Widespread occurrence of ranavirus in pond-breeding amphibian populations.

Ranaviruses are an emerging threat for many amphibian populations, yet their distribution in amphibian communities and the association of infection with possible stressors and species is not fully understood due to historically sparse surveillance. Agricultural practices that reduce the water quality of amphibian breeding habitats (e.g., cattle access to wetlands) and environmental stressors (e.g., lower temperatures) may contribute to ranavirus emergence. We tested larval amphibians for ranavirus infection across four seasons in farm ponds (n = 40) located in Tennessee, USA. Cattle at various densities were allowed access to half of the sampled ponds. Ranavirus infections were detected in nine species and in 33 of the sampled ponds (83%), illustrating widespread occurrence of the pathogen. Species within the family Ranidae were the most frequently infected. In 13 of the ponds containing infected individuals, prevalence exceeded 40% during at least one season. Infections were detected in multiple seasons in 20 of the sampled ponds containing infections, suggesting that ranaviruses are relatively persistent in these systems. Cattle had negative effects on water quality (turbidity and ammonia) and there was a positive association between cattle abundance and ranavirus prevalence in the summer. Counter to previous field studies in North America, we found a significant positive association between water temperature and ranavirus prevalence in the fall sampling events. Despite these findings, the influences of cattle and temperature on ranavirus prevalence were not consistent across seasons. As such, the mechanisms driving high ranavirus prevalence across the landscape and over time remain unclear. Given the widespread occurrence of ranaviruses in wild amphibians, we encourage the implementation of surveillance programs to help identify potential drivers of emergence. Sites with high ranavirus prevalence should be monitored annually for outbreaks, and the long-term effects on population size determined.