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West Nile virus has been detected in a dead bird in Menlo Park, according to a May 15 press release from the San Mateo County Mosquito and Vector Control District. The district is encouraging residents to dump all standing water in their yards to prevent mosquito problems around their homes, report biting mosquitoes and report dead birds so they can be tested for the virus.
Dr. Rachel Curtis-Robles, the district’s public health education and outreach officer, said that this is the second bird with West Nile virus that has been found in Menlo Park this year.
“The previous bird was collected in mid-April. The test result from the first bird indicated it was unlikely to have been recently infected,” said Curtis-Robles. “In contrast, the test result from this second bird indicated it may have been recently infected.”
There have also been two West-Nile-virus-positive dead birds found in Foster City, two in Palo Alto and one in Mountain View, according to a report from the California Department of Public Health.
This year’s wet winter has led to additional habitat for mosquitoes to breed, as there is more stagnant water. According to the press release, California experienced “very high West Nile virus activity in 2023, and district staff are concerned that this year could be another long mosquito season.”
There have been 14 cases of West Nile virus detected in dead birds throughout California as of May 10. That represents almost five times more detections than at the same time last year.
The virus can be transmitted to humans through bites from infected mosquitoes. Though it is the leading cause of mosquito-borne disease in California, most people bitten by mosquitoes will not be infected, and most people infected will not feel symptoms. About 1 in 5 people who are infected will develop a fever and other mild symptoms, and 1 in 150 people who are infected will develop serious, sometimes fatal symptoms.
Dead birds are often the first sign of West Nile virus being present in an area, according to the district.
“The finding of West Nile virus in a dead bird can be a sign that West Nile virus is circulating in local mosquitoes,” said Curtis-Robles. “However, because birds can fly quite far, it may also be that the bird was infected by a mosquito elsewhere.” For this reason, our staff do additional work to determine whether there are infected mosquitoes in the area near where the positive dead bird was found.”
This additional work includes setting mosquito surveillance traps in the area near where the bird was found, and an extra check of potential mosquito breeding sources in the immediate vicinity.
A San Mateo County Mosquito and Vector Control District employee checking for standing water. Courtesy San Mateo County Mosquito and Vector Control District. 
A San Mateo County Mosquito and Vector Control District employee conducts mosquito surveillance in June 2023. Courtesy San Mateo County Mosquito and Vector Control District.
“The mosquitoes in those traps are tested for West Nile virus. If mosquitoes are positive for West Nile virus, this indicates a local threat to public health, and additional mosquito control actions would be pursued,” said Curtis-Robles. “In this particular case, we tested mosquitoes from the additional traps, and none of the mosquitoes were positive for West Nile virus.”
The mosquitoes that carry West Nile virus mostly bite in the evening, night and early morning.
“Residents should use protective clothing (long pants and long sleeves) and an EPA-registered insect repellent if outdoors during those times,” said Curtis-Robles.
If you are being bitten by mosquitoes on your property in San Mateo County, you can request a visit from Vector Control District staff. They can help you identify the source of the mosquitos, and suggest mitigation measures.
“Dumping standing water is a key action for residents to take to reduce mosquitoes in their area,” said Curtis-Robles. “In cases where the water is unable to be dumped or drained, we would like people to report standing water so that our staff can assess the situation and determine how best to control mosquito breeding. In addition, we would like to remind people to report when mosquitoes are biting them in their homes and yards — public reports are a main way for us to detect mosquito breeding in an area.”
The district is asking the public to report dead birds to the statewide West Nile Virus Call Center by phone at 877-WNV-BIRD (877-968-2473) or online at westnile.ca.gov.
