COLUMBIA — Three more Columbia residents have contracted West Nile virus, and local health officials expect more cases before cold weather sets in.
In the meantime, the Columbia/Boone County Health Department will be spraying insecticide three times in the next six days to control adult mosquitoes in two areas of Columbia.
“We’ll be expecting more cases probably until the end of October or until the first freeze,” health department spokeswoman Deidre Wood said.
The three infected people — two 60-year-old men and a 45-year-old man — raised the number of cases in Boone County to eight, doubling the total from 2006. There have been no fatal cases of West Nile virus in Boone County this year. Statewide, 48 cases, including one fatality, have been reported.
The increase in tick-borne disease in Boone County this year could be the reason why more cases of West Nile have been reported, said Karen Yates, vector-borne disease program coordinator for the Missouri Department of Health.
The symptoms of both diseases are similar and doctors order tests to find the cause.
“Cases may be being identified that might have sort of gone under the radar in previous years,” Yates said.
The first confirmed West Nile virus case this year, a 50-year-old man from eastern Boone County, was reported Aug. 17. The virus normally starts spreading in August. Mosquitoes feed on infected birds as they nest during the spring. When birds are more active during the summer, mosquitoes seek other mammals, like humans, for a food source and the number of cases goes up.
“The majority of cases that we see are generally being infected two weeks before Labor Day and two weeks after Labor Day,” Yates said.
Typically it takes three days to two weeks from the time of infection to show symptoms. The transmission cycle ends in fall. “It takes several days of hard, cold freezing to stop active mosquitoes,” Yates said.
The insecticide being sprayed in Columbia, called Anvil 2+2, targets adult mosquitoes. It is recommended for use in areas surrounding residences of individuals infected with the West Nile virus and approved by the Environmental Protection Agency. Missouri is part of the EPA’s seventh region, along with Iowa, Kansas and Nebraska.
Craig Thomson, Iowa’s pesticide program manager, said Anvil 2+2 is a “widely used” substance. Though the EPA cannot label any insecticide as 100 percent safe, he said it poses a relatively low risk to people.
Yates said research has concluded that the risk of West Nile infection was greater than the risk of harm from pesticide exposure.
“At full strength, (the insecticides are) toxic,” Yates said, but at the concentration at which they will be applied, the chemicals are non-toxic to humans.
A marked truck and low-volume sprayer will apply the insecticide to a half-mile area between Stadium Boulevard to the west and south, West Worley Street to the north and Glenwood Avenue to the south; and a half-mile area between Clinkscales Road to the west, Rollins Road to the south, Providence Road to the east and Sexton Road to the north.
The homes of the three Columbia residents infected with West Nile virus are within those areas, which will be sprayed between 4 and 8 a.m. The early morning hours are peak hours for mosquito activity this time of year, Yates said.
Thomson said these hours coincide with the time of day when people are less likely to be outdoors.
He recommends residents stay indoors when the truck drives through the neighborhood. The product is applied in very fine droplets, similar to a fog. The droplets cling to foliage and dissipate quickly.
Thomson said if residents do come in contact with the substance, they should remove and wash the outer layer of clothing and rinse skin and eyes.
According to the Centers for Disease Control and Prevention, only about 20 percent of those infected with West Nile experience the symptoms, which may include fever, headache and tiredness. One in 150 people will develop a severe illness, and 80 percent will not show symptoms at all. Because West Nile is a virus As with all viruses, only the symptoms can be treated, Yates said.
“The body will eliminate the virus,” she said.