Ranavirus infection in northern leopard frogs: the timing and number of exposures matter

Transmission is a central feature of pathogen fitness and influences host population dynamics. The form and magnitude of transmission rates determine whether a pathogen establishes itself in a host population and the proportion of a population that becomes infected. While the effects of environmental variation on pathogen transmission dynamics have received substantial attention, the time and number of pathogen exposure in relation to host ontogeny has been relatively less investigated. Such understanding is particularly important in host species exhibiting distinct life-history stages such as amphibians as this temporal variation in infection modulates transmission trends at the population level. We investigated the role of the timing and number of ranavirus (FV3) exposures on infection rate and mortality patterns in Lithobates pipiens tadpoles in a two-step laboratory experiment with four treatments: individuals exposed as hatchlings but not as tadpoles, individuals not exposed as hatchlings but exposed as tadpoles, individuals exposed both as hatchlings and tadpoles and individuals that were never exposed. Our results indicate that individuals exposed twice presented higher infection and mortality rates over individuals exposed only once (infection: 40 vs. 16%; mortality: 16.8 vs. 8.1%, respectively). Among individuals exposed only once, but at different time, no difference in mortality was observed; yet, dissimilar infection rates indicate a differential capacity to carry and transmit virions among various developmental stages suggesting distinct epidemiological roles. Our results stress the importance of considering stage-dependent host susceptibility to better understand infection patterns and transmission dynamics and advocate its incorporation in models and field study designs.