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April 2022 Newsletter

Black icon of a bird lying on its back with feet in the air (meant to represent a dead bird)
Dead birds can indicate West Nile virus in an area - Report dead birds to help us track disease spread.

West Nile Virus Season Starts

West Nile virus (WNV) is endemic in California, meaning it regularly occurs and is established in a community. In San Mateo County, detection of WNV in birds has been rare in recent years. When certain birds become infected with WNV, they carry a high level of virus in their blood. If a mosquito bites an infected bird and feeds on its blood, the mosquito can become infected and pass the virus on to people or other animals that they bite. Not all birds that are infected with WNV will get sick, but WNV can make some birds very sick and even cause death. Signs of WNV in birds may include uncoordinated movement, a lack of energy, and difficulty breathing.

As of April 11th, 2022 the California WNV Call Center has reactivated for calls about dead birds. If a dead bird is reported in San Mateo County, our District will pick it up and test it for WNV. If a bird is positive, we set up mosquito traps around the location the bird was found to determine if there are WNV positive mosquitoes in the area. The best way to protect yourself from WNV is to reduce mosquito habitats in and around your home by removing standing water, and wearing long, loose clothing and insect repellent when you visit mosquito heavy areas. 

To learn more about West Nile virus, visit


Mosquito Surveillance

Adult Mosquitoes

Although Culex pipiens has dropped in abundance after unusually high numbers in February associated with a sewer plant, counts of other species are beginning to rise, as expected in spring. Overall numbers remain low, with fewer than five per trap of any species. Aedes sierrensis, a tree-hole breeding species, was detected for the first time this year in March. Adult Aedes sierrensis will remain present through the summer, with a peak typically in April. Aedes sierrensis males, which do not bite, are more active during this time of year, while the females are more active in the summer months. Read more about mosquito surveillance here.

Larval Mosquitoes

This March, 258 larval samples were collected in the field by vector control technicians and submitted to the lab. A District vector control technician uses a dipper to take a sample of the water and visually inspects it for mosquitoes present, the technician transports the sample to the laboratory for the larvae to be counted and identified. Read more about mosquito surveillance here.

Service Requests

The most common service requests in March 2022 were for mosquitoes and rodent inspections. Mosquitofish were also commonly requested, and yellowjacket nest calls are increasing as usual for this time of year.

Read more about service requests here.

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