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Seasonality in infectious disease prevalence is predominantly attributed to changes in exogenous risk factors. For vectored pathogens, high abundance, activity, and/or diversity of vectors can exacerbate disease risk for hosts. Conversely, many host defenses, particularly immune responses, are seasonally variable. Seasonality in host defenses has been attributed, in part, to the proximate (i.e., metabolic) and ultimate (i.e., reproductive fitness) costs of defense. In this study, our goal was to discern whether any seasonality is observable in how a common avian host, the house sparrow (Passer domesticus), copes with a common zoonotic arbovirus, the West Nile virus (WNV), when hosts are studied under controlled conditions. We hypothesized that if host biorhythms play a role in vector-borne disease seasonality, birds would be most vulnerable to WNV when breeding and/or molting (i.e., when other costly physiological activities are underway) and thus most transmissive of WNV at these times of year (unless birds died from infection). Overall, the results only partly supported our hypothesis. Birds were most transmissive of WNV in fall (after their molt is complete and when WNV is most prevalent in the environment), but WNV resistance, WNV tolerance, and WNV-dependent mortality did not vary among seasons. These results collectively imply that natural arboviral cycles could be partially underpinned by endogenous physiological changes in hosts. However, other disease systems warrant study, as this result could be specific to the nonnative and highly commensal nature of the house sparrow or a consequence of the relative recency of the arrival of WNV to the United States. [ABSTRACT FROM AUTHOR]

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The extent to which host group size affects the hosts' exposure to parasites and pathogens has been explored by behavioral ecologists for almost 50 years, and we know that host and parasite taxa, mobility of host and parasite, and the extent of spatial structure within groups all affect the group-size relationship. Here we examine how the prevalence of an arthropod-borne viral pathogen changes with host group size in a host-parasite system recently invaded by an introduced species that also serves as a host for the virus. Infection by the alphavirus, Buggy Creek virus (BCRV), in swallow bugs (Cimex vicarius) increased with colony size of the cliff swallow (Petrochelidon pyrrhonota; the bugs' principal host) in the absence of invasive house sparrows (Passer domesticus) but decreased with swallow colony size in the presence of sparrows. The sparrow-adapted lineage A of BCRV declined to near extinction in the largest cliff swallow colonies, regardless of sparrow presence. The results may reflect BCRV's divergence into a lineage (B) that amplifies mostly in bugs and thus is transmitted more effectively in large cliff swallow colonies that maintain high numbers of the blood-feeding bugs, whereas the other lineage (A) is adapted to house sparrows and does not require large numbers of bugs and cliff swallows for effective transmission and persistence. The results show that an alternative host can modify the group-size consequences for the original host and illustrate another complexity in analyzing the costs and benefits of coloniality whenever invasive species are present. Significance statement: Predicting the spread of viral pathogens can be important in assessing threats to wildlife or humans, and the size of a host's social group is regarded as a potentially important determinant of pathogen exposure. Invasive species can sometimes profoundly alter disease dynamics when they enter a host-parasite system and serve as alternative hosts, thereby either increasing or decreasing the original host's exposure to pathogens. For an arthropod-borne virus that is vectored by a blood-feeding bug that parasitizes a colonially breeding bird, we found that virus infection in bugs increased with host (bird) group size when an invasive bird species was absent but decreased with host group size when the invasive was present. The study indicates that the presence of alternative hosts is another variable to consider in trying to predict how pathogen exposure is affected by host group size. [ABSTRACT FROM AUTHOR]

Many of the arthropod-borne viruses (arboviruses) show extensive genetic variability and are widely distributed over large geographic areas. Understanding how virus genetic structure varies in space may yield insight into how these pathogens are adapted to and dispersed by different hosts or vectors, the relative importance of mutation, drift, or selection in generating genetic variability, and where and when epidemics or epizootics are most likely to occur. However, because most arboviruses tend to be sampled opportunistically and often cannot be isolated in large numbers at a given locale, surprisingly little is known about their spatial genetic structure on the local scale at which host/vector/virus interactions typically occur. Here, we examine fine-scale spatial structure of two sympatric lineages of Buggy Creek virus (BCRV, Togaviridae), an alphavirus transmitted by the ectoparasitic swallow bug ( Oeciacus vicarius) to colonially nesting cliff swallows ( Petrochelidon pyrrhonota) and invasive house sparrows ( Passer domesticus) in North America. Data from 377 BCRV isolates at cliff swallow colony sites in western Nebraska showed that both virus lineages were geographically structured. Most haplotypes were detected at a single colony or were shared among nearby colonies, and pair-wise genetic distance increased significantly with geographic distance between colony sites. Genetic structure of both lineages is consistent with isolation by distance. Sites with the most genetically distinct BCRV isolates were occupied by large numbers of house sparrows, suggesting that concentrations of invasive sparrows may represent foci for evolutionary change in BCRV. Our results show that bird-associated arboviruses can show genetic substructure over short geographic distances. [ABSTRACT FROM AUTHOR]

Determining the effect of an invasive species on enzootic pathogen dynamics is critical for understanding both human epidemics and wildlife epizootics. Theoretical models suggest that when a naive species enters an established host-parasite system, the new host may either reduce ('dilute') or increase ('spillback') pathogen transmission to native hosts. There are few empirical data to evaluate these possibilities, especially for animal pathogens. Buggy Creek virus (BCRV) is an arthropod-borne alphavirus that is enzootically transmitted by the swallow bug (Oeciacus vicarius) to colonially nesting cliff swallows (Petrochelidon pyrrhonota). In western Nebraska, introduced house sparrows (Passer domesticus) invaded cliff swallow colonies approximately 40 years ago and were exposed to BCRV. We evaluated how the addition of house sparrows to this host-parasite system affected the prevalence and amplification of a bird-associated BCRV lineage. The infection prevalence in house sparrows was eight times that of cliff swallows. Nestling house sparrows in mixed-species colonies were significantly less likely to be infected than sparrows in single-species colonies. Infected house sparrows circulated BCRV at higher viraemia titres than cliff swallows. BCRV detected in bug vectors at a site was positively associated with virus prevalence in house sparrows but not with virus prevalence in cliff swallows. The addition of a highly susceptible invasive host species has led to perennial BCRVepizootics at cliff swallow colony sites. The native cliff swallow host confers a dilution advantage to invasive sparrow hosts in mixed colonies, while at the same sites house sparrows may increase the likelihood that swallows become infected. [ABSTRACT FROM AUTHOR]