Dead Bird Surveillance
Did you know that dead birds can indicate local West Nile virus activity?
become infected with West Nile virus when they are bitten by a mosquito
carrying the virus. Mosquitoes that bite birds infected with West Nile
virus become infected and can pass the infection to humans, birds, and
other animals. Birds from the Corvidae family, like crows, ravens, and
jays, are susceptible to West Nile virus and may die. Other
birds, like chickens, may be infected but do not become sick or die
from West Nile virus.
2021 West Nile virus data
Image from the California Department of Public Health (https://westnile.ca.gov/)
of August 5, 2021, there have been 205 dead birds reported in San Mateo
County. Of those, 61 have been suitable for testing and all have tested
negative for West Nile virus (WNV). To date, there have been no West
Nile virus detections in mosquitoes or sentinel chickens in San Mateo
County in 2021.
there have been 8 human cases of WNV in 2021 compared to
a five-year average of 8.4 cases at this date. Throughout California,
3,538 dead birds have been reported and 1,069 have been
tested with 109 (10%) dead birds positive for WNV. Additionally,
770 mosquito samples have tested positive for WNV compared to a
five-year average of 1,166 at this date. Ten sentinel chickens
have tested positive, which is also below the five year average of 30.6.
erythrothorax, a mosquito that breeds in lakes and ponds with tules,
was the most frequently collected mosquito in July. The high number of
tule mosquitoes this month reflects a large, seasonal emergence of this
species in Pacifica. Adults of this species are usually present from
around April through October. Seasonal helicopter treatments for this
mosquito began in July, which modestly reduced total abundance since
June, but counts remain high. Culex pipiens, the northern house
mosquito and typically the most abundant mosquito in San Mateo County,
increased in numbers during July, but abundance remains below average
for this time of year, likely because of dry weather and cool nighttime
temperatures in San Mateo County.
total number of service requests is close to average for July (527
compared to an average of 540). Yellowjacket and other wasp requests
increased dramatically for the second month in a row, from 34 in May to
174 in June to 367 in July. Yellowjacket and wasp requests are common
in the summer and will likely remain numerous until the weather cools
in autumn. The number of mosquito service requests is lower than
average, likely because of a low abundance of Culex pipiens, which
usually prompts the most mosquito requests. Dead bird requests have
increased as well, from nine in June to sixteen in July, as West Nile
virus has become more seasonally active in California. Fortunately,
West Nile virus has not been detected in San Mateo County or any
neighboring counties this year through the month of July. The category
of “other” included a variety of request types, such as bumblebees,
fleas, cockroaches, flies, mites, and ticks.