An enzootic vector-borne virus is amplified at epizootic levels by an invasive avian host

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 BCRV epizootics 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.

Isolation of Buggy Creek virus (Togaviridae: Alphavirus) from field-collected eggs of Oeciacus vicarius (Hemiptera: Cimicidae)

Alphaviruses (Togaviridae) rarely have been found to be vertically transmitted from female arthropods to their progeny. We report two isolations of Buggy Creek virus (BCRV), an ecologically unusual alphavirus related to western equine encephalomyelitis virus, from field-collected eggs of cimicid swallow bugs (Oeciacus vicarius Horvath), the principal vector for BCRV. Ten percent of egg pools were positive for BCRV, and we estimated minimum infection rates to be 1.03 infected eggs per 1,000 tested. The results show potential vertical transmission of BCRV, represent one of the few isolations of any alphavirus from eggs or larvae of insects in the field, and are the first report of any virus in the eggs of cimicid bedbugs. The specialized ecological niche of BCRV in swallow bugs and at cliff swallow (Petrochelidon pyrrhonota Vieillot) nesting sites may promote vertical transmission of this virus.

Persistence of Buggy Creek virus (Togaviridae, Alphavirus) for two years in unfed swallow bugs (Hemiptera: Cimicidae: Oeciacus vicarius)

Alphaviruses (Togaviridae) have rarely been found to persist for long in the adult insects that serve as their vectors. The ectoparasitic swallow bug (Hemiptera: Cimicidae: Oeciacus vicarius Horvath), the vector for Buggy Creek virus (BCRV; Togaviridae, Alphavirus), lives year-round in the mud nests of its host, the cliff swallow (Petrochelidon pyrrhonota Vieillot). We measured the prevalence of BCRV in swallow bugs at sites with cliff swallows present and at the same sites after cliff swallows had been absent for 2 yr. We collected bugs directly from cliff swallow nests in the field and screened bug pools with BCRV-specific real-time-polymerase chain reaction (RT-PCR) and plaque assay. At two colony sites last occupied by birds 2 yr earlier, we found 12.5 and 55.6% of bug pools positive for BCRV RNA by RT-PCR. Infection rates (per 1,000 bugs) for these sites were 1.32 and 7.39. RNA prevalence in the unfed bugs was not significantly different from that in fed bugs 2 yr earlier at the same sites. The RNA-positive samples from unfed bugs failed to yield cytopathic BCRV by Vero-cell plaque assay. However, viral RNA concentrations did not differ between unfed bugs and bugs at active sites, and over 84% of positive bug pools were cytopathic to Vero cells 4-5 wk later, after cliff swallows moved into one of the colony sites. These data demonstrate the persistence of potentially infectious BCRV in unfed swallow bugs for at least 2 yr in nature.