Anuran susceptibilities to the emerging amphibian pathogen Ranavirus

Background/Question/Methods   The global emergence of infectious diseases in humans, livestock, wildlife, and plants has generated interest in understanding the factors that drive host-pathogen interactions. Disease emergence may be driven by multiple factors, including differences in species susceptibility and whether pathogens are novel or endemic. By developing a mechanistic understanding of how these factors drive disease dynamics, we can predict outbreaks and potentially manage or reduce the negative consequences associated with disease. Emerging infectious diseases have been implicated as a contributing factor to amphibian population declines across the globe. In particular, ranaviruses, viruses that infect amphibians, reptiles and fish, have been linked to amphibian die-off events worldwide with the greatest number of reported mortality events occurring in the United States. A hypothesized factor in the emergence of ranaviruses is novel strain introduction by humans (i.e., pathogen pollution). While pathogen pollution has been identified as a significant concern, studies are needed that compare the relative susceptibility of amphibian species to novel versus endemic strains. To date, only a few species have been tested for their susceptibility to ranaviruses under controlled laboratory conditions. Our goal was to compare susceptibilities of 9 anuran species from 5 families for two Ranavirus isolates: endemic frog virus 3 (FV3) and a novel isolate from an American bullfrog culture facility. Tadpoles of each anuran species were exposed to one randomly assigned isolate for 3 days and survival monitored for 21 days under laboratory conditions.

Results/Conclusions   We found that wood frogs, southern leopard frogs, and Cope's gray tree frogs were the most susceptible species, with mortality rates ranging from 60 – 100%. In contrast, American toads, eastern spadefoots, and eastern narrow-mouthed toads experienced <30% mortality. All 9 species experienced mortality following virus exposure and tested positive for ranavirus infection suggesting a broad host range for the pathogen. Importantly, we found that the novel Ranavirus isolate was on average 2.3X more lethal to tadpoles compared to FV3. Our results reveal a significant conservation concern for native amphibians if infected tadpoles from ranaculture facilities are intentionally or accidentally released, or if contaminated water from captive facilities is not properly treated prior to environmental discharge. Given that Ranavirus isolates from captive facilities can be highly pathogenic to native amphibians, we recommend implementation of the World Organization for Animal Health guidelines in the United States, which require “ranavirus-negative” certification prior to the commercial shipment of amphibians.