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.
The District works to control mosquitoes in our county because
they are disease vectors and aggressive biting pests. However,
there are several mosquito species that are not known to bite
people. One of these infrequently-encountered mosquitoes is
called Orthopodomyia signifera. It breeds in the holes
of trees that hold water year-round and feeds exclusively on
birds.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of May 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 below
average for Culex pipiens at this time of year, but
consistent with the five-year average for other species. Adult
mosquito abundance will likely continue to rise as the daily
temperatures increase in the summer months.
As of May 31, 2020, there have been 68 dead birds reported in San
Mateo County. Of those, 23 have been suitable for testing and all
23 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.
The following table and graphs show the average number of
mosquitoes collected per CO2 trap per night during the
month of April 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. April was the first month
in 2020 the Aedes sierrensis, the western treehole
mosquito was collected. This mosquito is a vector of dog
heartworm and adults are usually present from spring to fall.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of March 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 numbers are expected
to remain low until after spring rains.
During early March, laboratory staff continued
surveillance for adult Ixodes pacificus (Western
black-legged ticks). Ticks are collected by dragging a 1 meter
square sheet of white flannel over the vegetation alongside
trails. Ixodes pacificus ticks will be tested for the
presence of bacteria Borrelia burgdorferi, the causative
agent of Lyme disease, as well as Borrelia miyamotoi and
Anaplasma phagocytophilum.
The following table and graph show the average number of
mosquitoes collected per CO2 trap per night during the
month of February 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 numbers are expected
to remain low until spring.
During February, laboratory staff continued surveillance for
adult Ixodes pacificus (Western black-legged ticks).
Ticks are collected by dragging a 1 meter square sheet of white
flannel over the vegetation alongside trails. Ixodes
pacificus ticks will be tested for the presence of bacteria
Borrelia burgdorferi, the causative agent of Lyme
disease, as well as Borrelia miyamotoi and
Anaplasma phagocytophilum.
District staff are assisting in a collaborative study on a
mosquito parasite that is spearheaded by researchers from
Stanford University and the University of Innsbruck in Austria.
The parasite is a ciliate type of protozoa, Lambornella
clarki, which is a single-celled animal that lives in water.
Lambornella clarki is found in water that collects in
tree holes, and can enter the body of the mosquito larvae of
Aedes sierrensis, the western tree-hole mosquito. Inside
the mosquito larvae, it multiplies and kills the hosts.
