Heat and risk of West Nile virus at normal levels

Scott Gordon

The risk of contracting West Nile virus is returning to normal levels with the heat after two of Cook County’s coolest summers ever, health agencies said this week.

The Illinois Department of Public Health announced that five samples of Cook County mosquitoes — two of them from Evanston — tested positive for the virus last week. So far, no humans have tested positive for the virus in Illinois.

Health officials across Illinois thought West Nile had disappeared after 2002, when the virus killed 66 people in Illinois — more than in any other state, said Lynn Haramis, an entymologist for the state health department.

“It didn’t go away any place. It’s still here,” Haramis said. “We had two cool summers in a row” in 2003 and 2004, but this year temperatures are back in the 90s — ideal conditions for the two species of mosquito that carry the virus to humans from infected birds.

The risk to humans is as unpredictable as the weather. Heat makes water stagnate, allowing mosquitoes to breed, and scientists think heat may also increase the number of individual virus organisms an infected mosquito is carrying.

On Wednesday, the North Shore Mosquito Abatement district sprayed pesticides to kill adult mosquitoes in Skokie and Evanston, according to an Evanston press release.

The district, which collects and tests mosquitoes daily in 13 north suburbs including Evanston, tries to stop the spread of West Nile by spraying for adult mosquitoes, treating street drains where water stagnates and telling people to wear long sleeves, long pants and insect repellent, said district spokesman David Zazara.

But it’s harder to wipe out mosquito breeding grounds on private property. These include backyard drains, puddles, ponds and old tires.

More mosquitoes doesn’t necessarily mean greater risk, Haramis said. Mosquitoes are most visible after heavy rains, but most of these are “floodwater” mosquitoes, which don’t feed on birds and are unlikely to carry West Nile, he said. “What people need to do is not count mosquitoes. They need to listen to their local health officials,” Haramis said.

The district, the state health department and Evanston’s health department all test crows or blue jays, the two kinds of birds most likely to die from the virus. The district gets test mosquitoes from 26 traps, four of which are in Evanston.

As of June 20, health agencies in 24 of Illinois’ 102 counties submitted mosquito-test results to the state health department, Haramis said. Of the 1,152 samples recorded so far, 11 have tested positive.

In 2002, about 80 percent of samples tested positive, Haramis said.

Reach Scott Gordon at [email protected].

Quick facts:

  • The West Nile Virus spreads through mosquitoes, which carry the virus after they feed on infected birds.
  • Severe symptoms include high fever, headache, neck stiffness,and muscle weakness.
  • To prevent the virus, health officials advise to use insect repellent and wear long sleeves, especially at dusk and dawn.

Source: CDC