About West Nile Virus
West Nile Virus (WNV) is the most common and serious vector-borne disease in California. Most people who become infected with WNV have only a mild illness or no symptoms, but in rare cases they can become seriously ill. The types of mosquitoes that can transmit WNV are common mosquitoes in Alameda County. There have been more than 7,000 human cases and over 300 deaths reported in California since 2003. Mosquitoes that spread WNV are found throughout California.
WNV is established as a seasonal epidemic that generally runs from June 1 to November 1 each year. Click on this interactive map to view reported WNV cases across California.
To report a dead bird or squirrel, call the West Nile Virus Dead Bird Hotline at (877) WNV-BIRD (877-968-2473). The Hotline is staffed with live operators. You may also use this online reporting form.
How WNV is Spread to Humans
WNV is transmitted to humans and animals through a mosquito bite. Mosquitoes become infected when they feed on infected birds. Human-to-human transmission of WNV does not occur. However, human WNV infections have been associated with blood transfusions and organ transplants.
Prevention
Control Activities in Alameda County
In 2025, California reported 154 dead birds and 2,759 mosquito pools that tested positive for WNV in 36 counties. There were also 113 human cases statewide of WNV with 11 deaths. In 2025 in Alameda County, 6 dead birds and 0 mosquito pools tested positive for WNV, with no human cases.
During WNV season, Alameda County’s Mosquito Abatement program works to limit risks to residents by:
- Monitoring ponds and other possible mosquito breeding sites
- Trapping to detect high numbers of mosquitoes
- Treating sewer catch-basins to prevent breeding
- Collecting birds for testing
- Educating residents and owners about removing standing water from private property to limit mosquito breeding and mosquito bites
Testing
The rationale for West Nile Virus testing:
While there is no specific treatment for WNV, there are a number of critical reasons to test patients with symptoms compatible with WNV disease, including:
- Differentiating WNV from other conditions such as enterovirus and other arboviral diseases.
- Patients can have lingering symptoms and identifying the underlying cause is helpful for the patient and family members.
- A positive patient may function as a local sentinel, indicating infected mosquitoes are in the area.
- Mosquito control and public health agencies can institute proper control measures to prevent further cases.
Mosquito Control
Alameda County Mosquito Abatement District (510) 783-7744 provides testing and surveillance of potentially disease-bearing mosquitoes in all parts of Alameda County.
The Abatement District offers a variety of health education and mosquito control services designed to protect to County residents from the spread of communicable diseases like West Nile Virus.