Factors affecting susceptibility to chytridiomycosis
Vazquez, Victoria M.
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Chytridiomycosis is an emerging infectious disease of amphibians that has caused population declines in North and Central America and Australia. The disease is caused by the fungus Batrachochytrium dendrobatidis. Factors contributing to susceptibility of amphibians to this disease are not well known. Temperature has been suggested as a key factor since chytridiomycosis is thought to occur more frequently at cool high-elevation sites where enigmatic amphibian declines have been observed. To study the effects of temperature on susceptibility to chytridiomycosis, we experimentally exposed Gastrotheca peruana at 18 and 23ºC and Plethodon metcalfi and Desmognathus monticola at 8 and 16ºC to B. dendrobatidis. Mortality of infected G. peruana was higher at 18°C than at 23°C. Mortality of P. metcalfi due to B. dendrobatidis was higher at 8°C than at 16°C. Therefore, temperatures at the low end of the range suitable to maintain the health of amphibians appear to increase susceptibility to the pathogen. Phylogeny is also a possible factor in susceptibility, since a few studies indicate salamanders may respond differently to infection with Batrachochytrium dendrobatidis. We compared infection of Plethodon metcalfi and Desmognathus monticola to that of anuran species. Infected salamanders exhibited few of the clinical signs associated with B. dendrobatidis infection; however, they exhibited histological signs of disease similar to those previously observed in other salamanders. Some salamanders may have cleared infection. The amphibian immune system likely plays a role in susceptibility to chytridiomycosis. Cutaneous peptides secreted from amphibian skin are effective at inhibiting growth of Batrachochytrium dendrobatidis in vitro. To examine the role of cutaneous peptides in vivo we depleted Rana catesbeiana of peptides and then exposed them to about 10 million B. dendrobatidis zoospores. All frogs became infected with B. dendrobatidis, as determined by PCR; however no clinical or histological signs of disease were observed. The natural mixtures of peptides of R. catesbeiana had low effectiveness against B. dendrobatidis in vitro. Therefore antimicrobial peptides are not the sole source of resistance to chytridiomycosis in R. catesbeiana. Further research is required on the factors discussed here and the many others likely involved in susceptibility to chytridiomycosis.