Predicting West Nile Virus Infection Risk From the Synergistic Effects of Rainfall and Temperature.

Mosquito-based surveillance is a practical way to estimate the risk of transmission of West Nile virus (WNV) to people. Variations in temperature and precipitation play a role in driving mosquito infection rates and transmission of WNV, motivating efforts to predict infection rates based on prior weather conditions. Weather conditions and sequential patterns of meteorological events can have particularly important, but regionally distinctive, consequences for WNV transmission, with high temperatures and low precipitation often increasing WNV mosquito infection. Predictive models that incorporate weather can thus be used to provide early indications of the risk of WNV infection. The purpose of this study was first, to assess the ability of a previously published model of WNV mosquito infection to predict infection for an area within the region for which it was developed, and second, to improve the predictive ability of this model by incorporating new weather factors that may affect mosquito development. The legacy model captured the primary trends in mosquito infection, but it was improved considerably when calibrated with local mosquito infection rates. The use of interaction terms between precipitation and temperature improved model performance. Specifically, temperature had a stronger influence than rainfall, so that lower than average temperature greatly reduced the effect of low rainfall on increased infection rates. When rainfall was lower, high temperature had an even stronger positive impact on infection rates. The final model is practical, stable, and operationally valid for predicting West Nile virus infection rates in future weeks when calibrated with local data.

Overwintering and establishment of Aedes albopictus (Diptera: Culicidae) in an urban La Crosse virus enzootic site in Illinois.

In 1997, Aedes albopictus (Skuse) was discovered in Peoria, IL, a known focus of La Crosse (LAC) virus transmission. This accidental introduction provided an opportunity to determine whether Ae. albopictus would reemerge in the spring or summer and, if successful overwintering occurred, to follow changes in the geographic range of Ae. albopictus, and to compare its distribution to that of the local treehole mosquito and LAC vector Aedes triseriatus (Say). In 1998, 25 oviposition traps were placed in and around the area of the initial finding of Ae. albopictus, with adult collections by aspirators and larval collections from water-holding containers used to identify areas of additional activity. Ae. albopictus successfully survived the mild 1997-1998 El Niño winter, and expanded its range during 1998. By September 1998, Ae. albopictus oviposited in all 25 traps, including traps near and in wooded sites. Intensity of oviposition activity (number of eggs per positive trap) ranged from 20-40, lower than the range for Ae. triseriatus, which was as high as 175 eggs per infested trap in mid-August. Prevalence of Ae. albopictus increased through September, where as the prevalence of Ae. triseriatus declined starting in mid-July. Although direct competition between the two mosquito species cannot be inferred based on this surveillance effort, a gradual range expansion was observed, and Ae. albopictus eggs were collected in traps where initially only Ae. triseriatus oviposition activity was detected.

Introduction of Aedes albopictus into a La Crosse virus--enzootic site in Illinois.

In late summer and fall 1997, Aedes albopictus mosquitoes were found in Peoria, Illinois, a long recognized focus of La Crosse virus transmission. Larvae were found in tires and other artificial containers, biting adults were recovered, and eggs were collected in oviposition traps within a 25-ha area. One chipmunk trapped < 0.25 km from the infested area tested positive for neutralizing antibodies against La Crosse virus.

Isolation of La Crosse, Cache Valley, and Potosi viruses from Aedes mosquitoes (Diptera: Culicidae) collected at used-tire sites in Illinois during 1994-1995.

Prospective studies were conducted at used-tire sites in Illinois during 1994-1995 in an effort to isolate arboviruses from mosquitoes, particularly Aedes albopictus (Skuse) and Aedes triseriatus (Say). Three isolates of Potosi virus were obtained from Ae. albopictus collected at a waste tire site in Jasper County during 1994 and 1995. Also, a single isolate of Cache Valley virus was obtained from Ae. albopictus collected at the Jasper County site during 1995. These are the first records of arbovirus isolations from Ae. albopictus in Illinois and the first isolate of Cache Valley virus from this mosquito species. During 1994, two isolates of La Crosse virus were made from Ae. triseriatus collected at a used-tire site in Peoria County in proximity to the residence of a human La Crosse encephalitis case. This is the first evidence in Illinois that indicates increased risk to humans living near used-tire sites, which may serve as foci for production of Ae. triseriatus, the vector of La Crosse virus. Tire removal and improved environmental sanitation at such sites may greatly reduce the abundance of vector mosquitoes, and, therefore, the risk of arbovirus transmission.

Spatial analysis of the distribution of LaCrosse encephalitis in Illinois, using a geographic information system and local and global spatial statistics.

The spatial and temporal distribution of LaCrosse encephalitis cases in Illinois was analyzed using a geographic information system (GIS) and spatial statistics. Case data were obtained from the Illinois Department of Public Health and mapped on the county, town, and address level. Human cases were concentrated in and around the city of Peoria in central Illinois. Local spatial statistics were used to identify hot spots where cases appear to be concentrated in the Peoria region. Several small towns surrounding the city of Peoria appeared as foci where cases were most common. Second-order spatial analysis of the case distribution was conducted on the address level. Cases were clustered within a range of 3.0 km in the city of Peoria. Since most cases appear to be associated with residential (peridomestic) exposure, and since several cases have been reported from neighboring addresses, transmission may be concentrated around specific sites (hardwood ravines, tire piles). The GIS and spatial analysis may be useful in identifying and targeting for intervention potential sites of enzootic transmission.

Gonotrophic cycle duration, population age structure, and onset of sugar feeding and insemination of Aedes triseriatus (Diptera: Culicidae).

During two successive summers, laboratory-reared Aedes triseriatus (Say) were marked and released into two woodlots in central Ohio. Several cohorts of two groups of mosquitoes were marked and released: blood-engorged, 10-12-d-old females and unfed, 2-d-old females. The parity of marked mosquitoes captured during daily biting collections were used to estimate the duration of the gonotrophic cycle. The parity rate of unmarked mosquitoes, in conjunction with the cycle estimate, yielded data on age structure and daily survival. Information on the onset of insemination and sugar feeding also was obtained. The mean and median duration of the first gonotrophic cycle, based on females released after blood feeding and later finding the collector, was 12 d. Unfed females required a median of about 8 d to feed on sugar, mate, and locate the collector, but required 22-32 d to sugar feed, mate, find a natural host, become parous, and then find the collector. The median duration of the natural gonotrophic cycle was estimated to take 14 d during warm (22 degrees C) weather. The parity rate of the natural population remained between 29 and 51% during the last 3 mo of each summer. The high proportion parous indicated that daily survival rate of unmarked females was above 0.9. Marked females moved between woodlots for distances as far as 400 m and were more likely to emigrate from the small woodlot than the large one.

Penetration rate of two pesticide carriers at a large used-tire storage facility in Chicago, Illinois.

During August 1987, a large and concentrated infestation of Aedes albopictus was discovered on the property of a tire recapper and gasket manufacturer in Chicago, IL, in a densely populated urban environment. The infestation called for special abatement procedures because of the large number of tires and the varied ways they were stacked. An effective method for delivering pesticides into the cavity of each tire is described. Corncob granules, when applied to stacked tires, effectively penetrated this larval habitat at rates of 85% in column-stacked piles, 93% in random-stacked piles and an average of 95% in shingle-stacked piles. By comparison, gypsum pellet carriers sustained penetration rates of 37% in shingle-stacked piles and 87% in random-stacked piles.