Ranavirus in Wood Frogs (Rana Sylvatica): Potential Sources of Transmission Within and Between Ponds

Abstract

ABSTRACT:Members of the genus Ranavirus (family Iridoviridae) can cause catastrophic mortality of pond-breeding amphibians and are associated with an emerging infectious disease that may be contributing to amphibian declines. We conducted three experiments to examine factors that may affect transmission both within and between local breeding populations of the wood frog (Rana sylvatica). In a laboratory study, when exposed to moribund tadpoles collected during a local ranaviral die-off, uninfected tadpoles died as soon as 4 days after exposure. The onset of death was accelerated when tadpoles were allowed to scavenge on carcasses of infected tadpoles. In a mesocosm experiment that was conducted in outdoor wading pools, die-offs of tadpoles began approximately 19 days after infected tadpoles were added to pools containing uninfected tadpoles. Mass die-offs with greater than 98% mortality occurred in all pools, regardless of the initial tadpole density. In a second mesocosm experiment, the addition of water and bottom sediments that were collected from a pond during a ranaviral die-off did not result in lower tadpole survival or growth relative to controls. Only a small percentage of tadpoles appeared to be sick, and most tadpoles survived until the first individuals began metamorphosing within a pool. However, tests for ranavirus using polymerase chain reaction were positive for most pools that received contaminated sediment, suggesting that some infections were sublethal. Our results indicate that transmission within ponds is enhanced by scavenging and that spread between local ponds could occur via the transport of contaminated sediment by animals or humans.

Frog Virus 3-Like Infections in Aquatic Amphibian Communities

“…It should be noted that the ponds were not monitored through tadpole metamorphosis. Furthermore, although infections were detected with PCR, we never observed visible signs of FV3/FV3-like infections in field-collected animals, although previous observations suggest that extensive mortality occurs in wood frog tadpoles infected with FV3/FV3-like viruses (Green et al, 2002;Harp and Petranka, 2006), including those studied at one of our study sites (Greer et al, 2005). Our results also concur with those of Harp and Petranka (2006) in that they indicate that in some cases, wood frog tadpoles can carry the virus without exhibiting signs of infection.…”

“…As the dispersal ability of adult wood frogs is poor (Homan et al, 2004), it is unlikely that the geographic movement of the virus occurs through movement of wood frogs. An alternative explanation, hypothesized by Harp and Petranka (2006), is that virus spread between ponds could occur through the movement of virus-contaminated sediment by humans or animals. Since we have found extensive infection of ambystomatid salamanders with FV3 or an FV3-like virus in our pond communities, it is possible that virus movement between ponds is mediated by salamanders or other amphibians living in these pond communities.…”

“…An emerging infectious disease is defined as a disease caused by a pathogen that is currently increasing in geographical range, is infecting an increased diversity of hosts, and/or has recently evolved (Daszak et al, 2000). Examples of diseases in amphibian populations around the world that have recently gained attention include chytridiomycosis, caused by Batrachochytrium dendrobatidis (Muths et al, 2003;Rachowicz et al, 2006), and the iridoviruses (family Iridoviridae) (Harp and Petranka, 2006). Iridoviruses have been associated with large-scale morbidity and mortality events of amphibians throughout North America and Europe.…”

“…In addition, salamanders that were exposed to ATV as larvae could become ATV carriers as adults and pass the virus on to uninfected adults (Brunner et al, 2004). A recent and similar study examined potential transmission mechanisms of ranaviruses between wood frogs inhabiting geographically isolated ponds (Harp and Petranka, 2006). In this study, transmission within ponds was enhanced by tadpoles scavenging diseased, moribund, or dead tadpoles, and the authors concluded that the virus could be spread between local ponds as a result of the movement of contaminated sediment by animals or humans (Harp and Petranka, 2006).…”

Evolved pesticide tolerance influences susceptibility to parasites in amphibians

“…Indeed, we found a 230% increase in viral loads from the least to most susceptible population. In nature, wood frog tadpoles interact with a diversity of other amphibian species (Werner et al., 2014) and wood frogs are often implicated as important species in ranavirus transmission (Harp & Petranka, 2006). As such, the wide variation in viral loads between constitutive versus inducible populations may have broad implications for understanding transmission dynamics of ranavirus within communities.…”