High levels of disease prevalence and intensity in Illinois amphibians: Rethinking enzootic infections

Background/Question/Methods

Surveys of Batrachochytrium dendrobatidis (Bd) in US amphibians generally show low infections among sites and species, which is typical of post-epizootic disease dynamics. In active declines, average population intensities > 10,000 zoospores are associated with mortality. We conducted a large-scale systematic survey throughout Illinois to identify the geographic distribution of Bd, its prevalence, and intensity patterns among sites, wetlands, and species. We hypothesized that Bd infection would be geographically patchy because different habitats and anuran communities are expected to dictate Bd dynamics. We also predicted that southern communities would have lower disease prevalences and intensities because the climate is unfavorable to Bd growth. We predicted related species would have analogous infection levels because of similar ecology. We hypothesized that the highest infection levels would be in Acris crepitans because of the widespread declines in northern Illinois populations of that species in the 1980’s. We sampled anurans from 20 sites, comprised of 45 wetlands, over 2 breeding seasons (2008, 2009). We used logistic regression and Tukey’s test to compare differences in prevalence followed by ANOVA to compare differences in intensity of infection among sites and species; Pearson’s product moment correlation coefficients were used to test latitudinal trends.

Results/Conclusions

We analyzed swabs from 1,689 individuals from three Families (Bufonidae, Hylidae, Ranidae). All 20 sites contained infected anurans, but prevalence differed among sites (Χ² = 206.06, df = 19, p < 0.0001) (25 – 91%) and wetlands (Χ² = 361.13, df = 25, p < 0.0001) (0 – 95%) with no latitudinal correlation (site: t = 0.753, p = 0.461). All species were infected, but Bd intensity differed among sites (F_18,1050 = 8.58; p < 0.0001) and species (F_6,807 = 20.74; p < 0.0001) (0 – 130,000 zoospores) with no latitudinal correlation (site: t = 0.541, p = 0.689). Despite high levels, we saw no mortality events, although data on population trends are lacking. Acris crepitans had the highest infection levels (μ = 3,204 zoospores) supporting the hypothesis that population declines may have been associated with Bd. Possible explanations for elevated infection in Illinois include current but unrecognized declines, less-virulent Bd strains, or native frog abilities to withstand typically lethal infection levels. Several factors could keep intensity levels below the 10,000 zoospore threshold, including increased frog activity at temperatures outside Bd optimum, decreased contact among infected individuals, the annual life cycle of some Illinois species, and use of habitats unfavorable to Bd growth.