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

We have provided epidemiologic evidence that Africa is the origin of the amphibian chytrid fungus. Support for six of the seven criteria proposed for the source of B. dendrobatidis has been demonstrated: 1) the major host (X. laevis) shows minimal or no apparent clinical effects, 2) site of the earliest global occurrence (1938), 3) this date precedes any amphibian declines in pristine areas, 4) the prevalence in the source host or hosts (Xenopus spp.) has been stable over time, 5) no geographic spreading pattern could be observed over time, and 6) a feasible means of global dissemination exists via the international trade in wild-caught X. laevis, which commenced in 1935 and continues today. Criterion 7, greater genetic diversity of B. dendrobatidis at the source, has not been investigated. A low level of genetic variation was shown for 35 strains of B. dendrobatidis and suggested that B. dendrobatidis was a recently emerged clone (39). The strains had been collected in North America, Australia, Panama, and Africa from wild and captive amphibians. Three strains isolated from captive X. tropicalis in United States had been imported from Ghana. Although these showed no significant differences from the U.S. strains (39), their assignment to Africa assumes no cross-infection had occurred within the importing facility. Future work on the genetic diversity of B. dendrobatidis in Africa compared with strains from regions outside Africa will add weight to the hypothesis if greater genetic diversity is found in African strains.

Table 1. Prevalence of chytridiomycosis in archived Xenopus spp. from
southern Africa (a)

Species          No. examined   % positive (95% CI)

Xenopus laevis       583           2.6 (1.5-4.2)
X. meulleri           53          3.8 (0.5-13.0)
X. gilli              61          3.3 (0.4-11.4)
Total                697                2.7

Species          Earliest positive detected     Country

Xenopus laevis              1938              South Africa
X. meulleri                 1991               Swaziland
X. gilli                    1943              South Africa
Total

(a) p = 0.7; CI, confidence interval.

Table 2. Prevalence of chytridiomycosis in archived Xenopus, by time
intervals (a)

Time interval   No. examined   No. positives   % positive (95% CI)

1871-1940            56              1            1.8 (0.0-9.6)
1941-1950            16              1           6.3 (0.2-30.2)
1951-1960            63              0            0.0 (0.0-5.7)
1961-1970            17              0           0.0 (0.0-19.5)
1971-1980           230              6            2.6 (1.0-5.6)
1981-1990           145              3            2.1 (0.4-5.9)
1991-2001           170              8            4.7 (2.0-9.0)
Total               697             19            2.7 (1.7-4.2)

(a) p=0.36, CI, confidence interval.

Acknowledgments

We thank Bayworld (Port Elizabeth), Natal Museum (Pietermaritzburg), National Museum (Bloemfontein), South African Museum (Cape Town), and Transvaal Museum (Pretoria) for making the material available.