Larval habitats of potential mosquito vectors of West Nile virus in the Florida Keys

The occurrence of larvae of two potential vectors of West Nile virus, Culex nigripalpus and Culex quinquefasciatus, was examined in the Florida Keys. About half of the aquatic habitats examined contained larvae of either one or both of the species. Culex quinquefasciatus was the most frequently encountered species, whereas only 9% of habitats sampled contained Culex nigripalpus. Over half of those samples that contained Culex nigripalpus also contained Culex quinquefasciatus. The two species utilize similar larval habitats in the Florida Keys, Monroe County, Florida, USA.     

Selected mosquito vectors of West Nile virus: comparison of their ecological dynamics in four woodland and marsh habitats in Delaware

We compared human landing, CO2-baited light trap, CO2-baited Omni-Fay trap, and gravid trap collections of mosquitoes at 4 woodland and marsh sites with varied vegetative habitats in Delaware during summer 2001. Landing collections provided more consistent, but sometimes smaller, collection numbers than light traps. Proportions of parous mosquitoes were also higher in landing than in light trap or Omni-Fay collections about 65% of the time. Circadian feeding rhythms were observed for 3 suspected vector species selected on the basis of 1999-2000 laboratory vector competence data and their presence at local habitats. These species included Aedes vexans, Culex salinarius, and Cx. pipiens pipiens. Aedes vexans fed after dusk, primarily before 2200 h and again around dawn (0600-0700 h). Both Cx. salinarius and Cx. p. pipiens exhibited peak feeding activity after dusk and in predawn periods, as well as extended feeding periods up until midnight. Time of feeding is an important factor in evaluating vector-host associations and risks of human outbreaks.     

Host-feeding patterns of suspected West Nile virus mosquito vectors in Delaware, 2001-2002

Paucity of data on host-feeding patterns and behavior of 43 mosquito species that are reported as suspected West Nile virus (WN) vectors has limited full evaluation of their vectorial capacity. Recent studies addressing this issue need additional confirmation and should also be expanded to include collections of species or subpopulations attracted to humans. We used 4 types of collection methods to collect mosquitoes, including omnidirectional Fay-Prince traps, Centers for Disease Control-type light traps, gravid traps, and human-landing collections. Mosquitoes were collected during 2 full WN transmission seasons in 2001 and 2002, at 9 different sites across Delaware. We collected from various habitats, including salt marshes, brackish water areas, woodlands, a tire dump, a racetrack, and a mushroom farm. Blood-meal analyses were performed on parous mosquitoes by using a sandwich-type enzyme-linked immunosorbent assay. We tested primarily for 5 common host species, including rabbits, dogs, deer, horses, and chickens. We obtained substantial host-feeding data from 8 mosquito species. The most mammalophilic species were Anopheles quadrimaculatus, Coquillettidia perturbans, and Aedes albopictus, whereas the most ornithophilic species was Culex pipiens. Aedes albopictus was the most anthropophilic species, whereas Ae. vexans and Cq. perturbans exhibited relatively low attraction to humans. Culex salinarius was the species with the most diverse host-feeding activity. Based on feeding behavior, Cx. pipiens and Cx. salinarius appear to be the most likely bridge vectors. Other species may have opportunities to be bridge vectors under special circumstances, as discussed in the paper.     

The potential for West Nile virus to establish outside of its natural range: a consideration of potential mosquito vectors in the United Kingdom

Outbreaks of West Nile virus (WNV) infection have occurred sporadically in Europe, apparently due to the migration of infected birds and the subsequent establishment of a transmission cycle involving culicine and anopheline mosquitoes. Both human and equine species become infected, but are considered as dead end hosts since they play an insignificant role in the maintenance of the cycle. Following the introduction of WNV into the United States in 1999 it is increasingly apparent that the virus has an extraordinary ability to infect a very broad range of arthropod species. Here we consider the potential for British mosquitoes to transmit WNV in the event that it is introduced into the UK.     

江苏省西尼罗病毒病潜在蚊媒库蚊发生情况调查研究

目的了解江苏省西尼罗病毒病潜在蚊媒库蚊的构成及发生消长情况,为及时有效地切断西尼罗病毒病传播途径提供科学依据。方法2004年4~11月在江苏省苏州、镇江和连云港3市的城区和郊区设点,采用人工小时法和帐诱法调查成蚊。结果全省西尼罗河病毒病潜在蚊媒库蚊由淡色库蚊、三带喙库蚊和二带喙库蚊等3种构成;全省库蚊平均发生密度野外高于室内,郊区室内高于城区室内。室内平均密度镇江显著高于其他两市,野外平均密度连云港最高。优势种淡色库蚊的消长为双峰型,次优势种三带喙库蚊的消长为单峰型。结论基本了解了江苏省西尼罗病毒病潜在蚊媒库蚊的群落构成及消长情况,尚需掌握流行病学证据证明确为西尼罗河病毒病蚊媒。     

Comparison of mosquito trapping method efficacy for West Nile virus surveillance in New Mexico

As part of the West Nile virus surveillance program for the state of New Mexico, 13 sites along the Rio Grande River were sampled for mosquitoes during spring and summer 2003. We evaluated 3 different trapping procedures for their effectiveness at capturing selected species of mosquitoes. The 3 methods used were a dry ice-baited Centers for Disease Control and Prevention (CDC) light trap set 1.5 m above the ground (standard method), a CDC light trap suspended within the forest canopy, and a gravid trap set on the ground. Thirteen sites were sampled for 10 1-night periods biweekly from May through September. The relative numbers of captured Culex tarsalis, Cx. salinarius, Cx. quinquefasciatus, and Aedes vexans as well as the numbers of total recorded captures of all species were compared for each trapping method. Significant differences were observed for each species by location and by trapping method. Culex tarsalis was most commonly caught in canopy or standard CDC traps, especially in cottonwood bosque. Culex salinarius was found most frequently in association with marshy water, and was most often caught in gravid or standard light traps. Culex quinquefasciatus was captured almost exclusively in gravid traps within urban areas. Aedes vexans was primarily sampled in standard CDC light traps and found most frequently in wooded areas near floodplains. With the exception of Cx. Quinquefasciatus, no species was collected significantly more frequently in gravid or canopy traps than in the standard CDC light trap. Our findings do not support altering the methods currently used in New Mexico, namely, the use of 1.5-m CDC light traps and gravid traps. An increased use of gravid traps seems to be warranted in monitoring urban vector populations (specifically Cx. quinquefasciatus and Cx. salinarius) that may be involved in human transmission.     

Potential mosquito vectors of arboviruses in Portugal: species, distribution, abundance and West Nile infection

Circulation of West Nile virus in Portugal was demonstrated by serological surveys, and the virus was isolated in 1969 from Anopheles maculipennis s.l. A survey of the whole country was carried out (2001-2004) to assess the abundance of mosquito species and to screen them for arbovirus infection. A total of 770 collections yielded 32460 mosquitoes of 15 species. The regions with the highest abundance of mosquitoes were the coastal and estuarine districts of Santarém, Setúbal and Faro. Culex pipiens s.l., An. maculipennis s.l., Cx. theileri and Ochlerotatus caspius were the most abundant and widespread, accounting for 92% of mosquitoes caught. Anopheles maculipennis s.l. and Cx. pipiens s.l. were present all over the country. Culex theileri and. Oc. caspius were more abundant in the southern and coastal areas, respectively. A total of 2355 mosquito pools were screened by RT-PCR for flaviviruses, of which 987 pools were also screened for bunyaviruses. Culex pipiens s.l. and Cx. univittatus collected in 2004 in the southern district of Faro were found to be infected with West Nile virus. The density and proximity of these mosquitoes to the human populations may constitute a public health threat in the case of involvement in arbovirus transmission cycles.     

A human-health risk assessment for West Nile virus and insecticides used in mosquito management

West Nile virus (WNV) has been a major public health concern in North America since 1999, when the first outbreak in the Western Hemisphere occurred in New York City. As a result of this ongoing disease outbreak, management of mosquitoes that vector WNV throughout the United States and Canada has necessitated using insecticides in areas where they traditionally have not been used or have been used less frequently. This has resulted in concerns by the public about the risks from insecticide use. The objective of this study was to use reasonable worst-case risk assessment methodologies to evaluate human-health risks for WNV and the insecticides most commonly used to control adult mosquitoes. We evaluated documented health effects from WNV infection and determined potential population risks based on reported frequencies. We determined potential acute (1-day) and subchronic (90-day) multiroute residential exposures from each insecticide for several human subgroups during a WNV disease outbreak scenario. We then compared potential insecticide exposures to toxicologic and regulatory effect levels. Risk quotients (RQs, the ratio of exposure to toxicologic effect) were < 1.0 for all subgroups. Acute RQs ranged from 0.0004 to 0.4726, and subchronic RQs ranged from 0.00014 to 0.2074. Results from our risk assessment and the current weight of scientific evidence indicate that human-health risks from residential exposure to mosquito insecticides are low and are not likely to exceed levels of concern. Further, our results indicate that, based on human-health criteria, the risks from WNV exceed the risks from exposure to mosquito insecticides.     

West Nile virus infection in organ donor and transplant recipients--Georgia and Florida, 2002

On August 23, 2002, the Georgia Division of Public Health (GDPH) and CDC were notified of two cases of unexplained fever and encephalitis in recipients of organ transplants from a common donor. An investigation has identified illness in two other recipients from the same donor: one with encephalopathy and the other with febrile illness. CDC, the Food and Drug Administration, GDPH, and the Florida Department of Health are conducting the investigation. This duster could possibly represent the first recognized transmission of West Nile virus (WNV) by organ donation.     

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.     

West Nile virus and the climate

West Nile virus is transmitted by urban-dwelling mosquitoes to birds and other animals, with occasional “spillover” to humans. While the means by which West Nile virus was introduced into the Americas in 1999 remain unknown, the climatic conditions that amplify diseases that cycle among urban mosquitoes, birds, and humans are warm winters and spring droughts. This information can be useful in generating early warning systems and mobilizing timely and the most environmentally friendly public health interventions. The extreme weather conditions accompanying long-term climate change may also be contributing to the spread of West Nile virus in the United States and Europe.     

West Nile virus expanding in Europe

The areas of Europe in which West Nile virus (WNV)-transmission to humans is observed have expanded over the last few years, with endemic circulation amongst animals of southern Europe. This situation calls for heightened vigilance to the clinical presentation of WNV infection in humans. The average incubation period lasts 2-6 days. Of those infected, 20% will experience a mild, non-specific disease presentation such as high fever, headache, myalgia, possibly with rash and lymphadenopathy; <1% will develop severe neurological symptoms. Rare complications include: myelitis, optic neuritis, rhombencephalitis, polyradiculitis, myocarditis, pancreatitis and fulminant hepatitis. Clinicians should take WNV infection into consideration when making a differential diagnosis for such symptoms in patients who have returned from areas with potential virus circulation. Given the increase in the spread of WNV within Europe, this now holds true for continental travellers as well as those destined for the Americas, Africa and Asia. It is important to include the patient's travel history, clinical symptoms and any occurrences of vaccination against viruses causing Japanese encephalitis, tick-borne encephalitis and yellow fever into the diagnostic workup, as the antibodies against these diseases show cross-reactivity.     

West Nile virus, Guadeloupe

To determine whether West Nile virus (WNV) had reached the archipelago of Guadeloupe, a serologic study in horses and birds was conducted in 2002. Immunoglobulin (Ig) G, IgM, enzyme-linked immunosorbent assay, and seroneutralization tests identified WNV infection in horses and chickens. Six months later, a high rate of seroconversion was observed in horses.