Parasite Infections Influence Immunological Responses But Not Reproductive Success of Male Hellbender Salamanders (<i>Cryptobranchus alleganiensis</i>)

The emergence and spread of infectious diseases is a significant contributor to global amphibian declines, requiring increased surveillance and research. We assessed host-vector-parasite dynamics using a population of eastern hellbender salamanders (<i>Cryptobranchus alleganiensis</i>) that harbor leeches (<i>Placobdella appalachiensis</i>) that transmit endoparasitic blood parasites (<i>Trypanosoma</i> spp) to the host, with coinfection frequently occurring. We centered our study on adult males throughout their extended 8-month paternal care period because recent research indicates that nest failure caused by lack of paternal care and filial cannibalism is contributing to hellbender population declines. Recognizing the potential for parasites to modulate host physiology and behavior, we explored how infection severity influences paternal health and reproductive success. We assessed white blood cell profiles of adult male hellbenders in response to parasites, coinfection, and seasonal temperature fluctuations, while also investigating whether parasite infection or coinfection was predictive of nest success. We found that hellbenders exhibited seasonal shifts in white blood cell indices; as temperatures increased across seasons (from 5°C to 20°C), the proportion of neutrophils and eosinophils decreased (by 14% and 46%, respectively) in circulation while the proportion of lymphocytes and basophils increased (by 8% and 101%, respectively). Moreover, the proportion of neutrophil precursors increased by 80% under colder temperatures, which signifies seasonal immune cell recruitment. We demonstrated that neutrophils and eosinophils increased while lymphocytes decreased in response to leech infection. However, as leech and trypanosome infection intensity increased together, the proportion of lymphocytes increased while neutrophils and eosinophils decreased, underscoring the complex interactions between coinfection and immune responses of hellbenders that warrant future research. Despite the influence of infection and coinfection on hellbender physiology, we detected no evidence to support the hypothesis that parasites influence the likelihood of nest failure or whole-clutch filial cannibalism. In light of amphibian declines being exacerbated by climate change and disease, our study emphasizes the need to establish hematological reference values that account for physiological adaptations to seasonal fluctuations in temperature and different life history stages and to study the physiological responses of imperiled amphibian species to parasites.