Chytrid Fungus

Background

Amphibian chytrid Batrachochytrium dendrobatidis was first identified in the late 1990s in both the Americas and Australia. It is believed to have emerged from Africa (as suggested by examinations of archive samples from the 1930s) due to global trade of clawed frogs Xenopus laevis for use in laboratories, for example in pregnancy testing.

This chytrid fungus has now been identified on all continents where amphibians occur. It is responsible for the disease chytridiomycosis, causing population declines and extinctions in at least 120 species of amphibians worldwide.

Effects of chytridiomycosis

It is not understood exactly how chytrid affects its amphibian hosts. It lives in the cells of keratinised skin. Hence it is possible that it interferes with the amphibian’s respiration and water balance both of which occur via the skin. Alternatively, the chytrid may release toxic products. Researchers have proposed the possibility of a neurotoxin, as paralysis of the hind legs has been noted in some severely infected animals.

Amphibians may be most susceptible to chytrid shortly after completion of metamorphosis. It has been responsible for mass mortalities of toadlets within 10-50 days after emergence from the water. Testing of archived samples suggests that it was responsible for ‘toadlet mortality syndrome’ noted in North America in the late 1970s.

However, chytrid does not always have noticeable effects. Even severely infected adult animals may show little or no symptoms.

Situation in the UK

In the UK the disease was first recorded in a population of introduced North American bullfrogs Lithobates catesbeianus/Rana catesbeiana, which had become established in south-east England. However a more worrying trend was found during 2006 when a volunteer involved with the HCT’s captive rearing programmes noticed unexplained mortality in captive reared natterjacks toadlets prior to their release. His vigilance and reporting led, in turn, to the discovery of infected natterjacks at a site in Cumbria.

Subsequent sampling by an HCT officer, volunteers and the Institute of Zoology identified five further infected natterjack sites in Cumbria, during the spring and summer 2007.

The HCT is a partner in the current national sampling effort, which is a direct result of the work from Cumbria, and is intended to establish whether chytrid occurs in other parts of England. Not only do we not know whether chytrid is widespread in England, but we also do not know what effects it might have, if any, on our native amphibians.

Related Links:-

Bullfrogs
Chytrid
The Institute of Zoology
ARG UK

Dave Coward (The HCT), Andrew Cunningham (IoZ) and Judit Hidalgo-Vila (IoZ) swab sampling natterjack toads in Cumbria.

The HCT, March 2008