SPRINGFIELD – While the number of mosquitoborne illnesses is on the rise nationwide, Illinois public health officials are watching the weather to gauge the risk of a West Nile virus outbreak.

Melaney Arnold with the Illinois Department of Public Health said that West Nile virus cases in Illinois are very dependent on weather.

“When you see a hot, dry summer, then you typically see more mosquitoes, more West Nile virus activity,” Arnold said. “When it’s cool and a little bit wetter, we don’t see as much West Nile virus activity.”

Chicago came in third on a list of worst U.S. cities for mosquitoes by pest control company Orkin. “Chicago is sometimes a hotbed of where we see them,” Arnold said. “Typically, the mosquito that carries West Nile virus likes to breed in stagnant waters, so you have the catch basin.”

Nationally, the CDC said illnesses from mosquito, tick and flea bites have tripled in the past 12 years. However, Arnold said the number of Illinois’ West Nile virus cases has varied widely over the past 15 years.

“To start, in 2002 we had the most West Nile virus cases in the country with 884 human cases,” she said. “Last year, we saw 90 human cases.”

Illinois reported more than 2,500 mosquitoborne disease cases between 2004 and 2016, according to the CDC. That landed the state in the top 20 percent for mosquito-borne illnesses. The CDC said local governments and health agencies across the country must be alert to help control mosquito populations.