Epidemiology of West Nile Virus in Connecticut: A Five-Year Analysis of Mosquito Data 1999–2003

Abstract

Two hundred and ten isolations of West Nile virus (WNV) were obtained from 17 mosquito species in six genera in statewide surveillance conducted in Connecticut from June through October, 1999-2003. Culex pipiens (86), Culex salinarius (32), Culex restuans (26), Culiseta melanura (32), and Aedes vexans (12) were implicated as the most likely vectors of WNV in the region based on virus isolation data. Culex pipiens was abundant from July through September and is likely involved in early season enzootic transmission and late season epizootic amplification of the virus in wild bird populations. Epidemic transmission of WNV to humans in urban locales is probable. The abundance of Cx. restuans in June and July and isolations of WNV in early July suggest that this species may play an important role as an enzootic vector involved in early amplification of WNV virus among wild birds. Its involvement as a bridge vector to humans is unlikely. Culex salinarius was the most frequently captured Culex species and was abundant in August and September when virus activity was at its height. Frequent isolations of WNV from this species in September when the majority of human cases were reported in union with its abundance at this time of the year, demonstrated vector competence, and broad feeding habits, make Cx. salinarius a likely bridge vector to humans, horses and other mammals. Multiple isolations WNV from Cs. melanura collected in more rural locales in late August and September, provide supportive evidence to suggest that this predominant avian feeder may play a significant role in epizootic amplification of the virus among wild bird populations in these environs. Aedes vexans was the only species of Aedes or Ochlerotatus from which multiple isolations of WNV were made in more than one year and was among the most frequently trapped and abundant species throughout the season. Since Ae. vexans predominately feeds on mammals it is unlikely to play a significant role in epizootic amplification of WNV, however, because of its abundance and aggressive mammalian and human biting behavior it must receive strong consideration as a bridge vector to humans and horses. The occasional virus isolations obtained from Aedes cinereus (4), Uranotaenia sapphirina (3), Ochlerotatus canadensis (2), Ochlerotatus trivittatus (2), Ochlerotatus sollicitans (2), Ochlerotatus sticticus (2), Psorophora ferox (2), Anopheles punctipennis, Anopheles walkeri, Ochlerotatus cantator, Ochlerotatus taeniorhynchus, and Ochlerotatus triseriatus in conjunction with their inefficient vector competency and host feeding preferences indicate that these species likely play a very minor role in either the enzootic maintenance or epizootic transmission of WNV in this region. The principal foci of WNV activity in Connecticut were identified as densely populated (>3,000 people/mi2) residential communities in coastal Fairfield and New Haven Counties, and in the case of 2002, similar locales in proximity of the city of Hartford in central Hartford County. In almost all insta...

Host Feeding Patterns ofCulexMosquitoes and West Nile Virus Transmission, Northeastern United States

“…pipiens in surveillance activities conducted over the last 6 years ( 6 , 10 , 14 , 15 ) clearly incriminates this species as the predominant mosquito vector in this region. However, while enzootic transmission to birds is strongly supported by a number of host-preference studies on regional populations ( 8 , 9 , 16 – 18 ), no consensus has been reached on the role of Cx.…”

“…This interpretation was viewed as consistent with the incidence of human cases in 3 densely populated urban areas of Connecticut in 2002, where most viral isolations (78%) were from Cx. pipiens ( 6 ). Kilpatrick et al ( 7 ), integrating WNV testing data from New York from 2000 to 2003 with information on mosquito abundance, infection prevalence, vector competence, and biting behavior, further suggested that Cx.…”

“…Mosquitoes were collected from 31 different sites in 6 counties in Connecticut during a 3-year period (June through October, 2002–2004) as part of a statewide surveillance program ( 6 ) and a focused trapping effort in Fairfield County ( 10 ). Most (71%) of the mosquito collection sites were located in densely populated residential locales along the urban/suburban corridor that extends from lower Fairfield and New Haven Counties, where high levels of WNV activity were recorded (Figure, Table 1).…”

“…restuans , and Cx. salinarius have been implicated as the principal vectors because they are physiologically competent ( 5 ), frequently infected with the virus in nature, and closely associated with WNV transmission foci ( 6 ). However, the precise role that each of these species plays in enzootic transmission among birds or epidemic transmission to humans is not entirely clear.…”

“…salinarius and prevalence of WNV infection in this species often approach those of Cx. pipiens ( 6 ). Observations in rural and urban sites in New York further indicate that Cx.…”