Vector competence of Culiseta incidens and Culex thriambus for West Nile virus

The vector competence of Culiseta incidens (Thomson) and Culex thriambus Dyar for West Nile virus (WNV) were compared to Cx. quinquefasciatus Say or Cx. tarsalis Coquillett and Cx. stigmatasoma Dyar collected concurrently in California. Culiseta incidens were less susceptible to oral infection than Cx. quinquefasciatus, but transmitted virus at a significantly higher rate, thereby yielding comparable population transmission rates. Culex thriambus was equally susceptible to oral infection and transmitted virus at rates comparable to Cx. tarsalis or Cx. stigmatosoma. A mammalian host selection pattern most likely precluded detection of natural infection in Cs. incidens, a fairly abundant peridomestic species. In contrast, an avian host selection pattern and efficient vector competence resulted in repeated detection of WNV in Cx. thriambus; however, limited abundance and restrictive riparian larval habitat requirements would seem to limit the involvement of Cx. thriambus in WNV epidemiology.     

Studies on the bioecological aspects of adult mosquitoes in the Prado Basin of southern California

Studies on the bionomics of adult mosquitoes were carried out in the Prado Basin of southern California during 1985-86. The faunal composition of mosquitoes caught by species was (in descending order) Culex quinquefasciatus, Cx. tarsalis, Cx. erythrothorax, Cx. stigmatosoma (formerly Culex peus) followed by Anopheles freeborni, Culiseta particeps, Cs. inornata and Cs. incidens. The number of mosquitoes per trap night was the lowest during December through February, and the highest during August through October. Depending on both intrinsic and extrinsic factors, adult mosquitoes were active at dusk and dawn. In spatial distribution studies, both adult and larval collections showed that Cx. quinquefasciatus and Cx. stigmatosoma were associated with dairy lagoons and Cx. erythrothorax with duck ponds and a nearby wooded area. Culex tarsalis was found in greater number at all habitats. Anopheles freeborni and Culiseta spp. were found around the wooded area. In vertical distribution studies, more mosquitoes were captured at the highest (6 m) level than at lower (0.6 and 3 m) levels which was probably due to the large percentage of parous females present at this site.     

Oviposition responses of gravid Culex quinquefasciatus and Culex tarsalis to bulrush (Schoenoplectus acutus) infusions

Laboratory bioassays demonstrated that fermented infusions of dried bulrushes (Schoenoplectus acutus) strongly attracted and stimulated oviposition by gravid female Culex quinquefasciatus and Culex tarsalis. The responses of the 2 species varied with the concentration and method of preparation of the infusions, with responses generally increasing with increasing concentration. No major differences were found in the responses of either species to infusions prepared with bulrushes alone, or with bulrushes plus lactalbumin hydrolysate and brewer's yeast. Infusions remained more attractive than distilled water controls to both species for up to 8 wk. Field tests corroborated the results from the laboratory bioassays. Culex quinquefasciatus, Cx. tarsalis, and Cx. stigmatosoma egg rafts were collected from water pan traps baited with bulrush infusions. A few Culiseta incidens eggs also were collected. In multiple-choice tests using gravid female or egg traps, Cx. quinquefasciatus preferred the most concentrated bulrush infusions, whereas Cx. tarsalis preferred intermediate concentrations. Female Cx. stigmatosoma and Culiseta incidens also were attracted. Overall, these results may provide new leads towards developing synthetic chemical baits to attract bloodfed mosquitoes.     

Detection of chicken antibodies to mosquito salivary gland antigens by enzyme immunoassay

Sentinel chickens are used to detect western equine encephalomyelitis, St. Louis encephalitis, and West Nile virus activity. Flocks that receive high mosquito exposure will be most effective for surveillance purposes. However, mosquito population indices at the flock sites may only provide an indirect measure of potential exposure. Therefore, we developed an indirect enzyme immunoassay to detect chicken antibodies to salivary gland antigens (SGAs) from Culex tarsalis, the primary arbovirus vector in California. Chickens fed upon by Cx. tarsalis developed an antibody response that was proportional to the amount of exposure. Cross-reactivity between sera from Cx. tarsalis-exposed chickens and SGAs from Culex pipiens quinquefasciatus, Culex pipiens pipiens, Ochlerotatus melanimon, and Ochlerotatus sierrensis was likely due to shared SGAs among these species. This serologic assay for mosquito exposure could be used to evaluate the sensitivity of sentinel flocks for detecting arboviral activity.     

An assessment of mosquito production and nonchemical control measures in structural stormwater best management practices in southern California

A 3-year study was conducted to assess mosquito production in structural stormwater Best Management Practices (BMPs) installed by the California Department of Transportation in San Diego and Los Angeles Counties. Thirty-seven BMPs were monitored weekly for presence and relative abundance of immature mosquitoes and for conditions conducive to mosquito production. Species identified were Aedes squamiger, Anopheles franciscanus, An. hermsi, Culex quinquefasciatus, Cx. stigmatosoma, Cx. tarsalis, Culiseta incidens, and Cs. inornata. Structures designed with accessible, permanent sources of standing water in sumps, vaults, or basins were observed to support immatures all year. In BMPs intended to drain rapidly and completely, observed larval habitats resulted from design features, component failure, construction flaws, and non-stormwater runoff flows. Specific nonchemical mitigation measures to minimize or eliminate mosquito production were developed, implemented, and evaluated. This study provides a 1st assessment of mosquito production, species composition, and nonchemical control measures in structural stormwater BMPs in southern California.     

West Nile virus in California

West Nile virus (WNV) was first isolated in California during July 2003 from a pool of Culex tarsalis collected near El Centro, Imperial County. WNV transmission then increased and spread in Imperial and Coachella Valleys, where it was tracked by isolation from pools of Cx. tarsalis, seroconversions in sentinel chickens, and seroprevalence in free-ranging birds. WNV then dispersed to the city of Riverside, Riverside County, and to the Whittier Dam area of Los Angeles County, where it was detected in dead birds and pools of Cx. pipiens quinquefasciatus. By October, WNV was detected in dead birds collected from riparian corridors in Los Angeles, west to Long Beach, and through inland valleys south from Riverside to San Diego County. WNV was reported concurrently from Arizona in mid-August and from Baja, Mexico, in mid-November. Possible mechanisms for virus introduction, amplification, and dispersal are discussed.     

Vector competence of selected North American Culex and Coquillettidia mosquitoes for West Nile virus.

To control West Nile virus (WNV), it is necessary to know which mosquitoes are able to transmit this virus. Therefore, we evaluated the WNV vector potential of several North American mosquito species. Culex restuans and Cx. salinarius, two species from which WNV was isolated in New York in 2000, were efficient laboratory vectors. Cx. quinquefasciatus and Cx. nigripalpus from Florida were competent but only moderately efficient vectors. Coquillettidia perturbans was an inefficient laboratory vector. As WNV extends its range, exposure of additional mosquito species may alter its epidemiology.     

First collection of Anopheles (Anopheles) punctipennis (Say) on Oahu, Hawaii: implications for the potential introduction of West Nile virus

A single adult female of Anopheles (Anopheles) punctipennis (Say) was collected in a New Jersey light trap in the port area of Honolulu, Oahu, HI, on December 8, 2003. This is the 1st record of a wild-collected, intact Anopheles mosquito, and the 1st detection of An. punctipennis in the Hawaiian Islands. Extensive larval surveillance and additional light trapping in the area of the collection site indicate that populations of this species have not become established on Oahu. The distribution of An. punctipennis largely coincides with the distribution of West Nile virus (WNV) in North America. Although An. punctipennis is not an important vector of WNV, Culex quinquefasciatus and Cx. tarsalis are important vectors of WNV and occur in areas of North America with active WNV transmission. Culex quinquefasciatus is established in Hawaii, and Cx. tarsalis has been detected on multiple occasions by quarantine. Increased quarantine inspections, disinsection of airline cargo holds, enhanced vector surveillance, and the development of sanitary corridors around airports and port facilities are necessary to reduce the introduction of vectors and pathogens.     

West Nile virus infection in mosquitoes, birds, horses, and humans, Staten Island, New York, 2000

West Nile (WN) virus transmission in the United States during 2000 was most intense on Staten Island, New York, where 10 neurologic illnesses among humans and 2 among horses occurred. WN virus was isolated from Aedes vexans, Culex pipiens, Cx. salinarius, Ochlerotatus triseriatus, and Psorophora ferox, and WN viral RNA was detected in Anopheles punctipennis. An elevated weekly minimum infection rate (MIR) for Cx. pipiens and increased dead bird density were present for 2 weeks before the first human illness occurred. Increasing mosquito MIRs and dead bird densities in an area may be indicators of an increasing risk for human infections. A transmission model is proposed involving Cx. pipiens and Cx. restuans as the primary enzootic and epizootic vectors among birds, Cx. salinarius as the primary bridge vector for humans, and Aedes/Ochlerotatus spp. as bridge vectors for equine infection.     

Ochlerotatus j. japonicus in Frederick County, Maryland: discovery, distribution, and vector competence for West Nile virus

Ochlerotatus japonicus japonicus is reported for the 1st time south of the Mason-Dixon Line, in Frederick County, Maryland. Fifty-seven oviposition trap samples were collected throughout the county between June 30 and August 24, 2000. From 971 larvae reared from the oviposition traps, 5 species were identified: Ochlerotatus triseriatus (45%), Oc. j. japonicus (43%), Aedes albopictus (7%), Culex pipiens (4%), and Toxorynchites ritulus septentrionalis (<1%). Ochlerotatus j. japonicus was found widely distributed over the area sampled. This is the 1st record of Ae. albopictus in the county as well. Vector competence studies indicated that Oc. j. japonicus is an efficient laboratory vector of West Nile (WN) virus. Depending on the viral titer at time of feeding, the estimated transmission rates for Oc. j. japonicus for WN virus were 2-4 times higher than that for Cx. pipiens. Studies of the viral titer in mosquitoes over time showed that titers in the bodies of infected Oc. j. japonicus reached their peak (approximately 10(6.5) plaque-forming units/mosquito) between 7 and 11 days after taking an infectious blood meal, and that virus became detectable in the legs (an indicator of disseminated infection) as early as 3 days after taking an infectious blood meal.     

Emergence patterns of Culex mosquitoes at an experimental constructed treatment wetland in southern California

The emergence patterns of mosquitoes inhabiting a 0.1-ha experimental wetland in southern California were monitored using emergence traps during the late summer and autumn of 1996. Culex erythrothorax was the largest contributor to emerging populations, comprising 94% of the total emerged adults with an average emergence rate of 59 adults/day/m2. None of the Culex species exhibited a pattern of emergence associated with water depth (range: 5-60 cm). Culex quinquefasciatus and Cx. tarsalis did not show a pattern of emergence associated with the inflow-outflow gradient; however, Cx. erythrothorax emerged in higher numbers along a transect at the middle of the wetland than from near the inflow and outflow. Additionally, the number of emerged Cx. erythrothorax was positively correlated with the density of vegetation below emergence traps. The comparatively large number of adults emerging from the middle of the wetland was most likely caused by a trade-off between an increasing gradient of resource abundance and a decreasing gradient of toxic compounds from the inflow to the outflow of the small wetland.     

上海口岸蚊虫与分种检索研究

目的了解上海口岸蚊虫种群和分布。方法灯诱法和捞勺法。结果2005—2007年间上海口岸共捕获蚊虫6属18种,其中按蚊属1种,伊蚊属3种,阿蚊属1种,库蚊属10种,曼蚊属1种,骚扰蚊属2种,背点骚扰蚊在上海首次捕获。广泛分布的有致倦库蚊、淡色库蚊、三带喙库蚊、中华按蚊和白纹伊蚊。结论致倦库蚊、淡色库蚊、三带喙库蚊、中华按蚊和白纹伊蚊是上海口岸可能骚扰人的常见蚊虫。     

Stormwater ponds, constructed wetlands, and other best management practices as potential breeding sites for West Nile virus vectors in Delaware during 2004

We performed longitudinal surveys of mosquito larval abundance (mean mosquito larvae per dip) in 87 stormwater ponds and constructed wetland in Delaware from June to September 2004. We analyzed selected water quality factors, water depth, types of vegetation, degree of shade, and level of insect predation in relation to mosquito abundance. The 2004 season was atypical, with most ponds remaining wet for the entire summer. In terms of West Nile virus (WNV) vectors, wetlands predominantly produce Aedes vexans, culex pipiens pipiens, and Culex restuans. Retention ponds generally produced the same species as wetlands, except that Cx. p. pipiens was more abundant than Cx. restuans in retention ponds. Aedes vexans and Culex salinarius were the most abundant species to Conservation Restoration Enhancement Program ponds. Sand filters uniquely produced high numbers of Cx. restuans, Cx. p. pipiens, and Aedes japonicus japonicus, a newly invasive vector species. Site that alternately dried and flooded, mostly detention ponds, forebays of retention ponds, and some wetlands often produced Ae. vexans, an occasional WNV bridge vector species. Overall, seasonal distribution of vectors was bimodal, with peaks occurring during early and late summer. Ponds with shallow sides and heavy shade generally produced an abundance of mosquitoes, unless insect predators were abundant. Bright, sunny ponds with steep sides and little vegetation generally produced the fewest mosquitoes. The associations among mosquito species and selected vegetation types are discussed.     

广东省清远市快速城市化过程中蚊虫群落动态变化规律

目的 研究城市化过程中病媒蚊虫群落特征和演替动态变化,了解主要传播媒介蚊虫种群动态及与城市化的关系.方法 采用人诱法,连续4年在清远市调查蚊虫种群密度.结果 此次调查清远市蚊虫有致倦库蚊、骚扰阿蚊、白纹伊蚊、中华按蚊和三带喙库蚊5种;致倦库蚊为优势蚊种,占群落种群数量的92.27％;其次为骚扰按蚊,占3.98％;白纹伊蚊和三带喙库蚊数量较少,各占1.62％;中华按蚊仅占0.51％.在研究期间,比较2006年与2008、2009年数据,结果表明群落构成发生显著变化,其中致倦库蚊在种群中所占比例逐渐减少,白纹伊蚊所占比例逐渐增加,中华按蚊在种群中逐渐消失,三带喙库蚊在群落中数量波动较大.不同蚊种在夜间呈现不同活动节律,致倦库蚊和三带喙库蚊19:00-22:00其密度逐渐增加,高峰主要在21:00之后;白纹伊蚊在夜晚各个时间段都有活动,活动高峰主要在20:00之前;骚扰阿蚊呈现与白纹伊蚊相似的活动规律.结论 清远市在城市化过程中,登革热媒介白纹伊蚊逐渐增加,而流行性乙型脑炎(乙脑)媒介三带喙库蚊则在不同年份中波动较大;传疟媒介中华按蚊逐渐减少,甚至消失.因此在城市化过程中,需要加强登革热和乙脑媒介的监测和防控.     

Development time of Culex mosquitoes in stormwater management structures in California

A widely recommended strategy to minimize mosquito production in structural stormwater Best Management Practices (BMPs) is to ensure they hold captured water for no more than 72 h. However, this standard may be overly conservative for many mosquito species found in urban environments and may impede or prevent the capacity of BMPs to fulfill more stringent water quality standards in environmentally sensitive areas. Egg-to-pupa development of Culex tarsalis, Cx. pipiens, and Cx. quinquefasciatus were examined during July, August, and September 2006 in stormwater management basins and in water collected from these basins in 3 climatically distinct regions of California: the Lake Tahoe Basin, Sacramento Valley, and Los Angeles Basin. The observed minimum times to pupal development were 6 days for Cx. tarsalis and Cx. quinquefasciatus and 8 days for Cx. pipiens. Multiple linear regression models were used to estimate minimum predicted development times under optimal conditions for each region. The results suggest that water residence times of up to 96 h will not significantly increase the potential for Culex mosquito production in stormwater BMPs in the 3 regions included in this study.     

The importance of agricultural tire habitats for mosquitoes of public health importance in New York State

The presence of mosquito species breeding in agricultural "bunker tires" on dairy farms in New York state was evaluated. Twelve species of mosquitoes (Aedes vexans, Anopheles barberi, An. punctipenriis, An. perplexens, Culex pipiens pipiens, Cx. restuans, Cx. salinarius, Cx. territans, Toxorhynchites rutilus septentrionalis, Ochlerotatus atropalpus, Oc. japonicus japonicus, and Oc. triseriatus) were collected from 8 New York state dairy farms in 2001 and from 17 dairy farms in 2002. All but 2 of these species have been found naturally infected with arboviruses and could be important vectors affecting human and animal health. Because of the potential public and animal health importance of the invasive species Oc. j. japonicus, active surveillance to identify breeding sites and distribution of this mosquito is essential. In 2001, Oc. j. japonicus was recovered from 5 of 8 central New York dairy farms surveyed. In 2002, Oc. j. japonicus was recovered from 4 of the same dairy farms plus an additional dairy out of 5 surveyed. This species appears to be established on dairy farms in the south, central, and eastern regions of New York state, with greatest abundance in the southeastern region. A single Oc. j. japonicus larva was collected from the northern region on the final sampling date in September 2002. Our data demonstrate that agricultural tire habitats can be productive breeding sites for arbovirus vectors. As a consequence, these habitats should not be ignored in vector control and surveillance programs.     

Experimental transmission of St. Louis encephalitis virus by Ochlerotatus j. japonicus

Ochlerotatus japonicus japonicus a newly discovered nonindigenous mosquito species in North America, and a colonized strain of Culex pipiens were compared for their vector competence for St. Louis encephalitis virus (SLE). Infection rates in Oc. j. japonicus were 0-33% after feeding on chickens with viremias between 10(4.1) and 10(4.7) plaque-forming units (PFU)/ml of blood. In comparison, infection rates were 12-94% for Cx. pipiens that fed on the same chickens. When fed on chickens with viremias between 10(5.3) and 10(5.6) PFU/ml of blood, infection rates for Oc. j. japonicus and Cx. pipiens were similar, 96% and 100%, respectively. After 12-14 days of extrinsic incubation at 26 degrees C, all 34 infected Oc. j. japonicus had a disseminated infection. In contrast, only 23 (43%) of 54 infected Cx. pipiens had a disseminated infection after feeding on the same chickens. If they developed a disseminated infection, both species efficiently transmitted (> or = 87%) SLE. Estimated transmission rates at viral doses sufficient to infect both of the tested species were 29-84% for Oc. j. japonicus and 30-50% for Cx. pipiens. Because of its continued geographic expansion, field and laboratory evidence incriminating it as a vector of the closely related West Nile virus, and its ability to transmit SLE in the laboratory, Oc. j. japonicus should be considered as a potential enzootic or epizootic vector of SLE.     

Improved high-throughput method for molecular identification of Culex mosquitoes

Culex pipiens, Cx. restuans, and Cx. salinarius play important and most likely different roles in transmission of West Nile virus (WNV) in the northeastern United States. While Cx. pipiens and Cx. restuans are considered the main enzootic vectors of WNV, Cx. salinarius may be involved in epizootic cycles due to its broader host preferences. Accurate morphological identification of field-collected Culex specimens may be difficult, and therefore the New York State Department of Health arbovirus surveillance program allows combined Cx. pipiens and Cx. restuans pools to be tested for WNV. We have developed a modified and improved DNA isolation protocol using proteinase K digestion without traditional mosquito trituration and nucleic acid extraction to permit high-throughput screening of a large number of Culex specimens for species identification using polymerase chain reaction (PCR). This method utilizes a 96-well-plate format and a novel 1-step crude extraction procedure using proteinase K to obtain genomic DNA template from 1 mosquito leg in sufficient quantity for at least 2 standard 50-microl PCR reactions. Proteinase K digestion of legs from individual Culex mosquitoes was performed and used for PCR amplification with previously described species-specific ribosomal DNA primers. Using these rDNA primers, our modified proteinase K method successfully identified 91% to 100% of the Culex samples.