West Nile fears spur spraying of pesticides

Molly Ball

Friday, Aug. 27, 2004 | 11:15 a.m.

For the first time, Clark County workers are spraying pesticides that kill adult mosquitoes to fight the spread of West Nile virus, officials said Thursday.

On Thursday a health district employee monitoring the Overton area caught 12,000 mosquitoes in a single trap, Daniel Maxson, an environmental health supervisor of the Clark County Health District, said.

"That's bad enough that we're going to spray adulticide out there," Maxson said.

Up to this point, officials have focused on preventing mosquitoes from breeding or from reaching adulthood. Spraying pesticide to kill adult mosquitoes is seen as a last resort because it is not as targeted a method, and some of the sprays are thought to have health effects on humans.

"Typically, we just try to do the larvicide, or source reduction, before they turn into adults," said Richard Hicks, head of Vector Control for the Clark County Public Works Department. But in this case, he said, killing adults "is what we felt we need to do to knock back the adult population."

One round of spraying was done Thursday and the second was planned for this morning, Hicks said. The method is a car-wash-style sprayer attached to a 100-gallon tank of a pesticide containing pyrethrin, mounted on a truck.

The mosquitoes trapped in Overton had not yet been tested for West Nile, and they are a different type than the mosquitoes that usually carry the virus. They may carry the disease but are not as likely to do so as some other mosquitoes, Hicks said.

But the sheer abundance of the bugs on Thursday was enough to spur the county to action, officials said. West Nile has also been detected in mosquitoes in Logandale, just down the road from Overton.

Mosquitoes are the way West Nile virus gets from the animal kingdom to humans. They suck blood from infected birds, then pass the disease on to humans they bite.

Mosquitoes breed in stagnant water. Health and public works officials have been traveling around the state identifying and eliminating or treating stagnant pools, which can be anything from a rural streambed to an abandoned baseball field. Many are unmaintained swimming pools.

In Overton, the culprit is an irrigation canal that flooded during recent rains, sending runoff down to pool in a spot near a railroad track, Hicks said.

The pesticide kills mosquitoes on contact but then dissipates quickly, Hicks said. Although there are homes in Overton across the railroad tracks from where the spraying was done, officials did not notify residents because the chemicals could not spread that far, he said.

To date, five cases of West Nile illness have been confirmed in Clark County and nine others are classed as probable, meaning they scored positive on an initial test but need more precise examination by specialists.

That marks one new case since the last time numbers were released a week ago.

This morning state health officials announced that two more human cases of West Nile virus were confirmed in Churchill County residents. That raised the total number of confirmed human cases in Nevada to 11 -- five in Clark County, one in Washoe County and five in Churchill County.

Of the 14 recorded Clark County West Nile victims, 10 have suffered from the most severe form of the disease: West Nile neuroinvasive disease, which involves swelling of the brain and spinal cord.

The neurological damage from this form of the disease can be permanent, according to the federal Centers for Disease Control. Three to 15 percent of those affected die.

This is the first year the virus has been detected in Nevada, and as of Thursday night there had been no reported deaths in Nevada from West Nile. Nationwide, however, 20 people have died from the disease in 2004, half of them in Nevada's neighboring states of Arizona and California.

But most people who get West Nile -- 80 percent -- don't know they have it.