New Zealand has four extant native species of frogs all in the genus Leiopelma, which are of great scientific interest because they retain primitive anatomic features reflecting the estimated 85-million-year isolation of the landmass since its separation from Gondwana. Three species of Australian frogs in the genus Litoria were introduced to New Zealand in the second half of the 19th century and are now widespread.
Conspicuous numbers of Litoria raniformis died at an ephemeral pond near the South Island city of Christchurch between November 1999 and February 2000. Organisms resembling the chytrid fungus Batrachochytrium dendrobatidis were observed in histologic sections of skin from each of five frogs that died. No gross changes were observed in skin except the accumulation of retained sloughs in some frogs. Microscopic skin lesions varied in appearance. Typically, there was little change evident in the dermis or stratum germinativum, with only the most superficial epidermal cells colonized by developing zoosporangia. Zoosporangia accumulated under the stratum corneum or within layers of retained keratinized cells and either contained several zoospores or were empty. In some areas, isolated epithelial cells were necrotic, or there was bacterial superinfection at the lesion’s surface. There were also erosions where the stratum corneum and superficial layers of keratinizing epithelium were lost, and no organisms were seen.
Chytridiomycosis has not previously been diagnosed in free-living frogs in New Zealand. Only a single colony of a common and widespread introduced species has been found infected, though only limited surveillance has been undertaken in other species or locations. Chytridiomycosis has been associated with mass mortality and population declines in frogs in Australia and South America. The implications of isolation of this emerging pathogen in New Zealand for the conservation management of native frog species may be serious.