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

Detection of haemosporidian parasites in wild and domestic birds in northern and central provinces of Iran: Introduction of new lineages and hosts

Haemosporidian parasites characterize multi-host and multi-parasite structures which are prevalent among wild bird populations. Here, determination of host records, estimation of the prevalence and diversity of haemosporidian lineages were performed in wild and domestic birds in 11 provinces in Iran. To our knowledge, for the first time in this region, molecular characterization of haemosporidians in migratory water birds, raptors, and domestic birds was carried out: blood or tissue samples were collected from 246 birds belonging to 36 species, 12 families, and 11 orders. The prevalence of Plasmodium, Haemoproteus, and Leucocytozoon were documented as 1.21%, 3.65%, and 0.4%, respectively. Of 36 birds' species inspected in this investigation, 13 individuals of 9 species were parasitized by blood parasites. To our knowledge, five lineages including hANACRE03, hAYTFER01, hAYTFER02, hAQUCYR01, and hSTAL06 were found as un-described lineages, while six known lineages of hLK03, pLK05, lTUSW04, pSW5, hMILANS02, and hHAECOL1 were recorded in hosts within novel geographical regions. Such results are required to fill the gaps in understanding the geographical distribution patterns of wildlife related vector-borne parasites in migratory birds as potential carriers, raptors with high vulnerability, and domestic birds as pet or with economic value.

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Molecular characterization of haemosporidians in migratory, raptors, and domestic birds.

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Eleven avian haemosporidian lineages discovered in new geographical regions of West Asia.

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Novelhost records of blood parasites in Iran.

Robust geographical determinants of infection prevalence and a contrasting latitudinal diversity gradient for haemosporidian parasites in Western Palearctic birds

Identifying robust environmental predictors of infection probability is central to forecasting and mitigating the ongoing impacts of climate change on vector-borne disease threats. We applied phylogenetic hierarchical models to a data set of 2,171 Western Palearctic individual birds from 47 species to determine how climate and landscape variation influence infection probability for three genera of haemosporidian blood parasites (Haemoproteus, Leucocytozoon, and Plasmodium). Our comparative models found compelling evidence that birds in areas with higher vegetation density (captured by the normalized difference vegetation index [NDVI]) had higher likelihoods of carrying parasite infection. Magnitudes of this relationship were remarkably similar across parasite genera considering that these parasites use different arthropod vectors and are widely presumed to be epidemiologically distinct. However, we also uncovered key differences among genera that highlighted complexities in their climate responses. In particular, prevalences of Haemoproteus and Plasmodium showed strong but contrasting relationships with winter temperatures, supporting mounting evidence that winter warming is a key environmental filter impacting the dynamics of host-parasite interactions. Parasite phylogenetic community diversities demonstrated a clear but contrasting latitudinal gradient, with Haemoproteus diversity increasing towards the equator and Leucocytozoon diversity increasing towards the poles. Haemoproteus diversity also increased in regions with higher vegetation density, supporting our evidence that summer vegetation density is important for structuring the distributions of these parasites. Ongoing variation in winter temperatures and vegetation characteristics will probably have far-reaching consequences for the transmission and spread of vector-borne diseases.

Cohort antler size signals environmental stress in a moderate climate

Research in northern latitudes confirms that climate teleconnections exert important influences on ungulate fitness, but studies from regions with milder climates are lacking. We explored the influence of the Pacific Decadal Oscillation (PDO), Northern Atlantic Oscillation (NAO), and El Niño-Southern Oscillation (ENSO) on male, 2.5-year-old white-tailed deer (Odocoileus virginianus) antler and body mass in Mississippi, USA, a region with mild winters and warm, humid summers. Explanatory variables were seasonal averages of each climate index extending back to 3 years prior to account for possible maternal and lag effects. Seasonal climate indices from the period of gestation and the first year of life were correlated with deer morphometrics. Reduced antler mass was largely correlated (R² = 0.52) with PDO values indicating dry conditions during parturition and neonatal development and NAO values indicating warmer than normal winters during gestation and the first year of life. Body mass was less correlated (R² = 0.16) to climate indices, responding negatively to warmer winter weather during the first winter of life. Climate may promote variable fitness among cohorts through long-term effects on male competition for dominance and breeding access. Because broad-scale climate indices simplify complex weather systems, they may benefit management at larger scales. Although this study compared climate with morphological variables, it is likely that demographic characteristics can likewise be modeled using climate indices. As climate change in this region is projected to include greater variability in summer precipitation, we may see concomitantly greater variability in fitness among cohorts of white-tailed deer.

A 15 Year Evaluation of West Nile Virus in Wisconsin: Effects on Wildlife and Human Health

West Nile virus (WNV) is the most important and widespread mosquito-borne virus in the United States (U.S.). WNV has the ability to spread rapidly and effectively, infecting more than 320 bird and mammalian species. An examination of environmental conditions and the health of keystone species may help predict the susceptibility of various habitats to WNV and reveal key risk factors, annual trends, and vulnerable regions. Since 2002, WNV outbreaks in Wisconsin varied by species, place, and time, significantly affected by unique climatic, environmental, and geographical factors. During a 15 year period, WNV was detected in 71 of 72 counties, resulting in 239 human and 1397 wildlife cases. Controlling for population and sampling efforts in Wisconsin, rates of WNV are highest in the western and northwestern rural regions of the state. WNV incidence rates were highest in counties with low human population densities, predominantly wetland, and at elevations greater than 1000 feet. Resources for surveillance, prevention, and detection of WNV were lowest in rural counties, likely resulting in underestimation of cases. Overall, increasing mean temperature and decreasing precipitation showed positive influence on WNV transmission in Wisconsin. This study incorporates the first statewide assessment of WNV in Wisconsin.

Landscape and Environmental Factors Influencing Stage Persistence and Abundance of the Bamboo Mosquito, Tripteroides bambusa (Diptera: Culicidae), across an Altitudinal Gradient

The bamboo mosquito, Tripteroides bambusa (Yamada) (Diptera: Culicidae), is a common insect across East Asia. Several studies have looked at the ecology of Tr. bambusa developmental stages separately, but little is known about the factors associated with the persistence (how often) and abundance (how many individuals) of Tr. bambusa stages simultaneously studied across a heterogeneous landscape. Here, we ask what environmental and landscape factors are associated with the persistence and abundance of Tr. bambusa stages across the altitudinal gradient of Mt. Konpira, Nagasaki City, Japan. During a season-long study we counted 8065 (7297 4th instar larvae, 670 pupae and 98 adults) Tr. bambusa mosquitoes. We found that persistence and abundance patterns were not associated among stages, with the exception of large (4th instar) and small (1st to 3rd instars) larvae persistence, which were positively correlated. We also found that relative humidity was associated with the persistence of Tr. bambusa aquatic stages, being positively associated with large and small larvae, but negatively with pupae. Similarly, landscape aspect changed from positive to negative the sign of its association with Tr. bambusa pupae and adults, highlighting that environmental associations change with life stage. Meanwhile, Tr. bambusa abundance patterns were negatively impacted by more variable microenvironments, as measured by the negative impacts of kurtosis and standard deviation (SD) of environmental variables, indicating Tr. bambusa thrives in stable environments, suggesting this mosquito species has a finely grained response to environmental changes.

West Nile virus illness in Ontario, Canada: 2017

Background

In Canada, the annual incidence rates of West Nile virus (WNV) illness have fluctuated over the last 15 years. Ontario is one of the provinces in Canada most affected by WNV and, as a result, has implemented robust mosquito and human surveillance programs.

Objective

To summarize and discuss the epidemiology of WNV illness in Ontario, Canada in 2017, with comparisons to previous years.

Methods

Case data were obtained from the provincial integrated Public Health Information System. Provincial and public health unit (PHU)-specific incidence rates by year were calculated using population data extracted from intelliHEALTH Ontario.

Results

In 2017, the incidence of WNV illness in Ontario was 1.1 cases per 100,000 population, with 158 confirmed and probable cases reported by 27 of the province’s 36 PHUs. This is the highest rate since 2013, but less than the rate in 2012 (2.0 cases per 100,000 population). Incidence rates in 2017 were highest in Windsor-Essex County and in PHUs in eastern Ontario. While the seasonality is consistent with previous years, the number of cases reported between July and September 2017 was above expected. Most cases were in older age groups (median: 58 years old) and males (59.5% of provincial total); cases with severe outcomes (neurological complications, hospitalizations, deaths) were also disproportionately in older males.

Conclusion

WNV illness continues to be an ongoing burden in Ontario. The increase in the number of cases reported in 2017, and the increased number of PHUs reporting cases, suggests changing and expanding risk levels in Ontario. Continued mosquito and human surveillance, increased awareness of preventive measures, and early recognition and treatment are needed to mitigate the impact of WNV infections.

Increased Adult Aedes aegypti and Culex quinquefasciatus (Diptera: Culicidae) Abundance in a Dengue Transmission Hotspot, Compared to a Coldspot, within Kaohsiung City, Taiwan

The assumption that vector abundance differences might drive spatial and temporal heterogeneities in vector-borne disease transmission is common, though data supporting it is scarce. Here, we present data from two common mosquito species Aedes aegypti (Linnaeus) and Culex quinquefasciatus Say, biweekly sampled as adults, from March 2016 through December 2017, with BG-sentinel traps in two neighboring districts of Kaohsiung City (KC), Taiwan. One district has historically been a dengue transmission hotspot (Sanmin), and the other a coldspot (Nanzih). We collected a total 41,027 mosquitoes, and we found that average mosquito abundance (mean ± S.D.) was higher in Sanmin (Ae. aegypti: 9.03 ± 1.46; Cx. quinquefasciatus: 142.57 ± 14.38) than Nanzih (Ae. aegypti: 6.21 ± 0.47; Cx. quinquefasciatus: 63.37 ± 8.71) during the study period. In both districts, Ae. aegypti and Cx. quinquefasciatus population dynamics were sensitive to changes in temperature, the most platykurtic environmental variable at KC during the study period, a pattern predicted by Schmalhausen’s law, which states that organisms are more sensitive to small changes in environmental variables whose average value is more uncertain than its extremes. Our results also suggest that differences in Ae. aegypti abundance might be responsible for spatial differences in dengue transmission at KC. Our comparative approach, where we also observed a significant increment in the abundance of Cx. quinquefasciatus in the dengue transmission hotspot, suggests this area might be more likely to experience outbreaks of other vector borne diseases and should become a primary focus for vector surveillance and control.

Culex Flavivirus During West Nile Virus Epidemic and Interepidemic Years in Chicago, United States

Culex flavivirus (CxFV) is an insect-specific flavivirus infecting Culex mosquitoes, which are important vectors of West Nile virus (WNV). CxFV and WNV cocirculate in nature and coinfect Culex mosquitoes, including in a WNV "hotspot" in suburban Chicago. We previously identified a positive association between CxFV and WNV in mosquito pools collected from suburban Chicago in 2006. To further investigate this phenomenon, we compared the spatial and temporal distribution of CxFV during an interepidemic year (2011) and an epidemic year (2012) for WNV. Both viruses were more prevalent in mosquito pools in 2012 compared to 2011. During both years, the CxFV infection status of mosquito pools was associated with environmental factors such as habitat type and precipitation frequency rather than coinfection with WNV. These results support the idea that WNV and CxFV are ecologically associated, perhaps because both viruses respond to similar environmental drivers of mosquito populations.

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.

West Nile virus infections in Greece: an update

Approximately 2 years have passed since the detection of the first human case of West Nile virus (WNV) infection in Greece, which was the starting signal of a large outbreak in 2010, followed by a second one in 2011. More than 250 neuroinvasive disease cases with 15% fatality were observed during the two WNV seasons. WNV lineage 2 sequences were obtained from blood donors, Culex mosquitoes, wild birds and sentinel chickens. The Greek WNV strain shows high genetic relatedness to the goshawk-Hungary/04 WNV strain; an amino acid substitution in nonstructural protein 3 (H249P) is observed, which has been previously associated with increased virus transmission. This article provides an overview of the WNV outbreaks in Greece and discusses the knowledge gained from these events.

West Nile virus and non-West Nile virus mortality and coinfection of American crows (Corvus brachyrhynchos) in California

American crows are acutely sensitive to West Nile virus (WNV) infection, and crow mortality has been used in WNV surveillance to monitor enzootic transmission. However, non-WNV sources of mortality could reduce the reliability of crow death as a surveillance tool. Here, using a combination of histopathologic, toxicologic, virologic, and molecular techniques we describe causes of mortality in 67 American crows (Corvus brachyrhynchos) that were collected from a population in the Sacramento Valley of California in 2012 and 2013. Evidence of infectious disease was detected in 70% (47/67) of carcasses. The majority of deaths were linked to a suite of non-WNV viral, bacterial, and fungal infections (39%; 23/59 cases), WNV (36%; 24/67 cases), and an acute toxic event (25%; 15/59 cases). Coinfections were detected in 20% (12/59) of birds and frequently were associated with WNV and poxviral dermatitis. Inferences about WNV activity based on crow mortality should be supported by laboratory confirmation because crow mortality frequently can be caused by other infectious diseases or toxic events.

Ecophysiology of Anopheles gambiae s.l.: persistence in the Sahel

The dry-season biology of malaria vectors is poorly understood, especially in arid environments when no surface waters are available for several months, such as during the dry season in the Sahel. Here we reappraise results on the dry-season physiology of members of the Anopheles gambiae s.l. complex in the broad context of dormancy in insects and especially in mosquitoes. We examine evidence on seasonal changes in reproduction, metabolism, stress tolerance, nutrition, molecular regulation, and environmental conditions and determine if the current results are compatible with dry-season diapause (aestivation) as the primary strategy for persistence throughout the dry season in the Sahel. In the process, we point out critical gaps in our knowledge that future studies can fill. We find compelling evidence that members of the An. gambiae s.l. complex undergo a form of aestivation during the Sahelian dry season by shifting energetic resources away from reproduction and towards increased longevity. Considering the differences between winter at temperate latitudes, which entails immobility of the insect and hence reliance on physiological solutions, as opposed to the Sahelian dry season, which restricts reproduction exclusively, we propose that behavioral changes play an important role in complementing physiological changes in this strategy.

British container breeding mosquitoes: the impact of urbanisation and climate change on community composition and phenology

The proliferation of artificial container habitats in urban areas has benefitted urban adaptable mosquito species globally. In areas where mosquitoes transmit viruses and parasites, it can promote vector population productivity and fuel mosquito-borne disease outbreaks. In Britain, storage of water in garden water butts is increasing, potentially expanding mosquito larval habitats and influencing population dynamics and mosquito-human contact. Here we show that the community composition, abundance and phenology of mosquitoes breeding in experimental water butt containers were influenced by urbanisation. Mosquitoes in urban containers were less species-rich but present in significantly higher densities (100.4±21.3) per container than those in rural containers (77.7±15.1). Urban containers were dominated by Culex pipiens (a potential vector of West Nile Virus [WNV]) and appear to be increasingly exploited by Anopheles plumbeus (a human-biting potential WNV and malaria vector). Culex phenology was influenced by urban land use type, with peaks in larval abundances occurring earlier in urban than rural containers. Among other factors, this was associated with an urban heat island effect which raised urban air and water temperatures by 0.9°C and 1.2°C respectively. Further increases in domestic water storage, particularly in urban areas, in combination with climate changes will likely alter mosquito population dynamics in the UK.

Modeling Culex tarsalis abundance on the northern Colorado front range using a landscape-level approach

Remote sensing and Geographic Information System (GIS) data can be used to identify larval mosquito habitats and predict species distribution and abundance across a landscape. An understanding of the landscape features that impact abundance and dispersal can then be applied operationally in mosquito control efforts to reduce the transmission of mosquito-borne pathogens. In an effort to better understand the effects of landscape heterogeneity on the abundance of the West Nile virus (WNV) vector Culex tarsalis, we determined associations between GIS-based environmental data at multiple spatial extents and monthly abundance of adult Cx. tarsalis in Larimer County and Weld County, CO. Mosquito data were collected from Centers for Disease Control and Prevention miniature light traps operated as part of local WNV surveillance efforts. Multiple regression models were developed for prediction of monthly Cx. tarsalis abundance for June, July, and August using 4 years of data collected over 2007-10. The models explained monthly adult mosquito abundance with accuracies ranging from 51-61% in Fort Collins and 57-88% in Loveland-Johnstown. Models derived using landscape-level predictors indicated that adult Cx. tarsalis abundance is negatively correlated with elevation. In this case, low-elevation areas likely more abundantly include habitats for Cx. tarsalis. Model output indicated that the perimeter of larval sites is a significant predictor of Cx. tarsalis abundance at a spatial extent of 500 m in Loveland-Johnstown in all months examined. The contribution of irrigated crops at a spatial extent of 500 m improved model fit in August in both Fort Collins and Loveland-Johnstown. These results emphasize the significance of irrigation and the manual control of water across the landscape to provide viable larval habitats for Cx. tarsalis in the study area. Results from multiple regression models can be applied operationally to identify areas of larval Cx. tarsalis production (irrigated crops lands and standing water) and assign priority in larval treatments to areas with a high density of larval sites at relevant spatial extents around urban locations.

Vector-virus interactions and transmission dynamics of West Nile virus

West Nile virus (WNV; Flavivirus; Flaviviridae) is the cause of the most widespread arthropod-borne viral disease in the world and the largest outbreak of neuroinvasive disease ever observed. Mosquito-borne outbreaks are influenced by intrinsic (e.g., vector and viral genetics, vector and host competence, vector life-history traits) and extrinsic (e.g., temperature, rainfall, human land use) factors that affect virus activity and mosquito biology in complex ways. The concept of vectorial capacity integrates these factors to address interactions of the virus with the arthropod host, leading to a clearer understanding of their complex interrelationships, how they affect transmission of vector-borne disease, and how they impact human health. Vertebrate factors including host competence, population dynamics, and immune status also affect transmission dynamics. The complexity of these interactions are further exacerbated by the fact that not only can divergent hosts differentially alter the virus, but the virus also can affect both vertebrate and invertebrate hosts in ways that significantly alter patterns of virus transmission. This chapter concentrates on selected components of the virus-vector-vertebrate interrelationship, focusing specifically on how interactions between vector, virus, and environment shape the patterns and intensity of WNV transmission.

Regional and seasonal response of a West Nile virus vector to climate change

Climate change will affect the abundance and seasonality of West Nile virus (WNV) vectors, altering the risk of virus transmission to humans. Using downscaled general circulation model output, we calculate a WNV vector's response to climate change across the southern United States using process-based modeling. In the eastern United States, Culex quinquefasciatus response to projected climate change displays a latitudinal and elevational gradient. Projected summer population depressions as a result of increased immature mortality and habitat drying are most severe in the south and almost absent further north; extended spring and fall survival is ubiquitous. Much of California also exhibits a bimodal pattern. Projected onset of mosquito season is delayed in the southwestern United States because of extremely dry and hot spring and summers; however, increased temperature and late summer and fall rains extend the mosquito season. These results are unique in being a broad-scale calculation of the projected impacts of climate change on a WNV vector. The results show that, despite projected widespread future warming, the future seasonal response of C. quinquefasciatus populations across the southern United States will not be homogeneous, and will depend on specific combinations of local and regional conditions.

Ecology of West Nile virus in North America

The introduction, dispersal and establishment of West Nile virus in North America were reviewed, focusing on factors that may have enhanced receptivity and enabled the invasion process. The overwintering persistence of this tropical virus within temperate latitudes was unexpected, but was key in the transition from invasion to endemic establishment. The cascade of temporal events allowing sporadic amplification to outbreak levels was discussed within a future perspective.

Environmental forcing shapes regional house mosquito synchrony in a warming temperate island

Seasonal changes in the abundance of exothermic organisms can be expected with climate change if warmer temperatures can induce changes in their phenology. Given the increased time for ectothermic organism development at lower temperatures, we asked whether population dynamics of the house mosquito, Culex pipiens s.l. (L.) (Diptera: Culicidae), in Jeju-do (South Korea), an island with a gradient of warming temperatures from north to south, showed differences in sensitivity to changes in temperature along the warming gradient. In addition, we asked whether synchrony, that is, the degree of concerted fluctuations in mosquito abundance across locations, was affected by the temperature gradient. We found the association of mosquito abundance with temperature to be delayed by 2 wk in the north when compared with the south. The abundance across all our sampling locations had a flat synchrony profile that could reflect impacts of rainfall and average temperature on the average of all our samples. Finally, our results showed that population synchrony across space can emerge even when abundance is differentially impacted by an exogenous factor across an environmental gradient.

Overwintering biology of Culex (Diptera: Culicidae) mosquitoes in the Sacramento Valley of California

At temperate latitudes, Culex (Diptera: Culicidae) mosquitoes typically overwinter as adult females in reproductive arrest and also may serve as reservoir hosts for arboviruses when cold temperatures arrest viral replication. To evaluate their role in the persistence of West Nile virus (WNV) in the Sacramento Valley of California, the induction and termination of diapause were investigated for members of the Culex pipiens (L.) complex, Culex tarsalis Coquillett, and Culex stigmatosoma Dyar under field, seminatural, and experimental conditions. All Culex spp. remained vagile throughout winter, enabling the collection of 3,174 females and 1,706 males from diverse habitats during the winters of 2010-2012. Overwintering strategies included both quiescence and diapause. In addition, Cx. pipiens form molestus Forskäl females remained reproductively active in both underground and aboveground habitats. Some blood-fed, gravid, and parous Cx. tarsalis and Cx. pipiens complex females were collected throughout the winter period. Under both field and experimental conditions, Cx. tarsalis and Cx. stigmatosoma females exposed to autumnal conditions arrested primary follicular maturation at previtellogenic stage I, with primary to secondary follicular ratios <1.5 (indicative of a hormonally induced diapause). In contrast, most Cx. pipiens complex females did not enter reproductive diapause and ovarian follicles matured to >or=stage I-II (host-seeking arrest) or were found in various stages of degeneration. Diapause was initiated in the majority of Cx. tarsalis and Cx. stigmatosoma females by mid-late October and was terminated after the winter solstice, but host-seeking seemed limited by temperature. An accrual of 97.52 +/- 30.7 and 162.85 +/- 79.3 degree-days after the winter solstice was estimated to be necessary for diapause termination in Cx. tarsalis under field and seminatural conditions, respectively. An increase in the proportion of blood-fed Culex females in resting collections occurred concurrently with diapause termination in field populations based on ovarian morphometrics. WNV RNA was detected in one pool of 18 males and in a single blood-fed female Cx. tarsalis collected during winter. Therefore, both vertically and horizontally infected Culex females may persist through winter and possibly transmit WNV after diapause termination in late winter or early spring in the Sacramento Valley of California.

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.

Autogeny in Culex pipiens complex mosquitoes from the San Francisco Bay Area

We surveyed the genetic ancestry and recorded the occurrence of autogeny, the developmental times, and survival rates in families of Culex pipiens in Santa Clara County, CA, at 37°N latitude. Females in 95% of the families produced fertile egg rafts without access to blood (= autogeny) after mating in stenogamous conditions. Developmental time, survival, and egg raft production were closely correlated to temperature. Male DV/D ratios overwhelmingly matched Cx. pipiens but a microsatellite analysis revealed these were Cx. pipiens form molestus hybridized with Culex quinquefasciatus and to a lesser extent to Cx. pipiens form pipiens, a genetic mix heretofore not recorded elsewhere. Greater DV/D ratios and larger proportions of genetic ancestry from Cx. quinquefasciatus were negatively correlated to autogeny. The combination of multiple overwintering strategies and widespread autogeny in females arising from aboveground larval sites supports the hypothesis that some North American populations of Cx. pipiens complex mosquitoes express unusual phenologies.

Dry season reproductive depression of Anopheles gambiae in the Sahel

The African malaria mosquito, Anopheles gambiae, is widespread south of the Sahara including in dry savannahs and semi-arid environments where no surface water exists for several months a year. Adults of the M form of An. gambiae persist through the long dry season, when no surface waters are available, by increasing their maximal survival from 4 weeks to 7 months. Dry season diapause (aestivation) presumably underlies this extended survival. Diapause in adult insects is intrinsically linked to depressed reproduction. To determine if reproduction of the Sahelian M form is depressed during the dry season, we assessed seasonal changes in oviposition, egg batch size, and egg development, as well as insemination rate and blood feeding in wild caught mosquitoes. Results from xeric Sahelian and riparian populations were compared. Oviposition response in the Sahelian M form dropped from 70% during the wet season to 20% during the dry season while the mean egg batch size among those that laid eggs fell from 173 to 101. Correspondingly, the fraction of females that exhibited gonotrophic dissociation increased over the dry season from 5% to 45%, while a similar fraction of the population retained developed eggs despite having access to water. This depression in reproduction the Sahelian M form was not caused by a reduced insemination rate. Seasonal variation in these reproductive parameters of the riparian M form population was less extreme and the duration of reproductive depression was shorter. Blood feeding responses did not change with the season in either population. Depressed reproduction during the dry season in the Sahelian M form of An. gambiae provides additional evidence for aestivation and illuminates the physiological processes involved. The differences between the Sahelian and riparian population suggest an adaptive cline in aestivation phenotypes between populations only 130 km apart.

The contribution of rodent models to the pathological assessment of flaviviral infections of the central nervous system

Members of the genus Flavivirus are responsible for a spectrum of important neurological syndromes in humans and animals. Rodent models have been used extensively to model flavivirus neurological disease, to discover host-pathogen interactions that influence disease outcome, and as surrogates to determine the efficacy and safety of vaccines and therapeutics. In this review, we discuss the current understanding of flavivirus neuroinvasive disease and outline the host, viral and experimental factors that influence the outcome and reliability of virus infection of small-animal models.

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.

The distribution of potential West Nile virus vectors, Culex pipiens pipiens and Culex pipiens quinquefasciatus (Diptera: Culicidae), in Mexico City

Background

Culex spp. mosquitoes are considered to be the most important vectors of West Nile virus (WNV) detected in at least 34 species of mosquitoes in the United States. In North America, Culex pipiens pipiens, Culex pipiens quinquefasciatus, and Culex tarsalis are all competent vectors of WNV, which is considered to be enzootic in the United States and has also been detected in equines and birds in many states of Mexico and in humans in Nuevo Leon. There is potential for WNV to be introduced into Mexico City by various means including infected mosquitoes on airplanes, migrating birds, ground transportation and infected humans. Little is known of the geographic distribution of Culex pipiens complex mosquitoes and hybrids in Mexico City. Culex pipiens pipiens preferentially feed on avian hosts; Culex pipiens quinquefasciatus have historically been considered to prefer mammalian hosts; and hybrids of these two species could theoretically serve as bridge vectors to transmit WNV from avian hosts to humans and other mammalian hosts. In order to address the potential of WNV being introduced into Mexico City, we have determined the identity and spatial distribution of Culex pipiens complex mosquitoes and their hybrids.

Results

Mosquito larvae collected from 103 sites throughout Mexico City during 2004-2005 were identified as Culex, Culiseta or Ochlerotatus by morphological analysis. Within the genus Culex, specimens were further identified as Culex tarsalis or as belonging to the Culex pipiens complex. Members of the Culex pipiens complex were separated by measuring the ratio of the dorsal and ventral arms (DV/D ratio) of the male genitalia and also by using diagnostic primers designed for the Ace.2 gene. Culex pipiens quinquefasciatus was the most abundant form collected.

Conclusions

Important WNV vectors species, Cx. p. pipiens, Cx. p. quinquefasciatus and Cx. tarsalis, are all present in Mexico City. Hybrids of Cx. p. pipiens and Cx. p. quinquefasciatus were also collected and identified. The presence and abundance of these WNV competent vectors is a cause for concern. Understanding the distribution of these vectors can help improve viral surveillance activities and mosquito control efforts in Mexico City.

Migratory birds and the dispersal of arboviruses in California

Each spring large numbers of neotropical migrants traversing the Pacific flyway pass through the Coachella Valley enroute to northern destinations, providing an opportunity to test the hypothesis that mosquito-borne encephalitis viruses are introduced annually into California by migratory birds. A total of 5,632 sera were collected from 43 species of migrants during spring (April-June), of which 34 (0.61%) comprised of 14 species tested positive by enzyme immunoassay; only 10 were confirmed by plaque reduction neutralization tests (PRNT). In addition, of 1,109 migrants comprised of 76 species that were reported dead by the public and necropsied, 126 (11%) were positive for West Nile virus (WNV) RNA; however, only three (0.7%) of 428 birds tested during the spring were positive. Limited experimental infection studies with WNV showed that Orange-crowned Warblers were highly susceptible and frequently died, whereas most Yellow Warblers survived. Our results indicated that birds entering California rarely exhibited a history of infection and that most birds probably became infected after entering California.