Phylogenetic analysis of Buggy Creek virus: evidence for multiple clades in the Western Great Plains, United States of America

Detection of Usutu virus in a house martin bug Oeciacus hirundinis (Hemiptera: Cimicidae): implications for virus overwintering in a temperate zone

The family Cimicidae comprises ectoparasites feeding exclusively on the blood of endothermic animals. Cimicid swallow bugs specifically target swallow birds (Hirundinidae) and their nestlings in infested nests. Bugs of the genus Oeciacus are commonly found in mud nests of swallows and martins, while they rarely visit the homes of humans. Although-unlike other cimicid species-the house martin bug Oeciacus hirundinis has never been reported as a vector of zoonotic pathogens, its possible role in arbovirus circulation in continental Europe is unclear. Samples of O. hirundinis were therefore collected from abandoned house martin (Delichon urbicum) nests in southern Moravia (Czech Republic) during the 2021/2022 winter season and checked for alpha-, flavi- and bunyaviruses by RT-PCR. Of a total of 96 pools consisting of three adult bugs each, one pool tested positive for Usutu virus (USUV)-RNA. Phylogenetic analysis showed that the virus strain was closely related to Italian and some Central European strains and corresponded to USUV lineage 5. The detection of USUV in O. hirundinis during wintertime in the absence of swallows raises the question for a possible role of this avian ectoparasite in virus overwintering in Europe.

Alphavirus Infection: Host Cell Shut-Off and Inhibition of Antiviral Responses

Alphaviruses cause debilitating disease in humans and animals and are transmitted by blood-feeding arthropods, typically mosquitoes. With a traditional focus on two models, Sindbis virus and Semliki Forest virus, alphavirus research has significantly intensified in the last decade partly due to the re-emergence and dramatic expansion of chikungunya virus in Asia, Europe, and the Americas. As a consequence, alphavirus–host interactions are now understood in much more molecular detail, and important novel mechanisms have been elucidated. It has become clear that alphaviruses not only cause a general host shut-off in infected vertebrate cells, but also specifically suppress different host antiviral pathways using their viral nonstructural proteins, nsP2 and nsP3. Here we review the current state of the art of alphavirus host cell shut-off of viral transcription and translation, and describe recent insights in viral subversion of interferon induction and signaling, the unfolded protein response, and stress granule assembly.

Evolutionary genetics and vector adaptation of recombinant viruses of the western equine encephalitis antigenic complex provides new insights into alphavirus diversity and host switching

Western equine encephalitis virus (WEEV), Highlands J virus (HJV), and Fort Morgan virus (FMV) are the sole representatives of the WEE antigenic complex of the genus Alphavirus, family Togaviridae, that are endemic to North America. All three viruses have their ancestry in a recombination event involving eastern equine encephalitis virus (EEEV) and a Sindbis (SIN)-like virus that gave rise to a chimeric alphavirus that subsequently diversified into the present-day WEEV, HJV, and FMV. Here, we present a comparative analysis of the genetic, ecological, and evolutionary relationships among these recombinant-origin viruses, including the description of a nsP4 polymerase mutation in FMV that allows it to circumvent the host range barrier to Asian tiger mosquito cells, a vector species that is normally refractory to infection. Notably, we also provide evidence that the recombination event that gave rise to these three WEEV antigenic complex viruses may have occurred in North America.

Immune responses of a native and an invasive bird to Buggy Creek Virus (Togaviridae: Alphavirus) and its arthropod vector, the swallow bug (Oeciacus vicarius)

Invasive species often display different patterns of parasite burden and virulence compared to their native counterparts. These differences may be the result of variability in host-parasite co-evolutionary relationships, the occurrence of novel host-parasite encounters, or possibly innate differences in physiological responses to infection between invasive and native hosts. Here we examine the adaptive, humoral immune responses of a resistant, native bird and a susceptible, invasive bird to an arbovirus (Buggy Creek virus; Togaviridae: Alphavirus) and its ectoparasitic arthropod vector (the swallow bug; Oeciacus vicarius). Swallow bugs parasitize the native, colonially nesting cliff swallow (Petrochelidon pyrrhonota) and the introduced house sparrow (Passer domesticus) that occupies nests in cliff swallow colonies. We measured levels of BCRV-specific and swallow bug-specific IgY levels before nesting (prior to swallow bug exposure) and after nesting (after swallow bug exposure) in house sparrows and cliff swallows in western Nebraska. Levels of BCRV-specific IgY increased significantly following nesting in the house sparrow but not in the cliff swallow. Additionally, house sparrows displayed consistently higher levels of swallow bug-specific antibodies both before and after nesting compared to cliff swallows. The higher levels of BCRV and swallow bug specific antibodies detected in house sparrows may be reflective of significant differences in both antiviral and anti-ectoparasite immune responses that exist between these two avian species. To our knowledge, this is the first study to compare the macro- and microparasite-specific immune responses of an invasive and a native avian host exposed to the same parasites.

Group size and nest spacing affect Buggy Creek virus (Togaviridae: Alphavirus) infection in nestling house sparrows

The transmission of parasites and pathogens among vertebrates often depends on host population size, host species diversity, and the extent of crowding among potential hosts, but little is known about how these variables apply to most vector-borne pathogens such as the arboviruses (arthropod-borne viruses). Buggy Creek virus (BCRV; Togaviridae: Alphavirus) is an RNA arbovirus transmitted by the swallow bug (Oeciacus vicarius) to the cliff swallow (Petrochelidon pyrrhonota) and the introduced house sparrow (Passer domesticus) that has recently invaded swallow nesting colonies. The virus has little impact on cliff swallows, but house sparrows are seriously affected by BCRV. For house sparrows occupying swallow nesting colonies in western Nebraska, USA, the prevalence of BCRV in nestling sparrows increased with sparrow colony size at a site but decreased with the number of cliff swallows present. If one nestling in a nest was infected with the virus, there was a greater likelihood that one or more of its nest-mates would also be infected than nestlings chosen at random. The closer a nest was to another nest containing infected nestlings, the greater the likelihood that some of the nestlings in the focal nest would be BCRV-positive. These results illustrate that BCRV represents a cost of coloniality for a vertebrate host (the house sparrow), perhaps the first such demonstration for an arbovirus, and that virus infection is spatially clustered within nests and within colonies. The decreased incidence of BCRV in sparrows as cliff swallows at a site increased reflects the "dilution effect," in which virus transmission is reduced when a vector switches to feeding on a less competent vertebrate host.

Prevalence of Buggy Creek virus (Togaviridae: Alphavirus) in insect vectors increases over time in the presence of an invasive avian host

Invasive species can disrupt natural disease dynamics by altering pathogen transmission among native hosts and vectors. The relatively recent occupancy of cliff swallow (Petrochelidon pyrrhonota) nesting colonies in western Nebraska by introduced European house sparrows (Passer domesticus) has led to yearly increases in the prevalence of an endemic arbovirus, Buggy Creek virus (BCRV), in its native swallow bug (Oeciacus vicarius) vector at sites containing both the invasive sparrow host and the native swallow host. At sites without the invasive host, no long-term changes in prevalence have occurred. The percentage of BCRV isolates exhibiting cytopathicity in Vero-cell culture assays increased significantly with year at sites with sparrows but not at swallow-only sites, suggesting that the virus is becoming more virulent to vertebrates in the presence of the invasive host. Increased BCRV prevalence in bug vectors at mixed-species colonies may reflect high virus replication rates in house sparrow hosts, resulting in frequent virus transmission between sparrows and swallow bugs. This case represents a rare empirical example of a pathogen effectively switching to an invasive host, documented in the early phases of the host's arrival in a specialized ecosystem and illustrating how an invasive species can promote long-term changes in host-parasite transmission dynamics.

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.

Winter ecology of Buggy Creek virus (Togaviridae, Alphavirus) in the Central Great Plains

A largely unanswered question in the study of arboviruses is the extent to which virus can overwinter in adult vectors during the cold winter months and resume the transmission cycle in summer. Buggy Creek virus (BCRV; Togaviridae, Alphavirus) is an unusual arbovirus that is vectored primarily by the swallow bug (Hemiptera: Cimicidae: Oeciacus vicarius) and amplified by the ectoparasitic bug's main avian hosts, the migratory cliff swallow (Petrochelidon pyrrhonota) and resident house sparrow (Passer domesticus). Bugs are sedentary and overwinter in the swallows' mud nests. We evaluated the prevalence of BCRV and extent of infection in swallow bugs collected at different times in winter (October-early April) in Nebraska and explored other ecological aspects of this virus's overwintering. BCRV was detected in 17% of bug pools sampled in winter. Virus prevalence in bugs in winter at a site was significantly correlated with virus prevalence at that site the previous summer, but winter prevalence did not predict BCRV prevalence there the following summer. Prevalence was higher in bugs taken from house sparrow nests in winter and (in April) at colony sites where sparrows had been present all winter. Virus detected by reverse transcription (RT)-polymerase chain reaction in winter was less cytopathic than in summer, but viral RNA concentrations of samples in winter were not significantly different from those in summer. Both of the BCRV lineages (A, B) overwintered successfully, with lineage A more common at sites with house sparrows and (in contrast to summer) generally more prevalent in winter than lineage B. BCRV's ability to overwinter in its adult vector probably reflects its adaptation to the sedentary, long-lived bug and the ecology of the cliff swallow and swallow bug host-parasite system. Its overwintering mechanisms may provide insight into those of other alphaviruses of public health significance for which such mechanisms are poorly known.

Factors associated with mosquito pool positivity and the characterization of the West Nile viruses found within Louisiana during 2007

Background

West Nile virus (WNV) is an arbovirus of public health importance in the genus Flavivirus, a group of positive sense RNA viruses. The NS3 gene has a high level of substitutions and is phylogenetically informative. Likewise, substitutions in the envelope region have been postulated to enable viruses to subvert immune responses. Analysis of these genes among isolates from positive mosquitoes collected in Louisiana illustrates the variation present in the regions and provides improved insight to a phylogenetic model. Employing a GIS eco-regionalization method, we hypothesized that WNV pool positivity was correlated with regional environmental characteristics. Further, we postulated that the phylogenetic delineations would be associated with variations in regional environmental conditions.

Results

Type of regional land cover was a significant effect (p < 0.0001) in the positive pool prediction, indicating that there is an ecological component driving WNV activity. Additionally, month of collection was significant (p < 0.0001); and thus there is a temporal component that contributes to the probability of getting a positive mosquito pool. All virus isolates are of the WNV 2002 lineage. There appears to be some diversity within both forested and wetland areas; and the possibility of a distinct clade in the wetland samples.

Conclusions

The phylogenetic analysis shows that there has been no reversion in Louisiana from the 2002 lineage which replaced the originally introduced strain. Our pool positivity model serves as a basis for future testing, and could direct mosquito control and surveillance efforts. Understanding how land cover and regional ecology effects mosquito pool positivity will greatly help focus mosquito abatement efforts. This would especially help in areas where abatement programs are limited due to either funding or man power. Moreover, understanding how regional environments drive phylogenetic variation will lead to a greater understanding of the interactions between ecology and disease prevalence.

Natural infection of vertebrate hosts by different lineages of Buggy Creek virus (family Togaviridae, genus Alphavirus)

Buggy Creek virus (BCRV; family Togaviridae, genus Alphavirus) is an arbovirus transmitted by the ectoparasitic swallow bug (Hemiptera: Cimicidae: Oeciacus vicarius) to cliff swallows (Petrochelidon pyrrhonota) and house sparrows (Passer domesticus). BCRV occurs in two lineages (A and B) that are sympatric in bird nesting colonies in the central Great Plains, USA. Previous work on lineages isolated exclusively from swallow bugs suggested that lineage A relies on amplification by avian hosts, in contrast to lineage B, which is maintained mostly among bugs. We report the first data on the BCRV lineages isolated from vertebrate hosts under natural conditions. Lineage A was overrepresented among isolates from nestling house sparrows, relative to the proportions of the two lineages found in unfed bug vectors at the same site at the start of the summer transmission season. Haplotype diversity of each lineage was higher in bugs than in sparrows, indicating reduced genetic diversity of virus amplified in the vertebrate host. BCRV appears to have diverged into two lineages based on different modes of transmission.

Ecological divergence of two sympatric lineages of Buggy Creek virus, an arbovirus associated with birds

Most arthropod-borne viruses (arboviruses) show distinct serological subtypes or evolutionary lineages, with the evolution of different strains often assumed to reflect differences in ecological selection pressures. Buggy Creek virus (BCRV) is an unusual RNA virus (Togaviridae, Alphavirus) that is associated primarily with a cimicid swallow bug (Oeciacus vicarius) as its vector and the Cliff Swallow (Petrochelidon pyrrhonota) and the introduced House Sparrow (Passer domesticus) as its amplifying hosts. There are two sympatric lineages of BCRV (lineages A and B) that differ from each other by > 6% at the nucleotide level. Analysis of 385 BCRV isolates all collected from bug vectors at a study site in southwestern Nebraska, USA, showed that the lineages differed in their peak times of seasonal occurrence within a summer. Lineage A was more likely to be found at recently established colonies, at those in culverts (rather than on highway bridges), and at those with invasive House Sparrows, and in bugs on the outsides of nests. Genetic diversity of lineage A increased with bird colony size and at sites with House Sparrows, while that of lineage B decreased with colony size and was unaffected by House Sparrows. Lineage A was more cytopathic on mammalian cells than was lineage B. These two lineages have apparently diverged in their transmission dynamics, with lineage A possibly more dependent on birds and lineage B perhaps more a bug virus. The long-standing association between Cliff Swallows and BCRV may have selected for immunological resistance to the virus by swallows and thus promoted the evolution of the more bug-adapted lineage B. In contrast, the recent arrival of the introduced House Sparrow and its high competence as a BCRV amplifying host may be favoring the more bird-dependent lineage A.

Stone Lakes virus (family Togaviridae, genus Alphavirus), a variant of Fort Morgan virus isolated from swallow bugs (Hemiptera: Cimicidae) west of the Continental Divide

Multiple isolates of an alphaviruses within the western equine encephalomyelitis-serocomplex that were related closely to Ft. Morgan and its variant Buggy Creek virus were made from swallow bugs, Oeciacus vicarius Horvath (Hemiptera: Cimicidae), collected from cliff swallow (Petrochelidon pyrrhonota) nests at the Stone Lakes National Wildlife Refuge, Sacramento County, CA, during the summers of 2005 and 2006. This virus (hereafter Stone Lakes virus, family Togaviridae, genus Alphavirus, STLV) was the first record of this viral group west of the Continental Divide. STLV replicated well in Vero and other vertebrate cell cultures but failed to replicate in C6/36 cells or infect Culex tarsalis Coquillett mosquitoes. STLV failed to produce elevated viremias in adult chickens or house sparrows and was weakly immunogenic. In addition, STLV was not isolated from cliff swallow nestlings nor was antibody detected in adults collected at mist nets. We suggest that STL and related swallow bug viruses may be primarily infections of cimicids that are maintained and amplified either by vertical or nonviremic transmission and that cliff swallows may primarily be important as a bloodmeal source for the bugs rather than as an amplification host for the viruses.

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.

Overwintering of infectious Buggy Creek virus (Togaviridae: Alphavirus) in Oeciacus vicarius (Hemiptera: Cimicidae) in North Dakota

Arboviruses have seldom been found overwintering in adult vectors at northern latitudes in North America. Buggy Creek virus (BCRV; Togaviridae, Alphavirus) is an ecologically unusual arbovirus vectored principally by the cimicid swallow bug (Oeciacus vicarius Horvath). The ectoparasitic bugs reside year-round in the mud nests of their host, the cliff swallow (Petrochelidon pyrrhonota Vieillot). We report successful overwintering of infectious BCRV in bugs at a field site in western North Dakota, where mid-winter temperatures routinely reach -11 to -15 degrees C. Approximately 21% of bug pools were positive for virus in early spring just before the cliff swallows' return to their nesting colonies; this proportion did not differ significantly from that in summer at active cliff swallow nesting colonies in the same study area. Fewer of the isolates in early spring were cytopathic on Vero cells, and those that were infectious showed less plaque formation than did summer samples. The results show that infectious BCRV commonly overwinters in the adult stages of its vector at northern latitudes in North America.

Phylogeographical structure and evolutionary history of two Buggy Creek virus lineages in the western Great Plains of North America

Buggy Creek virus (BCRV) is an unusual arbovirus within the western equine encephalitis complex of alphaviruses. Associated with cimicid swallow bugs (Oeciacus vicarius) as its vector and the cliff swallow (Petrochelidon pyrrhonota) and house sparrow (Passer domesticus) as its amplifying hosts, this virus is found primarily in the western Great Plains of North America at spatially discrete swallow nesting colonies. For 342 isolates collected in Oklahoma, Nebraska, Colorado and North Dakota, from 1974 to 2007, we sequenced a 2076 bp region of the 26S subgenomic RNA structural glycoprotein coding region, and analysed phylogenetic relationships, rates of evolution, demographical histories and temporal genetic structure of the two BCRV lineages found in the Great Plains. The two lineages showed distinct phylogeographical structure: one lineage was found in the southern Great Plains and the other in the northern Great Plains, and both occurred in Nebraska and Colorado. Within each lineage, there was additional latitudinal division into three distinct sublineages. One lineage is showing a long-term population decline. In comparing sequences taken from the same sites 8-30 years apart, in one case one lineage had been replaced by the other, and in the other cases there was little evidence of the same haplotypes persisting over time. The evolutionary rate of BCRV is in the order of 1.6-3.6x10(-4) substitutions per site per year, similar to that estimated for other temperate-latitude alphaviruses. The phylogeography and evolution of BCRV could be better understood once we determine the nature of the ecological differences between the lineages.

Host and vector movement affects genetic diversity and spatial structure of Buggy Creek virus (Togaviridae)

Determining the degree of genetic variability and spatial structure of arthropod-borne viruses (arboviruses) may help in identifying where strains that potentially cause epidemics or epizootics occur. Genetic diversity in arboviruses is assumed to reflect relative mobility of their vertebrate hosts (and invertebrate vectors), with highly mobile hosts such as birds leading to genetic similarity of viruses over large areas. There are no empirical studies that have directly related host or vector movement to virus genetic diversity and spatial structure. Using the entire E2 glycoprotein-coding region of 377 Buggy Creek virus isolates taken from cimicid swallow bugs (Oeciacus vicarius), the principal invertebrate vector for this virus, we show that genetic diversity between sampling sites could be predicted by the extent of movement by transient cliff swallows (Petrochelidon pyrrhonota) between nesting colonies where the virus and vectors occur. Pairwise F(ST) values between colony sites declined significantly with increasing likelihood of a swallow moving between those sites per 2-day interval during the summer nesting season. Sites with more bird movement between them had virus more similar genetically than did pairs of sites with limited or no bird movement. For one virus lineage, Buggy Creek virus showed greater haplotype and nucleotide diversity at sites that had high probabilities of birds moving into or through them during the summer; these sites likely accumulated haplotypes by virtue of frequent virus introductions by birds. Cliff swallows probably move Buggy Creek virus by transporting infected bugs on their feet. The results provide the first empirical demonstration that genetic structure of an arbovirus is strongly associated with host/vector movement, and suggest caution in assuming that bird-dispersed arboviruses always have low genetic differentiation across different sites.

Bird Movement Predicts Buggy Creek Virus Infection in Insect Vectors

Predicting the spatial foci of zoonotic diseases is a major challenge for epidemiologists and disease ecologists. Migratory birds are often thought to be responsible for introducing some aviozoonotic pathogens such as West Nile and avian influenza viruses to a local area, but most information on how bird movement correlates with virus prevalence is anecdotal or indirect. We report that the prevalence of Buggy Creek virus (BCRV) infection in cimicid swallow bugs (Oeciacus vicarius), the principal invertebrate vector for this virus, was directly associated with the likelihood of movement by cliff swallows (Petrochelidon pyrrhonota), an amplifying host for the virus, between nesting colonies. The prevalence of BCRV in bugs was also directly correlated with the number of swallows immigrating into a site. Birds that move into a site are often transient individuals that may have more often encountered virus elsewhere. These results indicate that the magnitude and direction of daily bird movement in a local area can accurately predict transmission foci for this virus and provide rare quantitative evidence that birds can play a critical role in the dispersal of certain vector-borne viruses.

Ecological correlates of buggy creek virus infection in Oeciacus vicarius, southwestern Nebraska, 2004

Buggy Creek virus (family Togaviridae, genus Alphavirus, BCRV) is an alphavirus within the western equine encephalitis virus complex whose primary vector is the swallow bug, Oeciacus vicarius Horvath (Hemiptera: Cimicidae), an ectoparasite of the colonially nesting cliff swallow, Petrochelidon pyrrhonota, that is also a frequent host for the virus. We investigated ecological correlates of BCRV infection in 100-bug pools at 14 different swallow colony sites in southwestern Nebraska from summer 2004, by using plaque assay on Vero cells to identify cytopathic virus and reverse transcription-polymerase chain reaction to identify noncytopathic viral RNA. We found 26.7% of swallow bug pools positive for BCRV, with 15.6% showing cytopathic ("infectious") virus and 11.0% noncytopathic ("noninfectious") viral RNA. The prevalence of cytopathic BCRV increased with cliff swallow colony size in the current year; the percentage of noncytopathic samples at a site did not vary with colony size in the current year but increased with the previous year's colony size at a site. Active colony sites (those used by swallows) had higher percentages of cytopathic BCRV in bug pools than at inactive colony sites, but the reverse held for noncytopathic viral RNA. Nests that were occupied by birds at some time in the season had more pools with cytopathic BCRV than did inactive nests. Colonies used by birds for the first or second time had less virus in bugs than did sites that had had a longer history of bird use. The percentage of pools with BCRV was affected by whether bugs were clustering at nest entrances or distributed elsewhere on a nest. The prevalence of cytopathic samples decreased at inactive colony sites and increased at active sites over the course of the summer, whereas the reverse pattern held for noncytopathic samples. Noncytopathic bug pools seem to reflect infection patterns from a previous year. The results suggest that the birds play an important role in amplification of the virus and that the spatial foci of BCRV occurrence can be predicted based on characteristics of cliff swallow colonies and the cimicid bugs that are associated with them.