The heavy rains this month have filled seasonal water sources
with rain water, making surveillance for mosquito larvae a top
priority. This January, field staff collected 145 larval samples
that were submitted to the laboratory for identification.
The most frequently occurring species in the larval samples was
Culiseta incidens, the cool weather mosquito.
Culiseta incidens larvae are found year-round in a
variety of freshwater sources.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of January compared to the five-year average for the six
most common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. The abundance of adult
mosquitoes is expected to remain low until spring.
Due to the increased rain this month, scientific staff
spent the time in the lab to optimize DNA extraction techniques.
The current laboratory tick extraction protocol allows staff to
extract 24 tick pools (five ticks per pool) at a time. A
different extraction kit has the ability to double that number to
48, but it utilizes different chemical reagents. Considering the
lab processes thousands of ticks each year, this shift should
significantly reduce extraction time.
In August 2020, several small fires in the Santa Cruz Mountains
merged into the SZU Lightning Complex, which ultimately burned
86,509 acres. In San Mateo County, the fire burned 22,755 acres,
and is now known as the largest fire in San Mateo history.
Laboratory staff are conducting research at Butano State Park, a
beautiful old-growth Redwood Forest near Pescadero that was
significantly affected. Fire damage, strong winds, and heavy
rains have rendered the trees unstable. The park is currently
closed to the public due to the inherent safety risks.
Laboratory staff continued surveillance for adult Ixodes
pacificus (Western black-legged ticks) in January. However,
field collection opportunities were limited by weather. Ticks
were collected along residential homes in Hillsborough (ticks
collected = 225), Costanoa near Pescadero (131), Montara Mountain
near Montara (310) and Skyline Ridge Open Space Preserve near
Portola Valley (581). In addition, tick surveillance and
environmental surveys were conducted at Butano State Park near
Pescadero (25).
Surveillance for mosquito larvae typically increases during the
month of December as seasonal rains begin and water sources begin
to accumulate. Through December 2020 there has not been a lot of
late autumn/early winter rain, so many impounds remain dry.
This December, 26 larval samples were submitted to the
laboratory. The most frequently occurring species in larval
samples was Culiseta incidens, the cool weather
mosquito, present in 9 of the 26 samples.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of December compared to the five-year average for the six
most common mosquito species in San Mateo County. Counts for all
species are averaging below five per trap night, which is
consistent with the five-year average. The abundance of adult
mosquitoes is expected to remain low until spring.
In the 2020 calendar year, the District closed out the second
year in a row of very low levels of West Nile virus (WNV). The
virus was only detected in the county once this year in any of
the standard WNV surveillance indicators. The positive detection
was a dead American crow from South San Francisco in September.
The District followed-up on the detection by trapping and testing
mosquitoes near the location of the bird, but all mosquitoes
tested negative for the virus.
On December 16, 2020, the District welcomed a new Vector
Ecologist, Arielle Crews. Arielle previously worked as a Vector
Biologist for the California Department of Public Health in the
Vector-Borne Disease Section. There, she conducted state-wide
laboratory disease surveillance for an array of pathogens
including West Nile, St. Louis encephalitis, Western Equine
Encephalitis, Lyme disease, and plague. Additionally, Arielle has
worked on a wide breadth of zoonotic disease research projects
from across the country.
As of October 31, 2020, there have been 204 dead birds reported
in San Mateo County. Of those, 62 have been suitable for testing
and one has tested positive for West Nile virus (WNV). The WNV
positive bird was an American crow collected from South San
Francisco in September. This has been the only detection of WNV
in San Mateo County in 2020. No mosquito samples or sentinel
chickens have been confirmed positive for WNV in San Mateo County
this year.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the month of
September compared to the five-year average for the six most
common mosquito species in San Mateo County. Culex erythrothorax
was the most frequently collected mosquito in September, seen in
higher numbers this year compared to the five-year average. Much
of this increase is due to significantly elevated numbers of
these mosquitoes in marsh areas on the coast, coupled with
improvements to targeted surveillance of this species this
season.
As of October 2, 2020, there have been 172 dead birds reported in
San Mateo County. Of those, 50 have been suitable for testing and
one has tested positive for West Nile virus (WNV). The WNV
positive bird was an American crow collected from South San
Francisco on September 22, 2020. This has been the only detection
of WNV in San Mateo County in 2020. No mosquito samples or
sentinel chickens have been confirmed positive for WNV in San
Mateo County this year.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of August compared to the five-year average for the six
most common mosquito species in San Mateo County. Culex
erythrothorax was the most frequently collected mosquito in
August, with numbers more than twice the five-year average. These
mosquitoes are collected in tule marshes, which will be treated
by helicopter. Culex pipiens, typically the most common
mosquito trapped in August, is at average levels this year.
The District conducts intensive surveillance for invasive Aedes
mosquitoes during the summer months. The surveillance program
mainly targets two container-breeding species, Aedes aegypti and
Aedes albopictus, which have been increasing their range within
California over the last several years. This summer, Ae. aegypti
and Ae. albopictus were detected for the first time in Shasta
County. The presence of these mosquitoes is highly undesirable
because they are aggressive human biters and also transmit
diseases including dengue, chikungunya, yellow fever and Zika.
As of August 31, 2020, there have been 140 dead birds reported in
San Mateo County. Of those, 38 have been suitable for testing and
all 38 have tested negative for West Nile virus (WNV). No
mosquito pools or sentinel chickens have been confirmed positive
for West Nile virus in San Mateo County in 2020.
As of August 4, 2020, there have been 120 dead birds reported in
San Mateo County. Of those, 38 have been suitable for testing and
all 38 have tested negative for West Nile virus (WNV). To date,
there have been no confirmed West Nile virus detections in
mosquitoes or sentinel chickens in San Mateo County in 2020.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of July compared to the five-year average for the six most
common mosquito species in San Mateo County. Culex
erythrothorax was collected in above average numbers in
July, whereas Culex pipiens, typically the most common
mosquito in July, is well below average this year. Cx.
erythrothorax, the tule mosquito, is largely treated by
helicopter because the sites are often inaccessible by foot.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of June compared to the five-year average for the six most
common mosquito species in San Mateo County. Culex
erythrothorax was the most frequently collected mosquito in
June, but numbers were only slightly above average. These
mosquitoes are collected in tule marshes, which will be treated
by helicopter. Culex pipiens, typically the most common
mosquito in June, is well below average this year.
As of June 30, 2020, there have been 95 dead birds reported in
San Mateo County. Of those, 29 have been suitable for testing and
all 29 have tested negative for West Nile virus (WNV). No
mosquito pools or sentinel chickens have tested positive for West
Nile virus in San Mateo County in 2020.
Beaches are popular among San Mateo County residents this summer,
including our local bees. District staff responded to a request
in June to identify bees nesting on a beach in Pacifica that were
flying in and out of small holes in the sand.
