

The new models showed that if the chytrid was able to live outside the host for a year, the impact on UK toads would be considerable, with severe declines in the numbers of toads and, in some cases, extinction in 10 years within infected areas.

Research has demonstrated that the chytrid is able to live for at least seven weeks outside the host. However, the rapid declines in amphibian numbers elsewhere in the world support the idea that the chytrid may be able to persist for a much longer period of time than has so far been demonstrated in the laboratory.

Dr Mat Fisher, corresponding author of the research from the Department of Infectious Disease Epidemiology at Imperial College, said: “We don’t know enough about the chytrid to predict how it is going to behave in the UK, but we start to see dramatic effects if the chytrid lives for longer than 7 weeks outside the host. We strongly suspect that it can live for longer because of the devastating effect it has had elsewhere, and the new models show that this would be very bad news for toads in this country.”

The models showed that there was little effect on the UK toad population size if the chytrid was only able to live outside the host for seven weeks.

Bd infects the skins of amphibians such as frogs, toads, salamanders and newts and is thought to interfere with their ability to absorb water. More than 140 species of amphibians are known to be affected by it. Some are very susceptible and die quickly while others, which are more resistant, are carriers of the pathogen. One lucky species, the common British frog (Rana temporaria), appears to be completely resistant to Bd.

Journal reference: Kate M. Mitchell, Thomas S. Churcher, Trenton W.J. Garner, Matthew C. Fisher, “Persistence of the emerging pathogen Batrachochytrium dendrobatidis outside the amphibian host greatly increases the probability of host extinction” Proceedings of the Royal Society B, 28 November 2007