Abstract

Chytridiomycosis is a recently-discovered fungal disease of amphibians responsible for mass mortality and population declines in a number of regions globally.¹ It appears to have recently expanded in geographic range and has therefore been classed as an emerging infectious disease (EID).² Since its discovery in 1998, a series of experimental and field studies have led to a hypothesis that anthropogenic introduction of chytridiomycosis is largely responsible for its recent emergence.³ First, the pattern of amphibian declines in Central America and Australia, and the biologic traits of the causative agent (Batrachochytrium dendrobatidis) are typical of an invasive pathogen moving through naïve hosts.³ Second, molecular studies of global isolates have shown that there is little variation between isolates from widely separated regions, suggesting a recent emergence event.⁴ Finally, this pathogen has been reported from amphibians traded nationally and internationally for food, as pets, for ornamental backyard release, for zoo exhibits and captive breeding programs, and in amphibians introduced for biocontrol.⁴

In this presentation, we present the following evidence from field and experimental data that implicate bullfrogs (Rana catesbeiana) in the introduction of chytridiomycosis in some regions:

• Experimental inoculation of bullfrogs led to infection in some cases, but no infected animals showed clinical signs of chytridiomycosis. The lesions were focal and not typical of animals which had died of chytridiomycosis.
• Bullfrogs farmed for food in Uruguay showed high prevalence of infection by B. dendrobatidis, but no clinical signs of chytridiomycosis.
• The causative agent of chytridiomycosis has been identified historically in bullfrogs from the Savannah River Site, where 25 yr of amphibian population data demonstrate the absence of long-term declines.
• In a population of recently-introduced bullfrogs in Venezuela, B. dendrobatidis was found in 48/48 individuals, none of which showed clinical signs of chytridiomycosis.

These studies, and recent work by other groups in the USA suggest that bullfrogs are efficient carriers and are involved to some extent in the introduction of chytridiomycosis into regions in the USA and abroad. We propose that the trade in bullfrogs for food, and their introduction into new regions or countries should be more closely monitored, and surveillance measures introduced for this disease.

Acknowledgments

This study is funded by NSF IRCEB award IBN-9977063 and NSF IRCEB award DEB-02133851 and by core funding to the Consortium for Conservation Medicine from the V. Kann Rasmussen Foundation.

Literature Cited

1.  Berger L, R Speare, P Daszak, DE Green, AA Cunningham, CL Goggin, R Slocombe, MA Gagan, AD Hyatt, KR McDonald, HB Hines, KR Lips, G Marantelli, H Parkes. 1998. Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proc. Natl. Acad. Sci. USA. 95, 9031–9036.

2.  Daszak P, Berger L, Cunningham AA, Hyatt AD, Green DE, Speare R. 1999. Emerging infectious diseases & amphibian population declines. Emerg. Infect. Dis. 5, 735–748.

3.  Morehouse EA, James TY, Ganley ARD, Vilgaly R, Berger L, Murphy PJ, Longcore JE. 2003. Multilocus sequence typing suggests the chytrid pathogen of amphibians is a recently emerged clone. Mol. Ecol. 12, 395–403.

4.  Daszak P, Cunningham AA, Hyatt AD. (2003). Infectious disease and amphibian population declines. Diver. Distr. 9, 141–150.