Mosquito indexing system could identify best time to battle West Nile virus

Researchers at the University of Texas Southwestern Medical Center found that a mosquito vector-index rating system can identify the best time to intervene in West Nile virus outbreaks, according to a recently published study.

The study, which was published in Wednesday's Journal of the American Medical Association, analyzed a decade of West Nile infections in Dallas County, Texas, and found the best way to avoid an outbreak was to determine a mosquito vector index. A vector index value is determined from the abundance of mosquitoes and the percentage of mosquitoes infected with the West Nile virus. The number of human cases are not factored into the system.

"When the vector index goes above 0.5 early -- in June or July -- large numbers of people are silently infected, and this is the best time to intervene," Robert Haley, the senior author of the study, said. "In years when the vector index did not rise until late July or August, impending outbreaks just sputtered -- in late summer mosquito abundance declines, and mosquitoes become less active and stop biting as much."

West Nile infections in humans can result in long-term neurological damage and death. The study found that determining the number of human West Nile virus infections is not the best way to figure out how to respond to an outbreak.

"After the infecting mosquito bite, it takes a week for the first symptoms to develop, a week to see people turning up at hospitals, and a week for laboratory confirmation of the diagnosis and reporting to health officials," Haley said. "That three-week time period is crucial. Acting early from the vector index rather than after human case reports and deaths mount up can nip an outbreak in the bud. However, if mosquito data are unavailable or a decision to intervene takes longer, later intervention may still be important to terminate the outbreak."

The study also showed that milder winters and unusually warm spring temperatures contributed to epidemic years and areas of higher housing density, property values and percentages of unoccupied homes were at greater risk for West Nile virus.

Haley and the other authors provided an instruction manual with the paper to allow other counties to determine the vector index from their own mosquito infection surveillance data.