Amphibian Chytridiomycosis as an Emerging Infectious Disease of Wildlife: What Can We Learn from the Earliest Diverging Fungi?

Abstract

Investigation by wildlife biologists into the cause of the decline of amphibian populations has revealed that many factors (e.g., pollution, habitat loss, and invasive species) are likely to be involved. As basal fungi, Chytridiomycetes have presumably retained many of the characteristics of the last common ancestor with other Fungi millions of years ago. Chytridiomycosis is now considered to be an emerging infectious disease afflicting a wide range of frog species. Two goals of population genetic studies are to determine whether the disease has emerged due to recent introduction and to determine where the fungus came from and how has it been dispersed. First, a global sample of 35 isolates revealed only three variable nucleotides among nearly 6,000 surveyed positions. Second, direct sequencing of PCR-amplified gene regions demonstrated multiple sequence types per strain at 4 of the 10 loci examined. The investigation of emerging infectious fungal diseases of wildlife can help elucidate principles shared among all pathogenic fungi. As a model system, Batrachochytrium dendrobatidis informs us about general principles of mycoses, such as invasive growth and evasion of host immunity. Chytridiomycosis has the added advantage that both host and parasite are amenable to experimentation. Quarantines are needed to protect wildlife resources and to prevent the spread of future zoonoses. Changes in the environment may trigger emerging infectious disease epidemics. Identification of asexual lineages can be facilitated by detecting fixed heterozygosity as in other parasitic protozoa and oomycetes.