West Nile virus epizootiology in the southeastern United States, 2001

Public health significance of the white-tailed deer (Odocoileus virginianus) and its role in the eco-epidemiology of tick- and mosquito-borne diseases in North America

White-tailed deer (Odocoileus virginianus) are a ubiquitous species in North America. Their high reproductive potential leads to rapid population growth, and they exhibit a wide range of biological adaptations that influence their interactions with vectors and pathogens. This review aims to characterize the intricate interplay between white-tailed deer and the transmission cycles of various tick- and mosquito-borne pathogens across their range in the eastern United States and southeastern Canada. The first part offers insights into the biological characteristics of white-tailed deer, their population dynamics, and the consequential impacts on both the environment and public health. This contextual backdrop sets the stage for the two subsequent sections, which delve into specific examples of pathogen transmission involving white-tailed deer categorized by tick and mosquito vectors into tick-borne and mosquito-borne diseases. This classification is essential, as ticks and mosquitoes serve as pivotal elements in the eco-epidemiology of vector-borne diseases, intricately linking hosts, the environment, and pathogens. Through elucidating these associations, this paper highlights the crucial role of white-tailed deer in the transmission dynamics of tick- and mosquito-borne diseases. Understanding the interactions between white-tailed deer, vectors, and pathogens is essential for effective disease management and public health interventions.

Evaluation of the In2care Mosquito Station against Culex quinquefasciatus mosquitoes (Diptera: Culicidae) under semifield conditions

Culex quinquefasciatus is an important mosquito vector responsible for the transmission of filarial worms, arthropod-borne viruses like Oropouche, St. Louis encephalitis, and West Nile and protozoans that cause avian malaria. Due to insecticide resistance documented in Cx. quinquefasciatus populations worldwide, integrated vector management programs can benefit from new strategies to control this species. The In2Care Mosquito Station (In2Care station), a commercially available dissemination station containing pyriproxyfen (PPF) and Beauveria bassiana spores, has been shown to be effective against skip-ovipositing Aedes aegypti and Aedes albopictus in previously conducted semifield and field trials. To determine the potential of Cx. quinquefasciatus adult females to autodisseminate PPF and if the In2Care station could be used for Cx. quinquefasciatus control, we assessed its efficacy in a semifield setting against wild Cx. quinquefasciatus. We found that the In2Care station was attractive to gravid Cx. quinquefasciatus females, with a significantly higher percentage of egg rafts laid in the In2Care station compared to alternative ovipots. Adult females successfully autodisseminated PPF from the In2Care station to surrounding ovipots, leading to a significant increase in mosquito emergence inhibition. Additionally, adult Cx. quinquefasciatus exposure to B. bassiana spores significantly reduced mosquito survivorship. These results suggest that the In2Care station may be effective against Cx. quinquefasciatus in addition to Ae. aegypti and Ae. albopictus. Additional field evaluations are needed to assess impacts at the population level.

Bioassay-guided isolation of insect repellent compounds from Ligusticum porteri root extract

In our program of screening natural products against the pests of medical and veterinary importance, ethanolic extract of the roots of Ligusticum porteri J.M.Coult. & Rose showed significant repellency against mosquitoes. The extract was then fractionated to test different fractions to identify the active repellent compounds. This testing resulted in the isolation of different compounds including (Z)-3-butylidenephthalide, (E)-3-butylidenephthalide, and a mixture of (E/Z)-3-butylidenephthalide. Biting deterrence of all these compounds was similar to N,N-diethyl-3-methylbenzamide (DEET) against Aedes aegypti (L.) in Klun and Debboun (K & D) bioassay. (E/Z)-3-butylidenephthalide which is a mixture of the two compounds was further tested in Ali & Khan (A & K) bioassay. Based on these data repellency of this compound was similar whereas the MED values of the mixtures of (E/Z)-3-butylidenephthalide with carotol were lower (6.25 + 6.25 = 12.5 µg/cm2) than individual treatments (25 µg/cm2). In in vivo (direct skin application bioassay), (E/Z)-3-butylidenephthalide showed excellent repellency. The residual repellency of (E/Z)-3-butylidenephthalide at 8 and 16 % application rates was 4.5 and 10-h respectively which was equal to or better than DEET with the residual time of 5 and 9-h, respectively. The mixture of (E/Z)-3-butylidenephthalide with carotol (8 + 8 %) increased the residual repellency by 2-h (44 %) as compared to (E/Z)-3-butylidenephthalide alone at a dose of 8 %. These data indicated that (E/Z)-3-butylidenephthalide is an effective mosquito repellent that is stable and has a long shelf life. The activity of this compound is extraordinary and residual time is comparable to DEET. In vivo data demonstrated an enormous potential of (E/Z)-3-butylidenephthalide as a repellent that can be developed for commercial use. However, (E/Z)-3-butylidenephthalate was found in lower amounts of the L. porteri essential oil.

A scoping review of waterborne and water-related disease in the Florida environment from 1999 to 2022

Florida's environments are suitable reservoirs for many disease-causing agents. Pathogens and toxins in Florida waterways have the potential to infect mosquito vectors, animals, and human hosts. Through a scoping review of the scientific literature published between 1999 and 2022, we examined the presence of water-related pathogens, toxins, and toxin-producers in the Florida environment and the potential risk factors for human exposure. Nineteen databases were searched using keywords relating to the waterborne, water-based toxins, and water-related vector-borne diseases which are reportable to the Florida Department of Health. Of the 10,439 results, 84 titles were included in the final qualitative analysis. The resulting titles included environmental samples of water, mosquitoes, algae, sand, soil/sediment, air, food, biofilm, and other media. Many of the waterborne, water-related vector-borne, and water-based toxins and toxin-producers of public health and veterinary importance from our search were found to be present in Florida environments. Interactions with Florida waterways can expose humans and animals to disease and toxins due to nearby human and/or animal activity, proximal animal or human waste, failing or inadequate water and/or sanitation, weather patterns, environmental events, and seasonality, contaminated food items, preference of agent for environmental media, high-risk populations, urban development and population movement, and unregulated and unsafe environmental activities. A One Health approach will be imperative to maintaining healthy waterways and shared environments throughout the state to protect the health of humans, animals, and our ecosystems.

Effects of Hurricane Irma on mosquito abundance and species composition in a metropolitan Gulf coastal city, 2016-2018

Mosquitoes are the most common disease vectors worldwide. In coastal cities, the spread, activity, and longevity of vector mosquitoes are influenced by environmental factors such as temperature, humidity, and rainfall, which affect their geographic distribution, biting rates, and lifespan. We examined mosquito abundance and species composition before and after Hurricane Irma in Miami, Dade County, Florida, and identified which mosquito species predominated post-Hurricane Irma. Our results showed that mosquito populations increased post-Hurricane Irma: 7.3 and 8.0 times more mosquitoes were captured in 2017 than at baseline, 2016 and 2018 respectively. Warmer temperatures accelerated larval development, resulting in faster emergence of adult mosquitoes. In BG-Sentinel traps, primary species like Ae. tortills, Cx. nigripalpus, and Cx. quinquefasciatus dominated the post-Hurricane Irma period. Secondary vectors that dominated post-Hurricane Irma include An. atropos, An. crucians, An. quadrimaculatus, Cx. erraticus, and Ps. columbiae. After Hurricane Irma, the surge in mosquito populations in Miami, Florida heightened disease risk. To mitigate and prevent future risks, we must enhance surveillance, raise public awareness, and implement targeted vector control measures.

Quantifying the invasion risk of West Nile virus: Insights from a multi-vector and multi-host SEIR model

The invasion of vector-borne diseases depends on the type of specific features of the vector and hosts at play. Within the Culex pipiens complex, differences in ecology, biology, and vector competence can influence the risk of West Nile virus (WNV) outbreaks. To determine which life-history traits affect WNV invasion into susceptible communities the most, we constructed an epidemiological Susceptible-Exposed-Infectious-Recovered model with three vector (eco)types, Culex pipiens pipiens, Cx. pip. molestus, and their hybrids, and two vertebrate hosts, birds (as amplifying hosts) and humans (as dead-end hosts). We investigated how differences in feeding preferences and transmission rates influenced WNV transmission across different habitats and two seasons (Spring versus Summer), to investigate the impact of increasing mosquitoes on the WNV transmission risk. Our results showed that vector feeding preferences and the transmission rate between mosquitoes and birds were the parameters that most influenced WNV invasion risk. Even though our model did not predict WNV invasion across any of the studied environments, we found that natural habitats displayed the highest susceptibility to WNV invasion. Pipiens (eco)type acted as the primary vector in all habitats. Hybrids, contrary to common opinion, showed minimal involvement in WNV transmission. However, it is important to interpret our study results with caution due to the possibility of idealized spring and summer seasons being reflected in the field-collected data. Our study could be a tool to enhance current vector surveillance and control programs by targeting specific vector types in specific environments, especially in natural habitat, which are most responsive to environmental shifts. The joint approach based on epidemiological modelling based on field collected data can help to reduce wasted time and economic costs while maximizing the efficiency of local public health authorities.

Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance

West Nile virus (WNV) neuroinvasive disease threatens the health and well-being of horses and humans worldwide. Disease in horses and humans is remarkably similar. The occurrence of WNV disease in these mammalian hosts has geographic overlap with shared macroscale and microscale drivers of risk. Importantly, intrahost virus dynamics, the evolution of the antibody response, and clinicopathology are similar. The goal of this review is to provide a comparison of WNV infection in humans and horses and to identify similarities that can be exploited to enhance surveillance methods for the early detection of WNV neuroinvasive disease.

Multi-Year Comparison of Community- and Species-Level West Nile Virus Antibody Prevalence in Birds from Atlanta, Georgia and Chicago, Illinois, 2005-2016

West Nile virus (WNV) is prevalent in the United States but shows considerable variation in transmission intensity. The purpose of this study was to compare patterns of WNV seroprevalence in avian communities sampled in Atlanta, Georgia and Chicago, Illinois during a 12-year period (Atlanta 2010-2016; Chicago 2005-2012) to reveal regional patterns of zoonotic activity of WNV. WNV antibodies were measured in wild bird sera using ELISA and serum neutralization methods, and seroprevalence among species, year, and location of sampling within each city were compared using binomial-distributed generalized linear mixed-effects models. Seroprevalence was highest in year-round and summer-resident species compared with migrants regardless of region; species explained more variance in seroprevalence within each city. Northern cardinals were the species most likely to test positive for WNV in each city, whereas all other species, on average, tested positive for WNV in proportion to their sample size. Despite similar patterns of seroprevalence among species, overall seroprevalence was higher in Atlanta (13.7%) than in Chicago (5%). Location and year of sampling had minor effects, with location explaining more variation in Atlanta and year explaining more variation in Chicago. Our findings highlight the nature and magnitude of regional differences in WNV urban ecology.

North American House Sparrows Are Competent for Usutu Virus Transmission

Usutu virus (USUV, Flaviviridae) is an emerging mosquito-borne virus that has been implicated in neuroinvasive disease in humans and epizootic deaths in wild birds. USUV is maintained in an enzootic cycle between ornithophilic mosquitoes, primarily Culex spp., and wild birds, predominantly passerine species. However, limited experimental data exist on the species competent for USUV transmission. Here, we demonstrate that house sparrows are susceptible to multiple USUV strains. Our study also revealed that Culex quinquefasciatus mosquitoes are susceptible to USUV, with a significantly higher infection rate for the Netherlands 2016 USUV strain compared to the Uganda 2012 USUV strain at 50% and 19%, respectively. To assess transmission between avian host and mosquito vector, we allowed mosquitoes to feed on either juvenile chickens or house sparrows inoculated with USUV. Both bird models transmitted USUV to C. quinquefasciatus mosquitoes. Linear regression analyses indicated that C. quinquefasciatus infection rates were positively correlated with avian viremia levels, with 3 to 4 log₁₀ PFU/mL representing the minimum avian viremia threshold for transmission to mosquitoes. Based on the viremia required for transmission, house sparrows were estimated to more readily transmit the Netherlands 2016 strain compared to the Uganda 2012 strain. These studies provide insights on a competent reservoir host of USUV. IMPORTANCE Usutu virus (USUV) is a zoonotic mosquito-borne virus that can cause neuroinvasive disease, including meningitis and encephalitis, in humans and has resulted in hundreds of thousands of deaths in wild birds. The perpetuation of USUV in nature is dependent on transmission between Culex spp. mosquitoes and various avian species. To date, few experimental data exist for determining which bird species are important for the maintenance of USUV. Our studies showed that house sparrows can transmit infectious Usutu virus, indicating their role as a competent host species. By identifying reservoir species of USUV, we can predict areas of USUV emergence and mitigate its impacts on global human and wildlife health.

Consumption of endemic arbovirus mosquito vectors by bats in the southeastern United States

Mosquitoes affect human health and well-being globally through their roles as disease-causing pathogen vectors. Utilizing genetic techniques, we conducted a large-scale dietary study of three bat species common to the southeastern U.S.A., Lasiurus seminolus (Seminole bat), Nycticeius humeralis (evening bat), and Myotis austroriparius (southeastern myotis). Through next-generation sequencing of a 180 bp portion of cytochrome oxidase subunit I (COI) of mitochondrial DNA from 180 bat guano samples, we documented consumption of 17 species of mosquitoes by bats, including six endemic arbovirus vectors. Culex quinquefasciatus, Culex coronator, Culiseta melanura, Culex salinarius, Culex erraticus, and Coquillettidia perturbans were consumed by 51.3%, 43.7%, 27.2%, 22.8%, 18.0%, and 12.7% of bats sampled, respectively. Consumption of two of these mosquito species was explained by spatial variables reflecting the prevalence of mosquito larval habitat, five were explained by bat traits (bat mass, bat species), and two were explained by these factors plus temporal variables (maximum daily temperature, time since sunset, date), making it challenging to offer specific guidance on how best to promote bats as a means of reducing arbovirus vector species. Our results show that common bat species of the southeastern U.S.A. consume endemic, but not exotic, arbovirus mosquito vectors. Future studies are needed to understand the impact of bat consumption on mosquito numbers and public health.

Bioassay-Guided Isolation of Iridoid Glucosides from Stenaria nigricans, Their Biting Deterrence against Aedes aegypti (Diptera: Culicidae), and Repellency Assessment against Imported Fire Ants (Hymenoptera: Formicidae)

In our natural product screening program, we screened natural products for their repellency and toxicity against insect vectors. Methanolic extract of aerial parts of Stenaria nigricans (Lam.), with no published chemistry, was tested for repellency against mosquitoes and imported hybrid fire ants. Methanolic extracts showed biting deterrence similar to DEET (N,N-diethyl-3-methylbenzamide) against Aedes aegypti L. Based on this activity, the crude extract was fractionated into chloroform, ethyl acetate, and methanol subfractions. The active methanolic subfraction was further fractionated into 13 subfractions. These fractions were tested for their biting deterrence against Ae. Aegypti. Active subfractions were further characterized to identify the compounds responsible for this activity. Four undescribed iridoid glucosides (1-4) and three previously reported compounds (5-7) were isolated from active subfractions and tested for their biting deterrent activity. Based on BDI values, compounds 2, 3, 6, and 7, with biting deterrence similar to DEET, showed the potential to be used as repellents against mosquitoes. In an in vitro digging bioassay, none of these compounds showed any repellency against hybrid imported fire ants at a dose of 125 µg/g. This is the first report of biting deterrence and repellency of S. nigricans extract and its pure compounds, iridoid glucosides against mosquitoes and imported fire ants. Further studies will be conducted to explore the repellent potential of these compounds in different formulations under field conditions.

Invasive Plants as Foci of Mosquito-Borne Pathogens: Red Cedar in the Southern Great Plains of the USA

West Nile virus (WNV) is the most significant mosquito-borne disease affecting humans in the United States. Eastern redcedar (ERC) is a native encroaching plant in the southern Great Plains that greatly alters abiotic conditions and bird and mosquito populations. This study tested the hypotheses that mosquito communities and their likelihood of WNV infection differ between ERC and other habitats in the southern Great Plains of the United States. We found support for our first hypothesis, with significantly more Culex tarsalis and Culex erraticus in ERC than deciduous and grass habitats. Mosquito communities in Central Oklahoma were more diverse (21 species) than western Oklahoma (11 species) but this difference was not associated with vegetation. Our second hypothesis was also supported, with significantly more WNV-infected Culex from ERC in both regions, as was our third hypothesis, with significantly more Culex tarsalis and Culex pipiens collected in ERC than other habitats in urban areas. The connection of mosquito-borne disease with invasive plants suggests that land management initiatives can affect human health and should be considered in light of public health impact. Evidence from other vector-borne disease suggests invasive plants, both in the Great Plains and globally, may facilitate the transmission of vector-borne pathogens.

A systematic review of waterborne and water-related disease in animal populations of Florida from 1999-2019

BACKGROUND

Florida's waters are a reservoir for a host of pathogens and toxins. Many of these microorganisms cause water-related diseases in people that are reportable to the Florida Department of Health. Our objective in this review was to ascertain which water-related pathogens and toxins of public health importance have been found in animal populations in Florida over the last twenty years.

METHODS

Nineteen databases were searched, including PubMed and Web of Science Core Collection, using keywords and search terms for the waterborne diseases, water-related vector-borne diseases, and water-based toxins reportable to the Florida Department of Health. For inclusion, peer-reviewed journal articles were to be written in English, published between January 1, 1999 and December 31, 2019, and contain primary research findings documenting at least one of the water-related pathogens or toxins of interest in an animal population within Florida during this same time frame.

RESULTS

Of over eight thousand initial search results, 65 studies were included for final analysis. The most common animal types implicated in the diseases of interest included marine mammals, fish and shellfish, wild birds, and livestock. Toxins or pathogens most often associated with these animals included toxin-producer Karenia brevis, vibriosis, Escherichia coli, and Salmonellosis.

DISCUSSION/CONCLUSION

Findings from this review elucidate the water-related disease-causing pathogens and toxins which have been reported within animal populations in recent Florida history. As most of these diseases are zoonotic, our results suggest a One Health approach is necessary to support and maintain healthy water systems throughout the state of Florida for the protection of both human and animal populations.

Feeding Habits of Vector Mosquitoes in Harris County, TX, 2018

Mosquito-borne pathogens contribute significantly to the global burden of infectious diseases and are a continuing public health concern in the United States. Blood feeding by vector mosquitoes is a critical step in the transmission of human pathogens. Continuous surveillance of mosquito feeding patterns, especially in major population centers, is necessary for sustainable, effective control strategies. To better understand female feeding habits in Harris County, TX, we trapped mosquitoes from various locations, distributed among urban and semi-urban environments. Bloodmeal hosts were determined using a cytochrome C oxidase I DNA barcoding strategy. We identified a diverse array of vertebrate hosts with a high degree of avian host utilization, most surprisingly from anthropophilic species like Aedes aegypti (L.). We also detected sequences from two different vertebrate hosts in about half of specimens examined, suggesting that multiple bloodmeals had been acquired in the same feeding cycle by a sizable fraction of females in both urban and semi-urban locations. The high proportion of feeding on domestic chickens may indicate that a significant number of homeowners are rearing chickens within close proximity to study trap sites. As non-amplifying hosts, chickens may have a diluting effect on West Nile virus, as well as a zooprophylactic effect in their immediate vicinities. Ultimately, spatial and temporal host utilization patterns add insight into potential disease transmission dynamics, thereby informing vector control strategies in Harris County and other metropolitan areas.

Bioassay guided isolation of mosquito biting deterrent compounds from Strumpfia maritima

BACKGROUND

In our natural products screening program against mosquitoes, we screened ethanolic extract of Strumpfia maritima Jacq. which showed high biting deterrent activity in Klun and Debboun (K&D) bioassays against Aedes aegypti L. Through bioguided fractionation we isolated six compounds from the active fractions. The extract and pure compounds were evaluated for larvicidal and biting deterrent activity against Aedes aegypti.

RESULTS

Ethanolic extract with proportion not biting (PNB) value of 0.8 ± 0.08 provided biting deterrent activity similar to N,N-dimethyl-m-toluamide (DEET) (PNB = 0.9 ± 0.05) against Ae. aegypti. Five out of eight fractions, prepared through bioassay-guided fractionation, showed biting deterrent activity similar to DEET. Six compounds were isolated from active fractions and characterized as squalene, ursolic acid, betulonic acid, cabraleone, ursolic acid acetate, and 27-p-E-coumaroyloxyursolic acid. The biting deterrent activity of ursolic acid acetate (biting deterrence index, BDI = 0.85), cabraleone (BDI = 0.79), and ursolic acid (BDI = 0.78) was similar to DEET against Ae. aegypti. Strumpfia maritima extract did not show larvicidal activity at the highest screening dose of 125 ppm in the larvicidal bioassay.

CONCLUSION

The ethanolic extract and pure compounds showed biting deterrent activity against Ae. aegypti. The high biting deterrent activity of ursolic acid acetate and cabraleone indicated a great potential for these natural compounds to be developed as mosquito repellents. © 2020 Society of Chemical Industry.

Bird Species Involved in West Nile Virus Epidemiological Cycle in Southern Québec

Despite many studies on West Nile Virus (WNV) in the US, including the reservoir role of bird species and the summer shifts of the Culex mosquito, feeding from birds to mammals, there have been few equivalent studies in the neighboring regions of Canada where WNV is endemic. Here, a priority list of bird species likely involved in WNV transmission in the greater Montréal area is constructed by combining three sources of data: (i) from WNV surveillance in wild birds (2002–2015); (ii) blood meal analysis of Culex pipiens–restuans (CPR), the primary enzootic vectors of WNV in the region, collected from surveillance in 2008 and 2014; (iii) literature review on the sero-prevalence/host competence of resident birds. Each of these data sources yielded 18, 23 and 53 species, and overall, 67 different bird species were identified as potential WNV amplifiers/reservoirs. Of those identified from CPR blood meals, Common starlings, American robins, Song sparrows and House sparrows ranked the highest and blood meal analysis demonstrated a seasonal shift in feed preference from birds to mammals by CPR. Our study indicates that there are broad similarities in the ecology of WNV between our region and the northeastern US, although the relative importance of bird species varies somewhat between regions.

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.

Repellent Activity of Carrot Seed Essential Oil and Its Pure Compound, Carotol, Against Mosquitoes

In our natural products screening program for mosquitoes, carrot seed essential oil showed high repellency. The gas chromatography (GC)/flame ionization detector and GC/mass spectrometry analysis of the essential oil revealed the presence of 47 compounds. Carotol was more than 75% w/w, followed by muurolene (4.86%), (Z)-β-farnesene (2.9%), and diepicedrene (1.1%). Systematic bioassay-guided fractionation of the essential oil was performed to identify active repellent compounds. In both Klun and Debboun (K&D) and Ali and Khan (A&K) bioassays, carotol showed biting deterrent activity similar to N,N-Diethyl-3-methylbenzamide (deet) and carrot seed essential oil against both Aedes aegypti and Anopheles quadrimaculatus, while in in vivo cloth patch bioassay, the minimum effective dose (MED) of deet was lower (12.5 μg/cm²) than the essential oil and carotol (25 μg/cm²) against Ae. aegypti. In the A&K bioassay, the MED values were similar, whereas the values of the mixtures of deet with essential oil and carotol was lower (6.25 + 6.25 = 12.5 μg/cm²) than their individual treatments (25 μg/cm²). In direct skin application bioassay, both the essential oil and carotol provided good repellency. The mixtures of deet and essential oil or carotol significantly increased the residual activity, indicating synergism. Mosquito repellency of the essential oil and carotol is reported for the 1st time. These data indicate the potential of these natural products to be developed as commercial repellents.

Focal amplification and suppression of West Nile virus transmission associated with communal bird roosts in northern Colorado

To explain the patchy distribution of West Nile virus (WNV), we propose that avian immunity encountered by Culex vectors regulates WNV transmission, particularly at communal bird roosts. To test this hypothesis, we selected two test sites with communally roosting American robins (Turdus migratorius) and two control sites that lacked communal roosts. The density of vector-vertebrate contacts, represented by engorged Culex pipiens, was 23-fold greater at test sites compared to control sites, and the density of blood-engorged Cx. pipiens measured in resting mosquito traps correlated positively with the presence of robins and negatively with the presence of other birds, confirming an attraction to robins for blood feeding. WNV transmission was alternately up-regulated (amplification) and down-regulated (suppression) at both test sites. At one test site, infection in resting Cx. pipiens surged from zero to 37.2 per thousand within four weeks, and robin immunity rose from 8.4% to 64% before reducing to 33%. At this site, ten potentially infectious contacts between vector and vertebrates (including nine robins and a mourning dove [Zenaida macroura]) were documented. Infectious vector-vertebrate contacts were absent from control sites. The use of infectious vector-vertebrate contacts, rather than infected mosquitoes, to evaluate a transmission focus is novel.

Mosquito community influences West Nile virus seroprevalence in wild birds: implications for the risk of spillover into human populations

Mosquito community composition plays a central role in the transmission of zoonotic vector-borne pathogens. We evaluated how the mosquito community affects the seroprevalence of West Nile virus (WNV) in house sparrows along an urbanisation gradient in an area with the endemic circulation of this virus. We sampled 2544 birds and 340829 mosquitoes in 45 localities, analysed in 15 groups, each containing one urban, one rural and one natural area. WNV seroprevalence was evaluated using an epitope-blocking ELISA kit and a micro virus-neutralization test (VNT). The presence of WNV antibodies was confirmed in 1.96% and 0.67% of birds by ELISA and VNT, respectively. The VNT-seropositive birds were captured in rural and natural areas, but not in urban areas. Human population density was zero in all the localities where VNT-positive birds were captured, which potentially explains the low incidence of human WNV cases in the area. The prevalence of neutralizing antibodies against WNV was positively correlated with the abundance of the ornithophilic Culex perexiguus but negatively associated with the abundance of the mammophilic Ochlerotatus caspius and Anopheles atroparvus. These results suggest that the enzootic circulation of WNV in Spain occurs in areas with larger populations of Cx. perexiguus and low human population densities.

Systematic Review: Land Cover, Meteorological, and Socioeconomic Determinants of Aedes Mosquito Habitat for Risk Mapping

Asian tiger and yellow fever mosquitoes (Aedes albopictus and Ae. aegypti) are global nuisances and are competent vectors for viruses such as Chikungunya (CHIKV), Dengue (DV), and Zika (ZIKV). This review aims to analyze available spatiotemporal distribution models of Aedes mosquitoes and their influential factors. A combination of five sets of 3-5 keywords were used to retrieve all relevant published models. Five electronic search databases were used: PubMed, MEDLINE, EMBASE, Scopus, and Google Scholar through 17 May 2017. We generated a hierarchical decision tree for article selection. We identified 21 relevant published studies that highlight different combinations of methodologies, models and influential factors. Only a few studies adopted a comprehensive approach highlighting the interaction between environmental, socioeconomic, meteorological and topographic systems. The selected articles showed inconsistent findings in terms of number and type of influential factors affecting the distribution of Aedes vectors, which is most likely attributed to: (i) limited availability of high-resolution data for physical variables, (ii) variation in sampling methods; Aedes feeding and oviposition behavior; (iii) data collinearity and statistical distribution of observed data. This review highlights the need and sets the stage for a rigorous multi-system modeling approach to improve our knowledge about Aedes presence/abundance within their flight range in response to the interaction between environmental, socioeconomic, and meteorological systems.

Evidence of exposure of laughing doves (Spilopelia senegalensis) to West Nile and Usutu viruses in southern Tunisian oases

It has previously been suggested that southern Tunisian oases may be suitable areas for the circulation of flaviviruses. In order to anticipate and prevent possible epidemiological spread of flaviviruses in humans and domestic animals, the ecology of their transmission in the oasis system needs to be better understood. Thus, the aim of this study was to assess the seroprevalence of anti-flavivirus antibodies in the laughing dove (Spilopelia senegalensis), an abundant resident bird in Tunisian oases. Anti-flavivirus antibodies were detected in 17% of sampled doves. Ten per cent of the total tested doves were West Nile virus (WNV) seropositive and 4% were Usutu virus (USUV) seropositive, which provides the first evidence of USUV circulation in Tunisian birds. We also found that the occurrence probability of anti-flavivirus antibodies in dove plasma increased with decreasing distance to coast, suggesting that doves inhabiting coastal oases were more exposed to flaviviruses compared with those inhabiting inland oases. We also found significantly higher antibody occurrence probability in adult doves compared with young doves, which underlines the effect of exposure time. Overall, our results suggest that the laughing dove may be used for WNV and USUV surveillance in southern Tunisia. They also stress the need for investigations combining data on birds and mosquitoes to better understand the ecological factors governing the circulation of flaviviruses in this area.

Larvicidal and Biting Deterrent Activity of Essential Oils of Curcuma longa, Ar-turmerone, and Curcuminoids Against Aedes aegypti and Anopheles quadrimaculatus (Culicidae: Diptera)

Essential oils and extract of Curcuma longa, ar-turmerone, and curcuminoids were evaluated for their larvicidal and deterrent activity against mosquitoes. Ar-turmerone and curcuminoids constituted 36.9, 24.9 and 50.6% of rhizome oil, leaf oil, and rhizome extract, respectively. Ar-turmerone was the major compound of the rhizome oil (36.9%) and leaf oil (24.9%). The ethanolic extract had 15.4% ar-turmerone with 6.6% bisdesmethoxycurcumin, 6.1% desmethoxycurcumin, and 22.6% curcumin. In in vitro studies, essential oils of the leaf (biting deterrence index [BDI] = 0.98), rhizome (BDI = 0.98), and rhizome ethanolic extract (BDI = 0.96) at 10 µg/cm(2) showed biting deterrent activity similar to DEET at 25 nmol/cm(2) against Aedes aegypti L. Among the pure compounds, ar-turmerone (BDI = 1.15) showed the biting deterrent activity higher than DEET at 25 nmol/cm(2) whereas the activity of other compounds was lower than DEET. In Anopheles quadrimaculatus Say, only ar-turmerone showed deterrent activity similar to DEET. In dose-response bioassay, ar-turmerone showed significantly higher biting deterrence than DEET at all the dosages. Ar-turmerone, at 15 nmol/cm(2), showed activity similar to DEET at 25 nmol/cm(2) and activity at 5 nmol/cm(2) was similar to DEET at 20 and 15 nmol/cm(2). Leaf essential oil with LC(50) values of 1.8 and 8.9 ppm against larvae of An. quadrimaculatus and Ae. aegypti, respectively, showed highest toxicity followed by rhizome oil and ethanolic extract. Among the pure compounds, ar-turmerone with LC(50) values of 2.8 and 2.5 ppm against larvae of An. quadrimaculatus and Ae. aegypti, respectively, was most toxic followed by bisdesmethoxycurcumin, curcumin, and desmethoxycurcumin.

Abundance and Diversity of Mosquito Species Collected From a Rural Area of Central Mississippi: Implications for West Nile Virus Transmission in Mississippi

To determine abundance and seasonality of potential West Nile virus (WNV) mosquito vectors in a forested area of central Mississippi, mosquitoes were collected weekly from a wildlife management area located approximately 10 mi from a local urban area known to have numerous human WNV cases. We were particularly interested in the presence or absence of Culex quinquefasciatus, the primary vector of WNV in Mississippi, although other Culex species were assayed. Two Centers for Disease Control and Prevention light traps baited with CO2 were set once a week from 2005 through 2006 in the Pearl River Wildlife Management Area (PRWMA), which consists of 6,925 acres primarily composed of bottomland hardwood forest with wetland areas. Traps were placed midafternoon and picked up the following morning. A total of 199,222 mosquitoes were collected during the 2-year study. No Cx. quinquefasciatus were collected throughout the entire study, although other health department surveys have indicated they are abundant just a few miles away. As for other potential WNV vectors, 1,325 (0.6%) Cx. nigripalpus, 1,804 (0.9%) Cx. restuans, and 6,076 (3.1%) Cx. salinarius were collected in the PRWMA over the 2-year period. These data suggest that Cx. quinquefasciatus is not usually found in remote forested environments, but is more associated with human habitation.

Does host receptivity or host exposure drives dynamics of infectious diseases? The case of West Nile Virus in wild birds

Infection is a complex biological process involving reciprocally both the intensity of host exposure to a pathogen as well as the host intrinsic "receptivity", or permissiveness to infection. Disentangling their respective contributions is currently seen as a fundamental gap in our knowledge. Here, we take the advantage of a rare semi-natural experiment context provided by the emergence of the West Nile Virus (WNV) in North America. Focusing on the pathogen emergence period, we combine datasets from (i) wild birds exposed to WNV in an urban zoo to evaluate the species intrinsic receptivity to WNV infection in an environment where exposure to WNV vectors can be assumed to be relatively homogenous for all captive species, and (ii) from free-ranging birds in their natural habitat where species ecological traits is expected to influence their exposure to WNV vectors. We show that ecological trait and intrinsic receptivity to infection both contribute similarly to the species variation in WNV seroprevalence, but considering only one of them can lead to erroneous conclusions. We then argue that degree of pathogen host specialization could be a fundamental factor for the respective contribution of species exposure and receptivity for numerous pathogens.

Reduced West Nile Virus Transmission Around Communal Roosts of Great-Tailed Grackle (Quiscalus mexicanus)

West Nile virus has caused several outbreaks among humans in the Phoenix metropolitan area (Arizona, southwest USA) within the last decade. Recent ecologic studies have implicated Culex quinquefasciatus and Culex tarsalis as the mosquito vectors and identified three abundant passerine birds-great-tailed grackle (Quiscalus mexicanus), house sparrow (Passer domesticus), and house finch (Haemorhous mexicanus)-as key amplifiers among vertebrates. Nocturnal congregations of certain species have been suggested as critical for late summer West Nile virus amplification. We evaluated the hypothesis that house sparrow (P. domesticus) and/or great-tailed grackle (Q. mexicanus) communal roost sites (n = 22 and n = 5, respectively) in a primarily suburban environment were spatially associated with West Nile virus transmission indices during the 2010 outbreak of human neurological disease in metropolitan Phoenix. Spatial associations between human case residences and communal roosts were non-significant for house sparrows, and were negative for great-tailed grackle. Several theories that explain these observations are discussed, including the possibility that grackle communal roosts are protective.

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.

The role of Australian mosquito species in the transmission of endemic and exotic West Nile virus strains

Recent epidemic activity and its introduction into the Western Hemisphere have drawn attention to West Nile virus (WNV) as an international public health problem. Of particular concern has been the ability for the virus to cause outbreaks of disease in highly populated urban centers. Incrimination of Australian mosquito species is an essential component in determining the receptivity of Australia to the introduction and/or establishment of an exotic strain of WNV and can guide potential management strategies. Based on vector competence experiments and ecological studies, we suggest candidate Australian mosquito species that would most likely be involved in urban transmission of WNV, along with consideration of the endemic WNV subtype, Kunjin. We then examine the interaction of entomological factors with virological and vertebrate host factors, as well as likely mode of introduction, which may influence the potential for exotic WNV to become established and be maintained in urban transmission cycles in Australia.

Ecology of potential West Nile virus vectors in Southeastern Louisiana: enzootic transmission in the relative absence of Culex quinquefasciatus

A study of West Nile virus (WNV) ecology was conducted in St. Tammany Parish, Louisiana, from 2002 to 2004. Mosquitoes were collected weekly throughout the year using Centers for Disease Control and Prevention (CDC) light traps placed at 1.5 and 6 m above the ground and gravid traps. A total of 379,466 mosquitoes was collected. WNV was identified in 32 pools of mosquitoes comprising four species; 23 positive pools were from Culex nigripalpus collected during 2003. Significantly more positive pools were obtained from Cx. nigripalpus collected in traps placed at 6 m than 1.5 m that year, but abundance did not differ by trap height. In contrast, Cx. nigripalpus abundance was significantly greater in traps placed at 6 m in 2002 and 2004. Annual temporal variation in Cx. nigripalpus peak seasonal abundance has important implications for WNV transmission in Louisiana. One WNV-positive pool, from Cx. erraticus, was collected during the winter of 2004, showing year-round transmission. The potential roles of additional mosquito species in WNV transmission in southeastern Louisiana are discussed.

The contribution of Culex pipiens complex mosquitoes to transmission and persistence of West Nile virus in North America

Mosquitoes within the Culex pipiens complex have been implicated as major vectors of West Nile virus (WNV) in North America due to their seasonal abundance, vector competence and high field infection rates. However, the role of Cx. p. pipiens complex mosquitoes in enzootic amplification of WNV among avian hosts and epidemic transmission to humans varies throughout its geographical distribution. In the northeastern United States, Cx. p. pipiens is recognized as the primary enzootic vector responsible for amplification of virus among wild bird populations. However, because this mosquito is strongly ornithophilic, its role in transmission to humans appears to be more limited in this region. In the north central and Mid-Atlantic States by contrast, Cx. p. pipiens shows an increased affinity for human hosts and has been incriminated as a key bridge vector. In southern regions of the United States, Culex p. quinquefasciatus are more opportunistic feeders, and are thought to be principal enzootic and epidemic vectors. In western regions of the United States where Culex tarsalis predominates, especially in rural areas, Cx. p. pipiens and Cx. p. quinquefasciatus play roles that are more limited and are recognized as secondary vectors. In the southwestern United States Cx. p. quinquefasciatus also appears to be the predominant vector in urban habitats, but only a secondary vector in more rural environs. The direct involvement of Cx. p. pipiens form molestus in WNV transmission is largely unknown, but human-biting Cx. p. pipiens are more likely to have a probability of genetic ancestry with Cx. p. pipiens form molestus. The detection of WNV from overwintering populations of diapausing Cx. p. pipiens and non-diapausing Cx. p. quinquefaciatus and their role in local overwintering of WNV are addressed.

The effect of exogenous corticosterone on West Nile virus infection in Northern Cardinals (Cardinalis cardinalis)

The relationship between stress and disease is thought to be unambiguous: chronic stress induces immunosuppression, which likely increases the risk of infection. However, this link has not been firmly established in wild animals, particularly whether stress hormones affect host responses to zoonotic pathogens, which can be transmitted to domesticated animal, wildlife and human populations. Due to the dynamic effects of stress hormones on immune functions, stress hormones may make hosts better or poorer amplifying hosts for a pathogen contingent on context and the host species evaluated. Using an important zoonotic pathogen, West Nile virus (WNV) and a competent host, the Northern Cardinal (Cardinalis cardinalis), we tested the effects of exogenous corticosterone on response to WNV infection. Corticosterone was administered at levels that individuals enduring chronic stressors (i.e., long-term inclement weather, food shortage, anthropogenic pollution) might experience in the wild. Corticosterone greatly impacted mortality: half of the corticosterone-implanted cardinals died between five - 11 days post-inoculation whereas only one of nine empty-implanted (control) birds died. No differences were found in viral titer between corticosterone- and empty-implanted birds. However, cardinals that survived infections had significantly higher average body temperatures during peak infection than individuals that died. In sum, this study indicates that elevated corticosterone could affect the survival of WNV-infected wild birds, suggesting that populations may be disproportionately at-risk to disease in stressful environments.

Culex flavivirus and West Nile virus in Culex quinquefasciatus populations in the southeastern United States

Little is known of the interactions between insect-only flaviviruses and other arboviruses in their mosquito hosts, or the potential public health significance of these associations. The specific aims of this study were to describe the geographic distribution, prevalence, and seasonal infection rates of Culex flavivirus (CxFV) and West Nile virus (WNV) in Culex quinquefasciatus Say in the Southeastern United States, investigate the potential association between CxFV and WNV prevalence in Cx. quinquefasciatus and describe the phylogenetic relationship among CxFV and WNV isolates from the Southeastern United States and around the world. Using ArboNET records, 11 locations were selected across Georgia, Mississippi, and Louisiana that represented a range of WNV human case incidence levels. Cx. quinquefasciatus were trapped weekly throughout the summer of 2009 and pools were screened for flavivirus RNA by reverse transcriptase polymerase chain reaction. Cx. quinquefasciatus from Georgia had significantly higher CxFV infection rates than either Mississippi or Louisiana. CxFV was not detected in Mississippi after July, and no CxFV was detected in Cx. quinquefasciatus in Louisiana. In Georgia, CxFV infection rates were variable between and within counties and over time. WNV infection rates were not significantly different across states or months, and WNV sequences from all three states were identical to each other in the envelope and NS5 gene regions. Phylogenetically, NS5 and E gene sequences from Georgia CxFV isolates clustered with CxFV from Japan, Iowa, and Texas. Multiple CxFV genetic variants were found circulating simultaneously in Georgia. No evidence was found supporting an association between WNV and CxFV infection prevalence in Cx. quinquefasciatus.

Weather and landscape factors associated with adult mosquito abundance in southwestern Georgia, U.S.A

Mosquito community composition and population dynamics were compared to weather variables and land use/cover data during 2008 to determine which variables affected population dynamics at the J.W. Jones Ecological Research Center in southwestern Georgia. Models relating adult mosquito distributions to weather variables and time of year were compared using Akaike's Information Criterion (AIC) model selection. Precipitation, temperature, humidity, and Keetch-Byram Drought Index were important factors correlated with mosquito abundance or presence/absence for the species considered. A cluster analysis, which grouped eight sites based on the percentages of land use/cover and hydric soils located in a 1-km radius surrounding collection sites, and an indicator species analysis were used to investigate the associations among 11 mosquito species and sites with similar land use/cover. Aedes albopictus (Skuse), Culex coronator Dyar & Knab, Culex quinquefasciatus Say, and Culex salinarius Coquillett were associated with sites that had the most anthropogenic influence, while Coquillettidia perturbans (Walker) and Psorophora ferox (von Humboldt) were associated with natural land cover such as wetlands and forested land. This study demonstrates that regional climate and land use/cover data can be predictive of the population dynamics of certain mosquito populations and is the first to examine how the distribution of Cx. coronator adults relate to land use/cover in the southeastern United States.

Impact of West Nile virus dose and incubation period on vector competence of Culex nigripalpus (Diptera: Culicidae)

Female Culex nigripalpus were fed blood containing a low dose (6.3±0.01 logs plaque-forming units (PFU)/mL) or high dose (7.3±0.1 logs PFU/mL) of West Nile virus (WNV) and maintained at 28°C for incubation periods (IPs) of 6 or 12 days. Vector competence was measured using rates of infection (% with WNV-positive bodies), dissemination (% infected with WNV-positive legs), and transmission (% infected with WNV-positive saliva). Infection rates were not influenced by dose or IP. Dissemination rates were significantly higher at the high dose, and this was dependent on IP. Despite 100% infection and 90% dissemination in the most permissive treatment of high dose and 12 days, only 11% transmission was observed. Virus titers in body and leg tissues were significantly lower at the low dose and the titers were not influenced by IP. We show that not all mosquitoes with infections and/or disseminated infections transmit WNV under the conditions of this test. Therefore, characterizing the transmission ability of a vector population using infection or dissemination as indicators of transmission may provide inaccurate information. The complex relationships between infection, dissemination, and transmission must be evaluated under a variety of biological and environmental conditions to begin to assess the epidemiological risk of natural mosquito populations.

Sentinel chicken seroconversions track tangential transmission of West Nile virus to humans in the greater Los Angeles area of California

In Los Angeles, California, West Nile virus (WNV) has followed a pattern of emergence, amplification, subsidence, and resurgence. A time series cross-correlation analysis of human case counts and sentinel chicken seroconversions revealed temporal concordance indicating that chicken seroconversions tracked tangential transmission of WNV from the basic passeriform-Culex amplification cycle to humans rather than antecedent enzootic amplification. Sentinel seroconversions provided the location and time of transmission as opposed to human cases, which frequently were reported late and were assumed to be acquired 2-14 days before disease onset at their residence. Cox models revealed that warming degree-days were associated with the increased risk of seroconversion, whereas elevated herd immunity in peridomestic birds dampened seroconversion risk. Spatially, surveillance data collected within a 5 km radius of flock locations 15-28 days before the bleed date were most predictive of a seroconversion. In urban Los Angeles, sentinel chicken seroconversions could be used as an outcome measure in decision support for emergency intervention.

Environmental and biological factors influencing Culex pipiens quinquefasciatus (Diptera: Culicidae) vector competence for West Nile Virus

Interactions between environmental and biological factors affect the vector competence of Culex pipiens quinquefasciatus for West Nile virus. Three age cohorts from two Cx. p. quinquefasciatus colonies were fed blood containing a low- or high-virus dose, and each group was held at two different extrinsic incubation temperatures (EIT) for 13 days. The colonies differed in the way that they responded to the effects of the environment on vector competence. The effects of mosquito age on aspects of vector competence were dependent on the EIT and dose, and they changed depending on the colony. Complex interactions must be considered in laboratory studies of vector competence, because the extent of the genetic and environmental variation controlling vector competence in nature is largely unknown. Differences in the environmental (EIT and dose) and biological (mosquito age and colony) effects from previous studies of Cx. p. quinquefasciatus vector competence for St. Louis encephalitis virus are discussed.

Evaluation of a new formulation of adulticide, DUET, against West Nile virus vector mosquitoes

DUET was evaluated against field-collected mosquito populations in St. John's County, FL, under laboratory and field conditions. Ten serial dilutions ranging from 22 to 2.2E-08 microg/ml of the product were tested against Aedes albopictus, Ae. taeniorhynchus, Culex quinquefasciatus, and Psorophora columbiae. At 0.0022 microg/ml, 100% mortality was recorded for all species except for Cx. quinquefasciatus. The 50% lethal concentration value for Cx. quinquefasciatus was 0.0029 microg/ml (CI = 0.00069 to 0.013). At the lowest dilution (2.2E-08 microg/ml) evaluated, Ae. albopictus, Ae. taeniorhynchus, and Ps. columbiae showed > 50% mortality. Ground ultralow-volume application of DUET resulted in overall average mortality of 87% of field-collected Cx. quinquefasciatus mosquitoes. There was a significant difference in mortality based on the distance of the caged mosquitoes (P < 0.05). However, mortality was > 70% at the farthest distance from the application (106 m).

Host feeding patterns of Culex mosquitoes (Diptera: Culicidae) in East Baton Rouge Parish, Louisiana

Host feeding patterns were examined for four species of Culex mosquitoes collected from 18 sites in or adjacent to East Baton Rouge Parish, LA, from November 2002 to October 2004. Host DNA from 37 bloodfed Culex coronator Dyar and Knab, 67 bloodfed Cx. salinarius Coquillett, 112 bloodfed Cx. nigripalpus Theobald, and 684 bloodfed Cx. quinquefasciatus Say were identified. The percentages of bloodmeals containing mammalian DNA were 94.6% for Cx. coronator, 82.1% for Cx. salinarius, 66.1% for Cx. nigripalpus, and 40.1% for Cx. quinquefasciatus. Human DNA was detected in 7% of the bloodmeals from Cx. quinquefasciatus and 2.7% of the bloodmeals from Cx. nigripalpus. The northern cardinal was the most frequent avian host of Cx. quinquefasciatus and Cx. nigripalpus. In 2003 and 2004, there was no significant relationship from May through October between the proportion of Cx. quinquefasciatus feeding on mammalian hosts and the date of collection. Of the six avian species most frequently fed on by Cx. quinquefasciatus, the northern cardinal, northern mockingbird, common grackle, and brown thrasher were fed on more frequently than expected based on their abundance. House sparrows were fed on less frequently than expected based on their abundance. These data support the conclusions of previous studies that Cx. quinquefasciatus is the most important vector for both the enzootic amplification and transmission of West Nile virus to humans in southern Louisiana.

Effects of West Nile virus dose and extrinsic incubation temperature on temporal progression of vector competence in Culex pipiens quinquefasciatus

Culex pipiens quinquefasciatus were fed blood containing either 7.0 +/- 0.1 logs plaque-forming units (pfu)/ml (high dose) or 5.9 +/- 0.1 logs pfu/ml (low dose) of West Nile virus and held at extrinsic incubation temperatures (EIT) of 28 degrees C or 25 degrees C. Approximately 20 mosquitoes per dose were collected after incubation periods (IP) of 4, 6, 8, and 12 days postinfection (dpi). Infection rates were influenced by EIT and virus dose but not by IP. Body titer was significantly higher for mosquitoes fed the high dose and held at 28 degrees C at the later IPs (6, 8, and 12 dpi). However, leg titer was significantly higher for mosquitoes at the later IPs but did not differ between EITs or doses. Because infection rates varied with EIT and dose, there is likely a midgut infection barrier influenced by these factors that is not influenced by IP. Dissemination rates were influenced by all 3 factors consistent with the presence of a midgut escape barrier. Dissemination rate, body titer, and leg titer were dependent on IP, indicating the need to investigate multiple time points in vector competence studies to elucidate critical events in infection and dissemination.

Effects of forced egg retention on the temporal progression of West Nile virus infection in Culex pipiens quinquefasciatus (Diptera: Culicidae)

Environmental factors that impact the biology of mosquito vectors can have epidemiological implications. Lack of oviposition sites facilitated by environmental factors such as temperature and drought can often force Culex spp. mosquitoes to retain their eggs. Culex pipiens quinquefasciatus Say were fed blood meals containing West Nile virus (WNV; family Flaviviridae, genus Flavivirus) and either allowed to oviposit or forced to retain their eggs through different time points postinfection (9, 13, 20, 27 d) at 28 degrees C. Oviposition status did not significantly affect rates of WNV infection (% with virus-positive bodies), dissemination (% with virus-positive legs), or transmission (% with virus-positive saliva) for any of the tested time points. As expected, WNV titers in bodies and legs were significantly (P < 0.05) higher at late time points compared with early time points. No significant differences were observed in WNV titers in saliva between time points. There were no significant effects of oviposition status on virus titers of bodies, legs, or saliva. However, we found that egg retention may increase vector competence at early and late time points after infection and that a single oviposition event may decrease vector competence, possibly by activating an immune response against the virus. Environmental changes that influence mosquito biology are important determinants of virus transmission, and further studies are needed to assess the effects of drought on virus transmission risk and how these interactions affect our interpretation of field data.

West Nile virus activity in mosquitoes and domestic animals in Chiapas, México

Prior to 2006, West Nile virus (WNV) had not been definitively detected in Chiapas, the southernmost state of Mexico, although it circulates elsewhere in Mexico and Central America. We collected over 30,000 mosquitoes and blood-sampled 351 domestic animals in Chiapas in search for evidence of current or recent transmission of WNV. Two mosquito pools tested positive for WNV RNA and 17 domestic animals tested positive for specific WNV-neutralizing antibodies, including young animals (<1 year old) in four of five sampled locations. The two WNV-positive mosquito pools were collected on the Pacific coastal plain of Chiapas in June, 2006, and included a pool of Culex nigripalpus, a suspected vector of WNV, and a pool of Cx. interrogator. The sequence of a 537-nucleotide portion of a cDNA amplicon derived from the WNV NS5 gene from the Cx. interrogator pool contained a single silent nucleotide substitution when compared to WNV strain NY99.

Spatially explicit West Nile virus risk modeling in Santa Clara County, California

A geographic information system model designed to identify regions at risk for West Nile virus (WNV) transmission was calibrated and tested with data collected in Santa Clara County, California. American Crows that died from WNV infection in 2005 provided spatial and temporal ground truth. When the model was run with parameters based on Culex tarsalis infected with the NY99 genotype of the virus, it underestimated WNV occurrence in Santa Clara Co. The parameters were calibrated to fit the field data by reducing the number of degree-days necessary to reach the mosquito's extrinsic incubation period from 109 to 76. The calibration raised model efficiency from 61% to 92% accuracy, and the model performed well the following year in Santa Clara Co.

Persistent West Nile virus infection in the house sparrow (Passer domesticus)

Long-term persistence of West Nile virus (WNV) infection within vertebrate reservoir hosts is a potential mechanism for overwintering of this (and other) arbovirus(es) at temperate latitudes. The house sparrow (Passer domesticus), an established amplifying host for WNV and other arboviruses, was used as a model to confirm chronicity of WNV infection in passerine birds and to evaluate the feasibility of two overwintering mechanisms: blood-borne infection of arthropod vectors (recrudescence) and oral infection of vertebrate reservoir hosts (ingestion of infected tissues through predation). WNV-inoculated sparrows were monitored for persistent infection for up to 2 years. Infectious virus persisted in tissues through 43 days, but not in sera beyond 6 days. Viral RNA persisted in tissues through 65 days. Chronicity of WNV infection in some tissues, but not blood, supports the predation mechanism of WNV overwintering, but not recrudescence. RNA persistence impacts interpretation and etiologic determination of avian mortality.

Seasonal blood-feeding behavior of Culex tarsalis (Diptera: Culicidae) in Weld County, Colorado, 2007

Studies on Culex tarsalis Coquillett in Colorado have shown marked seasonal variation in the proportion of blood meals from birds and mammals. However, limitations in the specificity of antibodies used in the precipitin test and lack of vertebrate host availability data warrant revisiting Cx. tarsalis blood feeding behavior in the context of West Nile virus (WNV) transmission. We characterized the host preference of Cx. tarsalis during peak WNV transmission season in eastern Colorado and estimated the relative contribution of different avian species to WNV transmission. Cx. tarsalis preferred birds to mammals each month, although the proportion of blood meals from mammals increased in July and August. The distribution of blood meals differed significantly across months, in part because of changes in the proportion of blood meals from American robins, a preferred host. The estimated proportion of WNV-infectious vectors derived from American robins declined from 60 to 1% between June and August. The majority of avian blood meals came from doves, preferred hosts that contributed 25-40% of the WNV-infectious mosquitoes each month. Active WNV transmission was observed in association with a large house sparrow communal roost. These data show how seasonal patterns in Cx. tarsalis blood feeding behavior relate to WNV transmission in eastern Colorado, with the American robin contributing greatly to early-season virus transmission and a communal roost of sparrows serving as a focus for late-season amplification.

Avian host community structure and prevalence of West Nile virus in Chicago, Illinois

Vertebrate host diversity has been postulated to mediate prevalence of zoonotic, vector-borne diseases, such that as diversity increases, transmission dampens. This "dilution effect" is thought to be caused by distribution of infective bites to incompetent reservoir hosts. We quantified avian species richness, avian seroprevalence for antibodies to West Nile virus (WNV), and infection of WNV in Culex mosquitoes, in the Chicago metropolitan area, Illinois, USA, a region of historically high WNV activity. Results indicated high overall avian seroprevalence and variation in seroprevalence across host species; however, there was no negative correlation between avian richness and Culex infection rate or between richness and infection status in individual birds. Bird species with high seroprevalence, especially northern cardinals and mourning doves, may be important sentinels for WNV in Chicago, since they were common and widespread among all study sites. Overall, our results suggest no net effect of increasing species richness to West Nile virus transmission in Chicago. Other intrinsic and extrinsic factors, such as variation in mosquito host preference, reservoir host competence, temperature, and precipitation, may be more important than host diversity for driving interannual variation in WNV transmission. These results from a fine-scale study call into question the generality of a dilution effect for WNV at coarser spatial scales.

Urban land use predicts West Nile virus exposure in songbirds

Urbanization is a widespread phenomenon that is likely to influence the prevalence and impact of wildlife pathogens, with implications for wildlife management and public health policies toward zoonotic pathogens. In this study, wild songbird populations were sampled at 14 sites along an urban rural gradient in the greater metropolitan Atlanta (Georgia, USA) area and tested for antibodies to West Nile virus (WNV). The level of urbanization among sites was quantitatively assessed using a principal component analysis of key land use characteristics. In total, 499 individual birds were tested during the spring and summer over three years (2004-2006). Antibody prevalence of WNV increased from rural to urban sites, and this trend was stronger among adult birds relative to juveniles. Furthermore, antibody prevalence among Northern Cardinals (Cardinalis cardinalis) was significantly higher than in other songbird species along the urban gradient. Findings reported here indicate that ecological factors associated with urbanization can influence infection patterns of this vector-borne viral disease, with likely mechanisms including changes in host species diversity and the tolerance or recovery of infected animals.

A method to increase efficiency in testing pooled field-collected mosquitoes

Testing field-caught mosquito collections can result in thousands of pools, and testing pools of 50 mosquitoes each can be both time consuming and cost prohibitive. Consequently, we have developed an alternative approach to testing mosquito pools for arboviruses, utilizing a superpool strategy. When mosquito samples are processed for extraction of viral RNA and subsequent virus testing via quantitative real-time polymerase chain reaction, each pool is tested individually. Using the method described here, 0.025 ml from each of 10 pools is combined into a superpool for RNA extraction and testing. When a virus-positive superpool sample is found, each of the original 10 pools that constitute this sample is tested individually in order to find the specific positive sample. By retesting the original samples after the initial superpool screen, we are still able to obtain reliable estimates for minimum infection rates or maximum likelihood estimations. To test this principle, we created controlled mosquito pools of known titer and subjected them to our superpool process. We were able to detect our entire range of laboratory-created pools as being West Nile virus (WNV) positive. In 2005, field surveillance efforts from our laboratory resulted in over 4,000 mosquito pools tested, with 8 resulting WNV-positive samples. We found that all of these field samples were detected as WNV positive using the superpool method and contained calculated virus titers from < 0.1 to 4.1 log10 plaque-forming units/ml WNV, indicating that the limit of superpool detection of WNV is below this point. These results reveal that the superpool method could be accurately used to detect WNV in field-collected specimens.

Arbovirus transmission by Culex nigripalpus in Florida, 2005

Understanding the transmission patterns of West Nile and St. Louis encephalitis viruses (family Flaviviridae, genus Flavivirus, WNV and SLEV) could result in an increased ability to predict transmission risk to humans. To examine transmission patterns between vector and host, we trapped mosquitoes in three Florida counties from June to November 2005 by using chicken-baited lard can mosquito traps. These traps were used to monitor for presence of WNV and SLEV in mosquitoes and subsequent transmission of these viruses to chickens. In total, 166,615 female mosquitoes were sorted into 4,009 pools based on species and bloodfed status, and they were tested for presence of WNV and SLEV. Sera from 209 chickens were tested for WNV and SLEV antibodies. We detected eight WNV-positive Culex nigripalpus Theobald mosquito pools; SLEV was not detected in any pools. Six positive pools were collected in August and September from Duval County, one pool in September from Manatee County, and one pool in November from Indian River County. Of the eight chickens potentially exposed to WNV, antibodies were detected in only one chicken, indicating a low rate of transmission relative to the observed mosquito infection rates. Low virus transmission rates relative to infection rates would suggest that using sentinel chicken seroconversion data as a means of arbovirus surveillance may underestimate the prevalence of WNV in the mosquito population. However, using mosquito infection rates may overestimate the risk of arboviral transmission. A variety of factors might account for the observed low level of transmission including a lack of viral dissemination in mosquito vectors.