(WISH) – Health authorities in Marion and Hamilton counties said Tuesday they have found mosquitoes with the West Nile virus for this first time this year.

Both counties said they will increase prevention activities in the areas where the mosquitoes were found.

About 1 in 5 people who are infected with West Nile develop a fever and other symptoms. About 1 out of 150 infected people develop a serious, sometimes fatal, illness.

Hamilton County said it has trucks with sprayers that go out after about 8:30 p.m. into the areas where the mosquitoes with West Nile were found.

“Please be aware that nuisance biting activity is increasing and that the Hamilton County Health Department only adulticide sprays for mosquitoes in known areas of positive West Nile Virus activity,” said the Hamilton County announcement.

Hamilton County said the mosquitoes testing positive were found Monday in Meadowlark Park in Carmel. Spraying began there on Tuesday.

Marion County did not disclose in a news release where its authorities found West Nile mosquitoes.

People can use products containing DEET during outdoor activities to deter the flying bugs.

Marion County also suggested people should stay indoors from dusk to dawn, wear long sleeves and long pants when outside from dusk to dawn and remove all standing water outside homes.

For questions about Marion County mosquito prevention, call the Marion County Public Health Department’s Mosquito Control program at (317) 221-7440.

The reports from Marion and Hamilton counties came a day after the Indiana State Department of Health reported mosquitoes in Elkhart and Clark counties tested positive for West Nile virus.

No human cases of West Nile virus disease have been detected so far in 2019. Information about West Nile virus is available at CDC.gov.

The state website keeps a map of counties where West Nile mosquitoes have been found.