Disentangling causes of seasonal infection prevalence patterns: tropical tadpoles and chytridiomycosis as a model system

INFECTION DYNAMICS OF BATRACHOCHYTRIUM DENDROBATIDIS IN TWO FROG SPECIES INHABITING QUITO'S METROPOLITAN GUANGÜILTAGUA PARK, ECUADOR

Batrachochytrium dendrobatidis (Bd) infection is one of the principal causes of amphibian declines worldwide. The presence of Bd has been determined in Gastrotheca riobambae tadpoles that inhabit ponds in Quito's Metropolitan Guangüiltagua Park, Ecuador. This study sought to determine whether these tadpoles are infected and to determine the presence of chytridiomycosis in another frog species, Pristimantis unistrigatus, which also inhabits the park and has different reproductive biology and distinct behavioral habits. We used end-point and real-time PCR techniques to detect and quantify Bd infection. At 1 yr, samples were taken from the skin of P. unistrigatus using swabs and were also taken from the mouthparts of G. riobambae tadpoles. It was found that the two species were infected with a Bd prevalence of 39% (53/135) in G. riobambae tadpoles and 15% (57/382) in P. unistrigatus frogs. The two types of samples (tissue and swabs) from mouthparts showed differences in the zoospores per microliter loads (x̄=1,376.7±3,450.2 vs. x̄=285.0±652.3). Moreover, a correlation (r2=0.621) was discovered between the monthly mean maximum temperature of the pond with disease prevalence in G. riobambae tadpoles. Infection levels in the P. unistrigatus population varied significantly over time, and distance to the pond was a determinant factor for infection intensity.

Thermal acclimation has little effect on tadpole resistance to Batrachochytrium dendrobatidis

Given that climate change is predicted to alter patterns of temperature variability, it is important to understand how shifting temperatures might influence species interactions, including parasitism. Predicting thermal effects on species interactions is complicated, however, because the temperature-dependence of the interaction depends on the thermal responses of both interacting organisms, which can also be influenced by thermal acclimation, a process by which organisms adjust their physiologies in response to a temperature change. We tested for thermal acclimation effects on Lithobates clamitans tadpole susceptibility to the fungus Batrachochytrium dendrobatidis (Bd) by acclimating tadpoles to 1 of 3 temperatures, moving them to 1 of 5 performance temperatures at which we exposed them to Bd, and measuring Bd loads on tadpoles post-exposure. We predicted that (1) tadpole Bd load would peak at a lower temperature than the temperature for peak Bd growth in culture, and (2) tadpoles acclimated to intermediate temperatures would have overall lower Bd loads across performance temperatures than cold- or warm-acclimated tadpoles, similar to a previously published pattern describing tadpole resistance to trematode metacercariae. Consistent with our first prediction, Bd load on tadpoles decreased with increasing performance temperature. However, we found only weak support for our second prediction, as acclimation temperature had little effect on tadpole Bd load. Our results contribute to a growing body of work investigating thermal responses of hosts and parasites, which will aid in developing methods to predict the temperature-dependence of disease.