Reducing West Nile Virus Risk Through Vector Management.

Over 50,000 human West Nile virus (WNV) (Flaviviridae: Flavivirus) clinical disease cases have been reported to the CDC during the 20 yr that the virus has been present in the United States. Despite the establishment and expansion of WNV-focused mosquito surveillance and control efforts and a renewed emphasis on applying integrated pest management (IPM) principles to WNV control, periodic local and regional WNV epidemics with case reports exceeding 2,000 cases per year have occurred during 13 of those 20 yr in the United States. In this article, we examine the scientific literature for evidence that mosquito control activities directed at either preventing WNV outbreaks or stopping those outbreaks once in progress reduce WNV human disease or have a measurable impact on entomological indicators of human WNV risk. We found that, despite a proliferation of research investigating larval and adult mosquito control effectiveness, few of these studies actually measure epidemiological outcomes or the entomological surrogates of WNV risk. Although many IPM principles (e.g., control decisions based on surveillance, use of multiple control methodologies appropriate for the ecosystem) have been implemented effectively, the use of action thresholds or meaningful public health outcome assessments have not been used routinely. Establishing thresholds for entomological indicators of human risk analogous to the economic injury level and economic thresholds utilized in crop IPM programs may result in more effective WNV prevention.

Gravid Culex pipiens Exhibit A Reduced Susceptibility to Ultra-Low Volume Adult Control Treatments Under Field Conditions.

In July and August of 2018, a field trial was conducted to examine the effectiveness of the North Shore Mosquito Abatement District's operational ultra-low volume (ULV) adulticide program. Two study sites were selected in Skokie, IL, and treated by truck-based ULV with d-phenothrin and prallethrin synergized with piperonyl butoxide over the course of a month. Natural mosquito populations were sampled via Biogents (BG)-counter baited with CO2 or Alfalfa infusion. The results from this study demonstrate that host-seeking mosquitoes were reduced by 65.3% after ULV treatment while gravid mosquitoes were reduced by only 29.2%. In addition, host-seeking mosquitoes rebounded dramatically (303.1%) 3 days posttreatment while gravid mosquitoes did not (5.7%). Based on the differential effect between gravid and host-seeking mosquitoes, we concluded that the gonotrophic cycle and timing of ULV adulticide operations are important factors affecting the resistance of West Nile virus vectors to pyrethroid exposures.

An Operational Evaluation of 3 Methoprene Larvicide Formulations for Use Against Mosquitoes in Catch Basins.

Effectiveness in controlling mosquitoes in storm water catch basins in the North Shore Mosquito Abatement District (northeastern Cook County, Illinois) was determined for 3 formulations of methoprene-based larvicides (Altosid XR 150-day Briquets, Altosid 30-day Pellets, Altosid 30-day Granules) in 2017 using a pass/fail evaluation criterion, in which emergence of a single adult from pupae collected from the basin constituted a control failure. Over the course of the 16-week study, basins receiving the 150-day briquets were treated once and basins receiving the pellet and granular formulations were treated every 4 weeks, with the first treatment occurring during the last week of May. Untreated basins were also observed for comparison with the treated basins. Over the course of the study, adult mosquitoes emerged from pupae collected in 94.2% of the untreated basins that contained pupae. All of the formulations evaluated in the study demonstrated some degree of control compared with the untreated basins, with pupae successfully emerging as adults in 64.6%, 55.5%, and 21.8% of samples from 150-day briquet, 30-day tablet, and 30-day pellet-treated basins that contained pupae, respectively. Pellets reapplied every 28 days provided significantly more effective control than the other formulations. The simple pass/fail criterion for evaluating control effectiveness proved to be a useful procedure for comparing effectiveness to untreated basins and among treatments.

Standardized Operational Evaluations of Catch Basin Larvicides from Seven Mosquito Control Programs in the Midwestern United States During 2017.

During June to September 2017, 7 mosquito control programs in the midwestern United States evaluated a total of 9 catch basin larvicide formulations using similar protocols. Treated basins were monitored among study sites to observe when larvicides failed to control mosquitoes in 25% or more basins within a site. Overall, when monitoring occurred within the maximum label duration of the larvicides, sites treated with a single larvicide tablet or briquet surpassed the 25% fail threshold more often than pellet and granular larvicide formulations. In 438 of the study basins, the depth from sump bottom to catch basin lid was measured. In basins that were deeper than 5 ft (1.5 m), larvicides failed to control mosquitoes significantly more often than those 5 ft or shallower.

Reduced productivity of Culex pipiens and Cx. restuans (Diptera: Culicidae) mosquitoes in parking area catch basins in the northeast Chicago metropolitan area.

From June to September, 2016, 100 catch basins in eight parking areas were monitored weekly for the presence of mosquito pupae in the operational area of the North Shore Mosquito Abatement District (NSMAD) located just north of Chicago, IL, U.S.A. Weekly results from these basins were compared to weekly samples taken from residential street catch basins, the most common type of catch basin treated seasonally by the NSMAD with larvicides. Over the 17 study weeks, residential street basins had a mean rate of productivity (pupae per basin-visit) 12 times that of parking area catch basins. The two parking area sites with the highest mean rate of productivity were associated with county forest preserves. Productivity in both street and parking area basins was positively associated with the presence of three or more deciduous trees within 20 m of basins and if they were located directly adjacent to curbs. Alternatively, productivity was negatively associated with the proportion of impervious surface within 10 m of basins and weekly rainfall. Findings suggest that reduced catch basin larvicide applications may be appropriate in many parking area sites.

Effectiveness of Five Products To Control Culex pipiens Larvae In Urban Stormwater Catch Basins.

Effectiveness in controlling mosquito larvae in stormwater catch basins in the North Shore Mosquito Abatement District (northeastern Cook County, IL) was determined for 2 extended-duration larvicides indicating up to 180 days of control on their labels (Natular™ XRT, FourStar® Briquet) and 3 larvicides indicating up to 30 days of control (Natular™ T30, Natular™ G30, and VectoLex® FG). Over the course of the 26-wk study, catch basins receiving the extended-release products were treated twice (an initial treatment in early April followed by a re-treatment after 16 wk), and catch basins receiving the shorter-duration products were treated every 28 days, with the 1st treatment occurring during the 1st week in April. Control in an individual catch basin was considered to have failed if late-stage larvae or pupae were found in 2-dip samples taken from the catch basin. Control for a treatment was considered to have failed if >25% of catch basins treated with the product failed at a given time period posttreatment. All of the products evaluated in the study demonstrated some degree of control; however, the Natular XRT-, FourStar Briquet-, and Natular T30-treated basins rarely achieved the effectiveness threshold of ≤25% of catch basins failing. By comparison, basins treated with Natular G30 were below that threshold for 3 of 4 wk every treatment round and VectoLex® FG was consistently below that threshold for all 4 wk posttreatment for every treatment round. Compared with untreated catch basins, the total season pupal production was reduced by approximately 48% in the Natular XRT-, FourStar Briquet-, and Natular T30-treated basins, and by 87% and 99% in the Natular G30- and VectoLex FG-treated basins, respectively. Operational quality control observations indicated that effective control (i.e., ≤25% of catch basins failing) ranged from 3 to 12 wk posttreatment for catch basins treated with Natular XRT and from 5 to 9 wk with VectoLex FG, and that there was considerable geographic variation in the duration of effectiveness. The results indicate that 30-day re-treatments with granular formulations in difficult-to-control areas may provide a more cost-effective outcome than using 1 or 2 applications of extended-duration larvicides.

Variable Efficacy of Extended-release Mosquito Larvicides Observed in Catch Basins in the Northeast Chicago Metropolitan Area.

Since the mid-1990s, the North Shore Mosquito Abatement District (NSMAD) has applied extended-release formulations of mosquito larvicides to approximately 50,000 catch basins in the suburbs north of Chicago, IL, USA. This is performed as part of NSMAD's efforts to reduce local populations of the West Nile virus vector, Culex pipiens. Analyses from NSMAD's monitoring of larvicide-treated basins throughout the District over the 2014 and 2015 seasons suggest that larvicides intended to provide extended durations of control (30-180 days) failed to provide control for the maximum duration specified on the product label in approximately 25% of the District's basins. For larvicides designed to last up to 180 days (or about 26 weeks), failures were found at 1-15 weeks after treatment with most found at five weeks posttreatment. For larvicides formulated to last up to 30 days, failures were found at one to four weeks after applications with most found at three weeks posttreatment. The highest percentages of failing basins (ie, containing late-stage mosquito larvae or pupae during the specified product effectiveness period) were found in communities on the eastern side of the District, bordering Lake Michigan. As the larvicides appeared to function properly in the majority of monitored basins, it appears that the failures likely resulted from basin-specific physical factors (ie, basin volume, sediment content, and hydrology) that cause either product removal or a reduction in the concentration of the larvicide's active ingredient below the effective levels in these basins.

Temporal and Spatial Variability of Entomological Risk Indices for West Nile Virus Infection in Northern Colorado: 2006-2013.

West Nile virus (WNV) is enzootic in northern Colorado. Annual surveillance activities in Fort Collins, CO, include collecting female Culex mosquitoes and testing them for the presence of WNV RNA in order to calculate 1) Culex female abundance, 2) WNV infection rate, and 3) the vector index (VI). These entomological risk indices inform public policy regarding the need for emergency adulticiding. Currently, these are calculated on a city-wide basis. In this study, we present descriptive data from historical surveillance records spanning 2006-2013 to discern seasonal and yearly patterns of entomological risk for WNV infection. Also, we retrospectively test the hypothesis that entomological risk is correlated with human transmission risk and is heterogeneous within the City of Fort Collins. Four logistically relevant zones within the city were established and used to test this hypothesis. Zones in the eastern portion of the city consistently had significantly higher Culex abundance and VI compared with zones in the west, leading to higher entomological risk indicators for human WNV infection in the east. Moreover, the relative risk of a reported human case of WNV infection was significantly higher in the eastern zones of the city. Our results suggest that a more spatially targeted WNV management program may better mitigate human risk for WNV infection in Fort Collins, and possibly other cities where transmission is enzootic, while at the same time reducing pesticide use.

Assessment of Arbovirus Surveillance 13 Years after Introduction of West Nile Virus, United States.

Before 1999, the United States had no appropriated funding for arboviral surveillance, and many states conducted no such surveillance. After emergence of West Nile virus (WNV), federal funding was distributed to state and selected local health departments to build WNV surveillance systems. The Council of State and Territorial Epidemiologists conducted assessments of surveillance capacity of resulting systems in 2004 and in 2012; the assessment in 2012 was conducted after a 61% decrease in federal funding. In 2004, nearly all states and assessed local health departments had well-developed animal, mosquito, and human surveillance systems to monitor WNV activity and anticipate outbreaks. In 2012, many health departments had decreased mosquito surveillance and laboratory testing capacity and had no systematic disease-based surveillance for other arboviruses. Arboviral surveillance in many states might no longer be sufficient to rapidly detect and provide information needed to fully respond to WNV outbreaks and other arboviral threats (e.g., dengue, chikungunya).

Meteorological conditions associated with increased incidence of West Nile virus disease in the United States, 2004-2012.

West Nile virus (WNV) is a leading cause of mosquito-borne disease in the United States. Annual seasonal outbreaks vary in size and location. Predicting where and when higher than normal WNV transmission will occur can help direct limited public health resources. We developed models for the contiguous United States to identify meteorological anomalies associated with above average incidence of WNV neuroinvasive disease from 2004 to 2012. We used county-level WNV data reported to ArboNET and meteorological data from the North American Land Data Assimilation System. As a result of geographic differences in WNV transmission, we divided the United States into East and West, and 10 climate regions. Above average annual temperature was associated with increased likelihood of higher than normal WNV disease incidence, nationally and in most regions. Lower than average annual total precipitation was associated with higher disease incidence in the eastern United States, but the opposite was true in most western regions. Although multiple factors influence WNV transmission, these findings show that anomalies in temperature and precipitation are associated with above average WNV disease incidence. Readily accessible meteorological data may be used to develop predictive models to forecast geographic areas with elevated WNV disease risk before the coming season.

Comparison of the efficiency and cost of West Nile virus surveillance methods in California.

Surveillance systems for West Nile virus (WNV) combine several methods to determine the location and timing of viral amplification. The value of each surveillance method must be measured against its efficiency and costs to optimize integrated vector management and suppress WNV transmission to the human population. Here we extend previous comparisons of WNV surveillance methods by equitably comparing the most common methods after standardization on the basis of spatial sampling density and costs, and by estimating optimal levels of sampling effort for mosquito traps and sentinel chicken flocks. In general, testing for evidence of viral RNA in mosquitoes and public-reported dead birds resulted in detection of WNV approximately 2-5 weeks earlier than serological monitoring of sentinel chickens at equal spatial sampling density. For a fixed cost, testing of dead birds reported by the public was found to be the most cost effective of the methods, yielding the highest number of positive results per $1000. Increased spatial density of mosquito trapping was associated with more precise estimates of WNV infection prevalence in mosquitoes. Our findings also suggested that the most common chicken flock size of 10 birds could be reduced to six to seven without substantial reductions in timeliness or sensitivity. We conclude that a surveillance system that uses the testing of dead birds reported by the public complemented by strategically timed mosquito and chicken sampling as agency resources allow would detect viral activity efficiently in terms of effort and costs, so long as susceptible bird species that experience a high mortality rate from infection with WNV, such as corvids, are present in the area.

Heartland virus-associated death in tennessee.

BACKGROUND: Heartland virus (HRTV) is a tick-borne phlebovirus recently described in Missouri that is associated with fever, leukopenia, and thrombocytopenia. The virus has also been detected in Ambylomma americanum ticks. METHODS: Here we report the first fatal case of HRTV disease in an 80-year-old Tennessee resident. He was hospitalized with fever, confusion, leukopenia, and thrombocytopenia and developed multiorgan failure and hemorrhage. A tick-borne illness was suspected and testing for ehrlichiosis was negative. He died on hospital day 15, and autopsy specimens were tested for various pathogens as part of an unexplained death evaluation. RESULTS: HRTV antigens were detected in postmortem spleen and lymph nodes by immunohistochemistry, and HRTV was detected in premortem blood by reverse transcription polymerase chain reaction and by isolation in cell culture. CONCLUSIONS: This case demonstrates that HRTV infection can cause severe disease and death and expands the geographic range of HRTV within the United States.

Effect of aerial insecticide spraying on West Nile virus disease--north-central Texas, 2012.

During 2012, four north-central Texas counties experienced high West Nile virus (WNV) disease incidence. Aerial insecticide spraying was conducted in two counties. To evaluate the effect of spraying on WNV disease, we calculated incidence rate ratios (IRRs) in treated and untreated areas by comparing incidence before and after spraying; for unsprayed areas, before and after periods were defined by using dates from a corresponding sprayed area. In treated areas, WNV neuroinvasive disease incidence before and after spraying was 7.31/100,000 persons and 0.28/100,000 persons, respectively; the IRR was 26.42 (95% confidence interval [CI]: 12.42-56.20). In untreated areas, the before and after incidence was 4.80/100,000 persons and 0.45/100,000 persons, respectively; the IRR was 10.57 (95% CI: 6.11-18.28). The ratio of IRRs was 2.50 (95% CI: 0.98-6.35). Disease incidence decreased in both areas, but the relative change was greater in aerial-sprayed areas.

Evaluation of a rapid analyte measurement platform and real-time reverse-transcriptase polymerase chain reaction assay West Nile virus detection system in mosquito pools.

We evaluated the commercially available Rapid Analyte Measurement Platform (RAMP) West Nile virus (WNV) antigen detection test for sensitivity and consistency with real-time reverse transcriptase polymerase chain reaction (RT-PCR) confirmation testing. Panels of samples consisting of WNV-spiked mosquito pools and negative control pools were sent to 20 mosquito abatement districts (MADs) that processed the pools using the RAMP assay. The samples were then sent to the reference laboratories used by the MADs for confirmation by real-time RT-PCR. Positive pools with virus titers of roughly 1-3 log10 PFU/ml had RAMP scores above the RAMP test positive cutoff score of 30 RAMP units, but these virus-positive samples could not be reliably confirmed by real-time RT-PCR testing. Pools with virus titers > or =4 log10 PFU/ml scored > or =50 RAMP units. Real-time RT-PCR results varied among the confirmation laboratories. With few exceptions, pools returning a RAMP score of > or =100 were confirmed with real-time RT-PCR, while pools returning a RAMP score of 50-99 appeared to be at the limit of real-time RT-PCR detection. Therefore, we recommend using a positive cutoff of 50 RAMP units with no real-time RT-PCR confirmation to maximize speed, efficiency, and economy of the RAMP assay. A more conservative approach would be to implement a "gray zone" range of 50-100 RAMP units. Pools scoring within the gray zone could be submitted for real-time RT-PCR confirmation with the understanding that positive pools may not confirm due to the inhibitory effect of the RAMP buffer on the real-time RT-PCR assay. We also conducted a series of experiments using laboratory-prepared mosquito pools spiked with WNV to compare mosquito homogenization buffers, pool sizes, and grinding methods in order to determine how these variables affect the RAMP and real-time RT-PCR assay results.

Mosquitoes of Western Yunnan Province, China: seasonal abundance, diversity, and arbovirus associations.

OBJECTIVE: The western borderland between Yunnan Province, China, and Myanmar is characterized by a climate that facilitates year-round production of mosquitoes. Numerous mosquito-transmitted viruses, including Japanese encephalitis virus circulate in this area. This project was to describe seasonal patterns in mosquito species abundance and arbovirus activity in the mosquito populations. METHODS: Mosquitoes were collected in Mangshi and Ruili cities of Dehong Prefecture near the border of China and Burma in Yunnan Province, the Peoples Republic of China in 2010. We monitored mosquito species abundance for a 12-month period using ultraviolet light, carbon dioxide baited CDC light and gravid traps; and tested the captured mosquitoes for the presence of virus to evaluate mosquito-virus associations in rural/agricultural settings in the area. RESULTS: A total of 43 species of mosquitoes from seven genera were collected, including 15 Culex species, 15 Anopheles spp., four Aedes spp., three Armigeres spp., one Mimomyia spp., two Uranotaenia spp. and three Mansonia spp.. Species richness and diversity varied between Mangshi and Ruili. Culex tritaeniorhynchus, Culex quinquefasciatus, Anopheles sinensis and Anopheles peditaeniatus were the most abundant species in both sampling sites. Ultraviolet light traps collected more specimens than CDC light traps baited with dry ice, though both collected the same variety of mosquito species. The CDC gravid trap was the most effective trap for capture of Culex quinquefasciatus, a species underrepresented in light trap collections. A total of 26 virus strains were isolated, which included 13 strains of Japanese encephalitis virus, four strains of Getah virus, one strain of Oya virus, one strain from the orbivirus genus, and seven strains of Culex pipien pallens densovirus. CONCLUSIONS: The present study illustrates the value of monitoring mosquito populations and mosquito-transmitted viruses year-round in areas where the climate supports year-round adult mosquito activity.

West Nile virus outbreak in Phoenix, Arizona--2010: entomological observations and epidemiological correlations.

In 2010, Arizona experienced an unusually early and severe outbreak of West Nile virus (WNV) centered in the southeast section of Maricopa County. Entomological data were collected before and during the outbreak, from May 25 through July 31, 2010, using the CO2-baited light trap monitoring system maintained by Maricopa County Vector Control. In the outbreak area, the most abundant species in the Town of Gilbert and in the area covered by the Roosevelt Water Conservation District was Culex quinquefasciatus, constituting 75.1% and 71.8% of the total number of mosquitoes collected, respectively. Vector index (VI) profiles showed that the abundance of infected Cx. quinquefasciatus peaked prior to human cases, suggesting that this species was involved in the initiation of the outbreak. In contrast, the VI profiles for Cx. tarsalis were consistently low, suggesting limited involvement in initiating and sustaining transmission. Taken together, the higher abundance and the VI profiles strongly suggest that Cx. quinquefasciatus was the primary vector for this outbreak. The VI profiles consistently showed that the abundance of infected mosquitoes peaked 1 to 2 wk before the peaks of human cases, suggesting that VI could have successfully been utilized to predict the WNV outbreak in Maricopa County, AZ, in 2010.

Sequence analyses of 2012 West Nile virus isolates from Texas fail to associate viral genetic factors with outbreak magnitude.

In 2012, Texas experienced the largest outbreak of human West Nile encephalitis (WNE) since the introduction of West Nile virus (WNV) in 2002. Despite the large number of WNV infections, data indicated the rate of reported WNE among human cases was no higher than in previous years. To determine whether the increase in WNV human cases could have been caused by viral genetic changes, the complete genomes of 17 isolates made from mosquito pools in Dallas and Montgomery Counties in 2012 were sequenced. The 2012 Texas isolates were found to be composed of two distinct clades, both circulating in Dallas and Montgomery Counties despite a 5-fold higher disease incidence in the former. Although minor genetic differences existed between Dallas and Montgomery WNV populations, there was weak support for population subdivision or adaptive changes. On the basis of these data, alternative explanations for increased WNV disease incidence in 2012 are proposed.