Virulent parasites cannot persist in small host populations unless the parasite also has a reservoir host. We hypothesize that, in hosts with complex life histories, one stage may act as an intraspecific reservoir for another. In amphibians, for example, larvae often occur at high densities, but these densities are ephemeral and fixed in space, whereas metamorphs are long-lived and vagile but may be very sparse. Parasite persistence is unlikely in either stage alone, but transmission between stages could maintain virulent parasites in seasonally fluctuating amphibian populations.We examined this hypothesis with a lethal ranavirus, Ambystoma tigrinum virus (ATV), that causes recurrent epidemics in larval tiger salamander populations, but which has no reservoir host and degrades quickly in the environment. Although exposure to ATV is generally lethal, larvae and metamorphs maintained sublethal, transmissible infections for Ͼ5 mo. Field data corroborate the persistence of ATV between epidemics in sublethally infected metamorphs. Three-quarters of dispersing metamorphs during one epidemic were infected, and apparently healthy metamorphs returning to breed harbored ATV infections. Our results suggest that larval epidemics amplify virus prevalence and sublethally infected metamorphs (re)introduce the virus into uninfected larval populations. Intraspecific reservoirs may explain the persistence of parasites in and declines of small, isolated amphibian populations.

An iridovirus was confirmed to be the cause of an epizootic in larval and adult tiger salamanders (Ambystoma tigrinum diaboli) from four separate ponds in southern Saskatchewan (Canada) during the summer of 1997. This organism also is suspected, based on electron microscopic findings, to be the cause of mortality of larval tiger salamanders in a pond over 200 km to the north during the same year. Salamanders developed a generalized viremia which resulted in various lesions including: necrotizing, vesicular and ulcerative dermatitis; gastrointestinal ulceration; and necrosis of hepatic, splenic, renal, lymphoid, and hematopoietic tissues. In cells associated with these lesions, large lightly basophilic cytoplasmic inclusions and vacuolated nuclei with marginated chromatin were consistently found. Virus was isolated from tissue homogenates of infected salamanders following inoculation of epithelioma papilloma cyprini (EPC) cells. The virus, provisionally designated Regina ranavirus (RRV), was initially identified as an iridovirus by electron microscopy. Subsequent molecular characterization, including partial sequence analysis of the major capsid protein (MCP) gene, confirmed this assignment and established that RRV was a ranavirus distinct from frog virus 3 (FV3) and other members of the genus Ranavirus. Intraperitoneal inoculation of 5 ϫ 10 6.23 TCID 50 of the field isolate caused mortality in inoculated salamanders at 13 days post infection. Field, clinical, and molecular studies jointly suggest that the etiological agent of recent salamander mortalities is a highly infectious novel ranavirus.

Transmission is central to pathogen fitness and strongly influences the impact of pathogens on host populations. Particularly important to transmission dynamics is the distinction between direct transmission requiring close physical contact (e.g. bumping, fighting, or coughing) and indirect transmission from environmental sources such as contaminated substrates. We present data from 4 experiments addressing the form, routes, and timing of transmission of Ambystoma tigrinum virus (ATV) among tiger salamanders Ambystoma tigrinum nebulosum. Our data suggest that ATV is efficiently transmitted by direct interactions between live animals (bumping, biting and cannibalism) as well as by necrophagy and indirectly via water and fomites. Determining which form of transmission is most important in nature is essential for understanding transmission at the population level. Our experiments also revealed an important temporal aspect of infectiousness: larval salamanders become infectious soon after exposure to ATV and their propensity to infect others increases with time. These results begin to clarify the mechanisms and dynamics of ATV transmission and lead to key questions that need to be addressed in future research.

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Emerging infectious diseases are a significant threat to global biodiversity. While historically overlooked, a group of iridoviruses in the genus Ranavirus has been responsible for die-offs in captive and wild amphibian, reptile and fish populations around the globe over the past two decades. In order to share contemporary information on ranaviruses and identify critical research directions, the First International Symposium on Ranaviruses was held in July 2011 in Minneapolis, MN, USA. Twenty-three scientists and veterinarians from nine countries examined the ecology and evolution of ranavirus–host interactions, potential reservoirs, transmission dynamics, as well as immunological and histopathological responses to infection. In addition, speakers discussed possible mechanisms for die-offs, and conservation strategies to control outbreaks.