West Nile virus appears to be on the decline in Michigan again this year, but health officials still urge residents to take precautions against the mosquito-borne illness.
Five human cases of West Nile have been reported in Michigan this season -- all in the southeast part of the state -- as of late last week. That's down from 10 cases statewide at the same point in 2007.
But it's relatively early in the West Nile season. The case count could rise later this month.
The virus's peak year in Michigan was 2002, with 644 human cases and 51 deaths. While the overall number of cases has varied by year, the general pattern has been toward fewer Michigan residents coming down with the virus.
Michigan reported 55 West Nile cases, including seven deaths, in 2006. There were 17 reported cases, including four deaths, in 2007.
Health officials can't attribute this year's decline in cases to any one factor.
"I don't think anyone has the total answer to it," said Mary Grace Stobierski, an epidemiologist with the Michigan Department of Community Health. "It's probably the combination of a lot of different factors."
Human cases this year have been confirmed in Wayne, Macomb and Lenawee counties.
West Nile is transmitted by mosquitoes, typically after biting infected birds or horses who carry the virus. It becomes more common in late summer and early autumn.
Michigan's abnormally cool and dry summer may have limited the mosquito population in some areas. Several communities may also be doing a better job of mosquito control than earlier this decade, and more people may be doing what they can to prevent bites.
There also could be cyclical factors at work with the bird population.
Most people bitten by an infected mosquito show no symptoms, but some become sick within a few days. About one in five infected persons will become mildly ill with fever, headache and body aches. Less common symptoms are a skin rash and swollen lymph glands.
Severe cases may result in encephalitis, meningitis or death.
People older than 50 or with weakened immune systems are among the most vulnerable to West Nile.
People can cut down on contact with mosquitoes by wearing long-sleeved shirts and long pants, applying insect repellent containing DEET and regularly draining standing water to prevent mosquito breeding.