Bionomics of Culex tarsalis (Diptera: Culicidae) in relation to arbovirus transmission in southeastern California

West Nile Virus Mosquito Vectors in North America

In North America, the geographic distribution, ecology, and vectorial capacity of a diverse assemblage of mosquito species belonging to the genus Culex determine patterns of West Nile virus transmission and disease risk. East of the Mississippi River, mostly ornithophagic Culex pipiens L. complex mosquitoes drive intense enzootic transmission with relatively small numbers of human cases. Westward, the presence of highly competent Culex tarsalis (Coquillett) under arid climate and hot summers defines the regions with the highest human risk. West Nile virus human risk distribution is not uniform geographically or temporally within all regions. Notable geographic 'hotspots' persist with occasional severe outbreaks. Despite two decades of comprehensive research, several questions remain unresolved, such as the role of non-Culex bridge vectors, which are not involved in the enzootic cycle, but may be involved in virus transmission to humans. The absence of bridge vectors also may help to explain the frequent lack of West Nile virus 'spillover' into human populations despite very intense enzootic amplification in the eastern United States. This article examines vectorial capacity and the eco-epidemiology of West Nile virus mosquito vectors in four geographic regions of North America and presents some of the unresolved questions.

Effects of Climate and Climate Change on Vectors and Vector-Borne Diseases: Ticks Are Different

There has been considerable debate as to whether global risk from vector-borne diseases will be impacted by climate change. This has focussed on important mosquito-borne diseases that are transmitted by the vectors from infected to uninfected humans. However, this debate has mostly ignored the biological diversity of vectors and vector-borne diseases. Here, we review how climate and climate change may impact those most divergent of arthropod disease vector groups: multivoltine insects and hard-bodied (ixodid) ticks. We contrast features of the life cycles and behaviour of these arthropods, and how weather, climate, and climate change may have very different impacts on the spatiotemporal occurrence and abundance of vectors, and the pathogens they transmit.

Projection of Climate Change Influences on U.S. West Nile Virus Vectors

While estimates of the impact of climate change on health are necessary for health care planners and climate change policy makers, models to produce quantitative estimates remain scarce. We describe a freely available dynamic simulation model parameterized for three West Nile virus vectors, which provides an effective tool for studying vector-borne disease risk due to climate change. The Dynamic Mosquito Simulation Model is parameterized with species specific temperature-dependent development and mortality rates. Using downscaled daily weather data, we estimate mosquito population dynamics under current and projected future climate scenarios for multiple locations across the country. Trends in mosquito abundance were variable by location, however, an extension of the vector activity periods, and by extension disease risk, was almost uniformly observed. Importantly, mid-summer decreases in abundance may be off-set by shorter extrinsic incubation periods resulting in a greater proportion of infective mosquitoes. Quantitative descriptions of the effect of temperature on the virus and mosquito are critical to developing models of future disease risk.

Modeling Occurrence of Urban Mosquitos Based on Land Use Types and Meteorological Factors in Korea

Mosquitoes are a public health concern because they are vectors of pathogen, which cause human-related diseases. It is well known that the occurrence of mosquitoes is highly influenced by meteorological conditions (e.g., temperature and precipitation) and land use, but there are insufficient studies quantifying their impacts. Therefore, three analytical methods were applied to determine the relationships between urban mosquito occurrence, land use type, and meteorological factors: cluster analysis based on land use types; principal component analysis (PCA) based on mosquito occurrence; and three prediction models, support vector machine (SVM), classification and regression tree (CART), and random forest (RF). We used mosquito data collected at 12 sites from 2011 to 2012. Mosquito abundance was highest from August to September in both years. The monitoring sites were differentiated into three clusters based on differences in land use type such as culture and sport areas, inland water, artificial grasslands, and traffic areas. These clusters were well reflected in PCA ordinations, indicating that mosquito occurrence was highly influenced by land use types. Lastly, the RF represented the highest predictive power for mosquito occurrence and temperature-related factors were the most influential. Our study will contribute to effective control and management of mosquito occurrences.

Extrinsic Incubation Rate is Not Accelerated in Recent California Strains of West Nile Virus in Culex tarsalis (Diptera: Culicidae)

The efficiency of West Nile virus (WNV) transmission by competent mosquito vectors is driven by temperature and defined, in part, by the extrinsic incubation period, which is the time from a mosquito's consumption of an infected bloodmeal until it becomes capable of transmitting the virus to the next vertebrate host. The extrinsic incubation period can be altered by a variety of factors involved in vector-pathogen interactions, and in North America, the WN02 strain of WNV emerged and displaced the founding NY99 strain reportedly because the duration of the extrinsic incubation period in Culex mosquitoes was shortened by a single positively selected mutation. However, recent work has suggested that this change is not universal and may depend on vector species or strain. In the current study, we estimated the extrinsic incubation periods at 22 and 30°C in Culex tarsalis Coquillett. We found that the time to transmission of the original North American WNV strain, NY99, was not different from two more recent California isolates of the WN02 genotype: one of the earliest California isolates from the southeastern deserts, and a more recent 2011 isolate from a hyperendemic region in the Central Valley. We conclude with a model-based assessment of the epidemiological effects of temperature on the duration of mosquitoes' infectious life, which estimated that most mosquitoes have an infectious life of only a few days, but its duration expands markedly at warmer temperatures.

Identifying avian malaria vectors: sampling methods influence outcomes

Background

The role of vectors in the transmission of avian malaria parasites is currently understudied. Many studies that investigate parasite-vector relationships use limited trapping techniques and/or identify potential competent vectors in the field in such ways that cannot distinguish between an infected or infectious vector. Without the use of multiple trapping techniques that address the specific biology of diverse mosquito species, and without looking at the infection status of individual mosquitoes, it is not possible to make dependable conclusions on the role of mosquitoes in the transmission of avian malaria parasites.

Methods

We conducted two years of mosquito collections at a riparian preserve in California where a wide diversity of species were collected with multiple trap types. We hypothesized that competent mosquito species can influence the distribution and diversity of avian malaria parasites by acting as a compatibility filter for specific Plasmodium species. To determine the infection status of all individual mosquitoes for Plasmodium species/lineages, amplification within the cytochrome b gene was carried out on over 3000 individual mosquito thoraxes, and for those that tested positive we then repeated the same process for abdomens and salivary glands.

Results

Our data show heterogeneity in the transmissibility of Plasmodium among ornithophillic mosquito species. More specifically, Culex stigmatosoma appears to not be a vector of Plasmodium homopolare, a parasite that is prevalent in the avian population, but is a vector of multiple other Plasmodium species/lineages.

Conclusions

Our results suggest that conclusions made on the role of vectors from studies that do not use different mosquito trapping methods should be re-evaluated with caution, as we documented the potential for trapping biases, which may cause studies to miss important roles of specific mosquito species in the transmission of avian malaria. Moreover, we document heterogeneity in the transmission of Plasmodium spp. by mosquitoes can influence Plasmodium diversity and prevalence in specific locations to Plasmodium-vector incompatibilities.

Culex tarsalis vitellogenin gene promoters investigated in silico and in vivo using transgenic Drosophila melanogaster

INTRODUCTION

Genetic modification, or transgenesis, is a powerful technique to investigate the molecular interactions between vector-borne pathogens and their arthropod hosts, as well as a potential novel approach for vector-borne disease control. Transgenesis requires the use of specific regulatory regions, or promoters, to drive expression of genes of interest in desired target tissues. In mosquitoes, the vast majority of described promoters are from Anopheles and Aedes mosquitoes.

RESULTS

Culex tarsalis is one of the most important vectors of arboviruses (including West Nile virus) in North America, yet it has not been the subject of molecular genetic study. In order to facilitate molecular genetic work in this important vector species, we isolated four fat body-specific promoter sequences located upstream of the Cx. tarsalis vitellogenin genes (Vg1a, Vg1b, Vg2a and Vg2b). Sequences were analyzed in silico to identify requisite cis-acting elements. The ability for promoter sequences to drive expression of green fluorescent protein (GFP) in vivo was investigated using transgenic Drosophila melanogaster. All four promoters were able to drive GFP expression but there was dramatic variation between promoters and between individual Drosophila lines, indicating significant position effects. The highest expression was observed in line Vg2bL3, which was >300-fold higher than the lowest line Vg1aL2.

CONCLUSIONS

These new promoters will be useful for driving expression of genes of interest in transgenic Cx. tarsalis and perhaps other insects.

The contrasting bionomics of Culex mosquitoes in western North America

Mosquitoes in the genus Culex are the primary enzootic maintenance and bridge vectors of the North American encephalitides, now including West Nile virus. This review briefly summarizes the biology of three key vector species in western North America, Culex tarsalis, Cx. pipiens complex and Cx. stigmatosoma, focusing on the long history of research done in California. Topics reviewed include population genetic structure, larval ecology, autogeny, mating behavior, host-seeking behavior, host-selection patterns, and overwintering strategies. These attributes collectively have allowed the successful exploitation of anthropogenically altered ecosystems and enabled the role of these species as maintenance and bridge vectors of arboviruses.

Rainfall influences survival of Culex pipiens (Diptera: Culicidae) in a residential neighborhood in the mid-Atlantic United States

Measurement of the survival and dispersal rates of mosquito vectors is an important step in designing and implementing control strategies. Vector survival plays a key role in determining the intensity of pathogen transmission, and vector movement determines the spatial scale on which control efforts must operate to be effective. We provide the first estimates of field survival and dispersal rates for Culex pipiens L. in North America, an important enzootic and bridge vector for West Nile virus (WNV). We conducted mark-release-recapture studies in a residential area near Washington, DC, in two consecutive years and fit nonlinear regression models to the recapture data that incorporate weather information into survival and recapture probabilities. We found that daily survival rates were not significantly different between the 2 yr but were negatively affected by rainfall. The daily survival rate was 0.904 +/- 0.037 (SE), which implies an average longevity of 10.4 d. As with other vector-borne pathogens, the measured survival rate suggests that at our site the majority of WNV-infected Cx. pipiens mosquitoes may perish before becoming infectious (being able to transmit WNV to hosts). We found relatively little evidence of dispersal after the initial night after release. Our results suggest that transmission of WNV and other pathogens transmitted by Cx. pipiens may be highly local and they highlight the importance of factors that influence survival of mosquito vectors.

Effects of temperature on emergence and seasonality of West Nile virus in California

Temperature has played a critical role in the spatiotemporal dynamics of West Nile virus transmission throughout California from its introduction in 2003 through establishment by 2009. We compared two novel mechanistic measures of transmission risk, the temperature-dependent ratio of virus extrinsic incubation period to the mosquito gonotrophic period (BT), and the fundamental reproductive ratio (R(0)) based on a mathematical model, to analyze spatiotemporal patterns of receptivity to viral amplification. Maps of BT and R(0) were created at 20-km scale and compared throughout California to seroconversions in sentinel chicken flocks at half-month intervals. Overall, estimates of BT and R(0) agreed with intensity of transmission measured by the frequency of sentinel chicken seroconversions. Mechanistic measures such as these are important for understanding how temperature affects the spatiotemporal dynamics of West Nile virus transmission and for delineating risk estimates useful to inform vector control agency intervention decisions and communicate outbreak potential.

Evaluating sampling method bias in Culex tarsalis and Culex quinquefasciatus (Diptera: Culicidae) bloodmeal identification studies

Determining the bloodmeal hosts of the Culex vectors of encephalitis viruses such as West Nile virus is essential for understanding the role of these mosquitoes in enzootic and epidemic transmission. Although molecular techniques have increased our knowledge of blood feeding patterns by allowing host identification to the species level, few studies have focused on the role that sampling methods may play in determining these patterns. In the current study, we identified 644 bloodmeals from Culex tarsalis Coquillett and Culex quinquefasciatus Say females collected in CO2 traps (dry ice-baited Center for Disease Control traps), in gravid traps, and aspirated from resting sites. There was no significant difference in the bloodmeal host apportionment in sampling methods such as gravid traps and resting collections that collected fully engorged females. However, CO2 traps that collected partially fed females had a significantly different apportionment of hosts than either gravid or resting collections. Bloodfed females from CO2 traps had either fed on only a small subset of available host species or were biased toward more mammalian and fewer nonpasserine avian feeds than females from other collections. Because both full and partial bloodmeals can contribute to viral transmission, obtaining Culex bloodmeal collections from multiple sampling methods may be important to fully interpret the role of these mosquitoes as maintenance and/or bridge vectors.

Mosquito host selection varies seasonally with host availability and mosquito density

Host selection by vector mosquitoes is a critical component of virus proliferation, particularly for viruses such as West Nile (WNV) that are transmitted enzootically to a variety of avian hosts, and tangentially to dead-end hosts such as humans. Culex tarsalis is a principal vector of WNV in rural areas of western North America. Based on previous work, Cx. tarsalis utilizes a variety of avian and mammalian hosts and tends to feed more frequently on mammals in the late summer than during the rest of the year. To further explore this and other temporal changes in host selection, bloodfed females were collected at a rural farmstead and heron nesting site in Northern California from May 2008 through May 2009, and bloodmeal hosts identified using either a microsphere-based array or by sequencing of the mitochondrial cytochrome c oxidase I (COI) gene. Host composition during summer was dominated by four species of nesting Ardeidae. In addition, the site was populated with various passerine species as well as domestic farm animals and humans. When present, Cx. tarsalis fed predominantly (>80%) upon the ardeids, with Black-crowned Night-Herons, a highly competent WNV host, the most prevalent summer host. As the ardeids fledged and left the area and mosquito abundance increased in late summer, Cx. tarsalis feeding shifted to include more mammals, primarily cattle, and a high diversity of avian species. In the winter, Yellow-billed Magpies and House Sparrows were the predominant hosts, and Yellow-billed Magpies and American Robins were fed upon more frequently than expected given their relative abundance. These data demonstrated that host selection was likely based both on host availability and differences in utilization, that the shift of bloodfeeding to include more mammalian hosts was likely the result of both host availability and increased mosquito abundance, and that WNV-competent hosts were fed upon by Cx. tarsalis throughout the year.

West Nile virus (WNV) is transmitted from one vertebrate host to another by the bite of a mosquito. The virus is maintained primarily in birds, but can also be transmitted to mammals such as horses and humans which may suffer severe neurological disease. Culex tarsalis is a primary mosquito vector of WNV in the western United States. Because this mosquito will bite a variety of host species, understanding bloodfeeding patterns and host selection is important for understanding WNV transmission. In our study, the bloodfeeding patterns of Cx. tarsalis varied markedly throughout the year. During summer nesting herons were utilized almost exclusively; avian host diversity increased in the fall, when an increase in the proportion of bloodfeeding on mammals was also observed. Yellow-billed Magpies and House Sparrows were common hosts in the winter, when no mammalian bloodmeals were detected. Seasonal shifts corresponded to both changes in host availability and mosquito density; however, WNV-competent hosts were fed upon throughout the year. This work supports the role of Cx. tarsalis as a vector of WNV to both avian and mammalian hosts and provides insight into seasonal changes in host selection that may influence the seasonality of WNV transmission to equines and humans.

Seasonal abundance of Culex tarsalis and Culex pipiens complex mosquitoes (Diptera: Culicidae) in California

Large-scale patterns in the seasonal abundance profiles of the arboviral mosquito vectors Culex tarsalis Coquillett and the species of the Culex pipiens complex were described based on a decade of counts from 868 New Jersey light traps located throughout the urban and agricultural areas of California. Mean seasonal abundance profiles varied markedly among hydrologic regions. For all species, abundance increased earlier and declined later in drier, warmer southern regions, but variation could not be explained fully by latitude. The observed patterns may be driven by temperature, availability of larval habitats, and for the Cx. pipiens complex, the taxonomic composition of local populations.

Temporal connections between Culex tarsalis abundance and transmission of western equine encephalomyelitis virus in California

Definition of targets for vector control requires an understanding of the relationship between vector abundance and the intensity of arbovirus transmission. Using an extensive surveillance dataset with observations from sentinel chicken flocks and mosquito traps paired in time and space, hierarchical autoregressive logistic regression models were developed to predict the probability of seroconversion in chickens for western equine encephalomyelitis virus (WEEV) based on the relative abundance of the principal vector, Culex tarsalis. After adjustments for confounders, the abundance of Cx. tarsalis 29-42 d before the date of chicken sampling was credibly associated with the risk of WEEV transmission in both the Central and Coachella Valleys, and a doubling of relative Cx. tarsalis abundance was associated with a 58% increase in the odds of seroconversion. The critical time windows identified in our study highlight the need for surveillance of vector populations and forecasting models to guide proactive vector control measures before the detection of transmission to sentinel chickens.

Effects of warm winter temperature on the abundance and gonotrophic activity of Culex (Diptera: Culicidae) in California

Culex tarsalis Coquillett, Cx. quinquefasciatus Say, and Cx. pipiens L. were collected during the warm winter of 2009 using dry ice-baited and gravid traps and walk-in red boxes positioned in desert, urban, and agricultural habitats in Riverside, Los Angeles, Kern, and Yolo Counties. Temperatures exceeded the preceding 50 yr averages in all locations for most of January, whereas rainfall was absent or below average. Abundance of Culex species in traps during January ranged from 83 to 671% of the prior 5 yr average in all locations. Few females collected resting were in diapause during January based on follicular measurements. Evidence for early season gonotrophic activity included the detection of freshly bloodfed, gravid, and parous females in resting collections, gravid oviposition site-seeking females in gravid female traps, and nulliparous and parous host-seeking females at dry ice-baited traps. Female Culex seemed to employ multiple overwintering strategies in California, including larval and adult quiescence, adult female diapause, and an intermediate situation with adult females collected with enlarged follicles, but without evident vitellogenesis. West Nile, St. Louis, or western equine encephalitis viruses were not detected in 198 pools of adults or 56 pools of adults reared from field-collected immatures collected during January and February 2009. Our preliminary data may provide insight into how climate change may extend the mosquito season in California.

Seasonal patterns for entomological measures of risk for exposure to Culex vectors and West Nile virus in relation to human disease cases in northeastern Colorado

We examined seasonal patterns for entomological measures of risk for exposure to Culex vectors and West Nile virus (family Flaviviridae, genus Flavivirus, WNV) in relation to human WNV disease cases in a five-county area of northeastern Colorado during 2006-2007. Studies along habitat/elevation gradients in 2006 showed that the seasonal activity period is shortened and peak numbers occur later in the summer for Culex tarsalis Coquillett females in foothills-montane areas >1600 m compared with plains areas <1600 m in Colorado's Front Range. Studies in the plains of northeastern Colorado in 2007 showed that seasonal patterns of abundance for Cx. tarsalis and Culex pipiens L. females differed in that Cx. tarsalis reached peak abundance in early July (mean of 328.9 females per trap night for 18 plains sites), whereas the peak for Cx. pipiens did not occur until late August (mean of 16.4 females per trap night). During June-September in 2007, which was a year of intense WNV activity in Colorado with 578 reported WNV disease cases, we recorded WNV-infected Cx. tarsalis females from 16 of 18 sites in the plains. WNV infection rates in Cx. tarsalis females increased gradually from late June to peak in mid-August (overall maximum likelihood estimate for WNV infection rate of 8.29 per 1000 females for the plains sites in mid-August). No WNV-infected Culex mosquitoes were recorded from sites >1600 m. The vector index for abundance of WNV-infected Cx. tarsalis females for the plains sites combined exceeded 0.50 from mid-July to mid-August, with at least one site exceeding 1.00 from early July to late August. Finally, we found that abundance of Cx. tarsalis females and the vector index for infected females were strongly associated with weekly numbers of WNV disease cases with onset 4-7 wk later (female abundance) or 1-2 wk later (vector index).

Persistent West Nile virus transmission and the apparent displacement St. Louis encephalitis virus in southeastern California, 2003-2006

West Nile virus (family Flaviviridae, genus Flavivirus, WNV) invaded the Colorado Desert biome of southern California during summer 2003 and seemed to displace previously endemic St. Louis encephalitis virus (family Flaviviridae, genus Flavivirus, SLEV, an antigenically similar Flavivirus in the Japanese encephalitis virus serocomplex). Western equine encephalomyelitis virus (family Togaviridae, genus Alphavirus, WEEV), an antigenically distinct Alphavirus, was detected during 2005 and 2006, indicating that conditions were suitable for encephalitis virus introduction and detection. Cross-protective "avian herd immunity" due to WNV infection possibly may have prevented SLEV reintroduction and/or amplification to detectable levels. During 2003-2006, WNV was consistently active at wetlands and agricultural habitats surrounding the Salton Sea where Culex tarsalis Coquillett served as the primary enzootic maintenance and amplification vector. Based on published laboratory infection studies and the current seroprevalence estimates, house sparrows, house finches, and several Ardeidae may have been important avian amplifying hosts in this region. Transmission efficiency may have been dampened by high infection rates in incompetent avian hosts, including Gamble's quail, mourning doves, common ground doves, and domestic pigeons. Early season WNV amplification and dispersal from North Shore in the southeastern portion of the Coachella Valley resulted in sporadic WNV incursions into the urbanized Upper Valley near Palm Springs, where Culex pipiens quinquefasciatus Say was the primary enzootic and bridge vector. Although relatively few human cases were detected during the 2003-2006 period, all were concentrated in the Upper Valley and were associated with high human population density and WNV infection in peridomestic populations of Cx. p. quinquefasciatus. Intensive early mosquito control during 2006 seemed to interrupt and delay transmission, perhaps setting the stage for the future reintroduction of SLEV.

Evaluation of pyrethrin aerial ultra-low volume applications for adult Culex tarsalis control in the desert environments of the Coachella Valley, Riverside County, California

Eliminating infected female mosquitoes by aerial applications of ultra-low volume adulticides is the intervention strategy currently recommended to interrupt the epidemic transmission of encephalitis viruses, including West Nile. The current research optimized pyrethrin formulations and evaluated their efficacy in the desert environment of the Coachella Valley, Riverside County, California. After seven trials during 2004, a 1:2 by volume mixture of Pyrenone 25-5 in BVA oil optimized particle size, droplet descent to ground level, and kill of sentinel mosquitoes. Three subsequent experiments used 3 aerial applications of the 1:2 Pyrenone 25-5:BVA oil mixture on alternate nights to suppress Culex tarsalis Coquillett host-seeking abundance over a 1-square-mile target area. Mortality patterns among caged sentinel mosquitoes varied among sites and replicate sprays, indicating variable particle dispersion at ground level within the target area. In addition, mortality was observed for sentinels up to 1 mile downwind from the target area, indicating considerable particle drift. Geometric mean abundance of host-seeking Cx. tarsalis females collected at dry ice-baited traps within each of 3 sprayed and 2 unsprayed negative control strata varied similarly over time, indicating that our sprays minimally impacted the target population or that drift combined with other factors led to widespread area control. Experiments during March and June when recruitment rates were minimal showed general area-wide suppression of abundance following spray, whereas an experiment during September when recruitment rates were high from newly flooded marshes failed to prevent an area-wide increase in abundance. Clearly additional research is needed to standardize the efficacy of aerial applications of pyrethrins in hot dry desert environments.

A GIS tool to estimate West Nile virus risk based on a degree-day model

West Nile virus (Flaviviridae: Flavivirus) is a serious infectious disease that recently spread across the North America continent. A spatial analysis tool was developed on the ArcMap 9.x platform to estimate potential West Nile virus activity using a spatially explicit degree-day model. The model identifies when the virus Extrinsic Incubation Period (EIP) is completed within the vector longevity during mid-summer months. The EIP is treated as a threshold indicator of the potential for virus emergence and activity. Comparing the number of West Nile virus cases in Wyoming reported from 2003 to 2005 with model results, actual cases and predicted events of West Nile virus activity match relatively well. The model represents a useful method for estimating potential West Nile virus activity in a large spatial scale.

Using fluorescently labelled M13-tailed primers to isolate 45 novel microsatellite loci from the arboviral vector Culex tarsalis

Culex tarsalis Coquillett (Diptera: Culicidae) is a highly efficient arbovirus vector. Spatial and temporal heterogeneity have been observed in Cx tarsalis for phenotypic traits including autogeny, virus susceptibility and host preference. Genetic differences between populations may in part explain these observations. Using the M13-tailed primer method, we identified 45 novel polymorphic microsatellite markers from microsatellite-enriched Cx tarsalis genomic libraries. The M13-tailed primer method was more efficient in identifying useful loci than traditional screening by acrylamide gel electrophoresis. These markers will be useful for investigating genetic questions in this important vector mosquito.

Effects of time after infection, mosquito genotype, and infectious viral dose on the dynamics of Culex tarsalis vector competence for western equine encephalomyelitis virus

The vector competence of Culex tarsalis Coquillett for the BFS 1703 strain of western equine encephalomyelitis virus (WEEV) changed significantly as a function of time after infection, mosquito genotype, and infectious virus dose. After ingesting a high virus dose (5 log10 plaque-forming units [PFU]/0.1 ml), female of the susceptible high virus producer (HVP) strain rapidly amplified the virus, developed a disseminated infection, and efficiently transmitted WEEV by 4 days postinfection (dpi). The quantity of virus expectorated peaked at 4 dpi (mean 3.4 log10 PFU), and the percentage of females transmitting per os peaked at 7 dpi (80%); both measures of transmission subsequently decreased to low levels throughout the remainder of infected life. HVP females imbibing a low virus dose (3 log10 PFU/0.1 ml) were infected less frequently and took longer to amplify virus to levels recorded for the high virus dose group and did not transmit virus efficiently, thereby indicating midgut infection and escape barriers were dose and time dependent. These data emphasized the importance of elevated avian viremias in Cx. tarsalis vector competence. Females from the WEEV-resistant (WR) strain and two wild-type strains from Kern and Riverside counties were significantly less susceptible to infection at both high and low doses than was the HVP strain. Overall, females with a high virus titer more frequently had a disseminated infection, but there did not seem to be a distinct threshold demarcating this relationship. In marked contrast, all infected females transmitting virus had body titers >4.3 log10 PFU, and most had titers >4.8 log10 PFU. These data indicated that not all females with a disseminated infection transmitted virus because of the presence of one or more salivary gland barriers.

Western equine encephalomyelitis virus infection affects the life table characteristics of Culex tarsalis (Diptera: Culicidae)

The life table attributes of Culex tarsalis Coquillett females infected experimentally by feeding on 4 and 6 log10 plaque-forming units (PFU) of western equine encephalomyelitis virus (WEEV) per milliliter of heparinized chicken blood were compared with an uninfected control group. Females continually were offered 10% sucrose and an oviposition substrate and daily a blood meal through a biomembrane feeder. Mortality (dead females) and fecundity (female eggs per female) were monitored daily until all females died. Overall, 94% of 198 females in the two virus-infected groups were positive for WEEV at death when tested by plaque assay; the average body virus titer at death did not differ between groups. WEEV infection significantly altered the life table characteristics of Cx. tarsalis. Life expectancy at infection in days (ex), reproductive effort in female eggs per female per generation (Ro), and generation time (T) in days for the infected cohorts were significantly lower than for the uninfected controls, whereas the reproductive rate (rc) in female eggs per female per day was higher for infected than uninfected cohorts. In agreement with the WEEV infection data that showed similar body titers, there were few differences between the life table parameters for the 4 and 6 log10 PFU treatment groups. Greatest differences were observed for survivorship between days 17-40 when virus titers in infected dying females were greatest. Our data extend recent studies that indicate mosquito infection with encephalitis viruses has a cost of reduced life expectancy and fitness.

Factors influencing the outcome of mark-release-recapture studies with Culex tarsalis (Diptera: Culicidae)

Three potentially important aspects of mark-release-recapture experimentation were addressed: 1) source of mosquitoes for release, 2) time of release, and 3) weather during recapture. Culex tarsalis Coquillett mosquitoes collected as adult host-seeking females from dry ice-baited traps (CO2 traps) operated within the study area (local) were recaptured more frequently than females collected from traps operated outside the study area (foreign) or reared from field-collected immatures (reared). These results supported published studies on Anopheles and Ochlerotatus that indicated mosquitoes may "memorize" flight paths within their environment. Releasing gravid females provided a potentially useful replacement for reared females, because these gravids oviposited at wetlands and then dispersed to upland traps. Releasing local, foreign, or reared mosquitoes just after sunrise or just before sunset did not alter recapture success or the distance dispersed. Elevated wind speeds inhibited dispersal from protected microhabitats with citrus orchards and resulted in most recaptures being found at the leeward portion of the orchard.

Persistence and amplification of St. Louis encephalitis virus in the Coachella Valley of California, 2000-2001

The introduction of a St. Louis encephalitis virus (SLE) genotype new to southeastern California during 2000 was followed by focal enzootic amplification in the Coachella Valley that was detected by seroconversions of 29 sentinel chickens in five of nine flocks of 10 chickens each, isolations of virus from 30 of 538 pools of 50 Culex tarsalis Coquillett females, and collection of 30 positive sera from 2,205 wild birds. This SLE strain over wintered successfully and then amplified during the summer of 2001, with 47 sentinel seroconversions in eight of nine flocks, 70 virus isolations from 719 pools of Cx. tarsalis and Cx. p. quinquefasciatus Say, and 40 positive sera from 847 wild birds. Human illness was not detected by passive case surveillance, despite issuance of a health alert during 2001. Virus amplification during both years was associated with above average temperatures conducive for extrinsic incubation and below average precipitation during spring associated with below average vector abundance. Seroconversions by sentinel chickens provided the timely detection of virus activity, with initial conversions detected before positive mosquito pools or wild bird infections. Vertical infection was not detected among Cx. tarsalis adults reared from immatures collected during the fall-winter of 2000, even though SLE over wintered successfully in this area. Early seroconversions by a sentinel chicken during February 2001 and a recaptured Gambel's quail in April 2001 provided evidence for transmission during winter and spring when ambient temperatures averaged below 17 degrees C, the threshold for SLE replication.

Nocturnal microhabitat distribution of adult Culex tarsalis (Diptera: Culicidae) impacts control effectiveness

Suction traps (30 cm diameter) were more effective for non-attractant sampling of flying adult Culex tarsalis Coquillett than were smaller CDC (5.5 cm diameter), Malaise or ramp traps. Comparative catch in suction traps operated in a variety of vegetation types indicated that females congregated along elevated ecotones and were significantly less abundant flying over low vegetation or under and over elevated vegetation. Most females taken at upland orchards or Tamarisk tree lines were unfed (97%, n = 5,278) and similar in reproductive condition to host-seeking females. Blood fed and gravid females and males were only abundant near emergence sites. Pyrocide 7396 (Pyrethrin 5%, PBO 25%) was applied at the label rate of 5 oz/min by truck mounted Pro-Mist ultra low volume (ULV) equipment and particle drift measured by bioassay. ULV particles dispersed well downwind over low vegetation, between citrus orchard rows, and under date orchard canopy, but did not penetrate citrus orchards or vineyards when rows were perpendicular to wind direction. Particles did move up and over vegetation contacting sentinel mosquitoes placed above the canopy. The congregation of adult mosquitoes at vegetative ecotones and within orchard vegetation may afford protection from ground applied ULV particles, negatively impacting control. These data may explain why repeated applications often fail to interrupt encephalitis virus transmission.

Climate variability and change in the United States: potential impacts on vector- and rodent-borne diseases

Diseases such as plague, typhus, malaria, yellow fever, and dengue fever, transmitted between humans by blood-feeding arthropods, were once common in the United States. Many of these diseases are no longer present, mainly because of changes in land use, agricultural methods, residential patterns, human behavior, and vector control. However, diseases that may be transmitted to humans from wild birds or mammals (zoonoses) continue to circulate in nature in many parts of the country. Most vector-borne diseases exhibit a distinct seasonal pattern, which clearly suggests that they are weather sensitive. Rainfall, temperature, and other weather variables affect in many ways both the vectors and the pathogens they transmit. For example, high temperatures can increase or reduce survival rate, depending on the vector, its behavior, ecology, and many other factors. Thus, the probability of transmission may or may not be increased by higher temperatures. The tremendous growth in international travel increases the risk of importation of vector-borne diseases, some of which can be transmitted locally under suitable circumstances at the right time of the year. But demographic and sociologic factors also play a critical role in determining disease incidence, and it is unlikely that these diseases will cause major epidemics in the United States if the public health infrastructure is maintained and improved.

Landscape affects the host-seeking patterns of Culex tarsalis (Diptera: Culicidae) in the Coachella Valley of California

Effective arbovirus transmission requires that the principal vertebrate hosts and vectors have frequent contact. Vegetation and other landscape features used by roosting or nesting birds at night dictate their exposure to nocturnally active host-seeking Culex tarsalis Coquillett and therefore to western equine encephalomyelitis and St. Louis encephalitis viruses. Precipitin tests on 645 Cx. tarsalis that were collected resting and host-seeking near the Salton Sea in Coachella Valley, CA, indicated that passeriform birds (64%) and rabbits (25%) were the most frequent bloodmeal hosts and that the percentage of females feeding on birds varied temporally as an inverse fuction of mosquito abundance. Blood meals were not taken from communally roosting water birds. The spatial distribution of host-seeking females then was investigated by deploying dry ice baited traps within seven sites representative of habitats found along the Salton Sea. Mosquito catch was greatest at traps within elevated vegetation such as Tamarisk, mesquite, cattails, and orchards and lowest at traps positioned at snags over water, sand bars, open fields, and within housing in a small rural community. These data indicate that host-seeking Cx. tarsalis females congregated at specific landscape features that were not necessarily associated with large concentrations of potential bloodmeal hosts.

Seasonal transmission of bluetongue virus by Culicoides sonorensis (Diptera: Ceratopogonidae) at a southern California dairy and evaluation of vectorial capacity as a predictor of bluetongue virus transmission

Vectorial capacity of Culicoides sonorensis Wirth & Jones for the transmission of bluetongue (BLU) virus was examined at a southern California dairy from January 1995 to December 1997. Insects were collected one to two times per week in five CDC-type suction traps (without light) baited with CO2 at a constant release rate of 1,000 ml/min. BLU virus was detected in midges collected from May through December with an estimated overall infection rate of 0.08%. The BLU virus infection rate of field-captured midges was not correlated with sentinel calf seroconversions to BLU virus. Sentinel calf seroconversions were highly seasonal, occurring from August through November with most calves seroconverting during September and October. Vector competence of field-collected nulliparous flies fed a locally acquired serotype of BLU virus in the laboratory was stable among years (17-23%). Vectorial capacity was strongly correlated with BLU virus transmission (measured by sentinel calf seroconversions) during 1995 and 1996, but not during 1997. Host biting rate estimated for traps nearest to the sentinel calves was the index best correlated with BLU virus transmission for all study years and was most highly correlated with sentinel seroconversions 4 wk later. The utility of vectorial capacity and its component variables is discussed for this system.

Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA

Temporal and spatial changes in the enzootic activity of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitored at representative wetland study sites in the Coachella, San Joaquin, and Sacramento valleys of California from 1996 to 1998 using three methods: (1) virus isolation from pools of 50 host-seeking Culex tarsalis Coquillett females, (2) seroconversions in flocks of 10 sentinel chickens, and (3) seroprevalence in wild birds collected by mist nets and grain baited traps. Overall, 74 WEE and one SLE isolates were obtained from 222,455 Cx. tarsalis females tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken flocks, and 143 and 27 of 20,192 sera tested from 149 species of wild birds were positive for antibodies to WEE and SLE, respectively. WEE was active in all three valleys, whereas SLE only was detected in Coachella Valley. Seroconversions in sentinel chickens provided the most sensitive indication of enzootic activity and were correlated with seroprevalence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity in chickens, because positive sera from after hatching year birds collected during spring most probably were the result of infections acquired during the previous season. Few seroconversions were detected among banded recaptured birds collected during spring and early summer. Age and resident status, but not sex, were significant risk factors for wild bird infection, with the highest seroprevalence rates among after hatching year individuals of permanent resident species. Migrants (with the exception of mourning doves) and winter resident species rarely were positive. House finches, house sparrows, Gambel's quail, California quail, common ground doves, and mourning doves were most frequently positive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these species in our study areas, perhaps indicating their role in virus amplification. Bird species most frequently positive roosted or nested in elevated upland vegetation, sites where Cx. tarsalis host-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies.