The chytrid fungus Batrachochytrium dendrobatidis (Bd) has been blamed for the decline of many amphibian species since its discovery in 1999. Bd is primarily a parasite of amphibians but may also be able to infect some birds and crustaceans which live in freshwater habitats. It has been recorded on all five continents where amphibians are known to occur.

The infectious stage of Bd can swim like a tadpole but it is too small to be seen by the naked eye. It embeds itself in the skin of an amphibian, where it reproduces before emerging into water to infect new hosts. Evidence suggests Bd causes death by preventing sufficient uptake of salts through the skin. Amphibians need salts for their circulation – without sodium, their hearts stop beating.

Recent research has shown that several strains of Bd exist and that some of these may be endemic to the areas in which they are found. Other research has identified a recently-evolved strain which may have spread rapidly around the globe with the help of human activities. This strain is known as the ‘global pandemic lineage’, or GPL. This strain may have caused the decline or even extinction of some species which did not have time to evolve infection resistance or tolerance.

All samples of Bd which have so far been tested in the UK belong to the GPL. Genetic work suggests the GPL could have evolved up to 150 years ago and although Bd was first recorded in the UK in 2004, it may have been present for decades. Non-native species such as African clawed toads, North American bullfrogs and alpine newts have been named as potential carriers or ‘vectors’ but it is now unlikely that the original source of the GPL will be identified.

A study of the effects of Bd upon natterjack toads in the UK has shown that this species can usually tolerate infection. Most natterjack populations which have been tested for Bd have tested positive. The effects of Bd on other UK species are less well understood. In mainland Europe, Bd has been linked to mass mortalities of species such as the European midwife toad. In the UK no mass mortalities have been linked to Bd but the effects may not be sudden or obvious.

The rapid spread of disease and invasive species in general could be limited by adopting a precautionary approach. ARG UK (2008) has produced an advice note for working with amphibians, including the disinfection of footwear and equipment before moving between sites.

Related Reading

ARG UK (2008) Amphibian and Reptile Groups of the UK Advice Note 4 (version 1): Amphibian disease precautions; a guide for UK fieldworkers. See publications section of www.arguk.org

Cunningham, A.A. and Minting, P. (undated). National survey of Batrachochytrium dendrobatidis infection in UK amphibians, 2008. Final report. Institute of Zoology.

Duffus, A.L.J. and Cunningham, A.A. (2010). Major disease threats to European amphibians. The Herpetological Journal 20, 117-127.