Seasonality in Competence to Transmit West Nile Virus for a Widespread Reservoir

All organisms face a central conflict; a limited supply of resources must beoptimally allocated to competing biological functions. Utilizing robust immune defenses can be energetically costly;, thus, immune function is often diminished when other processes, such as reproduction, molt, or migration demand energy. As wild animals residing in temporally dynamic environments face seasonal patterns in resource availability, infectious disease risk, and environmental suitability, many species have evolved endogenous biological rhythms so that the performance of reproduction, molt, or migration and the associated trade-off with immune function be completed when doing so best promotes fitness. In turn, this demand to optimize physiology to intra-annually variable conditions should give rise to wild animals varying seasonally in host competence, or the capacity to transmit a pathogen to others. Seasonality in host competence could have broad implications for zoonotic disease spillover. However, few studies have characterized how zoonotic disease reservoirs vary seasonally in traits that directly reflect competence. Here, I performed a series of experimental West Nile virus (WNV) infections in a geographically widespread host, the house sparrow (HOSP; Passer domesticus), to determine whether endogenous biological rhythms are generating intra-annual variation in host competence over the course of the year. This work revealed that HOSP do indeed vary seasonally in competence, maintaining infectious WNV viral titers 51% longer in the fall when compared to other times of the year. This prolonged infectious period was the result of a tendency for fall HOSP to possess higher WNV titers early in infection while tending to survive longer when compared to conspecifics at other times of the year. This period of increased competence in the fall may be the result of a delayed-tradeoff with molt, as the accumulation of the energetic demands imposed by molting and the inability to compensate for these demands by providing sufficient nutrients for a robust immune response leads to altered host-pathogen dynamics. Overall, the realization that HOSP are more competent during the peak period of WNV transmission suggests that zoonotic disease reservoir seasonality is important to consider when attempting to predict and prevent spillover. Further efforts to describe seasonality for other reservoir species in different environmental contexts could provide useful insights into zoonotic disease management