Global warming causing parasitic outbreaks in frogs

27/09/2009 23:44:52 Increase in deformities linked to parasites

September 2009. How climate change may affect parasites and infectious diseases is an important question in amphibian decline research. Researchers at the Angelo Coast Range Reserve in California have found data that supports a link between periods of unusually warm summer water temperatures during 2006 and 2008 in a northern California river, outbreaks of a copepod parasite and malformations in tadpoles and young of Foothill Yellow-legged Frogs (Rana boylii).

Water temperature increase
Relative to baseline data gathered since 1989, both 2006 and 2008 had significantly longer periods when daily mean water temperatures exceeded 20°C compared to years without copepod outbreaks. At breeding sites of R. boylii with copepods in 2006, infestation ranged from 2.9% of individuals upstream to 58.3% downstream. In 2008, copepods were absent from the most upstream sites and infested up to 28.6% of individuals sampled at downstream locations.

Copepods were most frequently embedded near a hind limb or the cloaca. Among individuals with parasites in 2006, 26.5% had morphological abnormalities compared to 1.1% of un-infested individuals. In 2008, when the infestation peak occurred later in tadpole's development, abnormalities were not associated with copepod infestation. In both years, recently metamorphosed frogs with copepods were, on average, slightly smaller than those not infested.

These occurrences represent a sudden increase in local prevalence atypical for this river ecosystem. The data suggest that increasing summer water temperatures, decreased daily discharge, or a combination of both, promote outbreaks of this non-native parasite on an indigenous host, and could present a threat to the long-term conservation of R. boylii under the scenarios predicted by climate change models.