Portal de Periódicos da CAPES

Mosquitoes and West Nile virus along a river corridor from prairie to montane habitats in eastern Colorado

2009; Wiley; Volume: 34; Issue: 2 Linguagem: Inglês

10.1111/j.1948-7134.2009.00036.x

1948-7134

Christopher M. Barker, Bethany G. Bolling, William C. Black, Chester G. Moore, Lars Eisen,

Insect symbiosis and bacterial influences

We conducted studies on mosquitoes and West Nile virus (WNV) along a riparian corridor following the South Platte River and Big Thompson River in northeastern Colorado and extending from an elevation of 1,215 m in the prairie landscape of the eastern Colorado plains to 1,840 m in low montane areas at the eastern edge of the Rocky Mountains in the central part of the state. Mosquito collection during June-September 2007 in 20 sites along this riparian corridor yielded a total of 199,833 identifiable mosquitoes of 17 species. The most commonly collected mosquitoes were, in descending order: Aedes vexans, Culex tarsalis, Ae. dorsalis, Ae. trivittatus, Ae. melanimon, Cx. pipiens, and Culiseta inornata. Species richness was higher in the plains than in foothills-montane areas, and abundances of several individual species, including the WNV vectors Cx. tarsalis and Cx. pipiens and the nuisance-biter and potential secondary WNV vector Ae. vexans, decreased dramatically from the plains (1,215-1,487 m) to foothills-montane areas (1,524-1,840 m). Ae. vexans and Cx. tarsalis had a striking pattern of uniformly high abundances between 1,200-1,450 m followed by a gradual decrease in abundance above 1,450 m to reach very low numbers above 1,550 m. Culex species were commonly infected with WNV in the plains portion of the riparian corridor in 2007, with 14 of 16 sites yielding WNV-infected Cx. tarsalis and infection rates for Cx. tarsalis females exceeding 2.0 per 1,000 individuals in ten of the sites. The Vector Index for abundance of WNV-infected Cx. tarsalis females during June-September exceeded 0.5 in six plains sites along the South Platte River but was uniformly low (0–0.1) in plains, foothills and montane sites above 1,500 m along the Big Thompson River. A population genetic analysis of Cx. tarsalis revealed that all collections from the ≈190 km riparian transect in northeastern Colorado were genetically uniform but that these collections were genetically distinct from collections from Delta County on the western slope of the Continental Divide. This suggests that major waterways in the Great Plains serve as important dispersal corridors for Cx. tarsalis but that the Continental Divide is a formidable barrier to this WNV vector. Northeastern Colorado emerged as a high-risk area for West Nile virus (WNV) disease after the introduction of WNV to the state in 2002. Human WNV disease incidence for counties in the eastern part of the state reached 151–517 and 33–65 cases per 100,000 person-years during the epidemic years of 2003 and 2007, respectively (calculated from data at http://diseasemaps.usgs.gov/wnv_historical.html). Furthermore, the three-county Boulder-Larimer-Weld area alone reported 1,903 WNV disease cases during 2003–2007 (http://www.cdphe.state.co.us/dc/zoonosis/wnv/), which accounts for 8.3% of 22,861 total reported U.S. cases in those years (http://www.cdc.gov/ncidod/dvbid/westnile/). This sparked a renewed interest in mosquitoes and mosquito-borne pathogens in Colorado, especially in the primary WNV disease focus in the northern Front Range and northeastern plains. Initial studies on potential mosquito WNV vectors focused on population centers in Larimer County, Fort Collins and Loveland, which were severely affected by the massive 2003 WNV disease outbreak. These Front Range cities are located at the western edge of the Great Plains at the foot of the Rocky Mountains. Studies conducted from 2003–2004 revealed that Culex tarsalis and Cx. pipiens are important local WNV vectors (Bolling et al. 2007, Gujral et al. 2007), which agrees with other studies in the western U.S. (e.g., Reisen et al. 2004, 2008, 2009; Bell et al. 2005; DiMenna et al. 2006, 2007; Nielsen et al. 2008; Kent et al. 2009). The study by Bolling et al. (2007) also revealed that the highly abundant human-biter Aedes vexans occasionally is infected with WNV in the Front Range. In 2006, we shifted our attention to the eastern slope of the Rocky Mountains to determine risk of exposure to Culex vectors in foothills and montane habitats which harbor large tracts of public land and are heavily used for recreational activities in the summer months. This showed that Cx. tarsalis and Cx. pipiens are abundant in the plains landscape at the foot of the Rocky Mountains but that their abundance decreases dramatically as one moves into montane habitats at higher elevations (Eisen et al. 2008, Winters et al. 2008). We also recorded dramatic changes in mosquito species richness, composition, and abundance along elevation gradients (1,500-2,400 m) in western Larimer County (Eisen et al. 2008). Our results indicate that the Continental Divide represents a formidable barrier to dispersal of Cx. tarsalis and Cx. pipiens from the plains in eastern Colorado to the mountain plateau in the western part of the state. The current study is, in part, a follow-up to the previous work along the 1,500-2,400 m high-elevation gradient where we now instead focus on a lower-elevation gradient ranging from 1,215-1,840 m and extending from prairie landscapes deep in the northeastern Colorado plains to low montane areas in the Rocky Mountains. The primary aims of the study were to: 1) describe patterns of mosquito species composition and abundance along an ≈190 km riparian elevation/habitat transect in eastern Colorado; 2) determine WNV infection rates and the Vector Index for Cx. tarsalis and Cx. pipiens along this transect; and 3) examine gene flow among Cx. tarsalis collections along the Big Thompson-South Platte river corridor compared to Delta County in western Colorado on the other side of the Continental Divide. Mosquito trapping was conducted from late June to early September 2007 along a riparian corridor following the Big Thompson River and the South Platte River in northeastern Colorado and including an elevation gradient from 1,215 – 1,840 m. In Colorado, elevation can change dramatically over short distances and is a convenient surrogate for many variables, such as temperature and land cover, that affect mosquito abundance and WNV transmission. We focused our collection efforts on the riparian habitat because it is a likely location for host-seeking, resting, and dispersal of adult mosquitoes that emerge from flooded riparian margins, adjacent suburban areas, and the extensive agricultural land that surrounds much of the riparian corridor. The Big Thompson River emerges from the Rocky Mountains in southeastern Larimer County and then merges with the South Platte River in Weld County as the South Platte crosses the northeastern Colorado plains (Figure 1). The sampling effort included 20 mosquito collection sites along the riparian corridor and spanned five counties: Larimer, Weld, Morgan, Washington, and Logan (Figure 1). In the plains, the rivers typically are bordered by a narrow band of forested riparian wetland, dominated by cottonwood (Populus spp.) and willow (Salix spp.), which in turn is commonly surrounded by irrigated agricultural land. In montane habitats, the Big Thompson River flows through a canyon landscape dominated by grass, shrub, conifers (primarily Ponderosa pine, Pinus ponderosa), and aspen (Populus tremuloides). Sampling site locations were mapped with a GPS receiver (Trimble Geo XT; Trimble Corp., Sunnyvale, CA) and visualized using ArcGIS 9.3 (ESRI, Redlands, CA). A) Mosquito sampling sites (circles with three-letter codes), population centers (in dark gray with names in italics), and selected major rivers (Big Thompson River and South Platte River) in the five-county study area. Forested riparian wetland is shown in medium gray (appearing as a narrow band along waterways) and irrigated agricultural land in light gray. The inset map shows the location of the five study counties in the northeast and of Delta County to the west on the other side of the Continental Divide; B) Location of eastern collection sites included in the population genetic analysis; C) UPGMA analysis of pairwise linear FST estimates among Cx. tarsalis collections from sites in northeastern Colorado and from Delta County in the west. The climate of the study area is characterized by cold winters and hot summers with low humidity. The average annual rainfall in Fort Collins in Larimer County from 1971–2000 was 393 mm (Mountain States Weather Services, Fort Collins, CO). Geographic coordinates and selected environmental site characteristics are provided in Table 1. Mosquitoes were collected using CO2-baited CDC miniature light traps (John W. Hock Company, Gainesville, FL) suspended ≈1.5 m above the ground and operated from afternoon (15:00-17:00) until morning (08:00-10:00). Each sampling site held two traps baited with ≈1 kg of dry ice and was located directly along the aforementioned rivers. Sampling was conducted on seven occasions per site, every two weeks, from 20 June to 13 September. Collected mosquitoes were examined on a chill table with a dissecting microscope and identified to species (Darsie and Ward 2005). Mosquito collections from 2007 used in gene flow studies included 11 collections along the South Platte River – Big Thompson River corridor, two other collections from the Fort Collins-Loveland area (NEW and FTC), and one collection from Delta County in western Colorado (Figure 1). The Delta County collection, which is separated from the 13 collection locations in the eastern Colorado plains by the high mountains of the Continental Divide, was comprised of mosquitoes collected near the towns of Paonia (collected on June 25th) and Hotchkiss (collected on August 2nd). The two collections from Delta County were combined in all genetic analyses after we confirmed that allele frequencies in the collections were similar. To analyze gene flow among collections of Cx. tarsalis, we examined variation at five microsatellite loci (CutC6, CutC12, CutD113, CutD107, CutD120: Rasgon et al. 2006) and at two single nucleotide polymorphism (SNP) loci (CutAMY162, GenBank accession #U01211 and CutTPI124, L07390). Genotypes at SNP loci were detected using allele-specific PCR. Genotypes were determined in a single-tube PCR using two different "allele-specific" primers, each of which contained a 3′ nucleotide corresponding to one of the two alleles and an opposite primer that amplified both alleles. Allele-specific primers were manufactured (Operon Inc., Huntsville, AL) with 5′ tails designed to allow discrimination between SNP alleles based on size or melting temperature. An intentional transversion mismatch was introduced three bases in from the 3′ end of allele-specific primers to improve specificity and each allele-specific primer differed by a transition at this site. Melting curve PCR was performed as previously described (Urdaneta-Marquez et al. 2008). The primers used were: CutAMY162f:5′-TATGGCYGCGATYAC TGG-3′; CutAMY162Cr:5′-GCGGGCAGGGCGGCGGGGGCGGGGCCTAARGTYTG TGAACTTCCCGTRCAC-3′; CutAMY162Tr:5′-GCGGGCTAARGTYTGTGAACTTC CCGTRTAT-3′; CutTPI2124Cf:5′-GCGGGCAGGGCGGCGGGGGCGGGGCCAAYG CCAGACAGTAAAGACATTGCC-3′; CutTPI2124Tf:5′-GCGGGCAAYGCCAGACAG TAAAGACATTACT-3′; and CutTPI2124r:5′-CGATGACTTATTCCGACCT-3′. Underlined nucleotides are the 5′ tail modifications that allow discrimination between SNP alleles based on melting temperature. All genotypes were entered into Arlequin 3.00 (Excoffier et al. 2005) to estimate linear FST values [FST/(1- FST)] (Slatkin 1993) and to perform an Analysis of Molecular Variance (AMOVA) (Excoffier et al. 1992). Linear FST values were entered into a distance matrix for analysis by NEIGHBOR in PHYLIP3.65 (Felsenstein 2005) which produced an unweighted pair-group method with arithmetic mean (UPGMA) (Sneath and Sokal 1962) analysis of pairwise linear FST values and produced a phylogenetic tree (Figure 1). The same matrix was used in a Mantel analysis to test for a positive correlation between geographic and genetic distances (Mantel 1967) to address the hypothesis that Cx. tarsalis collections along the South Platte River – Big Thompson River corridor were genetically isolated by distance. Arlequin also computed the significance of the variance components associated with each level of genetic structure by a nonparametric permutation test with 100,000 pseudoreplicates. Wright's F-statistics were calculated using the method described by Weir and Cockerham (1984). FIS was calculated to test for conformity to Hardy-Weinberg proportions. When FIS= 0, Hobs (the observed number of heterozygotes) = Hexp (the expected number of heterozygotes assuming Hardy-Weinberg proportions). When Hobs < Hexp then FIS > 0 and when Hobs > Hexp then FIS < 0. The null hypothesis FIS= 0 was tested using the formula (Black and Krafsur 1985): where n is the number of mosquitoes in a collection, pi is the frequency of allele i, and Hexp at a locus with a alleles. Culex mosquitoes were examined for presence of WNV RNA following Bolling et al. (2007) with the modifications outlined below. Mosquitoes were identified on a chill table and placed in pools of one to 50 by species, sex, site, trap, and date. Mosquito pools were then stored at -70° C until processed for viral RNA detection. Each pool was triturated for 45 s with a vortex mixer in a 5-ml round-bottom polypropylene tube (Becton Dickinson, Franklin Lakes, NJ) using 1.5 ml of diluent (1 × minimum essential medium containing 2% fetal bovine serum, 100 μg/ml penicillin/streptomycin, supplemented with L-glutamine and nonessential amino acids) and four copper-coated steel shot (4.5-mm diameter; 0.177″ caliber). Suspensions were then centrifuged at 3,000 rpm for 10 min at 4° C. Total RNA was extracted from 140 μl of the supernatant using the QIAamp viral RNA Mini kit (Qiagen Inc., Valencia, CA). RNA was then eluted in 60 μl of nuclease-free water (Ambion Inc., Austin, TX). Reverse transcription-PCR was used to detect viral RNA in the samples. Mosquito pools were first tested using universal flavivirus primers targeting a portion of the NS5 gene (forward MAMD: 5′-AACATGATGGGRAARAGRGARAA-3′, reverse cFD2: 5′-GTGTCCCAGCCGGCGGTGTCATCAGC-3′) (Scaramozzino et al. 2001). Pools testing positive for flavivirus RNA were then tested for WNV using primers developed and recommended by the CDC for use in WNV surveillance (forward WN212: 5′-TTGTGTTGGCTCTCTTGGCGTTCTT-3′, reverse WN619c: 5′-CAGCCGACAGCACTGGACATTCATA-3′) (Gubler et al. 2000, Lanciotti et al. 2000). PCR products were visualized following electrophoresis on a 1% agarose gel stained with ethidium bromide. Negative (no template) and positive controls were included in each RT-PCR run. Infection rates per 1,000 individuals were calculated as bias-corrected Maximum Likelihood Estimates using the Excel Add-In PooledInfRate, version 3.0 (Biggerstaff 2006). The Vector Index (Gujral et al. 2007) for the mean abundance of WNV-infected Cx. tarsalis or Cx. pipiens females was calculated, for each mosquito sampling site, as the mean number of females per trap night over the June-September sampling period times the overall proportion of WNV-infected females during the same period. To estimate the elevations at which the abundance of mosquito species and the Vector Index declined, non-linear regression models were fitted using a four-parameter logistic model (Ritz and Streibig 2008): where count is the abundance or Vector Index, depending on the quantity being modeled, and e is the elevation at the inflection point of the sigmoidal curve. Other parameters are related to the slope (b) and asymptotes (c and d) of the curve and were not of direct interest for the purposes of this study. These models were fitted using the nls() function in R version 2.7.1 (R Development Core Team; http://www.r-project.org/). For mosquito species that did not follow a sigmoidal abundance pattern with respect to elevation, second-order local polynomial regression models were fitted using the loess() function in R (degree = 2, span = 0.65), resulting in smooth curves that aided in visualization of abundance patterns. Other statistical tests used are indicated in the text. Results were considered significant when P < 0.05. Mosquito collection during 2007 in 20 sites in northeastern Colorado yielded a total of 199,833 identifiable mosquitoes of 17 species: Aedes vexans (66.6% of total collected), Culex tarsalis (19.3%), Aedes dorsalis (7.0%), Aedes trivittatus (3.8%), Aedes melanimon (1.5%), Culex pipiens (1.1%), Culiseta inornata (0.4%), Aedes increpitus (0.3%), Aedes hendersoni (0.1%), Aedes nigromaculis (<0.1%), Anopheles earlei (<0.1%), Culiseta incidens (<0.1%), Coquilletidia perturbans (<0.1%), Anopheles punctipennis (<0.1%), Anopheles hermsi (<0.1%), Culiseta impatiens (<0.1%), and Psorophora signipennis (<0.1%) (Tables 1–3). Mosquito species composition did not differ dramatically along the plains section of the riparian corridor but there were dramatic changes in both species composition and abundance of individual species for the montane sites in the Big Thompson Canyon (NRW, VSP, and IDY; above 1,600 m). In these montane sites, several species that were common in lower elevation sites in the plains were not found (Cx. pipiens, Ae. dorsalis, and Ae. melanimon), and other species were found but in much lower abundance relative to the plains sites (Ae. vexans, Cx. tarsalis, Cs. inornata, Ae. increpitus, and Ae. trivittatus) (Tables 2–4). Another noteworthy collection was a single Cs. impatiens found at the NRW site in the lower part of the Big Thompson Canyon (Table 2); this species previously was recorded from Rocky Mountain National Park at the upper end of the Big Thompson Canyon (Eisen et al. 2008). Anopheles species, Cq. perturbans, Ae. nigromaculis, and Ps. signipennis all were collected in low numbers and only below 1,550 m (Tables 2-3). Mosquito species richness was higher in the plains section of the riparian corridor (n = 16 sites; mean of 9.6 ± 0.3 species) than in the foothills-montane section (n = four sites; mean of 5.8 ± 0.7 species) (Table 1; ANOVA: F1, 18= 27.28, P < 0.001). For mosquito diversity, there was no significant difference between sites located in the plains (mean Shannon–Wiener index of 0.93 ± 0.06) relative to those located in foothills-montane habitat (1.04 ± 0.11) (Table 1; ANOVA: F1, 18= 0.84, P= 0.37). Abundance for individual, commonly collected mosquito species changed dramatically over the elevation/habitat gradient (Table 4). Two species, Ae. vexans and Cx. tarsalis, had a striking pattern of uniformly high abundances between 1,200-1,450 m followed by a gradual decrease in abundance above 1,450 m to reach very low numbers above 1,550 m (inflection points estimated from sigmoidal non-linear regression curves = 1,496 and 1,544 m, respectively; Figure 2). Other common species followed a similar pattern of decreasing abundance with increasing elevation, especially above 1,500 m, but abundances were lower and more variable at elevations below 1,500 m (Figure 3). Cx. pipiens abundance was uneven between 1,200-1,450 m (Figure 3). Other species had distinct peaks in abundance along different parts of the plains portion of the sampling transect. These included peaks at approximately 1,300 m for Ae. increpitus, 1,350 m for Ae. melanimon, and 1,400 m for Cs. inornata (Figure 3). Fitted sigmoidal non-linear regression curves for abundance of Ae. vexans and Cx. tarsalis females, and for the Vector Index for abundance of WNV-infected Cx. tarsalis females, in relation to elevation along the South Platte River - Big Thompson River corridor in northeastern Colorado, June-September 2007. Inflection points for the curves and geometric means at each elevation are indicated by squares for Ae. vexans, diamonds for Cx. tarsalis, and inverted triangles for the Vector Index for Cx. tarsalis females. Geometric mean abundance of Cx. pipiens, Cs. inornata, Ae. increpitus, and Ae. melanimon in relation to elevation along the South Platte River - Big Thompson River corridor in northeastern Colorado, June-September 2007. Curves were fitted using local polynomial regression to aid in visualization of patterns (see Materials and Methods for a more detailed description of this method). Abundance during the trapping period (June-September) was greater in the 16 plains sites than in the four sites in foothills-montane areas for six of the seven most commonly collected species: Ae. vexans, Cs. inornata, Cx. tarsalis, Ae. dorsalis, Ae. melanimon, and Ae. trivittatus (Table 4; Wilcoxon ranked sums test with chi square approximation: χ2≥ 6.38, df = 1, P < 0.02 in all cases), whereas abundance of Ae. increpitus was similar in plains and foothills-montane sites (P= 0.35). In the 16 sites located in the plains portion of the sampling transect, peak numbers of females collected per trap night commonly exceeded 1,000 for Ae. vexans (10/16 sites) and 400 for Cx. tarsalis (10/16 sites) (Table 4). In striking contrast, peak numbers of females collected per trap night for the three sites located in montane habitats in the Big Thompson Canyon did not exceed two for Ae. vexans or 10 for Cx. tarsalis. Culex species were commonly infected with WNV in the plains portion of the riparian corridor in 2007, whereas no infected Culex mosquitoes were collected from foothills-montane habitats above 1,550 m (Table 5). In the plains, 14 of 16 sites produced WNV-infected Cx. tarsalis females and four of 16 sites yielded WNV-infected Cx. pipiens females. Maximum likelihood estimates (MLEs) for infection rates of WNV in Cx. tarsalis females in the plains ranged from 0 to 10.17 per 1,000 females, with infection rates exceeding 1.0 in all 14 sites with infected females and 5.0 in three sites. Infection rates with WNV in Cx. tarsalis females were higher in the 16 plains sites [overall MLE (95% CI) of 2.77 (2.25–3.38) for the plains sites; total of 35,425 tested females] than in the four foothills-montane sites (infection rate of 0.0; total of 101 tested females). For the four sites where WNV-infected Cx. pipiens females were recorded, MLE values for infection rates ranged from 1.69 to 17.36 per 1,000 females. The Vector Index for abundance of WNV-infected Cx. tarsalis females during June-September ranged from 0–1.0 for plains sites along the South Platte River, with the Vector Index exceeding 0.5 in six of the 12 sites and 0.3 in another two sites (Table 5). The Vector Index for Cx. tarsalis females was lower along the eight Big Thompson River sites (range, 0-0.36), especially for the foothills-montane sites (0 in all four cases). In relation to elevation, the Vector Index for Cx. tarsalis females was variable below 1,500 m but uniformly low (0–0.07) above this elevation threshold (Figure 2). The inflection point estimated from the sigmoidal non-linear regression curve was 1,465 m (Figure 2). The Vector Index for abundance of WNV-infected Cx. pipiens females during June-September did not exceed 0.1 in any of the 20 sampling sites (Table 5). The frequency of alleles at the seven loci (CutC6, CutC12, CutD113, CutD107, CutD120, CutAMY162, and CutTPI124) are listed in Table 6. FIS was computed for each locus in each collection. Among the 70 (five loci * 14 collections) analyses for microsatellite loci, FIS= 0 in 90% (63/70). In the seven analyses where FIS≠0, there was a consistent excess of homozygotes (FIS > 0). This is usually attributed to the existence of a null allele at that locus in that collection (Chybicki and Burczyk 2009). In contrast, among the 25 (two loci * 14 collections – three collections in which one allele was fixed) analyses for SNP loci, FIS= 0 in only 36% (9/25). In the 16 analyses where FIS≠0, there was a consistent excess of homozygotes (FIS > 0). To test the hypothesis that the melting curve PCR assay was inaccurate in genotyping, we sequenced two randomly chosen homozygotes in every collection in which FIS > 0. In every case, sequences agreed with the genotype determined by melting curve PCR. However, there was a large amount of variation in the flanking sequences and this could have compromised amplification of one allele in the melting curve PCR assay. This would be manifest as an excess of homozygotes. A molecular analysis of variance (AMOVA) was performed to partition the variation in frequencies into: 1) the variation between the South Platte River and Big Thompson River collections in the eastern plains vs the collection from Delta County on the western side of the Continental Divide; 2) the variation among the South Platte River and Big Thompson River collections; and 3) the variation among mosquitoes in a collection (Table 7). The AMOVA showed that 32% of the overall variation in allele frequencies arose between the collections that were separated by the Continental Divide. Most of this was contributed by variation at the CutAMY162 SNP locus. Only 0.2% of the variation arose among the South Platte River-Big Thompson River collections. Most of this was contributed by variation at the CutD113 SNP locus. We also tested for isolation by distance among the 13 collections along the South Platte River - Big Thompson River corridor in the plains. The Mantel correlation was 0.094 and was not significant (P= 0.29) Finally, the overall pattern of variation revealed by the AMOVA was confirmed by the UPGMA analysis which indicated that the 13 collections of Cx. tarsalis from the eastern Colorado transect along the South Platte River and Big Thompson River were genetically uniform while the collections from Delta County on the western side of the Continental Divide were genetically distinct (Figure 1B-C). Studies on mosquito arbovirus vectors were scarce in the Great Plains in the two decades prior to the recent (2002-2003) introduction of WNV to this region and the subsequent emergence of the Great Plains as a persistent focus of WNV disease. Only a few studies on mosquito vectors in the Great Plains were published in the 1990s. These focused on seasonality of Cx. tarsalis abundance and western equine encephalitis virus infection rates in eastern Colorado (Smith et al. 1993) and seasonal abundance patterns and geographical variation of mosquitoes in Nebraska (Janousek and Kramer 1999). This dearth of studies from the Great Plains was unfortunate because information on mosquitoes and arboviruses provided in older studies conducted from 1950–1990 (e.g., Cockburn et al. 1957, Beadle 1959, Blackmore et al. 1962, Edman and Downe 1964, Hess and Hayes 1967, 1970, Tempelis et al. 1967, Hayes et al. 1976, Rapp 1985, Jakob et al. 1989) needs to be updated due to dramatic changes in land and water use patterns that have occurred in many parts of the Great Plains in recent decades. For example, rapid human population growth in the last two decades in the Colorado Front Range has transformed large tracts of naturally dry prairie landscapes into irrigated urban and semi-urban environments bordering Cx. tarsalis-producing agricultural land. Recent studies on mosquito vectors and WNV in the Great Plains have commonly focused on small, high-risk areas for WNV disease such as the cities of Fort Collins and Loveland in Colorado (Bolling et al. 2007, Gujral et al. 2007) or the city of Grand Forks in North Dakota (Bell et al. 2005). This is now being complemented by studies on mosquito distribution and abundance patterns, mosquito blood-feeding behavior, and enzootic WNV transmission in a wider range of ecological settings including the prairie landscape of the plains (current study; Janousek and Kramer 1999, Kent et al. 2009) and foothills and montane habitats at the eastern edge of the Rocky Mountains (Eisen et al. 2008, Winters et al. 2008). The current study focused on mosquitoes and WNV along a ≈190 km riparian corridor following the South Platte River and Big Thompson River in northeastern Colorado and extending from plains to foothills and low montane habitats. Key findings include that: 1) the key WNV vector, Cx. tarsalis, and the important nuisance-biter and potential secondary WNV vector, Ae. vexans, both have a striking pattern of uniformly high abundances between 1,200-1,450 m followed by a gradual decrease in abundance above 1,450 m to reach very low numbers above 1,550 m; 2) infection with WNV was common in Cx. tarsalis in the plains portion of the riparian corridor (14 of 16 sites; MLEs of infection rates ≈1-10 per 1,000 females) but was not found in higher-elevation foothills and montane sites; and 3) Cx. tarsalis collected from the ≈190 km riparian transect in northeastern Colorado east of the Continental Divide were genetically uniform but were genetically distinct from a collection from Delta County, CO, on the western side of the Continental Divide. Our finding that mosquito species richness was higher in the plains than in foothills-low montane areas agrees with previous reports from Larimer County and neighboring Boulder County to the south (Baker 1961, Eisen et al. 2008). As expected from previous studies in the Front Range area on host-seeking mosquitoes (Smith et al. 1993, Bolling et al. 2007, Eisen et al. 2008), collections were dominated by Ae. vexans followed by Cx. tarsalis, Cx. pipiens, and several other Aedes species. We did, however, collect two species not encountered in these previous studies: Anopheles punctipennis and Psorophora signipennis. Both species were collected in the plains; the two specimens of An. punctipennis came from the easternmost site (OVE) in Logan County and the single specimen of Ps. signipennis from the SOO site in Morgan County. These species were recorded previously from Weld County and other counties in the eastern Colorado plains (Harmston and Lawson 1967). Anopheles punctipennis also was collected commonly along the Platte River and Missouri River in eastern Nebraska (Janousek and Kramer 1999). Notably, collections during 1994–1995 along the Platte River in Scotts Bluff County in far western Nebraska, where the South Platte and North Platte tributaries merge, produced several species not collected b