Fort Collins confirmed its first human West Nile infection of the season this week after a blood donor tested positive for the mosquito-transmitted virus.

At this point, the southeast Fort Collins donor is asymptomatic, according to county health officials. About 75 percent of people infected with West Nile virus don't develop any symptoms.

Blood donations are screened prior to distribution to ensure recipient safety. Blood donations that test positive for West Nile are discarded and not used for transfusions, county health officials said.

The donor's case is the first West Nile infection of the season in Larimer County, which regularly ranks among the top 10 counties in the nation for West Nile virus.

The confirmation isn't enough to trigger city-ordered mosquito-spraying in Fort Collins. City officials only consider spraying when two human West Nile cases are confirmed in one week and a certain proportion of infected mosquitoes are trapped within city limits, although homeowners associations and county officials can spray within city limits.

Larimer County logged 32 confirmed human cases of West Nile virus last season. Less than 1 percent of people infected with West Nile virus develop a more severe, neuroinvasive form, which can lead to hospitalization, critical illness, chronic disability or death.

This article includes a correction: The original article included an inaccurate tally of West Nile virus infections in Larimer County last season. The county logged 32 confirmed human cases last season.

How to prevent West Nile

To avoid infection, follow the 4 D's:

Use DEET or other effective mosquito repellent: Use an EPA-registered mosquito repellent that has been proven to be effective against West Nile virus-carrying mosquitoes. DEET, Picaridin, Oil of Lemon Eucalyptus (also called p-menthane-3,8-diol or PMD) and IR3535 are good choices.

Dusk to dawn: Avoid exposure during peak Culex mosquito feeding times, from dusk through dawn.

Dress - Wear long sleeves and pants to keep mosquitoes from biting.

Drain – Remove standing water in your yard or garden to minimize mosquito breeding areas.

Source: Larimer County Department of Health and Environment