Ranavirus infection-induced avoidance behaviour in wood frog juveniles: do amphibians socially distance?

Hosts may limit exposure to pathogens through changes in behaviour, such as avoiding infected individuals or contaminated areas. Here, we tested for a behavioural response to ranavirus infection in juvenile wood frogs (Rana sylvatica) because the majority of dispersal between populations occurs during this life stage. We hypothesized that if infections are transmissible and detectable at this life stage, then susceptibles would display avoidance behaviours when introduced to an infected conspecific. Despite no apparent signs of infection, we observed a greater distance between susceptible-infected pairs, compared to pairs of either two infected or two susceptible animals. Further, distances between susceptible-infected pairs were positively related to the infection intensity of the focal exposed frog, suggesting the cue to avoid infected conspecifics may become more detectable with more intense infections. Although we did not quantify whether the transmission was affected by their distancing, our findings suggest that juvenile frogs have the potential to reduce terrestrial transmission of ranaviruses through avoidance behaviours.

Seasonal dynamics and potential drivers of ranavirus epidemics in wood frog populations.

Epidemics in wildlife populations often display a striking seasonality. Ranaviruses can cause rapid, synchronous mass mortality events in populations of wood frog (Rana sylvatica) larvae in the summer. While there are several possible explanations for this pattern-from seasonal introductions of the virus to environmental stressors to windows of susceptibility to mortality from infection during development-most studies have focused on single factors in laboratory settings. We characterized the time course of ranavirus epidemics in eight ephemeral ponds in Connecticut, USA, measuring the prevalence and intensity of infections in wood frog larvae and Ranavirus DNA in water samples using environmental DNA methods. We found little evidence that the timing of pathogen introduction affected the timing of epidemics (rising prevalence) or the resulting die-offs. Instead, we observed a pulse in transmission asynchronous with die-offs; prevalence reached high levels (≥ 50%) up to 6 weeks before mortality was observed, suggesting that die-offs may be uncoupled from this pulse in transmission. Rather, mortality occurred when larvae reached later stages of development (hind limb formation) and coinciding water temperatures rose (≥ 15 °C), both of which independently increase pathogenicity (i.e., probability of host mortality) of infections in laboratory experiments. In summary, the strong seasonality of die-offs appears to be driven by development- and/or temperature-dependent changes in pathogenicity rather than occurring chronologically with pathogen introduction, after a pulse in transmission, or when susceptible host densities are greatest. Furthermore, our study illustrates the potential for eDNA methods to provide valuable insight in aquatic host-pathogen systems.

Roles of corticotropin-releasing factor, neuropeptide Y and corticosterone in the regulation of food intake in Xenopus laevis.

In mammals, hypothalamic control of food intake involves counterregulation of appetite by an orexigenic peptides such as corticotropin-releasing factor (CRF), and orexigenic peptides such as neuropeptide Y (NPY). Glucocorticoids also stimulate food intake by inhibiting CRF while facilitating NPY actions. To gain a better understanding of the diversity and evolution of neuroendocrine feeding controls in vertebrates, we analysed the effects of CRF, NPY and glucocorticoids on food intake in juvenile Xenopus laevis. We also analysed brain CRF and NPY mRNA content and plasma corticosterone concentrations in relation to nutritional state. Intracerebroventricular (i.c.v.) injection of ovine CRF suppressed food intake while CRF receptor antagonist alpha helical CRF(9-41) significantly increased food intake relative to uninjected and placebo controls. By contrast, i.c.v. injection of frog NPY and short-term corticosterone treatment increased food intake. Semi-quantitative reverse transcription-polymerase chain reaction analyses showed that CRF and NPY mRNA fluctuated with food intake in the brain region containing the mid-posterior hypothalamus, pretectum, and optic tectum: CRF mRNA decreased 6 h after a meal and remained low through 31 days of food deprivation; NPY mRNA content also decreased 6 h after a meal, but increased to prefeeding levels by 24 h. Plasma corticosterone concentration increased 6 h after a meal, returned to prefeeding levels by 24 h, and did not change with prolonged food deprivation. This postprandial increase in plasma corticosterone may be related to the subsequent increase in plasma glucose and body water content that occurs 24 h postfeeding. Overall, our data support the conclusion that, similar to other vertebrates, CRF is anorexigenic while NPY is orexigenic in X. laevis, and CRF secretion modulates food intake in the absence of stress by exerting an inhibitory tone on appetite. Furthermore, the stress axis is activated in response to food intake, but in contrast to mammals and birds is not activated during periods of food deprivation.