Laboratory transmission potential of British mosquitoes for equine arboviruses

BACKGROUND

There has been no evidence of transmission of mosquito-borne arboviruses of equine or human health concern to date in the UK. However, in recent years there have been a number of outbreaks of viral diseases spread by vectors in Europe. These events, in conjunction with increasing rates of globalisation and climate change, have led to concern over the future risk of mosquito-borne viral disease outbreaks in northern Europe and have highlighted the importance of being prepared for potential disease outbreaks. Here we assess several UK mosquito species for their potential to transmit arboviruses important for both equine and human health, as measured by the presence of viral RNA in saliva at different time points after taking an infective blood meal.

RESULTS

The following wild-caught British mosquitoes were evaluated for their potential as vectors of zoonotic equine arboviruses: Ochlerotatus detritus for Venezuelan equine encephalitis virus (VEEV) and Ross River virus (RRV), and Culiseta annulata and Culex pipiens for Japanese encephalitis virus (JEV). Production of RNA in saliva was demonstrated at varying efficiencies for all mosquito-virus pairs. Ochlerotatus detritus was more permissive for production of RRV RNA in saliva than VEEV RNA. For RRV, 27.3% of mosquitoes expectorated viral RNA at 7 days post-infection when incubated at 21 °C and 50% at 24 °C. Strikingly, 72% of Cx. pipiens produced JEV RNA in saliva after 21 days at 18 °C. For some mosquito-virus pairs, infection and salivary RNA titres reduced over time, suggesting unstable infection dynamics.

CONCLUSIONS

This study adds to the number of Palaearctic mosquito species that demonstrate expectoration of viral RNA, for arboviruses of importance to human and equine health. This work adds to evidence that native mosquito species should be investigated further for their potential to vector zoonotic mosquito-borne arboviral disease of equines in northern Europe. The evidence that Cx. pipiens is potentially an efficient laboratory vector of JEV at temperatures as low as 18 °C warrants further investigation, as this mosquito is abundant in cooler regions of Europe and is considered an important vector for West Nile Virus, which has a comparable transmission ecology.

A Survey for Aedes aegypti in Delaware and the Virus-Positive Pool Rates of Aedes albopictus and Aedes triseriatus for West Nile and Zika Viruses

The introduction of Zika virus to the USA in 2015 engendered heightened interest in its known vectors. Aedes aegypti is the primary vector, with Ae. albopictus considered a potential secondary vector, together with several other possible marginal vectors. In Delaware, Ae. aegypti has been collected rarely, but no breeding populations were detected during past intensive statewide surveillance efforts. However, there is an abundance of Ae. albopictus statewide. Both species are container breeders and are peri-domestic-increasing the risk for virus transmission to humans. From July through September 2017, Delaware Mosquito Control conducted surveillance in 16 container-breeding hot spots to search for Ae. aegypti, and also ascertain the virus-positive pool rates of Ae. albopictus and Ae. triseriatus for West Nile virus (WNV) and Zika virus (ZIKV). The survey concluded that there were no known breeding populations of Ae. aegypti in Delaware, and no WNV- or ZIKV-positive pools were detected among pools of mosquitoes of the aforementioned species.

Comparative Susceptibility of Ochlerotatus japonicus, Ochlerotatus triseriatus, Aedes albopictus, and Aedes aegypti (Diptera: Culicidae) to La Crosse Virus

Invasive mosquito species can increase the transmission risk of native mosquito-borne diseases by acting as novel vectors. In this study, we examined the susceptibility of three exotic invasive mosquito species Aedes aegypti (L.), Ae. albopictus (Skuse), and Ochlerotatus japonicus (Theobald) to La Crosse virus (LACV) relative to the native primary vector Ochlerotatus triseriatus (Say). Adult females of the four mosquito species were orally challenged with LACV; incubated for 3, 5, 7, 9, or 11 d; and their midgut infection rates, dissemination rates, and effective vector competence were determined. Overall, Oc. japonicus (2.92) had the highest effective vector competence values, followed by Ae. albopictus (1.55), Ae. aegypti (0.88), and Oc. triseriatus (0.64). In addition, we assessed the relationship between mosquito size and LACV susceptibility for field-collected Oc. triseriatus and Oc. japonicus We hypothesized that smaller adults would be more susceptible to LACV; however, our results did not support this hypothesis. Infected Oc. triseriatus tended to be larger than exposed but uninfected females, while infected and uninfected Oc. japonicus were similarly sized. These findings suggest that Oc. japonicus, Ae. albopictus, and Ae. aegypti have significant potential to transmit LACV and more research is needed to uncover their potential role in LACV epidemiology.

Detection and genome analysis of a novel (dima)rhabdovirus (Riverside virus) from Ochlerotatus sp. mosquitoes in Central Europe

During an investigation for potential arboviruses present in mosquitoes in Hungary (Central Europe) three highly similar virus strains of a novel rhabdovirus (family Rhabdoviridae) called Riverside virus (RISV, KU248085-KU248087) were detected and genetically characterized from Ochlerotatus sp. mosquito pools collected from 3 geographical locations using viral metagenomic and RT-PCR methods. The ssRNA(-) genome of RISVs follows the general genome layout of rhabdoviruses (3'-N-P-M-G-L-5') with two alternatives, small ORFs in the P and G genes (Px and Gx). The genome of RISVs contains some unusual features such as the large P proteins, the short M proteins with the absence of N-terminal region together with the undetectable "Late budding" motif and the overlap of P and M genes. The unusually long 3' UTRs of the M genes of RISVs probably contain a remnant transcription termination signal which is suggesting the presence of an ancestral gene. The phylogenetic analysis and sequence comparisons show that the closest known relative of RISVs is the recently identified partially sequenced mosquito-borne rhabdovirus, North Creek virus (NOCRV), from Australia. The RISVs and NOCRV form a distinct, basally rooted lineage in the dimarhabdovirus supergroup. The host species range of RISVs is currently unknown, although the presence of these viruses especially in Ochlerotatus sp. mosquitoes which are known to be fierce biting pests of humans and warm-blooded animals and abundant and widespread in Hungary could hold some potential medical and/or veterinary risks.

Container Type Influences the Relative Abundance, Body Size, and Susceptibility of Ochlerotatus triseriatus (Diptera: Culicidae) to La Crosse Virus

Ochlerotatus triseriatus (Say), the primary vector of La Crosse virus (LAC), develops in a variety of natural and artificial aquatic containers where it often co-occurs with larvae of other mosquito species. We conducted a field study at two woodlots (South Farms and Trelease Woods) in Urbana, IL, to examine how container type influences vector abundance, body size, and susceptibility to LAC. Mosquito pupae were collected from tree holes, plastic bins, and waste tires, and eclosing adults were identified to species morphologically. Oc. triseriatus and Ochlerotatus japonicus (Theobald) females were orally challenged with LAC and midgut infection rate, disseminated infection rate, and body titer were determined by reverse-transcriptase real-time PCR. Oc. triseriatus was the dominant species collected in tree holes while Oc. japonicus and Culex restuans (Theobald) were mostly dominant in artificial containers. Female Oc. triseriatus and Oc. japonicus collected from plastic bins were significantly larger than those collected from tree holes or waste tires. Oc. japonicus females from South Farms were also significantly larger than those from Trelease Woods. Oc. triseriatus females collected from plastic bins and waste tires were significantly more susceptible to LAC infection relative to females collected from tree holes. In addition, Oc. triseriatus females from waste tires had significantly higher LAC titer relative to Oc. triseriatus from tree holes. For each container type and study site, wing length was not correlated to infection or dissemination rates. These findings suggest that the container type in which Oc.triseriatus develop may contribute to the spatial and temporal dynamics of LAC transmission.

Evaluation of a temperate climate mosquito, Ochlerotatus detritus (=Aedes detritus), as a potential vector of Japanese encephalitis virus

The U.K. has not yet experienced a confirmed outbreak of mosquito-borne virus transmission to people or livestock despite numerous autochthonous epizootic and human outbreaks of mosquito-borne diseases on the European mainland. Indeed, whether or not British mosquitoes are competent to transmit arboviruses has not been established. Therefore, the competence of a local (temperate) British mosquito species, Ochlerotatus detritus (=Aedes detritus) (Diptera: Culicidae) for transmission of a member of the genus Flavivirus, Japanese encephalitis virus (JEV) as a model for mosquito-borne virus transmission was assessed. The JEV competence in a laboratory strain of Culex quinquefasciatus (Diptera: Culicidae), a previously incriminated JEV vector, was also evaluated as a positive control. Ochlerotatus detritus adults were reared from field-collected juvenile stages. In oral infection bioassays, adult females developed disseminated infections and were able to transmit virus as determined by the isolation of virus in saliva secretions. When pooled at 7-21 days post-infection, 13% and 25% of O. detritus were able to transmit JEV when held at 23 °C and 28 °C, respectively. Similar results were obtained for C. quinquefasciatus. To our knowledge, this study is the first to demonstrate that a British mosquito species, O. detritus, is a potential vector of an exotic flavivirus.

Molecular identification of Aedes triseriatus and Aedes hendersoni by a novel duplex polymerase chain reaction assay

Aedes triseriatus is the principal vector of La Crosse virus (LACv), which is the most common cause of pediatric arboviral encephalitis in North America. Here we report a novel species-specific polymerase chain reaction (PCR) assay that differentially identifies Ae. triseriatus and Ae. hendersoni. Because these 2 sibling species differ in their abilities to transmit LACv, accurate identification is critical for surveillance, research, and control programs. This duplex assay can detect the presence of both species in a single PCR reaction and is therefore simpler and faster than previously reported methods.

Effect of oral infection of La Crosse virus on survival and fecundity of native Ochlerotatus triseriatus and invasive Stegomyia albopicta

Arboviruses can have benign, deleterious, or beneficial effects on the vector. We tested the hypothesis that oral infection with La Crosse virus (LACV) will have little to no effect on mosquito longevity and fecundity, a prediction of low virulence selected in a system with frequent vertical transmission. We tested the effects of infection in native Ochlerotatus triseriatus Say and invasive Stegomyia albopicta Skuse (Diptera: Culicidae). We artificially fed adult female mosquitoes of each species with either LACV-infected or uninfected bovine blood and determined adult longevity and fecundity. For females fed LACV-infected blood, bodies and legs, respectively, were separately homogenized and assayed by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) to determine the LACV infection and dissemination rates. Ochlerotatus triseriatus had a higher infection and dissemination rate than St. albopicta. For both species, female size had no effect on infection status. Infection status also had no effect on longevity or fecundity for both species. We suggest that the high frequency of vertical transmission may have selected for strains of the virus with low virulence in two vectors, in spite of their different evolutionary histories with the virus.

Is New Zealand prepared to deal with arboviral diseases?

To now, New Zealand has been spared arboviral diseases, except for the odd imported infection. However, two exotic vector mosquitoes (Ochlerotatus camptorhynchus and Ochlerotatus notoscriptus) have become established in New Zealand and the routine arrival of infected people from overseas may soon lead to local arboviral transmission. Large populations of wild wallabies and Brushtail possums could serve as reservoirs of Ross River virus and other arboviruses. Several other exotic mosquito species, including important disease vectors such as Aedes albopictus and Aedes aegypti, have already been intercepted in New Zealand. It is clear that increased border surveillance is necessary if we are to interdict the establishment of such species. We recommend several measures that should be adopted, including increased training and awareness of public health officials, clinicians and the general public, and taking appropriate steps to make New Zealand self-sustainable in terms of arboviral surveillance, diagnosis, and prevention.

West Nile virus in host-seeking mosquitoes within a residential neighborhood in Grand Forks, North Dakota

West Nile virus (WNV) was first recovered in North Dakota near the city of Grand Forks in June 2002. During 2002, 2003, and 2004, we collected mosquitoes from Grand Forks using Mosquito Magnet traps and tested them for WNV. The seasonal abundance, species composition, and reproductive status of female mosquitoes were correlated with local environmental temperature and state surveillance data on WNV to determine the factors affecting local transmission of WNV. Over 90% of the mosquitoes collected were Aedes vexans, Ochlerotatus dorsalis, and Culex tarsalis, but WNV was detected only in Cx. tarsalis. Average summertime temperatures and relative abundance of mosquitoes were highest in 2002 but no WNV-positive mosquitoes were detected until the following summer. In 2003, nulliparous Cx. tarsalis appeared in mid-June (first summer brood), and parous Cx. tarsalis appeared in mid-July. The first WNV-positive pool occurred 21 July, and minimum daily infections rates increased thereafter until 27 August. The minimum infection rate (MIR) for Cx. tarsalis during the season was 5.7 infected mosquitoes per 1,000 tested, with the highest infection rates occurring at the end of the season as Cx. tarsalis populations started to decline. Mid-to-late August was identified as the period of highest risk for being bitten by a WNV-infected mosquito in Grand Forks during 2003. In 2004, viral activity in Grand Forks was low, due to very cool temperatures throughout the summer. To examine the genetic diversity of the 2003 WNV isolates from Grand Forks, we sequenced a 366-nucleotide region of the capsid and premembrane gene. Thirteen (46%) of the 28 WNV isolates contained at least one nucleotide substitution when compared to the homologous region of the progenitor WN NY-99 strain, and seven of these 13 substitutions coded for amino acid changes. Thus, WNV is established in North Dakota, it appears to be evolving and it is vectored primarily by Cx. tarsalis.

Early warning of Ross River virus epidemics: combining surveillance data on climate and mosquitoes

BACKGROUND

Ross River virus disease is spread by mosquitoes, and an average of 5000 people are infected each year in Australia. It is one of the few infectious diseases for which climate-based early warning systems could be developed. The aim of this study was to test whether supplementing routinely collected climate data with mosquito surveillance data could increase the accuracy of disease prediction models.

METHODS

We focused on a temperate region of Western Australia between July 1991 and June 1999. We developed "early" and "later" warning logistic regression models to test the sensitivity of data on climate (tide height, rainfall, sea surface temperature) and mosquito counts for predicting epidemics of disease.

RESULTS

Climate data on their own were moderately sensitive (64%) for predicting epidemics during the early warning period. Addition of mosquito surveillance data increased the sensitivity of the early warning model to 90%. The later warning model had a sensitivity of 85%.

CONCLUSIONS

We found that climate data are inexpensive and easy to collect and allow the prediction of Ross River virus disease epidemics within the time necessary to improve the effectiveness of public health responses. Mosquito surveillance data provide a more expensive early warning but add substantial predictive value.

West Nile virus surveillance in mosquitoes in New York State, 2000-2004

A West Nile virus (WNV) surveillance system was created and implemented in New York State (NYS) in 2000 and described previously (White et al. 2001). We examine and evaluate the results of mosquito and virus surveillance for 2000 through 2004 exclusive of New York City. Forty-nine counties submitted 1,095,426 mosquitoes in 35,280 pools for WNV assay. Specimens of 47 species were tested, with Culex species accounting for 47.6% of all pools tested. WNV was detected in 814 pools from 10 species, with combined Culex pipiens/Culex restuans pools accounting for 90.8% of all detections. Pools submitted from gravid traps were 5.7 times more likely to be positive than submissions from carbon dioxide-baited light traps. Most human WNV cases resided in counties that conducted mosquito surveillance. Local health departments' use of mosquito surveillance information often led to an enhanced disease prevention response. In NYS, Cx. pipiens/Cx. restuans groups are most likely vectors of WNV. Future efforts to improve system efficacy are discussed.

Does 1-octen-3-ol enhance trap collections of Japanese encephalitis virus mosquito vectors in northern Australia?

The responses of Japanese encephalitis virus (JEV) mosquito vectors to 1-octen-3-ol (octenol) and CO2 were evaluated using Centers for Disease Control (CDC) light traps at 3 sites in northern Australia. There was no significant difference between the number of Culex sitiens subgroup mosquitoes or Cx. gelidus collected in CDC light traps baited with either CO2 alone or CO2 + octenol on Badu Island. At both mainland locations, using octenol in combination with CO2 significantly increased collections of Cx. sitiens subgroup mosquitoes. Collections of nontarget species, such as Ochlerotatus spp., Anopheles spp., and Verrallina spp. were also significantly increased with the addition of octenol. At all 3 locations, reducing collections of nontarget mosquitoes by not using octenol increased the proportion of Culex spp. collected, thus potentially reducing the time and resources required to sort and process collections for JEV detection. Our results also indicate that trials into the efficacy of using octenol as an attractant should be carried out in each area prior to the implementation of a mosquito-based arbovirus surveillance system.

Vector competence of Peruvian mosquitoes (Diptera: Culicidae) for a subtype IIIC virus in the Venezuelan equine encephalomyelitis complex isolated from mosquitoes captured in Peru

We evaluated mosquitoes collected in the Amazon Basin, near Iquitos, Peru, for their susceptibility to a subtype IIIC strain of the Venezuelan equine encephalomyelitis complex. This virus had been previously isolated from a pool of mixed Culex vomerifer and Cx. gnomatos captured near Iquitos, Peru, in 1997. After feeding on hamsters with viremias of about 10(8) plaque-forming units of virus per ml, Cx. gnomatos was the most efficient vector. Other species, such as Ochlerotatus fulvus and Psorophora cingulata, although highly susceptible to infection, were not efficient laboratory vectors of this virus due to a significant salivary gland barrier. The Cx. (Culex) species, consisting mostly of Cx. (Cux.) coronator, were nearly refractory to subtype IIIC virus and exhibited both midgut infection as well as salivary gland barriers. Additional studies on biting behavior, mosquito population densities, and vertebrate reservoir hosts of subtype IIIC virus are needed to determine the role that these species play in the maintenance and spread of this virus in the Amazon Basin region.

Comparison of carbon dioxide- and octenol-baited encephalitis virus surveillance mosquito traps at the Shoal Water Bay Training area, Queensland, Australia

The use of octenol in combination with carbon dioxide (CO2)-baited encephalitis virus surveillance (EVS) mosquito traps was evaluated under simulated wartime operational conditions during Operation Tandem Thrust (TT01) at the Shoalwater Bay Training area, Queensland, Australia in 2001. A greater number of mosquito species were captured in traps baited with octenol plus CO2 than those baited with CO2 or octenol in the saltwater marsh, Freshwater Beach. In the inland environments of Camp Growl and Raspberry Creek, the addition of octenol did not significantly increase the numbers of mosquito species captured. Trap treatment (octenol only, CO2 only, or octenol plus CO2) influenced the species captured at Freshwater Beach. More Ochlerotatus vigilax, Mansonia uniformis, and Coquillettidia xanthogaster were captured in traps baited with octenol plus CO2, and more Anopheles were captured in traps baited with CO2 only. The most commonly captured (83%) mosquito species in the Shoalwater Bay Training Area during TT01 was the salt marsh breeder and Ross River virus vector, Oc. vigilax.

Evaluation of methods to assess transmission potential of Venezuelan equine encephalitis virus by mosquitoes and estimation of mosquito saliva titers

Determining the dose of an arbovirus transmitted by a mosquito is important to design transmission and pathogenesis studies simulating natural infection. Several different artificial infection and transmission methods used to assess vector competence and to estimate the dose injected during mosquito feeding have not been fully evaluated to determine whether they accurately reflect natural transmission. Additionally, it is not known whether different mosquito vectors transmit similar amounts of a given virus. Therefore, we compared three traditional artificial transmission methods using Venezuelan equine encephalitis virus (VEEV) and Aedes albopictus and Ochlerotatus taeniorhynchus mosquitoes. Both the mosquito species and the infection route used affected the amount of virus detected in the saliva after a 10-day extrinsic incubation period. Median titers of virus detected in saliva of Ae. albopictus and Oc. taeniorhynchus mosquitoes ranged from 0.2 to 1.1 log(10) (mean 0.7-1.4 log(10)) and 0.2 to 3.2 log(10) (mean 1.0-3.6 log(10)) plaque-forming units, respectively. The results of this study will aid in the design of transmission and pathogenesis studies involving arboviruses.

A comparision of West Nile Virus transmission by Ochlerotatus trivittatus (COQ.), Culex pipiens (L.), and Aedes albopictus (Skuse)

Transmission of West Nile virus (WNV) by Ochlerotatus trivittatus, Culex pipiens, and Aedes albopictus were compared 14 days after taking blood meals from viremic chickens with titers ranging from 10(2.5) to 10(9.5) cell infective dose (50)s (CID50s)/mL serum. Transmission occurred in one of four (25%) Oc. trivittatus and one of 25 (4%) Cx. pipiens that fed on chickens with titers of 10(5.5) CID50s/mL. No transmission occurred among two of 16 (13%) Oc. trivittatus or one of 25 (4%) Cx. pipiens that became infected after blood meals with titers of 10(5.0) and 10(4.5) CID50s/mL, the next lowest blood meal titers evaluated. Seventeen of 28 (61%) Ae. albopictus transmitted WNV after blood meals with titers of 10(7.0) CID50s/mL, but no infection or transmission was observed among 21 Ae. albopictus that fed on chickens with titers of 10(5.0) CID50s/mL, the next lowest titer evaluated. Transmission by all three species increased dramatically after blood meals with WNV titers of > or = 10(5.5) CID50s/mL. No significant differences occurred in dissemination and transmission rates of the three species after taking blood meals with titers of > 10(7.0) CID50s/mL. The cumulative mean +/- SE transmission rates of Oc. trivittatus, Cx. pipiens, and Ae. albopictus after blood meals with titers of > or = 10(7.0) CID50s/mL were 45.5 +/- 4.1%, 46.8 +/- 4.5%, and 72.4 +/- 5.5%. The cumulative mean dissemination rates of the three species were 78.3 +/- 6.7%, 74.8 +/- 2.6%, and 88.6 +/- 2.1%. The rates of transmission by the three species that developed disseminated infections after blood meals with titers of > or = 10(7.0) CID50s/mL were 58.8 +/- 4.4%, 62.6 +/- 5.8%, and 81.6 +/- 5.4%, respectively. In a previous study, we found that susceptibility of the three species to WNV was essentially the same when fed on chickens with WNV titers of > 10(7.0) CID50s/mL, but Oc. trivittatus and Cx. pipiens were more susceptible than Ae. albopictus to WNV at lower virus titers. The current study strongly suggests that Ae. albopictus is a more efficient vector than Oc. trivittatus and Cx. pipiens when fed blood meals with titers of > 10(7.0) CID50s/mL. However, Oc. trivittatus and Cx. pipiens might be more efficient as vectors when infected by blood meals with titers of < 10(7.0) CID50s/mL.

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.

Quantification of West Nile virus in vector mosquito saliva

Saliva was collected from 4 species of mosquitoes intrathoracically inoculated with West Nile virus (WNV). The amount of infectious virus in the saliva was quantified by plaque assay and the number of WNV genomic equivalents (GE) was measured by reverse transcriptase-polymerase chain reaction. Ochlerotatus triseriatus had the greatest mean amount of infectious virus per saliva collection, followed by Aedes albopictus, Culex pipiens, and Cx. quinquefasciatus. The mean GE/saliva collection was also greatest in Oc. triseriatus, followed by Cx. quinquefasciatus, Cx. pipiens, and Ae. albopictus. The variance of log GE/saliva collection for Ae. albopictus was significantly lower than the variance for the other 3 species. This study provides a basis for comparing this component of vector competence and for determining the amounts of virus inoculated into vertebrates in experimental host competence studies.

First report of West Nile virus in mosquitoes from Lubbock County, Texas

Since July 2002, ongoing surveillance efforts have been conducted to determine potential vectors of West Nile virus (WNV) and Saint Louis encephalitis virus (SLEV) in the mosquito population occurring in Lubbock County, Texas. Adult mosquitoes collected in Lubbock County during 2002 and 2003 represented 7 genera, with Culex tarsalis and Ochlerotatus sollicitans being the predominant species collected. Mosquitoes were initially screened for WNV and SLEV by using the VecTest antigen panel assay. Positive VecTest results were confirmed by reverse transcriptase-polymerase chain reaction. West Nile virus-positive pools of mosquitoes were detected in 2002 and 2003, with the majority of the positive pools consisting of Cx. tarsalis. None of the mosquito pools tested positive for SLEV.

Quantitative genetics of vector competence for La Crosse virus and body size in Ochlerotatus hendersoni and Ochlerotatus triseriatus interspecific hybrids

La Crosse virus is a leading cause of pediatric encephalitis in the United States. The mosquito Ochlerotatus triseriatus is an efficient vector for La Crosse virus, whereas the closely related O. hendersoni transmits only at very low rates. Quantitative trait loci (QTL) affecting the ability to orally transmit this virus and adult body size were identified in 164 F(2) female individuals from interspecific crosses of O. hendersoni females and O. triseriatus males using a combination of composite interval mapping (CIM), interval mapping (IM) for binary traits, and single-marker mapping. For oral transmission (OT), no genome locations exceeded the 95% experimentwise threshold for declaring a QTL using IM, but single-marker analysis identified four independent regions significantly associated with OT that we considered as tentative QTL. With two QTL, an increase in OT was associated with alleles from the refractory vector, O. hendersoni, and likely reflect epistatic interactions between genes that were uncovered by our interspecific crosses. For body size, two QTL were identified using CIM and a third tentative QTL was identified using single-marker analysis. The genome regions associated with body size also contain three QTL controlling OT, suggesting that these regions contain either single genes with pleiotropic effects or multiple linked genes independently determining each trait.

New mosquito records of an area known for Japanese encephalitis hyperendemicity, Gorakhpur District, Uttar Pradesh, India

During long-term ecological studies carried out between 1990 and 2000, a total of 67 mosquito species were collected from Gorakhpur District, Uttar Pradesh, India, of which 43 were documented for the 1st time. The genus Aedes was represented by 15 species, Anopheles by 15, Culex by 24, Mansonia by 3, and Mimomyia, Ochlerotatus, and Verrallina by 2 each. The genera Aedeomyia, Armigeres, Coquillettidia, and Uranotaenia were represented by single species. The impact of ecological changes on mosquito fauna in Gorakhpur District is discussed. The role of culicine mosquitoes in disease transmission in the area is highlighted.

Susceptibility of Ochlerotatus trivittatus (Coq.), Aedes albopictus (Skuse), and Culex pipiens (L.) to West Nile Virus Infection

The susceptibility of Ochlerotatus trivittatus (Coq.) to West Nile virus (WNV) was assessed by comparing it to the susceptibility of Aedes albopictus (Skuse), a likely bridge vector, and Culex pipiens (L.), a primary WNV amplifying species. The three species were infected with WNV (NY crow-1999) by feeding on 2-3-day-old chickens with serum virus titers ranging from 10(2.5) to 10(9.5) cell culture infective dose (CID) 50s/mL. The lowest infective titer for Oc. trivittatus and Cx. pipiens was 10(4.5) CID50s/mL. Thirteen percent (4/32) and 2% (1/45) of each species became infected postprandially. Infection rates of the two species increased to 43% (6/14) and 15% (6/40) after blood meals with a titer of 10(5.5) CID50s/mL. In contrast no infection was observed in nine Ae. albopictus that fed among three chickens with titers of 10(4.5) CID50s/mL nor in 41 Ae. albopictus that fed among three chickens with titers of 10(5.0) CID50s/mL. The infective dose 50s for Oc. trivittatus, Cx. pipiens and Ae. albopictus were 10(6.0), 10(6.2), and 10(6.6) CID50s/mL, respectively. Collectively these observations suggest that Oc. trivittatus and Cx. pipiens are more susceptible than Ae. albopictus to WNV when they feed on hosts with WNV titers of <10(7.5) CID50s/mL, but nearly as susceptible with blood meal titers of > or =10(7.5) CID50s/mL. Unpublished studies in our laboratory showed that cottontail rabbits fed on by WNV-infected Oc. trivittatus developed viremias as high as 10(5.5) CID50s/mL serum which exceeds 10 (4.2 (3.4-4.6)) CID50s/mL, the predicted ID10+/-95% CI of Oc. trivittatus. Consequently this mosquito, which also feeds on humans and birds has the potential to serve as a bridge vector and as a maintenance vector among mammals.

Susceptibility of Ochlerotatus taeniorhynchus (Diptera: Culicidae) to infection with epizootic (subtype IC) and enzootic (subtype ID) Venezuelan equine encephalitis viruses: evidence for epizootic strain adaptation

To test the hypothesis that adaptation to epizootic mosquito vectors mediates emergence of Venezuelan equine encephalitis virus (VEEV) from enzootic progenitors, experimental infection studies were conducted to determine the susceptibility of Ochlerotatus taeniorhynchus (Wiedemann) to epizootic and enzootic strains. Artificial blood meals containing epizootic subtype IC strains isolated during the 1962-1964, 1992-1993, and 1995 Venezuelan/Colombian epizootics and closely related Venezuelan enzootic subtype ID strains were used to compare infectivity and transmission potential. Their greater infectivity and replication suggested that adaptation of epizootic strains to Oc. taeniorhynchus may have enhanced epizootic transmission during the 1962-1964 and 1995 IC coastal epizootics. However, strains from the small 1992-1993 Venezuelan outbreak that did not extend to coastal regions do not seem to infect this species better than closely related subtype ID strains. Adaptation of VEEV to epizootic vectors such as Oc. taeniorhynchus mosquitoes may be a determinant of some but not all VEE emergence events and may influence spread into coastal regions.

Venezuelan equine encephalitis emergence: enhanced vector infection from a single amino acid substitution in the envelope glycoprotein

In 1993 and 1996, subtype IE Venezuelan equine encephalitis (VEE) virus caused epizootics in the Mexican states of Chiapas and Oaxaca. Previously, only subtype IAB and IC VEE virus strains had been associated with major outbreaks of equine and human disease. The IAB and IC epizootics are believed to emerge via adaptation of enzootic (sylvatic, equine-avirulent) strains for high titer equine viremia that results in efficient infection of mosquito vectors. However, experimental equine infections with subtype IE equine isolates from the Mexican outbreaks demonstrated neuro-virulence but little viremia, inconsistent with typical VEE emergence mechanisms. Therefore, we hypothesized that changes in the mosquito vector host range might have contributed to the Mexican emergence. To test this hypothesis, we evaluated the susceptibility of the most abundant mosquito in the deforested Pacific coastal locations of the VEE outbreaks and a proven epizootic vector, Ochlerotatus taeniorhynchus. The Mexican epizootic equine isolates exhibited significantly greater infectivity compared with closely related enzootic strains, supporting the hypothesis that adaptation to an efficient epizootic vector contributed to disease emergence. Reverse genetic studies implicated a Ser --> Asn substitution in the E2 envelope glycoprotein as the major determinant of the increased vector infectivity phenotype. Our findings underscore the capacity of RNA viruses to alter their vector host range through minor genetic changes, resulting in the potential for disease emergence.

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.

Field detection of eastern equine encephalitis virus in the Amazon Basin region of Peru using reverse transcription-polymerase chain reaction adapted for field identification of arthropod-borne pathogens

In support of efforts to develop rapid diagnostic assays for use in the field, reverse transcription-polymerase chain reaction (RT-PCR) assays were developed to detect arboviruses circulating in the Amazon Basin region of Peru. Previous knowledge of arthropod/pathogen relationships allowed a focused evaluation to be conducted in November 2000 that assessed the feasibility and reliability of a mobile, rapid, field-expedient RT-PCR diagnostic system aimed at detecting eastern equine encephalitis virus (EEEV) in Culex (Melanoconion) pedroi mosquitoes. Modifications were made to a commercially available mobile molecular laboratory kit and assay procedures were tailored for use under harsh environmental conditions with field-collected and field-processed mosquitoes. From CO2 baited mosquito light traps, 3,227 Cx. (Mel.) pedroi mosquitoes were collected and sorted into 117 pools. The pools were processed and assayed in the field by RT-PCR and five of those pools were found positive for EEEV. Laboratory sequence analysis confirmed the presence of two distinct subtypes of EEEV.

Flavivirus isolations from mosquitoes collected from western Cape York Peninsula, Australia, 1999-2000

After the 1st appearance of Japanese encephalitis virus (JE) on mainland Australia in 1998, a study was undertaken to investigate whether JE had become established in enzootic transmission cycles on western Cape York Peninsula. Adult mosquitoes were collected during the late wet season from Kowanyama and Pormpuraaw in April 1999, and Pormpuraaw and Barr's Yard in April 2000. Despite processing 269,270 mosquitoes for virus isolation, no isolates of JE were obtained. However, other flaviviruses comprising Murray Valley encephalitis virus, Kunjin virus, Alfuy virus, and Kokobera virus (KOK) were isolated. Isolates of the alphaviruses Ross River virus, Barmah Forest virus (BF), and Sindbis virus (SIN) also were obtained. The majority (88%) of isolates were from members of the Culex sitiens subgroup. Single isolates of KOK, BF, and SIN were obtained from Ochlerotatus vigilax, Oc. normanensis, and Anopheles bancroftii, respectively. The isolations of flaviviruses during the late wet season indicate that conditions were suitable for flavivirus activity in the area. No evidence was found to suggest that JE has become established in enzootic transmission cycles on western Cape York, although study sites and field trips were limited.

Habitat preferences and phenology of Ochlerotatus triseriatus and Aedes albopictus (Diptera: Culicidae) in southwestern Virginia

Recently, the number of reported human cases of La Crosse encephalitis, an illness caused by mosquito-borne La Crosse virus (LAC), has increased in southwestern Virginia, resulting in a need for better understanding of the virus cycle and the biology of its vectors in the region. This study examined the spatial and temporal distributions of the primary vector of LAC, Ochlerotatus triseriatus (Say), and a potential secondary vector, Aedes albopictus (Skuse). Ovitrapping surveys were conducted in 1998 and 1999 to determine distributions and oviposition habitat preferences of the two species in southwestern Virginia. Mosquitoes also were collected for virus assay from a tire dump and a human La Crosse encephalitis case site between 1998 and 2000. Oc. triseriatus and Ae. albopictus were collected from all ovitrap sites surveyed, and numbers of Oc. triseriatus eggs generally were higher than those of Ae. albopictus. Numbers of Oc. triseriatus remained high during most of the summer, while Ae. albopictus numbers increased gradually, reaching a peak in late August and declining thereafter. In Wise County, relative Ae. albopictus abundance was highest in sites with traps placed in open residential areas. Lowest numbers of both species were found in densely forested areas. Ovitrapping during consecutive years revealed that Ae. albopictus was well established and overwintering in the area. An oviposition comparison between the yard and adjacent forest at a human La Crosse encephalitis case site in 1999 showed that Ae. albopictus preferentially oviposited in the yard surrounding the home, but Oc. triseriatus showed no preference. LAC isolations from larval and adult collections of Oc. triseriatus females from the same case site indicated the occurrence of transovarial transmission.