Introduction of Ranavirus to Isolated Wood Frog Populations Could Cause Local Extinction

Abstract

Amphibian declines and extinction have been attributed to many causes, including disease such as chytridiomycosis. Other pathogens may also contribute to declines, with ranavirus as the most likely candidate given reoccurring die-offs observed in the wild. We were interested in whether it is possible for ranavirus to cause extinction of a local, closed population of amphibians. We used susceptibility data from experimental challenges on different life stages combined with estimates of demographic parameters from a natural population to predict the likelihood of extinction using a stage-structured population model for wood frogs (Lithobates sylvaticus). Extinction was most likely when the larval or metamorph stage was exposed under frequent intervals in smaller populations. Extinction never occurred when only the egg stage was exposed to ranavirus. Under the worst-case scenario, extinction could occur in as quickly as 5 years with exposure every year and 25-44 years with exposure every 2 years. In natural wood frog populations, die-offs typically occur in the larval stage and can reoccur in subsequent years, indicating that our simulations represent possible scenarios. Additionally, wood frog populations are particularly sensitive to changes in survival during the pre-metamorphic stages when ranavirus tends to be most pathogenic. Our results suggest that ranavirus could contribute to amphibian species declines, especially for species that are very susceptible to ranavirus with closed populations. We recommend that ranavirus be considered in risk analyses for amphibian species.

Ranavirus Ecology and Evolution: From Epidemiology to Extinction

“…In one recent report, at least 200,000 larvae died within just 24 h (Wheelwright et al 2014 ). There are similar reports of rapid, seasonal outbreaks in wild amphibians in Europe (e.g., Ariel et al 2009a ;Kik et al 2011 ), South and Central America (Fox et al 2006 ;Stark et al 2014 ), and Asia (e.g., Une et al 2009 ), and widespread reports from captive populations that follow similar patterns (Duffus et al 2015 ). Price et al ( 2014 ) recently reported declines in amphibian communities at multiple sites in Spain after the apparent introduction of a novel ranavirus.…”

“…Most of what is known about the epidemiology, geography, and host range of ranaviruses comes from investigations of obvious die-offs, sporadic surveillance efforts in small numbers of populations and time points, and a few larger-scale surveillance efforts focused on a handful of species of economic importance or conservation interest (Grizzle and Brunner 2003 ;Gray et al 2009a ;Whittington et al 2010 ;Miller et al 2011 ;Duffus et al 2015 ). Duffus et al ( 2015 ) reviewed the known distribution and host range of ranaviruses.…”

“…Duffus et al ( 2015 ) reviewed the known distribution and host range of ranaviruses. In total, six species of Ranavirus are recognized, causing infection or overt disease in at least 175 species (52 families) of ectothermic vertebrates from 32 countries on six continents (Duffus et al 2015 ).…”

“…Ranavirus infections and related mortality events have been reported in amphibians from North and South America, Europe, Africa, and Asia (Duffus et al 2015 ). In North America, 43-57 % of amphibian mortality events were attributed to viral infections, presumably ranaviruses (Green et al 2002 ;Muths et al 2006 ).…”

“…This view changed with increasing evidence that ranaviruses were responsible for widespread epidemics and mortality in several fi shes and later amphibians (Ahne et al 1997 ;Chinchar 2002 ;Williams et al 2005 ). The growing interest in this genus of viruses is fueled by the apparent increases in geographic range, as well as evidence of population declines coming from a wide range of ectothermic vertebrates around the world (Duffus et al 2015 ). Ranavirus die-offs in the wild and in captive settings are often marked by a rapid onset and high mortality, but less obvious ranaviral infections in natural populations can occur.…”