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.     

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.     

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.     

Bird-baited traps for surveillance of West Nile mosquito vectors: effect of bird species, trap height, and mosquito escape rates

Host-seeking mosquitoes were sampled in bird-baited traps at four sites in New York state in 2003-2004. Trap placement and efficacy of chickens, Gallus gallus domesticus L., as bait compared with house sparrows, Passer domesticus L., an important reservoir of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), was evaluated. Each site had a chicken-baited trap near ground level (approximately 1.5 m) and in the tree canopy (approximately 9 m), and a house sparrow-baited trap at ground level and canopy level. Each trap allowed mosquito access to birds on one end, and an inner mesh screen blocked bird access on the other end. The two most abundant mosquitoes, Culex restuans Theobald and Culex pipiens pipiens L., were differentiated using molecular characters. In 2003, Cx. restuans and Cx. p. pipiens made up 88% of total mosquito catch. In 2004, Cx. restuans comprised 43% of total catch and Cx. p. pipiens comprised 33%. The remaining species representing at least 1% of total catch were Ochlerotatus trivittatus (Coquillett), Coquilletidia perturbans (Walker), and Culiseta morsitans (Theobald). Capture rates were similar for chicken and house sparrow-baited traps; however, significantly more mosquitoes were captured in the canopy for both bird species. Cx. restuans preferred canopy traps, whereas equal numbers of Cx. p. pipiens were captured at ground and canopy levels. Mosquitoes were more likely to escape (74%) when excluded from birds than when allowed free access to birds (54%). Sentinel bird surveillance for WNV can be improved by trapping in the tree canopy in addition to ground level to capture the most important avian vectors.     

Blood-feeding patterns of the Culex pipiens complex in Sacramento and Yolo Counties, California

Mosquitoes in the Culex pipiens complex are competent vectors of West Nile virus (WNV; family Flaviviridae, genus Flavivirus) in the laboratory, and field-collected mosquitoes have tested positive for the virus in California and elsewhere. A better understanding of Cx. pipiens complex blood-feeding patterns will help define the threat that these mosquitoes pose to human health and their role in WNV amplification in northern California. We collected blood-engorged Cx. pipiens complex mosquitoes from resting sites near and away from human habitation in Sacramento and Yolo Counties. Cytochrome c oxidase 1 gene sequences were used to identify the vertebrate species from which blood meals were taken. Of 330 engorged mosquitoes collected at 28 sites from June through August 2007 and May through August 2008, >99% fed on an avian host. Three mosquitoes contained bovine blood and none had fed on a human. American Robins (Turdus migratorius) were bitten most often, and the proportion of American Robin blood meals increased significantly over the summer. Other important avian hosts included House Finches (Carpodacus mexicanus), Barn Swallows (Hirundo rustica), Western Meadowlarks (Sturnella neglecta), and Mourning Doves (Zenaida macroura). In rural areas, Barn Swallows, Brewer's Blackbirds (Euphagus cyanocephalus), and House Sparrows (Passer domesticus) were frequent hosts. In settings near human habitation, Mourning Doves and Western Meadowlarks were common hosts. Our data indicate that in north central California mosquitoes in the Cx. pipiens complex may be more important as epiornitic than epidemic vectors of WNV.     

Entomological study on transmission of avian malaria parasites in a zoological garden in Japan: bloodmeal identification and detection of avian malaria parasite DNA from blood-fed mosquitoes

Several species of captive and wild birds have been found to be infected with various avian blood protozoa in Japan. We investigated the prevalence and transmission of avian malaria parasite and determined the bloodmeal hosts of mosquitoes collected in a zoological garden in Tokyo, Japan, by using the polymerase chain reaction. In total, 310 unfed and 140 blood-fed mosquitoes of seven species were collected by using sweep nets and CDC traps. Bloodmeal identification indicated that mosquitoes had fed on 17 avian and five mammalian species, including captive animals. The results of avian malaria parasite detection from mosquitoes with avian bloodmeals indicated that Culex pipiens pallens Coquillet is a main vector of avian Plasmodium in the current study site and that some captive and wild birds could be infected with avian malaria parasites. Furthermore, the distances between the collection site of blood-fed mosquitoes and the locations of their blood-source captive animals were estimated. Most females with fresh bloodmeals were found within 40 m of caged animals, whereas half-gravid and gravid females were found between 10 and 350 m from caged host animals. We demonstrated that blood-fed mosquitoes can provide useful information regarding the mosquito vector species of avian malaria parasites and allows for noninvasive detection of the presence of avian malaria parasites in bird populations.     

Natural infection of Culex theileri (Diptera: Culicidae) with Dirofilaria immitis (Nematoda: Filarioidea) on Madeira Island, Portugal

Field and laboratory studies were performed to verify whether Culex theileri Theobald functions as a natural vector of Dirofilaria immitis (Leidy) on Madeira Island, Portugal. CO2-baited light traps (EVS traps) were use to sample mosquitoes monthly basis between February 2002 and February 2003 in the area of Quebradas (Funchal). Three mosquito species were captured, including 58 Culex pipiens L., 790 Cx. theileri, and three Culiseta longiareolata (Macquart). Only C. theileri tested positive for D. immitis. The presence of this filarial worm was detected by direct observation, infectivity assay dissection technique, and polymerase chain reaction methods. Infected mosquitoes were recovered in October and December 2002 and January 2003. These data provide evidence that Cx. theileri could be the main vector of D. immitis in Funchal, Madeira.     

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.     

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.     

Host feeding of mosquitoes (Diptera: Culicidae) associated with the recurrence of Rift Valley fever in Egypt

In 1993, Rift Valley fever (RVF) virus reappeared in Egypt. We determined the prevalence and feeding patterns of mosquitoes in 5 villages where the virus was active. Of 10 species recovered, Aedes caspius (Pallas), Culex pipiens L., Cx. antennatus (Becker), and Cx. perexiguus Theobald constituted 99% of > 35,000 mosquitoes captured in dry ice-baited CDC light traps. Ae. caspius was most prevalent, except at Nag' El Hagar where it was replaced by Cx. perexiguus. Cx. pipiens ranked 2nd, except at Nag' El Ghuneimiya, where it was replaced by Cx. antennatus. Most blood meals analyzed by an enzyme-linked immunosorbent assay reacted to > or = 1 antiserum. Cx. pipiens was mainly anthropophagic, and therefore may have been the main vector of RVF virus among humans. Ae. caspius feeds were chiefly from humans, bovines, and equines. Cx. antennatus and Cx. perexiguus fed generally on bovines. Mixed blood meals from humans and RVF virus susceptible animals were identified in the predominant mosquitoes. Prevalence and host selection, as well as predicted probability for a blood meal being interrupted, indicated that Ae. caspius may have served as a bridge vector between humans and bovines in 4 of the villages. Cx. perexiguus may have played this role at Nag' El Hagar. Because potential vectors are abundant, susceptible domestic animals are associated closely with humans, and surveillance of imported livestock is not systematic, we conclude that RVF virus sporadically will recur in Egypt.     

Attractiveness of vertebrate hosts to Culex pipiens (Diptera: Culicidae) and other mosquitoes in Israel.

Host preferences of mosquitoes, mainly Culex pipiens L. and Aedes caspius (Pallas), were studied in Israel using live baits, chemical attractants, light, and suction traps. The feeding habits of Cx. pipiens were opportunistic. This species used a wide range of mammalian and avian hosts; a calf, chickens, turkeys, and sheep were the most attractive baits. A. caspius fed mainly on large mammals and was most attracted to calf and sheep baits. Although sheep were attractive, few Cx. pipiens and no Ae. caspius engorged after entering the traps. In addition to defensive behavior and the mechanical protection afforded by the wool, sheep may possess a close-acting repellent that deters mosquitoes from biting. CO2 was the best attractant for host-seeking Cx. pipiens and Ae. caspius, Aedes detritus (Haliday), Culiseta annulata (Schrank), Culiseta longiareolata (Macquart), and Uranotaenia unguiculata Edwards were caught in small numbers, and their bloodmeal hosts were recorded.     

Natural vectors of dirofilariasis in rural and urban areas of the Tuscan region, central Italy

Entomological investigations by means of dog- and human-baited traps were carried out in summers 2000-2002 in urban and rural areas of the Tuscan region in central Italy. The aim of the study was to define the mosquito species involved in the transmission of Dirofilaria nematodes and to assess the risk that their presence might represent for animal and human health. Nocturnal fieldwork on host-seeking activity and feeding preferences was followed by microscopic identification of the mosquito species attracted and by molecular identification of Dirofilaria parasites in mosquitoes. In total, 3,611 mosquito females belonging to 12 species, largely represented by Culex pipiens L. and Aedes caspius (Pallas), were caught. Some females of each species collected fed on the dogs, indicating their possible role as an intermediate host, but filarial DNA was found only in Cx. pipiens, Anopheles maculipennis s.l. (Meigen), and Coquillettidia richiardii (Ficalbi). In rural environments, the DNA evidence indicated the presence of infective larvae of Dirofilaria immitis, whereas in urban areas, infective larvae of Dirofilaria repens were present. The role of Cx. pipiens as a vector for heartworm disease and subcutaneous infections in natural and artificial environments was confirmed, whereas Ae. caspius seemed refractory to the infection. The different role of the collected species is discussed. The vector competence of An. maculipennis and Cq. richiardii needs further investigation, because the importance of these species poorly represented, and the role of species such as Aedes albopictus (Skuse), characterized by a dominant diurnal activity pattern, has to be evaluated.     

Blood meal identification and prevalence of avian malaria parasite in mosquitoes collected at Kushiro wetland, a subarctic zone of Japan

In Japan, the prevalence of avian Plasmodium in birds and mosquitoes has been partially examined in the temperate and subtropical zones; however, mosquitoes in the Japanese subarctic zone have not been adequately investigated. In this study, mosquito collections and avian Plasmodium detections from the mosquito samples were carried out to demonstrate the avian Plasmodium transmission between vector mosquitoes and birds inhabiting in Kushiro Wetland, subarctic zone of Japan. A total of 5657 unfed mosquitoes from 18 species and 320 blood-fed mosquitoes from eight species was collected in summer 2008, 2009, and 2010. Three Aedes esoensis that fed on Hokkaido Sika Deer and one unfed Culex pipiens group were found to be positive for avian Plasmodium by polymerase chain reaction. This is the first report of the detection of avian Plasmodium DNA from mosquitoes distributing in the subarctic zone of Japan. The blood meals were successfully identified to captive or wild animals, including seven mammalian species, four bird species, and one amphibian species. These results indicated that infected birds with avian Plasmodium inhabited and direct contacts occurred between the infected birds and mosquitoes in Kushiro Wetland, Hokkaido, Japan.     

Culex restuans (Diptera: Culicidae) relative abundance and vector competence for West Nile Virus

The abundance and vector competence of Culex restuans Theobald and Culex pipiens L. were compared to determine the relative importance of these species as West Nile virus (WNV) vectors in the northeastern United States. Abundance was estimated from egg raft collections at 12 sites in Albany, Suffolk, and Richmond counties, New York, during July, August, and September 2002 and 2003. Cx. restuans was more abundant than Cx. pipiens in both urban and rural areas, comprising 86% of 1,623 egg rafts collected. Vector competence for WNV was estimated after feeding on an artificial bloodmeal and in vitro transmission assays. The vector competence of the two species for WNV was similar, but the dynamics of infection seems to be mosquito species dependent. These findings suggest an important role for Cx. restuans in WNV transmission cycles in New York.     

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.     

Spatial variation in host feeding patterns of Culex tarsalis and the Culex pipiens complex (Diptera: Culicidae) in California

West Nile virus (family Flaviviridae, genus Flavivirus, WNV) is now endemic in California across a variety of ecological regions that support a wide diversity of potential avian and mammalian host species. Because different avian hosts have varying competence for WNV, determining the blood-feeding patterns of Culex (Diptera: Culicidae) vectors is a key component in understanding the maintenance and amplification of the virus as well as tangential transmission to humans and horses. We investigated the blood-feeding patterns of Culex tarsalis Coquillett and members of the Culex pipiens L. complex from southern to northern California. Nearly 100 different host species were identified from 1,487 bloodmeals, by using the mitochondrial gene cytochrome c oxidase I (COI). Cx. tarsalis fed on a higher diversity of hosts and more frequently on nonhuman mammals than did the Cx. pipiens complex. Several WNV-competent host species, including house finch and house sparrow, were common bloodmeal sources for both vector species across several biomes and could account for WNV maintenance and amplification in these areas. Highly competent American crow, western scrub-jay and yellow-billed magpie also were fed upon often when available and are likely important as amplifying hosts for WNV in some areas. Neither species fed frequently on humans (Cx. pipiens complex [0.4%], Cx. tarsalis [0.2%]), but with high abundance, both species could serve as both enzootic and bridge vectors for WNV.     

Vector movement underlies avian malaria at upper elevation in Hawaii: implications for transmission of human malaria

With climate warming, malaria in humans and birds at upper elevations is an emerging infectious disease because development of the parasite in the mosquito vector and vector life history are both temperature dependent. An enhanced-mosquito-movement model from climate warming predicts increased transmission of malaria at upper elevation sites that are too cool for parasite development in the mosquito vector. We evaluate this model with avian malaria (Plasmodium relictum) at 1,900-m elevation on the Island of Hawaii, with air temperatures too low for sporogony in the vector (Culex quinquefasciatus). On a well-defined site over a 14-year period, 10 of 14 species of native and introduced birds became infected, several epizootics occurred, and the increase in prevalence was driven more by resident species than by mobile species that could have acquired their infections at lower elevations. Greater movement of infectious mosquitoes from lower elevations now permits avian malaria to spread at 1,900 m in Hawaii, in advance of climate warming at that elevation. The increase in malaria at upper elevations due to dispersal of infectious mosquitoes is a real alternative to temperature for the increased incidence of human malaria in tropical highlands.     

Impact of trap elevation on estimates of abundance, parity rates, and body size of Culex pipiens and Culex restuans (Diptera: Culicidae)

Trapping success, abundance, parity rate, and body size indices of Culex pipiens (L.) and Culex restuans Theobald, important vectors of West Nile virus (family Flaviviridae, genus Flavivirus, WNV), were determined for specimens captured from both ground level (approximately 1-m) and elevated (approximately 6- to 7-m) CO2-baited CDC miniature light traps. Mosquitoes were collected from six study sites in Albany County, New York, from mid-May to mid-October 2004. There was no significant difference in abundance or mean parity for either Cx. pipiens or Cx. restuans between the elevated and the ground-level traps. Mosquitoes collected in elevated traps tended to be larger than mosquitoes collected at ground level. Elevated traps captured significantly fewer mosquitoes per trap-night than did ground traps, but a greater proportion of those captured were Culex spp. Therefore, elevated traps more efficiently captured Cx. pipiens and Cx. restuans with similar reproductive status and body size and reduced the time necessary to sort and identify collections. These findings may be beneficial to researchers or surveillance programs focusing on Cx. pipiens or Cx. restuans.     

Swedish Culex torrentium and Cx. pipiens (Diptera: Culicidae) as experimental vectors of Ockelbo virus

The ability of the sibling species Culex pipiens (L.) and Culex torrentium (Martini) from central Sweden to transmit Ockelbo (OCK) virus was determined. Both species became infected after ingesting OCK virus from a viremic chicken; they transmitted this virus to chickens after 21-28 d of extrinsic incubation. In Cx. torrentium, infection rates were 90% or higher, and all 10 refeeding mosquitoes transmitted virus after feeding on chickens with a viremia of at least 10(3.0) plaque-forming units (PFU)/ml. In contrast, only 1 of 28 (4%) of the Cx. pipiens that ingested blood containing 10(3.0-3.9) PFU/ml became infected, and none of 16 refeeding mosquitoes transmitted virus. However, 98 of 184 (53%) of the Cx. pipiens that ingested a blood meal containing at least 10(6.0) PFU/ml became infected. Transmission rates in Cx. pipiens increased with increasing virus concentration in the blood meal to a maximum of 37% in mosquitoes that ingested greater than 10(8.0) PFU/ml. Based on these data, both Cx. pipiens and Cx. torrentium are capable of transmitting OCK virus in an enzootic cycle involving birds as hosts. However, Cx. torrentium appears to be physiologically a more efficient vector than Cx. pipiens.     

Hematophagous Diptera collected from a horse and paired carbon dioxide-baited suction trap in southern California: relevance to West Nile virus epizootiology

Hematophagous Diptera landing on a horse were removed by vacuum, and their numbers were related to a paired carbon dioxide-baited suction trap at three locations in southern California where West Nile virus activity was high during the preceding year. Insects collected from the horse included mosquitoes (nine species), biting midges (Culicoides sonorensis Wirth & Jones), and black flies (Simulium bivittatum Malloch). Mosquitoes were predominantly collected from the head, crest, withers, neck, chest, and shoulders of the horse, whereas biting midges and black flies were predominantly collected from the ventral midline of the horse. Culex erythrothorax Dyar was by far the most abundant mosquito species collected overall. Frequency of engorgement for mosquitoes captured from the horse ranged by species from zero to 58.3%, with Culex pipiens quinquefasciatus Say having the lowest value (16.7% or one of six mosquitoes) of species that fed on the horse. The number of insects captured at the horse and paired CO2-baited suction trap was not different for Anopheles franciscanus McCracken, Culex tarsalis Coquillett, and S. bivittatum. Cx. p. quinquefasciatus was captured in greater numbers in the CO2-baited suction trap, whereas Anopheles hermsi Barr & Guptavanji, Cx. erythrothorax, Culiseta inornata (Williston), and Culiseta particeps (Adams) were captured in greater numbers from the horse. The horse biting rate was very low for Cx. p. quinquefasciatus, intermediate for Cx. tarsalis, and very high for Cx. erythrothorax. Both Cx. tarsalis and Cx. erythrothorax should be considered likely epizootic vectors of West Nile virus to horses in rural southern California.