Origin of the Amphibian Chytrid Fungus

Ché Weldon,Rick Speare,Louis H Du Preez,Reinhold Muller,Alex D Hyatt

doi:10.3201/eid1012.030804

Abstract

The sudden appearance of chytridiomycosis, the cause of amphibian deaths and population declines in several continents, suggests that its etiologic agent, the amphibian chytrid Batrachochytrium dendrobatidis, was introduced into the affected regions. However, the origin of this virulent pathogen is unknown. A survey was conducted of 697 archived specimens of 3 species of Xenopus collected from 1879 to 1999 in southern Africa in which the histologic features of the interdigital webbing were analyzed. The earliest case of chytridiomycosis found was in a Xenopus laevis frog in 1938, and overall prevalence was 2.7%. The prevalence showed no significant differences between species, regions, season, or time period. Chytridiomycosis was a stable endemic infection in southern Africa for 23 years before any positive specimen was found outside Africa. We propose that Africa is the origin of the amphibian chytrid and that the international trade in X. laevis that began in the mid-1930s was the means of dissemination.

Highlights

The sudden appearance of chytridiomycosis, the cause of amphibian deaths and population declines in several continents, suggests that its etiologic agent, the amphibian chytrid Batrachochytrium dendrobatidis, was introduced into the affected regions
If B. dendrobatidis emerged in this fashion, we hypothesize that the source would meet the following criteria: 1) the hosts would show minimal or no apparent clinical effects, 2) the site would be the place of the earliest known global occurrence, 3) the date of this occurrence would precede any amphibian declines in pristine areas, 4) the prevalence in the source host or hosts would be stable over time, 5) no geographic spreading pattern would be observed over time in the region, 6) a feasible means of global dissemination of Batrachochytrium from the region of origin would be identified, and 7) B. dendrobatidis would show a greater genetic variation in the host region than in more recently invaded regions
The earliest date for a chytridiomycosis-positive specimen was 1938 in an X. laevis collected from the Western Cape coastal lowland

Summary

Introduction

The sudden appearance of chytridiomycosis, the cause of amphibian deaths and population declines in several continents, suggests that its etiologic agent, the amphibian chytrid Batrachochytrium dendrobatidis, was introduced into the affected regions. B. dendrobatidis is common in African frogs from Ghana, Kenya, South Africa, and Western Africa [12,15] and declines in frog populations are poorly documented in Africa [7,16] These factors, combined with the global trade in X. laevis and X. tropicalis, prompted us to investigate the likelihood that Africa was the origin of Batrachochytrium and that the trade in Xenopus spp. played a key role in its global dissemination. Chytridiomycosis refers to infection of amphibians by B. dendrobatidis

Methods

Results

Conclusion