Reports of West Nile virus continue to rise.

So far, 18 cases are reported in humans. That's up four cases from the previous reporting period. The latest report from the North Dakota Department of Health is as of the week ending Aug. 18.

"It's definitely different than 2007," said Michelle Feist, West Nile virus surveillance coordinator.

In 2007, there were 83 human cases of West Nile reported to the state. At this time last year, the season also peaked. But the peak of infections also has occurred late in August or early in September.

Part of the reason for the low rate of infection is the low mosquito counts. The female culex tarsalis mosquito accounts for 3 percent of the mosquitoes caught in state surveillance traps. This species is the primary cause of West Nile in humans, Feist said.

The mosquitoes get the virus from infected birds. The infected mosquitoes then transmit it to other birds, horses and humans, Feist said.

West Nile can cause headache, body ache, fever and a rash. In more severe cases, it can affect the central nervous system. In extreme cases, it can lead to death.

To prevent infection, people should prevent mosquito bites and control mosquito habitat. This is done with insect repellent and loose clothing and by getting rid of standing water and keeping grass trimmed around the home.

For more information visit www.ndhealth.gov/wnv.