Mechanisms Contributing to the Emergence of Ranavirus in Ectothermic Vertebrate Communities

Brenes, Roberto

Research includes work with the Smoky mountain (SM) isolate ranavirus, an isolate obtained in 2009 from a dead marble salamander (Ambystoma opacum) larva collected during a die-off in Gourley pond at the Smoky Mountains National Park (Todd-Thompson et al. 2009). Ranaviruses have been linked to amphibian die-off events in ectothermic vertebrates worldwide. Differences in susceptibility and capacity of transmission among and within classes are poorly understood. My goal was to determine possible mechanisms influencing susceptibility to ranavirus infection in amphibian species and other aquatic vertebrate taxa, as well as the capacity of transmission between classes and the effects of amphibian community composition on ranavirus transmission. I tested 16 amphibian species from USA, Europe, and the pet trade, expanding an existing database developed by the Center for Wildlife Health to 35 amphibian species from 9 families. I also tested the susceptibility of 5 fish and 3 turtle species by exposure to a panel of ranaviruses from amphibian, fish and reptilian hosts under laboratory conditions. I used outdoor aquatic mesocosms to explore if certain species functioned as amplification hosts in a semi-natural environment. All vertebrate classes tested (amphibian, reptile, and fish) presented variability in susceptibility. Amphibians were most susceptible to ranavirus, but no phylogenetic relationship with susceptibility was detected. Susceptibility was related to life history characteristics of amphibian hosts. Fast-developing species that bred in temporary wetlands during spring showed higher susceptibility to ranavirus. Further, for one of the isolates, pathogenicity increased as distance between host population and isolate location increased. Fish and turtle species showed low susceptibility to ranavirus, but could function as reservoirs for ranavirus due to documentation of subclinical infections. Transmission experiments demonstrated that ranavirus could be transmitted between classes, with greatest mortality when infected turtles or fish transmitted the virus to amphibians. Finally, I showed that community composition affects ranavirus transmission and mortality in larval amphibians. Wood frog larvae functioned as amplification hosts to spotted salamander (Ambystoma maculatum) and caused an outbreak in chorus frog (Pseudacris feriarum). My results demonstrate that ranaviruses can infect multiple hosts from different classes with different susceptibilities, contributing to its persistence in the environment and recurrent outbreaks. My results can be used to identify potential species of high risk to ranaviral disease and highlight the need to understand host community to predict ranavirus outbreaks and develop conservation strategies to mitigate emergence of ranaviral disease.