Origin of the amphibian chytrid fungus

Emerging Infectious Diseases, Dec, 2004 by Che Weldon, Louis H. du Preez, Alex D. Hyatt, Reinhold Muller, Rick Speare

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.

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One of the biggest threats facing amphibian species and population survival worldwide is the disease chytridiomycosis, caused by the chytrid fungus, Batrachochytrium dendrobalidis (1,2). Chytridiomycosis was proposed as the cause of death in frog populations in the rain forests of Australia and Panama and was associated with the decline of frog populations in Ecuador, Venezuela, New Zealand, and Spain (3-6). Evidence for a countrywide decline in frog populations in South Africa is lacking (7), and local declines of several species have been ascribed to two main threats, habitat destruction and pollution (8). Chytridiomycosis is known in South Africa from infections in Xenopus laevis, Afrana fuscigula, and Strongylopus grayii (9-11). Through surveys of extant and archived specimens, Batrachochytrium has been found in every continent that has amphibians, except Asia (6,9,12,13). Since B. dendrobatidis has been recognized as an emerging pathogen, whose spread is facilitated by the international and intranational movement of amphibians (1), identifying its origin will be useful.

Some emerging infectious diseases arise when pathogens that have been localized to a single host or small geographic region go beyond previous boundaries (14). 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 (i.e., late 1970s), 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.

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. Within the Xenopus genus, X. laevis is distributed over the greatest area in sub-Saharan Africa. X. laevis occupies most bodies of water in savannah habitats from the Cape of Good Hope to Nigeria and Sudan (17, 18).

We report the earliest case of the amphibian chytrid found in any amphibian and present epidemiologic evidence to support the hypothesis that B. dendrobatidis originated in Africa. In this article, chytridiomycosis refers to infection of amphibians by B. dendrobatidis.

Materials and Methods

A retrospective survey was conducted on archived specimens of the genus Xenopus housed in five southern Africa institutions, Bayworld (Port Elizabeth), Natal Museum (Pietermaritzburg), National Museum (Bloemfontein), South African Museum (Cape Town), and Transvaal Museum (Pretoria). Specimens in these museums had been collected for archiving by a large number of persons for various purposes and had not been selected for a systematic survey of amphibian disease. Specimens were collected mainly from South Africa, Lesotho, and Swaziland. A piece (3 x 3 mm) of the interdigital webbing was removed from one hind foot of each specimen of X. gilli, X. muelleri, and X. laevis. Tissue was prepared for histologic examination with routine techniques (19). Sections were cut at 6 [micro]m and stained with hematoxylin and eosin. Chytridiomycosis was diagnosed by using described criteria (20). Sections from the two specimens diagnosed as having chytridiomycosis with hematoxylin and eosin before 1971 (one collected in 1938, the other in 1943) were confirmed with the more specific immunoperoxidase test (21) to increase the confidence of the diagnosis. Measurements of sporangia were performed with a calibrated eyepiece and expressed as mean [ or -] standard deviation (SD). Histologic slides were examined "blind," without reference to dates that the frogs were collected, to decrease any opportunity for bias in diagnosis.