National Fish Health Research Laboratory, U.S. Geological Survey, Kearneysville, WV, USA
Tadpole edema virus (TEV) is a ranavirus pathogenic to amphibians. Ranaviruses have been isolated from non-diseased, clinically diseased and recently deceased amphibians, sometimes in association with epizootic events. Some ranaviruses are believed to be relatively ubiquitous in the environment, yet factors that influence virulence, host susceptibility and pathogenesis are not well described. Here, we investigate the comparative virulence of TEV to selected aquatic life stages (hatch to metamorphosis) of the Fowler's toad, a common anuran species. A single mass of fertilized Fowler's toad eggs was field collected, and the hatched tadpoles reared under laboratory conditions. Replicate groups of eggs and tadpoles were exposed to TEV (aqueous exposures of 105 TCID50/ml for 24h) and maintained for 14 days. Trials involving the following approximated life stages were done: Gosner stage 20 (hatchlings), Gosner stage 25 (mouth parts developed), Gosner stage 36 (rear limbs developing), and Gosner stage 42 (metamorphosis). Mortality was recorded daily, and dead tadpoles and representative survivors (at the end of the 14 days) were evaluated for presence and titer of TEV using a quantal viral infectivity assay to determine 50% endpoint in cell culture. Mortality was negligible in control groups, and no virus was detected among any control tadpoles or the pre-screened eggs and embryos. Mortality and associated recovery of virus varied among the (4) TEV-challenged life stages studied. High mortality associated with TEV (89% or greater) was observed among the earliest and latest life stages (Gosner 20 and Gosner 42) while lower mortality levels (32% and below) were observed from the intermediate aquatic life stages examined. Viral titers among exposed survivors and tadpoles dying with TEV ranged from 103 to 106 TCID50/g. Virus-associated mortality appeared greatest during the periods of heightened physiological stress near hatch and metamorphosis.