Escape from the pond: Stress and developmental responses to ranavirus infection in wood frog tadpoles

1.Animal populations exhibit considerable variation in their susceptibility to infection by emerging diseases, yet it is poorly understood how environmental and intrinsic factors contribute to these patterns. Considering that intrinsic factors (e.g. life history stage, nutritional state) can impact immune function, knowledge of the physiological mechanisms that mediate susceptibility to infection may improve our understanding of the emergence of disease in natural populations. 2.Ranavirus outbreaks have been associated with die-offs of amphibians worldwide. While the ecological factors associated with epidemics have been widely studied, little is known about how physiological factors mediate amphibian responses to ranavirus infection. 3.The neuroendocrine hypothalamus-pituitary-interrenal axis (HPI) is a physiological system central to coordinating energy balance and development. It is known to both stimulate and inhibit immune function in vertebrates in different contexts. We hypothesized that the HPI axis would also mediate responses to ranavirus infection. We used wood frog (Rana sylvatica) larvae and ranavirus isolated from recent die-offs of local wood frog populations to examine the physiological responses to infection. 4.In addition to increasing odds of death with increasing doses of virus in an LD₅₀ study, we saw a 1·7-fold increase in the odds of death with each increase in Gosner stage at the time of infection. 5.We then examined the HPI stress response of prometamorphic tadpoles exposed to a lethal dose of ranavirus. Infected tadpoles exhibited significantly elevated corticosterone levels, more rapid developmental changes, and a greater decrease in body weight relative to controls over 6days after exposure. 6.Although elevated corticosterone mobilizes resources and enhances immunity, its acceleration of metamorphosis may be maladaptive in response to ranavirus infection, because it can draw energy away from expensive immune responses. These findings provide insight into how the balance of energy between development and immune function may contribute to patterns of ranavirus infection in pre-metamorphic amphibians.