Operational Activities Before and After West Nile Virus in Harris County, TX: 2002-2011

Due to the nature of WNV, the Harris County Public Health and Environmental Services (HCPHES) Mosquito Control Division (MCD) has implemented and/or enhanced strategies in order to minimize WNV exposure to the local residents. Some of these strategies are: (1)-comprehensive countywide surveillance in mosquito populations; (2)-timely disease diagnostics; (3)-insecticide resistance surveillance and management; (4)-vehicle tracking and chemical usage monitoring of ground ULV operations; and (5)-area profiling.


Due to the dispersal nature of WNV, surveillance of mosquito populations has gradually evolved to current weekly countywide coverage during the high risk period (May-October). In 2002, the county was subdivided into three groups- areas within the I610 Loop; areas between I610 and the Sam Houston Toll Road, and areas outside of the Sam Houston Toll Road. Surveillance was conducted weekly, bi-weekly, and monthly, respectively, within the three sectors. The number of sites monitored increased from 100 in 2002 to 200, 224, and 268 sites in 2003, 2004, and 2005, respectively. Trapping methodologies also were modified. Gravid traps were incorporated into the surveillance program along with underground storm sewer traps. Both of the traps are very attractive to Cx. quinquefasciatus; gravid traps use highly organic water to attract this species and storm sewers are attractive to this species due to the stable environment during the summer and winter months. Over ninety six percent of the collections during 2002-2011 was comprised of Cx. quinquefasciatus.

Disease Diagnostics:

Testing methodologies were modified and enhanced to include WNV in the process. In the old process, mosquito sample submission to and confirmatory testing in the virology laboratory was 1 day/week (Monday) but increased to 3 days/week in the current process. Test results were reported 1-2 days/week and increased to 3 days/week in the current process. The number of Cx. quinquefasciatus pools tested increased from approximately 7000 in 2002 to 16000 in 2011.

Insecticide Resistance Surveillance and Management:

Due to the detection of insecticide resistance in Cx. quinquefasciatus populations to malathion an extensive resistance surveillance program was developed and launched in 2005. Initially Cx. quinquefasciatus populations were collected from 15 operational areas and multiple field cage tests were conducted throughout the season. Mosquitoes from 3 operational areas were tested alongside a laboratory strain during each test event to test for resistance of insecticides used in ULV ground operations. Seven areas were selected as permanent areas in order to detect changes in insecticide resistance to verify the impact of the rotational strategy. The number of operational areas tested was reduced to 9 due to budgetary constraints in 2010 but the 7 permanent sites were maintained. A rotational treatment strategy using an organophosphate and a synthetic pyrethroid has been utilized throughout the county since insecticide resistance was detected. Data indicates that this strategy has been effective in curtailing insecticide resistance in Cx. quinquefasciatus populations.

Vehicle Tracking and Chemical Usage Monitoring System:

Because of the size of Harris County it is important to provide security and accountability when applicators conduct evening spray operations. Prior to the detection of WNV in Harris County, hand made maps were used to provide the applicators with information about their spray routes. Data was manually documented and entered into computer databases. Due to the increase in ULV ground treatments because of WNV activity a vehicle tracking/chemical usage monitoring system was installed on the spray units. Real time vehicle tracking provides a significant degree of security for the applicators as they travel throughout the county, oftentimes in high risk areas. This system also provides public accountability and transparency by providing verification of the spray operations. Data integrity is also enhanced because upon completion of the route the data is automatically downloaded to the departmental server providing more efficient data capture and management. This system also provides valuable documentation on chemical usage which is vital in order to comply with the Clean Water Act through the Texas Pollutant Discharge Elimination System (TPDES).

Area Profile:

Due to the need to better address endemic or emerging mosquito borne disease threats, the area profile project was initiated in 2007. A centralized data base was created in order to capture the large amount of historical and current data collected from the 268 operational areas in Harris County. To date, 164 of the 268 areas have been completed and includes but is not limited to entomological, environmental, epidemiological, insecticidal, and infrastructural data. Currently, the MCD is collaborating with the Centers for Disease Control and the University of Southern California to evaluate and develop a tool to conduct mosquito borne disease risk assessments for each operational area.

The enhanced strategies have enabled the MCD to monitor and mitigate WNV in a timely and efficient manner and will serve as a model for other mosquito borne diseases which threaten Harris County.

ULV Treatment Before WNV (Single Chemical)

ULV Treatment Before WNV (Single Chemical)

ULV Treatment After WNV (Rotational Strategy)

ULV Treatment After WNV (Rotational Strategy)