Amphibian decline and mass mortality: the value of visualizing ranavirus in tissue sections.

Ranaviruses negatively impact amphibian populations throughout the world and have been associated with population fluctuations and mass mortality events (Collins and Storfer, 2003; Daszak et al., 2003). Amphibian ranaviruses infect multiple species, with host susceptibility differing among species and developmental stages (Brunner et al., 2005; Duffus et al., 2008; Gray et al., 2007; Robert et al., 2007; Schock et al., 2008). One of the dilemmas that researchers, diagnosticians and pathologists face is determining what cell types are targeted by the virus. Existing reports suggest that multiple cell types are targeted by ranaviruses (Bollinger et al., 1999; Burton et al., 2008; Cunningham et al., 2008; Docherty et al., 2003; Gantress et al., 2003; Jancovich et al., 1997). However, published studies identifying target tissues are limited, because the testing required to document the presence of virus can be costly or requires specialized equipment (such as electron microscopy) and virions can be easily missed if only small tissue sections are available for examination. The article by Balseiro and colleagues published in this issue of The Veterinary Journal demonstrates the use of a common diagnostic tool (immunohistochemistry, IHC) tovisualize ranaviruswithincells (Balseiro et al., 2009). This approachallows researchers anddiagnosticians to link the presence of virus with histological changes. The authors compare the cell tropism of the virus in two different species, the common midwife toad (Alytes obstetricans) and the alpine newt (Mesotriton alpestris cyreni). Their work provides insight into host responses to ranavirus and provides a useful approach for linking viral infection with pathological changes. Identifying the cell types targeted by ranavirus is important for several reasons. First, it can lead to a better understanding of the possible routes of viral transmission. For example, is the virus shed into the intestinal lumen or are germ cells affected? Swab specimens collected from the cloacae of subclinically-infected hosts have revealed positive test results for ranavirus suggesting that environmental shedding is possible by hosts displaying no apparent signs of illness (Driskell et al., 2009; Gray et al., 2009). Identifying the source of the shed virions (e.g., renal vs. intestinal vs. cutaneous) can help manage this pathogen in captive environments. Similarly, it is unknown if infection of eggs or sperm during gametogenesis is possible (Duffus et al., 2008). Rather, post-gametogenesis exposure (e.g., in the cloaca or in the environment) of gametes or embryos has been theorized. The ability to visualize ranavirus within an egg or spermatid would provide insight into the possibility of vertical transmission. One of the most useful applications of identifying the cell types infected by ranavirus is relating the presence of the virus with the tissue changes (such as cellular degeneration or necrosis), or deter-