Health officials have identified West Nile Virus in mosquitos in 17 California counties, including Fresno, Madera, Merced and Tulare. The state's first West Nile fatality this year took place last month in Tulare County.

West Nile virus is one of several diseases that can be transmitted to humans through mosquito bites. Mosquitoes become infected with the virus when they feed on infected birds. The risk of human infection from mosquito bites is highest in the summer when the insects become more active. 

Given the serious threat, prevention is key. Mosquitoes can reproduce in standing water basins as small as a bottle cap. Residents are encouraged to check and empty buckets, plant saucers, pet dishes, bird baths, gutters, toys, lawn drains and any receptacle that can hold water. Dump the standing water immediately; mosquitoes can go from egg to adult in just 5 days.

When outside, use an EPA-approved mosquito repellent that contains DEET, Picaridin or oil of lemon eucalyptus. DEET is the most effective repellent if you will be outside for long periods. Be sure to follow all label instructions. Visit the EPA website to learn how to use repellents safely and choose the right one for you.

To prevent mosquitoes from coming inside the house, make sure that doors and windows have tight-fitting screens. Repair or replace screens that have tears or holes. Ensure dog doors and other entrances to the outside are properly sealed. If you leave shoes outside on the porch, move them away from the door. If they are close to the door, the human scent can attract mosquitoes, which may enter when doors open.

To learn more about West Nile virus, visit the California West Nile Virus website. For information on mosquito management and prevention, see Pest Notes: Mosquitoes from UC IPM. For local information and support, contact the mosquito abatement district for your area. (Map to find the district.)

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This article, written by Lauren Fordyce, originally appeared in the UC IPM Pests in the Urban Landscape blog. The photo is Culex tarsalis, a vector of West Nile Virus, by James Gathany, CDC.