Associations between two mosquito populations and West Nile virus in Harris County, Texas, 2003-06.

Associations between Culex quinquefasciatus, Aedes albopictus and West Nile virus (WNV) activity, temperature, and rainfall in Harris County, Texas 2003-06 are discussed. Human cases were highly correlated to Cx. quinquefasciatus (r = 0.87) and Ae. albopictus (r = 0.78) pools, blue jays (r = 0.83), and Ae. albopictus collected (r = 0.71), but not Cx. quinquefasciatus collected (r = 0.45). Human cases were associated with temperature (r = 0.71), not rainfall (r = 0.29), whereas temperature correlated with Ae. albopictus and Cx. quinquefasciatus collections (r = 0.88 and 0.70, respectively) and Cx. quinqueftsciatus pools (r = 0.75), but not Ae. albopictus pools (r = 0.55). Both species (collections and pools) and blue jays were weakly correlated (r 5 0.41) with rainfall, but blue jays were better correlated with Cx. quinquefasciatus pools (r = 0.87), compared with Ae. albopictus pools (r = 0.67), Ae. albopictus collections (r = 0.69), and Cx. quinquefasciatus collections (r = 0.46). Peak minimum infection rate for Cx. quinquefasciatus (4.55), and Ae. albopictus (4.41) was in August with highest human cases (17.87), blue jays (55.58), and temperature (29.01 degrees C). Between both species, blood meal analysis indicated 68.18% of Cx. quinquefasciatus mammalian hosts were dog, while 22.72% were human, whereas Ae. albopictus had higher human (44.44%) but fewer dog hosts (22.22%). Ten bird species were identified as hosts for Cx. quinquefasciatus, with northern cardinal and blue jay representing 26.66% and 20.00%, respectively. No bird feeding activity was observed in Ae. albopictus. The earliest and latest human blood meal occurred in May (Ae. albopictus) and November (Cx. quinquefasciatus); 66.66% of human host identifications between both species occurred in October-November, after the seasonal human case peak. Based upon our data, WNV activity in both mosquito species warrants further investigation of their individual roles in WNV ecology within this region.

Demographic and spatial analysis of West Nile virus and St. Louis encephalitis in Houston, Texas.

This descriptive prevalence study describes the relationships between mosquito density and the presence of arboviruses (in mosquitoes and humans) with various socioeconomic and environmental factors present near the time of teh arbovirus outbreak in Harris County, Texas, in 2002. This study suggests that mosquito density increased if the trap was located in an area with a large number of containers that may inadvertently retain rainwater (P = 0.056). When considering only virus-positive mosquitoes, significant relationships were observed if the trap was located near waste materials (P < 0.001) or near containers that may inadvertently retain rainwater (P = 0.037). Furthermore, the presence of arbovirus activity (in mosquitoes or humans) in a geographic area tended to be associated with the socioeconomic status of the local community. Although the results of socioeconomic comparisons were not significant, they were suggestive, demonstrating an interesting trend. Compared with communities where virus activity was not observed, the socioeconomic status of the arbovirus-positive community was consistently lower. Specifically, results showed that the populations residing in virus-positive census tracts attained less education, earned less income per household, and were more likely to be below the poverty level. In addition, this study found that virus-positive mosquitoes were randomly distributed throughout the study area, whereas severe human infection cases were clustered. Based on the results of this study, we conclude that the health outcome of a local community as it relates to West Nile virus and St. Louis encephalitis is dependent on many factors, including eh socioeconomic and environmental characteristics of the community.

Estimation of aerosol droplet sizes by using a modified DC-III portable droplet measurement system under laboratory and field conditions.

Modification of the DC-III portable droplet measurement system, permitting its use under field conditions, is described. Under laboratory conditions, the system effectively sampled water droplets from aerosols produced by a dry ice/water generator and high-pressure syringe. Seven droplet sizes, totaling 71,053 droplets within 22 tests (dry ice method), consisted of 1-, 2-, 6-, 11-, 18-, 25-, and 34-microm droplets with individual (rounded) percentages of 45.25, 37.22, 13.85, 3.17, 0.45, 0.02, and 0.005, respectively, for each size. Cumulatively, 1-microm droplets accounted for ca. 45.25% of the droplets sampled; combined with 2-microm (ca. 82.48% together), 6-microm (ca. 96.33% together), and 11-microm droplets, yielded ca. 99.51% of the droplets sampled. The syringe produced 12 droplet sizes, with 4,121 droplets sampled, consisting of 1, 2, 6, 11, 18, 25, 34, 45, 56, 69, 83, and 99 microm with individual percentages of 15.43, 21.91, 24.58, 17.30, 10.62, 4.65, 2.93, 1.33, 0.63, 0.33, 0.16, 0.07, respectively, for each size. The 6-microm droplets contributed the highest individual percentage, and cumulatively, these droplets combined with 1- and 2-microm droplets, yielding 61.93%, whereas 11- to 45-microm droplets contributed 36.83%, for a total of 98.76%. Droplets measuring 56-99 microm accounted for ca. 1.24% of droplets sampled. Hand-fogger oil aerosols produced 12 droplet sizes (1-38 microm) at test distances of 7.6 and 15.2 m, with 1,979 and 268 droplets sampled, respectively, during 10 tests at each distance. With analysis of variance of transformed individual percentages for each size at both distances, no significant differences were observed for 7.6 and 15.2 m. Cumulatively, 1-, 2-, 3-, and 5-microm droplets contributed 82.87 and 80.97%, whereas 8-, 11-, 14-, and 18-microm droplets added 14.55% to totals at both 7.6 and 15.2 m, respectively. Droplets measuring 22, 27, 32, and 38 microm contributed 2.57% and 4.47% to samples obtained at 7.6 and 15.2 m. The average mass median diameter (MMD) of mineral oil aerosols obtained at 7.6 and 15.2 m were 19.55 +/- 1.62 and 15.49 +/- 1.35, respectively. Positioned at 15 m (50 ft) within a field cage test plot during 12 separate tests, the MMD values obtained downrange were less than calibration MMD values in 5 tests (10-93% of calibration values), but 1.1 to 4.4 times greater in 7 remaining tests. With exception of 2 tests, susceptible Sebring strain Culex quinquefasciatus mortality at the 31-m (100-ft) stake, 15 m (50 ft) behind the DC-III, was consistently 100%. The average MMD values for malathion, permethrin, and resmethrin (at 0.003 and 0.007 rates) at this distance were 13.24 +/- 6.08, 7.48 +/- 5.69, 16.64 +/- 5.54, and 15.48 +/- 2.89, respectively.

A comparison of seven traps used for collection of Aedes albopictus and Aedes aegypti originating from a large tire repository in Harris County (Houston), Texas.

Among 7 traps tested, significantly higher (P < 0.01) mean numbers of Aedes albopictus (269) and Aedes aegypti (55) females were collected within the Mosquito Magnet Liberty trap compared with the remaining traps. The second highest mean captures for both species were obtained from omnidirectional Fay-Prince (77 Ae. albopictus) and Dragonfly (13 Ae. aegypti) traps, which were not significantly different (P > 0.01) from an experimental moving-target trap that produced mean captures of 40 Ae. albopictus and 6 Ae. aegypti (alpha = 0.01). In terms of Ae. albopictus capture, no significant differences (P > 0.01) existed between Dragonfly, CDC without light (CDC -), and CDC with light (CDC +) captures, which were significantly different (P < 0.01) from Mosquito Deleto. No statistical significance existed between moving-target, omnidirectional, CDC +, CDC -, and Mosquito Deleto traps in terms of Ae. aegypti capture (P > 0.01), individual trap positions, or number of Ae. albopictus and Ae. aegypti females collected throughout the 21-day test (P > 0.05). Mosquito Magnet Liberty collected 7,208 Ae. albopictus, 1467 Ae. aegypti, and 13 other species representing 5 genera, which comprised the largest total (9662) and percentage (62.5%) of mosquitoes collected by all traps combined. Omnidirectional and moving-target traps captured 1941 and 1050 Ae. albopictus, 138 and 220 Ae. aegypti, and 2171 (14.0%) and 1397 (9.0%) of the total mosquitoes captured by all traps, with 8 and 10 species representing 5 genera, respectively, included in these collections. The Dragonfly captured 476 Ae. albopictus, 376 Ae. aegypti, and 1008 total specimens (6.5%) representing 8 species and 4 genera in these collections. CDC + and CDC - traps collected nearly identical numbers of Ae. albopictus (431, 450) and Ae. aegypti (71, 71) with 537 (3.4%) and 551 (3.5%) total specimens, respectively. Eight species representing 5 genera were captured from CDC +, whereas CDC - captured 6 species representing 4 genera. Mosquito Deleto captured 118 mosquitoes, including 19 Ae. albopictus and 62 Ae. aegypti females (0.7%), with 6 species representing 4 genera. Battery-powered traps with contrasting color schemes and movement worked considerably better than stationary CDC miniatures without color or movement. Omnidirectional Fay-Prince and moving-target traps without octenol captured Ae. albopictus and Ae. aegypti females as frequently as some commercial traps. Additionally, costs incurred per mosquito trapped, future trap design, and important consumer-centered issues are briefly discussed.

Phylogenetic analysis of North American West Nile virus isolates, 2001-2004: evidence for the emergence of a dominant genotype.

The distribution of West Nile virus has expanded in the past 6 years to include the 48 contiguous United States and seven Canadian provinces, as well as Mexico, the Caribbean islands, and Colombia. The suggestion of the emergence of a dominant genetic variant has led to an intensive analysis of isolates made across North America. We have sequenced the pre-membrane and envelope genes of 74 isolates and the complete genomes of 25 isolates in order to determine if a dominant genotype has arisen and to better understand how the virus has evolved as its distribution has expanded. Phylogenetic analyses revealed the continued presence of genetic variants that group in a temporally and geographically dependent manner and provide evidence that a dominant variant has emerged across much of North America. The implications of these findings are discussed as they relate to transmission and spread of the virus in the Western Hemisphere.

Emergence of attenuated West Nile virus variants in Texas, 2003.

In order to understand how West Nile virus (WNV) has evolved since its introduction into North America, we have studied the genetic and phenotypic variation among WNV isolates collected in various areas during consecutive transmission seasons. The present report describes for the first time phenotypic changes occurring in the North American WNV population. Several isolates collected in Texas during 2003 display a small plaque (sp) and temperature sensitive (ts) phenotype, as well as reduced replication in cell culture, in comparison to isolates collected in 2002 and New York in 1999. Studies of mouse neuroinvasiveness/neurovirulence also indicate that several of these isolates were attenuated in neuroinvasiveness, but not for neurovirulence. The complete genome and deduced amino acid sequences of several of these isolates have been determined in order to map the mutations responsible for this phenotypic variation. These data indicate microevolution of WNV and the emergence of isolates exhibiting phenotypic variation.

Limited evolution of West Nile virus has occurred during its southwesterly spread in the United States.

Analysis of partial nucleotide sequences of nine West Nile virus strains isolated in southeast Texas during June-August 2002 revealed a maximum of 0.35% nucleotide variation from a New York 1999 strain. Two sequence subtypes were identified that differed from each other by approximately 0.5%, suggesting multiple introductions of virus to this area. Analysis of sequences from cloned PCR products for one strain revealed up to 0.6% divergence from the consensus sequence at the subpopulation level. The presence of unique patterns of small numbers of mutations in North American West Nile strains studied to date may suggest the absence of a strong selective pressure to drive the emergence of dominant variants.