Amphibian diseases: Pesticide immunotoxicity and chytridiomycosis in larval Rana catesbeiana and ranaviral disease in Rana sylvatica tadpoles of central Ontario

Amphibian populations worldwide have been declining for decades due to a variety of reasons including pesticides and emerging infectious diseases. Two diseases of amphibians, chytridiomycosis and ranaviral disease, have been found in Ontario Rana catesbeiana and Rana sylvalica tadpoles respectively.;Kingscote Lake, Algonquin Park was home to the first confirmed outbreak in Ontario of Batrachochvtrium dendrobatidis. The chytrid fungus was confirmed in bullfrog tadpoles from the 2004 and 2005 die-offs at Kingscote Lake; however, positive asymptomatic populations suggest that an additional factor is responsible for the observed mortality. A map is presented for the presence of ranavirus in 14 of the 25 R. sylvatica tadpole populations tested in 2004. Asymptomatic infections and long-tern monitoring of two ranavirus-impacted wild populations argue for adaptive immunity in these amphibians. Many pesticides have been show to be immunotoxic in that they disrupt some parameter of the immune system through suppression or enhancement. The pesticide immunotoxicity hypothesis may explain why some tadpole populations experience mortality. The organophosphate insecticide malathion was immunotoxic in a 96-hour exposure and subsequent chytrid challenge. Prior exposure to the pesticide favorably impacted the tadpoles' ability to avoid or clear chytridiomycosis.