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

[FIGURE 2 OMITTED]

Discussion

Our study has extended the date for the earliest case of chytridiomycosis in wild amphibians by 23 years. The next earliest case outside South Africa was found in Rana clamitans from Saint-Pierre-de-Wakefield, Quebec, Canada, in 1961 (22). After the case in Canada, the earliest cases from other countries follow sequentially over a period of 38 years from 1961 to 1999 (Figure 3).

[FIGURE 3 OMITTED]

X. laevis in the wild does not show clinical signs, nor has it experienced any sudden die-offs. Moreover, only subclinical chytrid infections have been observed among captive colonies of X. laevis (26,27). A frog of a related species, X. tropicalis, died in captivity from chytridiomycosis, it was suspected of having contracted the fungus from X. laevis (27). An ideal host for transmission of chytridiomycosis through international translocation would be a species of amphibian that does not become diseased or die from the infection; hence, X. laevis could take on the role of a natural carrier.

The sudden appearance of chytridiomycosis can best be explained by the hypothesis that B. dendrobatidis was recently introduced into new regions and subsequently infected novel host species (1). Dispersal of B. dendrobatidis between countries is most likely by the global transportation of amphibians (1,2,23,28,29). The World Organization for Animal Health has recently placed amphibian chytridiomycosis on the Wildlife Diseases List in recognition of this risk. If Africa is the source of B. dendrobatidis, a feasible route of dissemination by infected amphibians needs to be identified. Some members of the family Pipidae have been exported, in particular Hymenochirus curtipes and X. laevis, to North America and Europe (30).

In terms of a most likely candidate for spread from Africa, the number of frogs and geographic dissemination favor X. laevis. Soon after discovery of the pregnancy assay for humans in 1934 (30), enormous quantities of the species were caught in the wild in southern Africa and exported around the world. The pregnancy assay is based on the principle that ovulation in X. laevis is induced by injection with urine from pregnant women because of high levels of gonadotropic hormones in the urine (31,32). X. laevis was selected as the most suitable amphibian for investigating the mechanism of the mating reflex because of the relative ease with which the animal can be maintained in captivity (33). For 34 years, the trade in X. laevis in South Africa was controlled by the then Cape of Good Hope Inland Fisheries Department (Western Cape Nature Conservation Board) at the Jonkershoek Fish Hatchery. As an indication of the numbers involved in this trade, 10,866 frogs were distributed in 1949, of which 3,803 (35%) were exported, and of the 20,942 frogs distributed in 1970, a total of 4,950 (24%) were shipped abroad (34,35). After the introduction of nonbiologic pregnancy tests, X. laevis became important as a model for the scientific study of immunity and later embryology and molecular biology. X. laevis could have carried the disease globally, particularly if the prevalence was similar to that seen in wild-caught X. laevis today. In the importing country, escaped frogs, the water they lived in (36), or both, could have come into contact with local amphibian species, and subsequent transmission of the disease could have followed. The establishment of feral populations of X. laevis in Ascension Island, the United Kingdom, the United States, and Chile in 1944, 1962, the 1960s, and 1985 (37), respectively, show that transmission could have become ongoing if these feral populations were infected.