As temperatures and North Dakota's mosquito counts drop, the state's reported cases of human West Nile virus continue to rise.
The state health department reported 133 new human cases Thursday, bringing the state's total to 426 this year.
Health officials again urged people to protect themselves from mosquito bites.
But many of Thursday's reported cases could have been contracted weeks ago and the number of cases seems to have reached a plateau, the department said.
Many North Dakotans are already donning sweaters and long pants because of recent 40-degree temperatures.
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But until the thermometer drops to the mid-20s, people should continue limiting their exposure to mosquitoes, even if they can't see them, said Mike Trythall, the state's virology supervisor.
The disease's incubation period ranges from three days to two weeks. Some currently reported victims waited until their symptoms worsened before getting tested, said Tracy Miller, the state's West Nile virus program manager.
The state health lab runs about 64 tests per day, seven days a week, Trythall said.
While the lab immediately tests severe cases, it has a backlog of four to five days on the other, less dire cases, he said.
Of the 149 completely investigated cases, 33 percent, or 49 people, have been hospitalized, Miller said.
The virus' spread "really stands out in recent history," said Kirby Kruger, an epidemiologist with the state health department.
So far this year, the virus has infected a reported 4,325 people throughout the country and has killed 81, the U.S. Centers for Disease Control and Prevention said on its Web site Thursday.
Many more people are likely to have been infected, but haven't been tested because of mild or no symptoms, said Dr. Craig Lambrecht, North Dakota's state medical officer.
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One North Dakota resident died from the disease earlier this month. The virus claimed two other lives this year in North Dakota hospitals.
Few people recall being bitten by mosquitoes, but people might have been paying less attention this year because of the dry weather conditions, Miller said.
More people know of West Nile's symptoms this year, which could be one reason for the drastic increase in human cases this year over last, she said.
But Lambrecht said patients seemed more insistent about being tested last year, while they've seemed more at ease with the disease this year.
Some people who contracted West Nile virus this year may have lingering symptoms of fatigue for several months, he said.
The number of human victims varies greatly from state to state.
Of the 149 confirmed cases in North Dakota, the health department reports 39 confirmed cases in Burleigh County and one confirmed case in the more-populated Cass County.
Readers can reach Forum reporter Lisa Schneider at (701) 241-5529
