Expected to begin this evening, mosquito spraying in Manchester took on heightened significance Wednesday when state health officials announced a city resident has been diagnosed with West Nile virus.

Officials would only describe the person as a Manchester adult. The person, who has recovered, is the first New Hampshire resident this year — and the first since September 2010 — to test positive for the mosquito-borne illness.

“This confirms that there is wide circulation of the virus, that the risk for human health is real,” said the state’s public health director, Dr. Jose Montero.

So far this year, the virus has been detected in 29 mosquito traps in Manchester. Nashua, Salem, Seabrook, North Hampton and Brentwood have also reported positive trappings, bringing the state total to 36.

For most people, West Nile virus is not serious.

The majority of infected people won’t even know they have it, said Tim Soucy, Manchester director of public health. A small percentage suffer from cold-like symptoms.

For a very few — about 1 in 150 infections — severe neurological diseases such as encephalitis develop, according to information distributed by the state Department of Health and Human Services.

Of the 1,118 people infected with the West Nile virus this year, 41 have died, according to the U.S. Centers for Disease Control and Prevention.

Manchester spraying is expected to begin at 5 p.m. tonight at most parks and schools.

Stratham-based Dragon Mosquito Control is slated to begin spraying in Rye on Sunday night. Also scheduled: Derry schools and parks starting at 5 p.m. Monday night. Raymond sprays on Wednesday night.

The threat exists until the first hard frost.

“When I first started in mosquito control, the season ended on Labor Day,” said Dragon Mosquito owner Sarah MacGregor. “Now, our season doesn’t end until Halloween. And it starts earlier in the season too.”

Nashua isn’t spraying

Meanwhile Nashua is taking a pass on spraying, despite the detection of the virus in mosquitoes trapped on Aug. 8.

“We would have to see more widespread activity than we’ve seen,” said Kerran Vigroux, the city’s public health director. “We might have to see different types of mosquitoes that were infected. Some mosquitoes are more vociferous human-biters than others. We might want to see more than one species coming up positive.”

She said the city continues to work with the state. And she said prevention at this point is the best strategy.

That includes dumping any stagnant water, which is where mosquitoes breed; repairing screens; using bug spray.

Montero said the state ranks mosquito risk in five categories, from remote to critical. As of Wednesday, areas in southeastern New Hampshire rated between low and moderate, though the numbers could change today.

Cases of the disease have been reported in every state except Alaska, Hawaii and Vermont. In Manchester, an investigation took place after officials realized a person was infected. But Soucy wouldn’t say where officials believe the person was infected, in part because he wants to encourage everyone to take precautions.

“That’s not the message we want to send — that if you live in this block of the city you’re at a greater risk,” Soucy said. “Everyone is at an equal level of risk in the state.”

In Derry, Pinkerton Academy officials rescheduled soccer games to accommodate the spraying. The academy pays for spraying on its grounds.

“The fields will be vacated by the time spraying begins,” said Pinkerton Academy Communications Director Chip Underhill. “Our athletic director and security will monitor the fields to make sure no one remains on them when spraying begins.”

Dragon Mosquito Control will use the insecticides, Anvil 10+10 (sumithrin), and CrossCheck (bifenthrin) in Derry.

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New Hampshire Union Leader Correspondent Adam Swift (aswift@newstote.com) contributed to this story. Simon Rios may be reached at srios@newstote.com. Mark Hayward may be reached at mhayward@unionleader.com.