West Nile Virus activity tends to increase as summer progresses. By late summer, mosquitoes have had months of warm weather to reproduce, and the virus has had time to circulate between mosquitoes and birds. That combination is why August and September are important months for West Nile Virus prevention in Los Angeles County.
What Happens During the Summer
West Nile Virus is maintained in a cycle between birds and mosquitoes. A mosquito becomes infected after feeding on an infected bird. That mosquito can later transmit the virus when it bites a person. As the season continues, more generations of mosquitoes can emerge and the virus has more opportunities to circulate.
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Mosquito Feeds on Infected Bird
Mosquito becomes infected with West Nile Virus
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Virus Circulates Between Birds
Multiple generations amplify virus presence
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Infected Mosquito Bites a Person
People are incidental hosts — not part of the ongoing cycle
Warm temperatures can also speed mosquito development when standing water is available. Around homes, even small amounts of water in plant saucers, buckets, birdbaths, clogged gutters, and other containers can support mosquito breeding.
West Nile Virus Is Still Being Monitored
West Nile Virus surveillance continues across Los Angeles County and California. The start of school or the end of summer does not mean mosquito activity has ended. In Southern California, mosquitoes can remain active into the fall while temperatures remain warm.
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Peak Risk Window
August and September are the most active months for West Nile Virus transmission in Southern California
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Season Extends Into Fall
Mosquitoes remain active as long as temperatures stay warm — not tied to the academic calendar
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Active Surveillance
Ongoing monitoring of mosquito populations, dead birds, and human cases across LA County
Keep Taking Precautions
Residents should continue taking the same precautions they used earlier in the summer. The habits that worked in June and July are the same ones that matter in August and September.
What Residents Can Do
1
Use EPA-Registered Insect Repellent
Follow the product label when spending time outdoors, particularly during periods of increased mosquito activity. Visit epa.gov/insect-repellents for approved product options.
2
Check for Standing Water Every Week
Dump, tip, or cover containers that can hold water. Pay attention to plant saucers, birdbaths, buckets, toys, gutters, and areas where irrigation water collects.
3
Report Persistent Standing Water
If you see standing water that you cannot eliminate — in alleys, storm drains, neglected lots, or near public spaces — contact Compton Creek Mosquito Abatement District. We investigate.
4
Report Dead Birds
Dead crows, ravens, jays, and other birds can provide important information for West Nile Virus surveillance. Report recently dead birds to the California Dead Bird Reporting Program within 24–48 hours.
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California Dead Bird Reporting Program
Report recently dead crows, ravens, jays, magpies, sparrows, finches, or hawks. Use gloves or a bag if asked to dispose of the bird yourself.
1-877-WNV-BIRD
(1-877-968-2473)
