Origin of the amphibian chytrid fungus

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.

Figure 1

Micrographs of immunoperoxidase stained sections through the interdigital webbing of Xenopus gilli, showing the morphologic features and size of zoosporangia consistent with Batrachochytrium dendrobatidis. A) Arrow a indicates localized hyperplastic epidermal response; arrow b indicates an uninfected region of the epidermis. B) Arrows indicate two zoosporangia with internal septa. Circle indicates location of the infection in the stratum corneum. Bar, 10 μm.

Figure 2

Historical time-trend of chytridiomycosis prevalence in southern Africa. No significant change was shown in the prevalence over time (p = 0.22, 95% confidence interval).

Figure 3

Time bar indicating when chytridiomycosis first appeared in the major centers of occurrence in relation to each other. Following a 23-year interruption in occurrences after the Xenopus laevis infection in 1938, records outside Africa appear with increasing frequency up until the present; North America (22), Australia (2,23), South America (5), Central America (24), Europe (6), Oceania (New Zealand) (25).