The 2012 vector index also began increasing earlier and reached a higher peak than in previous seasons. "Sequential increases in the weekly vector index early in the 2012 season significantly predicted the number of patients with onset of symptoms of West Nile disease in the subsequent 1 to 2 weeks," the investigators said.
For predicting human illness, the vector index was superior to "other entomologic risk measures" such as mosquito abundance or mosquito infection rates, they added.
In the 2012 epidemic, the vector index passed a threshold of 0.5 by the end of June, at which time only three cases of West Nile disease had been reported. Taking action at this point instead of waiting for a sufficient number of cases to be reported would have averted much morbidity and mortality.
The researchers also addressed the concern that augmented spraying of insecticide at the onset of an outbreak of West Nile would harm residents. They analyzed the daily incidence of emergency department visits for skin rashes and acute upper respiratory distress and found no increase in these conditions during the 8-day period of ultralow-volume aerial spraying of minimally toxic pyrethroid insecticides approved by the Environmental Protection Agency for this purpose.
No financial conflicts of interest were reported.