Host-feeding patterns of Culex pipiens and other potential mosquito vectors (Diptera: Culicidae) of West Nile virus (Flaviviridae) collected in Portugal

The host blood-feeding patterns of mosquito vectors affects the likelihood of human exposure to zoonotic pathogens, including West Nile Virus (family Flaviviridae, genus Flavivirus, WNV). In Portugal, data are unavailable regarding the blood-feeding habits of common mosquito species, including Culex pipiens L., considered the primary vector of WNV to humans. The sources of bloodmeals in 203 blood-fed mosquitoes of nine species collected from June 2007 to November 2010 in 34 Portuguese counties were analyzed by sequencing cytochrome-b partial fragments. Cx. pipiens was the most common species collected and successfully analyzed (n = 135/78). In addition, blood-fed females of the following species were analyzed: Ochlerotatus caspius Pallas (n = 20), Culex theileri Theobald (n = 16), Anopheles maculipennis s.l. Meigen (n = 10), Culiseta longiareolata Macquart (n = 7), Aedes aegypti L. (n = 6), Culex perexiguus Theobald (n = 3), Culiseta annulata Schrank (n = 3), and Ochlerotatus detritus Haliday (n = 3). The Cx. pipiens mosquitoes fed predominantly on birds (n = 55/78, 70.5%), with a high diversity of avian species used as hosts, although human blood was identified in 18 specimens (18/78, 23.1%). No significant differences were found between the host-feeding patterns of blood-fed Cx. pipiens collected in residential and nonresidential habitats. The occurrence of human derived blood meals and the presence of a mix avian-human bloodmeal accordingly suggest this species as a potential vector of WNV. Therefore, in Portugal, Cx. pipiens may play a role both in the avian-to-avian enzootic WNV cycle and in the avian-to-mammal transmission. In this context, the identity of Cx. pipiens (considering the forms molestus and pipiens) and the potential consequence on feeding behavior and WNV transmission are discussed.     

West Nile virus from female and male mosquitoes (Diptera: Culicidae) in subterranean, ground, and canopy habitats in Connecticut

In total, 93,532 female mosquitoes (Diptera: Culicidae) were captured in traps placed in subterranean (catch basin), ground (approximately 1.5 m above ground), and canopy (approximately 7.0 m above ground) habitats in Stamford and Stratford, CT, during 2003-2005. Culex pipiens L. was the most abundant (64.8%) of the 31 species identified. Significantly greater numbers of Cx. pipiens were captured in canopy-placed mosquito magnet experimental traps, and significantly greater numbers were collected in catch basin-placed (Centers for Disease Control) CDC traps than in CDC traps placed elsewhere. Culex restuans Theobald was captured in significantly greater numbers in traps placed in catch basins. Aedes vexans (Meigen), Aedes cinereus Meigen, and Aedes cantator (Coquillett) were significantly more abundant in ground traps. In total, 429 isolations of West Nile virus (WNV) were made from seven species of mosquitoes from late June through the end of October during 2003 through 2005. Three hundred ninety-eight (92.8%) isolates were from Cx. pipiens. Others were from Cx. restuans (n = 16), Culex salinarius Coquillett (n = 5), Ae. vexans (n = 4), Ae. cantator (n = 3), Aedes triseriatus (Say) (n = 2), and Ae. cinereus (n = 1). Multiple isolates from Cx. pipiens were made each week, primarily during the later part of July through the end of September. Weekly minimum infection rates (MIRs) were lower in 2004 (highest weekly MIR = 7.1) when no human cases were reported in Connecticut in comparison with 2003 and 2005 (highest weekly MIR = 83.9) when human cases were documented. Frequencies of infected pools were significantly higher in Cx. pipiens captured in traps in the canopy and significantly higher in catch basin placed traps than in traps at ground level. The physiological age structure of Cx. pipiens captured in the canopy was significantly different from that of Cx. pipiens collected in catch basins. Invariably, Cx. pipiens captured in the canopy were nulliparous or parous with ovaries in Christophers' stage 2, whereas 58.7% of the females captured in catch basins possessed ovaries filled with mature oocytes in Christophers' stage 5. Our results suggest that females in the canopy are seeking hosts, and after digestion of the bloodmeal and development of mature oocytes, they descend to catch basins for shelter and deposition of eggs. WNV was isolated from three, one, and two pools of male Cx. pipiens captured in catch basin-, ground-, and canopy-placed traps, respectively, and from six nulliparous Cx. pipiens females collected in the canopy. Weekly MIR ranged from 1.2 to 31.1 per 1,000 male specimens. These data show that mosquitoes become infected by means other than by blood feeding, possibly by transovarial transmission. The placement of traps in tree canopies and in catch basins can be used to augment current practices of placement of traps near the ground for surveillance of mosquitoes infected with WNV and for studies of the ecology of WNV.     

Evaluation of potential West Nile virus vectors in Volgograd region, Russia, 2003 (Diptera: Culicidae): species composition, bloodmeal host utilization, and virus infection rates of mosquitoes

Potential West Nile virus (family Flaviviridae, genus Flavivirus, WNV) vectors were assessed during 2003 at indoor and outdoor collection sites in urban Volgograd, Russia, and in three nearby towns and surrounding rural areas. In total, 9,182 female mosquitoes comprising 13 species in six genera were collected. Relative abundance and bloodmeal host utilization differed temporarily and spatially. During June and July in Volgograd, Aedes vexans (Meigen) (85.4%) and Culex p. pipiens L. (7.6%) were the two most abundant species collected indoors, whereas during August, Cx. p. pipiens was the dominant species, accounting for 87.9% of specimens collected. Two WNV-positive mosquito pools were detected in August: one pool was composed of Cx. p. pipiens and the other pool of Culex modestus Ficalbi. Anopheles messeae Falleroni, Aedes caspius (Pallas), Ae. vexans, Cx. modestus, and Cx. p. pipiens used both humans and birds as bloodmeal sources. In urban areas, 20.4% of the Cx. p. pipiens fed on humans, 58.1% fed on chickens, and six specimens were positive for both chicken and human blood. Culex p. pipiens collected from flooded basements were predominantly autogenous (91.7%), whereas adult females resting in buildings with dry basements were composed of 67.5% anautogenous and 32.5% autogenous specimens. Our data suggest that the primary WNV vectors in the Volgograd region were Cx. p. pipiens and Cx. modestus and that intense transmission of WNV to humans in urban areas during the epidemic of 1999 may have been facilitated by the abundance and concentration of anautogenous Cx. p. pipiens in multistory buildings. The role of autogenous Cx. p. pipiens in urban transmission remains unresolved.     

Host-feeding habits of Culex and other mosquitoes (Diptera: Culicidae) in the Borough of Queens in New York City,with characters and techniques for identification of Culex mosquitoes

The host-feeding patterns of mosquitoes (n = 247) collected in the Borough of Queens in New York City in July and August 2000 were investigated using an indirect ELISA and a polymerase chain reaction (PCR)-heteroduplex assay. Culex pipiens L. and Cx. restuans Theobald fed primarily on birds, and their feeding habits support their implication as enzootic vectors of West Nile virus. Culex salinarius Coquillett and Coquillettidia perturbans (Walker) fed mainly on mammals, with fewer blood meals taken from birds, and these two species are potential bridge vectors of West Nile virus. Culex mosquitoes took blood meals (n = 54) from 11 different avian species. Only the northern cardinal (Cardinalis cardinalis), American robin (Turdus migratorius), and Brown-headed cow bird (MolIothrus ater) were fed upon by all three Culex species. Multiple blood feedings on avian hosts were detected in Cx. pipiens and Cx. restuans. Species identifications of Culex mosquitoes made using morphological characteristics were confirmed with a PCR assay that employed species-specific primers. All Cx. pipiens (n = 20) and Cx. salinarius (n = 10) specimens were correctly identified, but three (20%) of 15 Cx. restuans were misidentified as Cx. pipiens.     

Eastern equine encephalitis in Tennessee: 2002-2008

Human and equine outbreaks caused by eastern equine encephalomyelitis virus (EEEV) typically occur in North America adjacent to coastal wetlands associated with the presence of Culiseta melanura (Coquillet) mosquitoes. Eastern equine encephalomyelitis (EEE) is an emerging disease in Tennessee, as the first records of equine disease began in 2002. In 2006 we trapped and tested mosquitoes for EEEV at hardwood swamps in western Tennessee that were at the epicenter of a multi-equine outbreak in fall of 2005. Additionally, the Tennessee Valley Authority tested mosquito pools collected in Tennessee swamps from 2000 to 2007 for the presence of arboviruses. Two pools of EEEV positive Culex erraticus (Dyer and Knab) mosquitoes were found (one each in 2003 and 2004) in a county adjacent to where the 2005 outbreak occurred. In 2008, another EEE outbreak involving multiple horses occurred in West Tennessee. A brain specimen was collected from a horse during this outbreak and the first isolate of EEEV from Tennessee was obtained. In total, 74,531 mosquitoes collected from 2000 to 2008 were tested via polymerase chain reaction and VecTest for EEEV. The traditional enzootic vector, Cs. melanura, was found in low numbers at all collection sites. Cx. erraticus, however, was consistently found in high numbers and was the only mosquito species in which EEEV was detected. We suggest that EEE transmission may be maintained by Cx. erraticus in a nontraditional cycle. We discuss the importance of a nontraditional cycle from the perspective of EEEV adaptation and emergence.     

Ecological characterization of the aquatic habitats of mosquitoes (Diptera: Culicidae) in enzootic foci of Venezuelan equine encephalitis virus in western Venezuela

We studied the aquatic mosquito habitats in and around enzootic foci of Venezuelan Equine Encephalitis virus (VEE) in western Venezuela. Specimens were sampled for 5 mo in three types of vegetation: tall lowland tropical forests, short inundated/secondary growth forests, and pastures/herbaceous vegetation around forests. Ground pools, flooded pastures, swamps, ponds, and canals predominated. We used a multivariate statistical approach to quantitatively assess the relationships of mosquito species with broad categories of the landscape, and with environmental variables within each aquatic habitat. Twenty-four mosquito species in the genera Aedes, Psorophora, Culex, Mansonia, and Uranotaenia were collected. Species richness was higher in the tall forests than in other types of vegetation. Discriminant Function Analysis showed a strong association between landscape category and mosquito species assemblage and identified Culex erraticus Dyar & Knab and Mansonia titillans Walker as indicator species of open areas, and Aedes serratus Theobald as an indicator of tall forests. M. titillans, Uranotaenia geometrica Theobald, Cx. erraticus, and Culex dunni Dyar were associated with unshaded, warm, vegetated waters in flooded pastures and swamps, whereas Ae. serratus, Aedes fulvus (Wiedemann), Psorophora albipes Theobald, Psorophora ferox (Humboldt), Culex caudelli Dyar & Knab, and Culex pedroi Sirivanakarn & Belkin were associated with small, shaded ground pools within the tall forests. Culex coronator Dyar & Knab was associated with partially exposed sites within short forests. These results allowed us to interpret better our previous studies on mosquito adult spread in the study area and their possible role as VEEV disseminators.     

Experimental studies on comparison of the potential vector competence of four species of Culex mosquitoes in China to transmit West Nile virus

To assess the risk that indigenous mosquitoes in China are capable of transmitting and sustaining West Nile virus (WNV), four important Culex mosquito species, Culex tritaeniorhynchus, Culex modestus, Culex pipiens pallens, and Culex pipiens quinquefasciatus, were allowed to feed on the artificial infectious blood meal with WNV dose of 10(6.8) plaque-forming unit/ml and tested approximately 2 wk later to determine infection and transmission rates. The results indicated that four Culex mosquitoes were competent laboratory vectors of WNV. The infection rates and transmission rates were statistical differences among different species of mosquito (chi2 = 20.620, P = 0.000; chi2 = 15.020, P = 0.005, respectively). The highest infection rate and transmission rate were obtained with Cx. tritaeniorhynchus (87.5 and 74.2%, respectively).     

Nocturnal activity of mosquitoes (Diptera: Culicidae) in a West Nile virus focus in Connecticut

Six species of mosquitoes (Diptera: Culicidae) were collected in sufficient numbers for analysis in segregating traps set at 2-h intervals by using CO2 and light as attractants in a West Nile virus (family Flaviviridae, genus Flavivirus, WNV) focus in Stratford, CT. The Kolmogorov-Smirnov one-sided test for two samples was used to analyze the data. Mosquito activity began shortly before sunset and continued until shortly after sunrise the next morning. All species had geometric means that were significantly higher during the 2-h period shortly after sunset compared with the 2-h collection before sunset. Species, known to be naturally infected with WNV, were often attracted to these traps in about equal numbers at 2-h intervals during an 8- to 10-h period commencing shortly after sunset. Differences of geometric means were not significant among the four or five 2-h collection periods commencing at sunset for Aedes vexans (Meigen), Culex salinarius Coquillett, and Aedes cinereus Meigen. Aedes cantator (Coquillett) had a significantly higher geometric mean for the 2-h period commencing at sunset, and Coquillettidia perturbans (Walker) was captured in significantly greater numbers during the 2-h period starting at sunset compared with periods commencing 6 h after sunset. Culex pipiens L. tended to have an activity pattern that was primarily nocturnal. Time of night, not meteorological conditions, was the most important factor in determining the nightly variation in the number of trapped mosquitoes. Parity rates of Cx. pipiens collected during specific periods of the night were not significant. In total, 39 isolations of WNV were made from seven species collected primarily during periods of total darkness. Humans are at risk of being bitten by infected mosquitoes throughout the night.     

Culex pipiens (Diptera: Culicidae): a bridge vector of West Nile virus to humans

Host-feeding patterns of Culex pipiens L. collected in southwest suburban Chicago in 2005 were studied using polymerase chain reaction (PCR) and DNA sequencing techniques. Culex spp. mosquitoes, most identified to Cx. pipiens and the remainder to Cx. restuans by PCR, had fed on 18 avian species, most commonly American robin (Turdus migratorious), house sparrow (Passer domesticus), and mourning dove (Zenaida macroura). Additional blood meals were derived from four mammal species, primarily humans and raccoons (Procyon lotor). During a West Nile virus (WNV) epidemic in 2005, West Nile virus (WNV) RNA was detected in heads and thoraces of five Cx. pipiens (n = 335, 1.5%) using quantitative PCR. The hosts of these virus-infected, blood-fed mosquitoes included two American robins, one house sparrow, and one human. This is the first report of a WNV-infected Cx. pipiens mosquito collected during an epidemic of WNV that was found to have bitten a human. These results fulfill a criterion for incrimination of Cx. pipiens as a bridge vector.     

Considerations for accurate identification of adult Culex restuans (Diptera: Culicidae) in field studies

Understanding the ecology and behavior of different mosquito species (Diptera: Culicidae) is essential for identifying their role in disease transmission cycles and public health risk. Two species of Culex mosquitoes in the northeastern United States, Culex pipiens L. and Culex restuans Theobald, have been implicated in enzootic transmission of West Nile virus (family Flaviviridae, genus Flavivirus, WNV). Despite the difficulty of differentiating these two species as adults, many public health workers and vector biologists collecting adults in the field separate these species based on external morphology. This approach is often used rather than examination of dissected male genitalia or polymerase chain reaction (PCR)-based diagnostics due to time or cost constraints. We evaluated the reliability of seven published morphological characters to differentiate adults of these species by comparing blindly scored morphology with PCR-based confirmations. Our study demonstrates that morphological identification of Cx. pipiens is marginal and often not reliable for Cx. restuans. We also examined error rates with molecular-based approaches. DNA samples were contaminated with as little as one leg from another species. We conclude that to fully understand the respective roles of Culex species in the epidemiology of WNV and other pathogens, more attention should be paid to these considerations for accurate species identification.     

Host-seeking heights, host-seeking activity patterns, and West Nile virus infection rates for members of the Culex pipiens complex at different habitat types within the hybrid zone, Shelby County, TN, 2002 (Diptera: Culicidae)

Host-seeking heights, host-seeking activity patterns, and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) infection rates were assessed for members of the Culex pipiens complex from July to December 2002, by using chicken-baited can traps (CT) at four ecologically different sites in Shelby County, TN. Host-seeking height was assessed by CT placed at elevations of 3.1, 4.6, and 7.6 m during one 24-h period per month. Host-seeking activity was assessed by paired CT placed at an elevation of 4.6 m. Can traps were sampled at one 10-h daytime interval and at seven 2-h intervals during the evening, night, and morning. Cx. pipiens complex mosquitoes accounted for 87.1% of collected mosquitoes. Culex (Melanoconion) erraticus (Dyar & Knab) accounted for 11.9% of specimens. The average number of Cx. pipiens complex mosquitoes collected per 24-h CT period from July to September was lowest at a rural middle income site (1.7), intermediate at an urban middle income site (11.3), and highest at an urban low income site (47.4). Can traps at the forested site failed to collect Cx. pipiens complex mosquitoes. From July to September at urban sites, Culex pipiens pipiens L. was the rarest of the three complex members accounting for 11.1-25.6% of specimens. At the rural site, Culex pipiens quinquefasciatus Say was the rarest member of the complex. Cx. p. pipiens was not collected after September. Mean abundance of Cx. pipiens complex mosquitoes was higher in traps at 7.6 m than in traps at 4.6 m. Abundances at 3.1 m were intermediate and not significantly different from abundances at the other heights. Initiation of host-seeking activity was associated with the end of civil twilight and activity occurred over an extended nighttime period lasting 8-10 h. All 11 WNV-positive mosquitoes were Cx. pipiens complex mosquitoes collected from urban sites in traps placed at elevations of 4.6 and 7.6 m. Infection rates were marginally nonsignificant by height. Infection rates, host-seeking heights, and activity patterns were not significantly different among members of the Cx. pipiens complex.     

Estimation of dispersal distances of Culex erraticus in a focus of eastern equine encephalitis virus in the southeastern United States

Patterns of mosquito dispersal are important for predicting the risk of transmission of mosquito-borne pathogens to vertebrate hosts. We studied dispersal behavior of Culex erraticus (Dyar & Knab), a potentially significant vector of eastern equine encephalitis virus (EEEV) that is often associated with foci of this pathogen in the southeastern United States. Using data on the relative density of resting adult female Cx. erraticus around known emergence sites in Tuskegee National Forest, Alabama, we developed a model for the exponential decay of the relative density of adult mosquitoes with distance from larval habitats through parameterization of dispersal kernels. The mean and 99th percentile of dispersal distance for Cx. erraticus estimated from this model were 0.97 and 3.21 km per gonotrophic cycle, respectively. Parameterized dispersal kernels and estimates of the upper percentiles of dispersal distance of this species can potentially be used to predict EEEV infection risk in areas surrounding the Tuskegee National Forest focus in the event of an EEEV outbreak. The model that we develop for estimating the dispersal distance of Cx. erraticus from collections of adult mosquitoes could be applicable to other mosquito species that emerge from discrete larval sites.     

West Nile virus investigations in South Moravia, Czechland

Seven virus isolates were obtained from 11,334 mosquitoes after the 1997 Morava River flooding in South Moravia (Czech Republic): 6 strains of Tahyna bunyavirus, California antigenic group (5 from Aedes vexans, 1 from Ae. cinereus), and 1 strain of West Nile flavivirus (WNV) from Culex pipiens. In 1999, one isolate of Tahyna virus from Ae. vexans and one isolate of WNV from Cx. pipiens were recovered from a total of 14,354 mosquitoes examined in the same area, whereas no virus was detected there in 1,179 overwintering mosquitoes (mostly Cx. pipiens) in March 2000. The infection rate of mosquitoes with arboviruses was significantly higher in 1997, the year of the flood and an enormously high population density of mosquitoes. Antibodies neutralizing WNV were detected in 13 of 619 (2.1%) hospitalized patients or persons seeking outpatient clinics of the area in 1997. Five of the seroreactors revealed clinical symptoms compatible with West Nile fever: in 2 of them (children), recent infection with WNV was confirmed by a significant increase of antibody titer between acute and convalescent serum samples.     

Vector competence of selected African mosquito (Diptera: Culicidae) species for Rift Valley fever virus

Outbreaks of Rift Valley fever (RVF) in Egypt, Yemen, and Saudi Arabia have indicated the potential for this disease to spread from its enzootic areas in sub-Saharan Africa. Because little is known about the potential for most African mosquito species to transmit RVF virus (family Bunyaviridae, genus Phlebovirus, RVFV), we conducted studies to determine the vector competence of selected African species of mosquitoes for this virus. All eight species tested [Aedes palpalis (Newstead), Aedes mcintoshi Huang, Aedes circumluteolus (Theobald), Aedes calceatus Edwards, Aedes aegypti (L.), Culex antennatus (Becker), Culex pipiens (L.), and Culex quinquefasciatus Say], were susceptible to infection, and all except Ae. calceatus, Ae. aegypti and Cx. quinquefasciatus transmitted RVFV by bite after oral exposure. Estimated transmission rates for mosquitoes that successfully transmitted RVFV by bite ranged from 5% for Ae. mcintoshi to 39% for Ae. palpalis for mosquitoes that fed on a hamster with a viremia > or = 10(8) plaque-forming units of virus/ml. We did not recover RVFV from any of 3,138 progeny of infected female mosquitoes. RVFV is unusual among arboviruses in that it has been isolated in nature from a large number of species and that numerous mosquitoes and other arthropods are able to transmit this virus in the laboratory. The recent introduction and spread of West Nile virus into the Americas and the spread of RVFV to the Arabian Peninsula illustrates the potential for viruses, once enzootic in Africa, to spread to other parts of the world.     

Oviposition activity patterns and West Nile virus infection rates for members of the Culex pipiens complex at different habitat types within the hybrid zone, Shelby County, TN, 2002 (Diptera: Culicidae)

Oviposition activity and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) infection rates were assessed for members of the Culex pipiens complex from July through December 2002 by using gravid traps placed at four ecologically different sites in the southern portion of the hybrid zone in Shelby County, TN. Molecular assays identified three members of the Cx. pipiens complex: Cx. pipiens pipiens L., Cx. p. quinquefasciatus Say, and Cx. p. pipiens-Cx. p. quinquefasciatus hybrids (hybrids). The Cx. pipiens complex accounted for 90% of mosquitoes collected in gravid traps. All 285 WNV-positive mosquitoes were Culex mosquitoes, and 277 (97%) were Cx. pipiens complex mosquitoes. Infection rates among members of the Cx. pipiens complex were not significantly different. Infection rates were significantly higher at two urban sites than at a rural site, and WNV was not detected at a forested site. At urban sites, abundances of members of the Cx. pipiens complex corresponded to a simple latitude model of the hybrid zone. Cx. p. quinquefasciatus was most abundant (46.4%), followed by hybrids (34.1%) and Cx. p. pipiens (19.5%). The relative abundances at a rural site were reversed with Cx. p. pipiens (48.4%) being most abundant. This demonstrates that spatial habitat variation may profoundly influence the distribution of members of the Cx. pipiens complex within the hybrid zone. Members of the Cx. pipiens complex did not display different oviposition patterns. However, oviposition patterns assessed hourly at urban and rural sites were significantly different. At urban sites, oviposition activity of Cx. pipiens complex mosquitoes was bimodal with an evening peak associated with sunset and a morning peak associated with sunrise. At the rural site, the evening peak was pronounced and the morning peak weak and similar to nighttime activity.     

Potential for North American mosquitoes (Diptera: Culicidae) to transmit rift valley fever virus

To determine which arthropods should be targeted for control should Rift Valley fever virus (RVFV) be detected in North America, we evaluated Culex erraticus (Dyar and Knab), Culex erythrothorax Dyar, Culex nigripalpus Theobald, Culex pipiens L., Culex quinquefasciatus Say, Culex tarsalis Coquillett, Aedes dorsalis (Wiedemann), Aedes vexans (Meigen), Anopheles quadrimaculatus Say, and Culicoides sonorensis Wirth and Jones from the western, midwestern, and southern United States for their ability to transmit RVFV. Female mosquitoes were allowed to feed on adult hamsters inoculated with RVFV, after which engorged mosquitoes were incubated for 7-21 d at 260C, then allowed to refeed on susceptible hamsters, and tested to determine infection, dissemination, and transmission rates. Other specimens were inoculated intrathoracically, held for 7 d, and then allowed to feed on a susceptible hamster to check for a salivary gland barrier. When exposed to hamsters with viremias > or =10(8.8) plaque-forming units/ml blood, Cx. tarsalis transmitted RVFV efficiently (infection rate = 93%, dissemination rate = 56%, and estimated transmission rate = 52%). In contrast, when exposed to the same virus dose, none of the other species tested transmitted RVFV efficiently. Estimated transmission rates for Cx. erythrothorax, Cx. pipiens, Cx. erraticus, and Ae. dorsalis were 10, 8, 4, and 2%, respectively, and for the remaining species were < or = 1%. With the exception of Cx. tarsalis and Cx. pipiens, all species tested had moderate to major salivary gland barriers. None of the C. sonorensis became infected and none of the An. quadrimaculatus tested transmitted RVFV by bite, even after intrathoracic inoculation, indicating that these species would not be competent vectors of RVFV. Although Ae. vexans from Florida and Louisiana were relatively efficient vectors of RVFV, specimens of this species captured in Colorado or California were virtually incompetent, illustrating the need to evaluate local population for their ability to transmit a pathogen. In addition to laboratory vector competence, factors such as seasonal density, host feeding preference, longevity, and foraging behavior should be considered when determining the potential role that these species could play in RVFV transmission.     

Horse-, bird-, and human-seeking behavior and seasonal abundance of mosquitoes in a West Nile virus focus of southern France

After 35 yr of disease absence, West Nile virus (family Flaviviridae, genus Flavivirus, WNV) circulation has been regularly detected in the Camargue region (southern France) since 2000. WNV was isolated from Culex modestus Ficalbi, which was considered the main vector in southern France after horse outbreaks in the 1960s. Recent WNV transmissions outside of the Cx. modestus distribution suggested the existence of other vectors. To study potential WNV vectors, horse- and bird-baited traps and human landing collections of mosquitoes were carried out weekly from May to October 2004 at two Camargue sites: one site in a wet area and the other site in a dry area, both chosen for their past history of WNV transmission. At the wet site, the most abundant species in bird-baited traps were Culex pipiens L. and Cx. modestus; both species also were found in lower proportions on horses and humans. The most abundant species in horse-baited traps and human landing collections were Aedes caspius (Pallas), Aedes vexans (Meigen), and Anopheles hyrcanus (Pallas) sensu lato; some of these species were occasionally collected with avian blood at the end of the summer. Anopheles maculipennis Meigen sensu lato was an abundant horse feeder, but it was rarely collected landing on human bait and never contained avian blood. At the dry site, Cx. pipiens was the most abundant species in bird- and horse-baited traps. The seasonal and circadian dynamics of these species are analyzed, and their potential in WNV transmission in Camargue discussed.