Indeed, comparative immunological studies between X. laevis and other anuran species are needed. The greatest mortality and infection prevalence occurred during the hatchling stage for S. holbrookii, which was a different trend among the species that we tested. Infection and mortality decreased during the larval and metamorph stages, suggesting that immune function increased through development for this species. Compromised immunity during early development may be a consequence of physiological trade-offs associated with rapid development in this species. Spadefoots are among the fastest developing anuran species due to their association with ephemeral breeding sites. Zettergren reported cells synthesizing immunoglobulins during embryogenesis and B lymphocytes circulating in pre-metamorphic L. pipiens at the onset of feeding. Leukocyte mobilization and anti-FV3 IgY antibody production have been reported as immune responses to ranavirus infection in X. laevis. We hypothesize that development of these components of the amphibian immune system is delayed in S. holbrookii due to rapid growth during the embryo and hatchling stages. Among species, L. sylvaticus was the most susceptible, with infection and mortality exceeding 80% in the hatchling, larval, and metamorph stages. These results support field observations for this species across its geographic range. To date, no studies have explored the immunological mechanisms underlying the high susceptibility of L. sylvaticus to ranavirus compared to other species, although see Warne et al.. Cotter et al. reported that poor lymphocyte production in the spleen was a mechanism driving high susceptibility of larval Ambystoma mexicanum to ranavirus. Significant increases in total leukocytes and natural killer cells are detected after 1 and 3 days post-infection with ranavirus, respectively, in X. laevis. Pre-metamorphic L. catesbeianus and X. laevis produce antibodies, and therefore may resist ranavirus infection. Thus, minimal innate and adaptive immune response to ranavirus infection may be mechanisms contributing to high infection and mortality rates in ranavirus-exposed L. sylvaticus. Our study is the first to report mortality of anuran hatchlings by ranavirus. The possibility for hatchling mortality from ranaviruses raises a significant conservation concern considering that detecting die-offs of hatchlings is extremely difficult in the wild. Differential susceptibility among developmental stages also indicates that studies that focus on one stage may provide narrow insight into species susceptibility. If testing only one stage is feasible, we recommend using the larval stage because mortality and infection prevalence were either greater or similar to hatchling and metamorph stages for most species. More research is needed investigating the role of immune function in regulating differences in susceptibility to ranavirus among anuran species. To date, few studies have quantified immune responses to ranavirus in pre-metamorphic amphibians. Identifying commonalities among immunogenetic, evolutionary and life history traits of susceptible species will improve our understanding of host-pathogen interactions, and help facilitate identification of amphibian communities at greatest risk of ranavirus epizootics. To this end, we recommend that additional amphibian species and ranavirus strains be tested for relative susceptibility. Abmole Fedratinib Various multivariate techniques exist that can elucidate patterns between host characteristics and indices of susceptibility.