Anuran susceptibilities to ranaviruses: role of species identity, exposure route, and a novel virus isolate

Ranaviruses are responsible for widespread amphibian die-offs, particularly with larval anurans. To understand the factors that may be contributing to the emergence of ranaviruses, we conducted 3 experiments that exposed 3 species of larval anurans to either endemic frog virus 3 (FV3) or an FV3-like isolate from a ranaculture facility. Our goals were to (1) determine the susceptibility of each species to each virus, (2) determine whether direct ingestion of virions or exposure to virions in a water bath were similarly lethal routes of transmission, and (3) quantify the effects of exposure duration on disease outcomes. We conducted our research in a controlled aquatic laboratory using a factorial combination of virus isolates, transmission routes, and exposure durations. While ranaviruses can affect many species, we found that larval anurans differ greatly in susceptibility to ranaviruses. Average mortality rates of Cope's gray tree frogs (66%) and pickerel frogs (68%) were similar but 3-fold higher than for eastern narrow-mouthed toads. Direct ingestion of the viruses increased mean infection and mortality rates by 30% and caused death about 2 times faster compared to water bath exposure. However, exposure duration did not impact mean infection or mortality rates. We also found that the ranaculture isolate increased mortality by >34% compared to FV3. Our results suggest that ranaviruses can rapidly infect and cause disease in multiple amphibian species. Given the risk associated with introducing novel ranaviruses from ranaculture facilities, we recommend that all nations adopt the protocol set forth by the World Organization for Animal Health for testing and certifying that amphibians that are commercially shipped are negative for ranavirus infection.