Crawfish Frog, Lithobates areolatus
Photo Credit: Stanley Trauth
Amphibian populations around the world have been decimated by the chytrid fungus, Batrachochytrium dendrobatidis (Bd), but not all species or individuals in all regions are equally susceptible. Vanessa Kinney and colleagues (2011) report the first case of Bd in Crawfish Frogs (Lithobates areolatus). But, more importantly, describe the nature and the course of this disease in Crawfish Frogs which has an unusual natural history. They investigated whether there is a life history pattern or a seasonal pattern to infection by this fungus, and if it is possible to determine when and where the infection is being acquired and shed. Given the concern for the conservation of Crawfish Frogs, they examined the potential of Bd to cause fatalities in this species and to determine if they are carriers of the fungus, as are other large North American ranids. Crawfish Frogs are a typical North American pond-breeding species that have explosive spring breeding aggregations in seasonal and semi-permanent wetlands. However, when they are not breeding Crawfish Frogs are solitary, isolated from other individuals in burrows dug by crayfish. The burrows penetrate the water table, providing a permanent aquatic habitat when the frogs are not breeding. Over the course of two years Kinney et al. sampled for the presence of Bd in Crawfish Frog adults. Sampling was conducted seasonally, as animals moved from post-winter emergence through breeding migrations, then back into upland burrow habitats. During the study, 53% of Crawfish Frog breeding adults tested positive for Bd in at least one sample; 27% entered breeding wetlands Bd positive; 46% exited wetlands Bd positive. Five emigrating Crawfish Frogs (12%) developed chytridiomycosis and died. In contrast, all 25 adult frogs sampled while occupying upland crayfish burrows during the summer tested Bd negative. One percent of postmetamorphic juveniles sampled were Bd positive. Zoospore equivalents/swab ranged from 0.8 to 24,436; five out of eight frogs with zoospore equivalents near or more than 10,000 died. The data suggest Crawfish Frogs acquire Bd during breeding activities. A Bd-positive female entered a breeding pond on 8 April, 2010 with a low infection intensity (20 zoospore equivalents) and exited 15 days later with a high infection intensity (8,607 zoospore equivalents). A similar situation occurred on 19 April 2010, when a Bd-positive subadult Crawfish Frog entered the breeding pond with 119 zoospore equivalents and exited 5 days later with 23,006 zoospore equivalents. Overall, 46% (42/91) of samples from Crawfish Frogs exiting breeding wetlands on our study site were Bd positive. Thus, infection rates in Crawfish Frog populations increased from near zero during the summer to over 25% following overwintering; rates nearly double again during and after breeding—when mortality occurs—before the infection wanes during the summer. Bd-negative postmetamorphic juveniles may not be exposed again to this pathogen until they take up residence in crayfish burrows, or until their first breeding, some years later.
Kinney VC, Heemeyer JL, Pessier AP, Lannoo MJ (2011) Seasonal Pattern of Batrachochytrium dendrobatidis Infection and Mortality in Lithobates areolatus: Affirmation of Vredenburg's “10,000 Zoospore Rule”. PLoS ONE 6(3): e16708. doi:10.1371/journal.pone.0016708