West Nile virus has now appeared in humans in Onondaga and Oswego counties — causing one death — and experts say the deadly disease transmitted by mosquitoes soon could spread to the north country.

Mosquito samples taken by health officials tested positive for the disease in numerous areas across the two counties. The Post-Standard of Syracuse reported that a child from the city of Oswego and an adult from Onondaga County are recovering from West Nile, while an elderly Onondaga County resident recently died of the virus.

Data collected through July show the number of serious illnesses from West Nile has been the greatest since 2004, according to the U.S. Department of Health. So far, 241 human cases and four deaths have been found in 22 states.

In New York, 605 mosquito pools have tested positive for the virus from Jan. 1 through Aug. 14, according to the state Department of Health. Nine pools have been found in Onondaga County and four pools in Oswego County.

This year’s mild winter, early spring and hot summer have provided conditions conducive to the spread of West Nile, said Jeffrey M. Leiendecker, vector control coordinator for the Jefferson County Public Health Department.

“If you have a warm spring earlier, it gives that much more of a jump on establishing a larger mosquito population with the virus,” he said. “It could have happened about four weeks earlier” this season.

Because it takes several months for the virus to build up its strength, Mr. Leiendecker said, reports of the virus usually peak in August and September. He said the high levels of activity reported in Central New York have increased the likelihood it could spread to the north country this fall.

While no cases have been reported yet in Jefferson, Lewis or St. Lawrence counties, it doesn’t mean they don’t exist. Unlike Lewis and St. Lawrence counties, the Jefferson County Public Health Department doesn’t have a mosquito surveillance program in which it regularly tests mosquitoes. Its program was started in 2001 and discontinued in 2006.

“September is the month where humans get it the most because of its natural buildup, but it’s hard to say if it’s going to get worse,” Mr. Leiendecker said. Historically, “the virus has been present in mosquitoes in every county across New York state.”

Because birds — especially crows — carry the disease for miles and pass it on to mosquitoes, it can spread rapidly from county to county. While humans can’t get the disease from birds, female mosquitoes transmit the virus to birds by biting them to get blood.

“Birds will fly a mile, give it to mosquitoes, which will give it to more birds,” Mr. Leiendecker said.

Experts also have speculated that this summer’s drought could have contributed indirectly to the spread of West Nile, he said. The most prevalent breed of mosquito that carries West Nile is the culex pipiens, which breeds in water in open containers outside people’s homes, such as flower pots, gutters and drip trays. Experts hypothesize that homeowners who have used more water to irrigate their gardens and lawns this summer could have contributed greatly to the problem.

Mosquitoes “haven’t been able to find water in the woods, but they’ve been able to find it at people’s homes,” he said.

Residents should ensure there are no standing containers of water outside their homes and check their gutters for water. Mosquitoes breed by laying eggs in the water that pupate into larvae, which look like small worms. Removing outdoor containers can prevent the spread of thousands of mosquitoes.

The Lewis County Public Health Department has been testing mosquito samples since June at a marshy area near Dadville but hasn’t found West Nile yet, said Carol A. Paluck, preventive health supervisor for the department, which tests for West Nile and Eastern equine encephalitis.

“Those of us that are still testing mosquitoes know West Nile is here, but we’re still doing surveillance because we’re also concerned with other diseases,” she said.

Last year in early September, mosquitoes at the site tested positive for West Nile.

The state Department of Health recommends taking the following steps against mosquito-borne illnesses:

■ Cover your skin as completely as possible when outside when mosquitoes are present and active. Wear long sleeves, pants and socks.

■ Use insect repellent on exposed skin and follow label directions.

■ Make sure there are screens in the home’s windows and doors. Make sure the screens are free of rips, tears and holes.

■ Eliminate all standing water around your home and property where mosquitoes can breed.

Thousands of readers like you have made Northern New York Newspapers the number one ranked website* for local news and sports coverage in the north country.

You can continue on to this article in a few seconds, or you can visit one of our websites:

• Watertown Daily Times
• The Ogdensburg Journal
• Daily Courier-Observer