Local and state health officials are warning about the risk of the sometimes-deadly West Nile virus to Ohio residents after seeing a 460 percent increase in mosquitoes carrying the virus during testing in pockets of the state.

The number of mosquito samples testing positive for West Nile virus to this point in the year boosted from 38 in 2011 to 213 this year, the most since 2002, according to the Ohio Department of Health. Twenty state agencies in 14 counties participate in the testing program, but ODH officials are stressing that residents in all counties should be more aware of mosquitoes because of the rising numbers.

“At this time, it is very important for residents of Ohio to take precaution against mosquito bites,” an ODH spokesperson wrote in an email.

There have been no human cases of the virus reported in Ohio, according to the department of health. Last year, one Ohio resident died as part of 21 reported cases of West Nile, which ranked the state 11th nationally in cases. There were 712 human cases reported nationally in 2011.

Montgomery County, the only area county to participate in the state’s mosquito testing, saw 35 positive West Nile results in 151 pools of captured mosquitoes, which totaled 3,980 in those pools. Statewide, the 213 positive results came from 3,760 pools containing 99,128 captured mosquitoes, according to ODH.

Officials stressed the need for using insect repellent, placing screens on open windows and wearing long sleeves or pants when outside near high-mosquito areas. Mosquitoes are most active at dawn or dusk, officials said, and breed in standing water.

“The environment contributes to the breeding,” said John Steele, a spokesman for Public Health - Dayton and Montgomery County. “We’ve had hot weather, and there have been stagnant areas of water not being washed out because there hasn’t been much rain. It seems to be the ideal condition to breed mosquitoes.”

Symptoms of West Nile virus include the basic fever, aches and swollen glands to the more serious neck stiffness, disorientation, tremors, vision loss and numbness. Officials said the number of reported cases is likely lower than the actual total because many who contract West Nile virus do not know they have the illness.

The virus boomed nationally in 2002, when there were 4,156 reported cases, including 284 deaths nationally. That year, 441 cases were reported in Ohio, a number that dropped to 108 in 2003 and has not reached triple digits since.

Locally, there were 64 reported human cases of West Nile virus from 2002-11 in Butler, Champaign, Clark, Greene, Miami, Montgomery and Warren counties. But those numbers decreased from 32 in 2002 to two last year, one case each in Montgomery and Miami counties.

“Part of it might be people had West Nile and were not even aware of it,” said Brian Williamson, chief of environmental services for the Butler County Board of Health, which last saw a human West Nile case in 2007. “The most efficient thing we can do is educate to make people aware of the symptoms and to limit their exposure to mosquitoes.”

Each of the region’s seven counties has seen at least one human West Nile case since 2003, led by Clark County with 24 cases, including 14 in 2003. Clark County’s last case came in 2005, and only Montgomery County (three) and Miami County (one) have reported cases since 2007 among the area’s counties.

“We’ve never really seen a description (of what causes a rise or fall in cases),” said Dan Chatfield, director of environmental health for the Clark County Combined Health District. “Back in the early 2000s it seemed to be very virulent with a lot of it around, starting on the East coast and moving this way. Now most of the activity seems to be west of Ohio.”

Health officials said they are prepared to offer aid if a human case of West Nile is reported in the region. The number of positive mosquito tests has varied in its relation to the number of human cases. In 2002, there was a reported human case in Ohio for each 3.8 positive mosquito tests. By 2009, there was one reported human case for each 121.5 positive mosquito tests in Ohio.

“We would immediately follow up to that area, go the property and find out what areas are most prominent for mosquitoes there,” said Dennis Murray, director of environmental health for the Warren County Combined Health District, which has seen four cases since 2003. “I think people are getting much better at doing what they need to do to protect themselves, which hopefully will offset any increased risk.”