North Dakota health officials are investigating a death that may be linked to the West Nile virus.

Epidemiologist Michelle Feist said Tuesday that officials are not releasing the person's name or other details. If confirmed, it would be the first death linked to the mosquito-borne virus this year.

Feist said Tuesday that the number of human West Nile virus cases in North Dakota was at 15 last week and could reach 25 this week.

Ten human deaths have been linked to the virus in North Dakota since 2002, Feist said. Last year, the state reported 137 confirmed human cases and one death.

Also on Tuesday, the South Dakota Health Department announced that a Hand County, S.D., resident 80 to 89 years old was the first South Dakotan to die this year of West Nile virus.

The department is prohibited from releasing more information because of patient confidentiality requirements.

Family members told Sioux Falls, S.D., television stations that the victim was Robert Ames, 86, of Miller, S.D., and that he died July 17.

There have been 28 human West Nile cases reported in South Dakota so far this year. In all of 2006, the state had 113 human cases and three deaths.

West Nile is transmitted from sick birds to humans through mosquito bites.

"Personal protection is important for all of us because anyone can get West Nile disease," South Dakota epidemiologist Lon Kightlinger said in a news release Tuesday. "It's also important that communities intensify their mosquito control measures, particularly when they host outdoor public events."

The virus-carrying species, Culex tarsalis, prefers to breed in intermittent water - areas that become wet and dry up within a few days, said Denise Van Roekel, coordinator of the mosquito control program in Sioux Falls.

Even though some cities have put lawn watering restrictions in place, residents may douse their yards more heavily when they do water, which can create pools that become breeding grounds for the bugs, she said. The same goes for farmers who irrigate their crops, Van Roekel said.

Most mosquito control programs focus on more permanent water supplies, she said.

"We're targeting our efforts to areas that can hold water all the time, like big detention ponds and sloughs or wetlands," Van Roekel said. "We're trying to stay on top of it and keep numbers as low as possible."

As of Tuesday morning, there had been 122 human cases of West Nile virus nationwide reported to the Centers for Disease Control and Prevention. The first death in the U.S. this year was reported June 7 by the Mississippi Department of Health.

West Nile virus is a seasonal epidemic and flares up in the summer. Most people infected don't feel any symptoms.

The virus peaked in 2003, when nearly 10,000 people were infected nationwide. According to weather records, 2003 was a drought year with weather similar to this year.