As of September 28, 2018, there have been 330 dead birds reported in San Mateo County. Of those, 103 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Two mosquito pools have tested positive for WNV, both from East Palo Alto during the month of August. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.
Every year the District collects ticks from recreational areas in the county to assess the risk of tick-borne disease. One way to measure this is by determining the minimum infection prevalence (MIP), which estimates what percentage of these ticks are expected to be carrying a given disease agent in a population. The laboratory has completed testing for two vector-borne diseases from ticks collected from November 2017 to May 2018.
Autumn is the time of year for termite swarms as adult winged termites (called alates), leave their colonies, mate, and establish new colony sites. Residents often see the swarms around their neighborhood or find discarded wings and dead termites scattered on the ground or caught in spider webs. Only a very small proportion of termites are successful in establishing a new colony.
As of August 31, 2018, there have been 291 dead birds reported in San Mateo County. Of those, 91 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Additionally, two mosquito pools have tested positive for WNV, both from East Palo Alto. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018, although a sentinel chicken in Santa Clara County, near the border with San Mateo County, tested positive in August.
The vast majority of mosquito control work at the District targets mosquitoes when they’re in the aquatic larval stage. However, occasionally the mosquitoes need to be controlled in their adult stage, such as when there is a human health risk from West Nile Virus. At this District, adult mosquitoes are treated with a backpack in small, isolated areas or by truck for larger sections of a neighborhood.
As of August 1, 2018, there have been 247 dead birds reported in San Mateo County. Of those, 66 have been suitable for testing and two have tested positive (3%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.
As of June 29, 2018, there have been 176 dead birds reported in San Mateo County. Of those, 52 have been suitable for testing and two have tested positive (4%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.
District laboratory staff conducted a rodent survey for hantavirus with California Department of Public Health (CDPH) staff on June 4-5 in La Honda. Peromyscus truei (pinyon mice) and Peromyscus californicus (parasitic mice) were trapped and blood samples were taken for testing. Kidney samples from any mice with positive blood samples will also be tested by the CDPH to be used for a study of the geographical distribution of different hantavirus strains in California.
During the summer months, the District receives numerous calls regarding yellowjackets and paper wasps. These social wasps both have yellow and black markings and are common in San Mateo County, but they have important behavioral differences.
It’s summer and that means peak season for the dog tick and Pacific coast tick (collectively called Dermacentor ticks)! These large, brown-colored ticks are very prevalent this time of year along the edge of trails and in brushy areas with tall grass all throughout California. Unfed, they can be nearly three times as large as the black-legged ticks that are prevalent during the winter and spring and can engorge to the size of a grape if left attached to repletion.
As of May 25, 2018, there have been 115 dead birds reported in San Mateo County. Of those, 25 have been suitable for testing and two have tested positive (8%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.
The laboratory began yearly surveillance for hantavirus with a rodent survey at San Bruno Mountain on May 2-3. Peromyscus maniculatus (deer mice) were trapped and blood samples were taken for testing. The mouse carcasses will be tested by the California Department of Public Health (CDPH) and positive samples will be used for a study of the geographical distribution of different hantavirus strains in California.
Laboratory staff continued collecting ticks in the nymph life stage during the month of May. Collections in May occurred at Laurelwood Park in San Mateo, Wunderlich County Park in Woodside, Thornewood Open Space Preserve near Woodside, Coal Creek Open Space Preserve near Portola Valley and Water Dog Lake Park in Belmont. Ixodes pacificus nymphs are active in the spring, and more difficult to collect in abundant numbers than adult ticks because they are typically on fallen logs or in leaf litter, whereas adult ticks are easily found on the edges of vegetation along trails.
During May, the District received several calls and requests for service regarding rat mites. Rat mites are ectoparasites that live in rodent nests and feed on their blood. These mites require rat blood to survive, but if rats have been recently eradicated from a home or the infestation is large, they will also bite people out of desperation. The most common rat mite San Mateo County is Ornithonyssus bacoti, the tropical rate mite, but there are several other species.
As of April 27, 2018, there have been 46 dead birds reported in San Mateo County. Of those, 13 have been suitable for testing and two have tested positive (15%) for West Nile virus (WNV). San Mateo and Santa Clara counties are the only counties with positive WNV dead birds (4) thus far this year. No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.
The nymph stage is the second of three stages in a tick’s life cycle. It is thought that nymphs are the most dangerous stage of ticks because they are more likely to be carrying bacteria that causes a tick-borne disease such as Lyme, and because they are significantly smaller and harder to see than adult ticks. Tick nymphs are most active in the spring months in California and can be found on tree stumps, downed logs, rocks, and in leaf litter.
Saint Louis encephalitis (SLE) is a mosquito-borne disease that was historically present in California, but became very rare after the arrival of West Nile virus in 2003. However, in the last two years there have been an increasing number of detections in mosquitoes and chickens and human disease cases from southern California. Continued surveillance for this virus is important because it might be resurging.
During March, laboratory staff continued winter surveillance for adult Ixodes pacificus (Western black-legged ticks). Ticks are collected by dragging a 1 meter square piece of white flannel over the vegetation alongside trails. Laboratory staff have begun testing collected Ixodes pacificus ticks for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum. Parks that were surveyed in March were Año Nuevo and Memorial Park near Pescadero, and Big Canyon Park and Eaton Park in San Carlos.
West Nile virus season begins in California in mid-April. The District protects residents from this mosquito-borne disease with control that targets mosquitoes in the larval stage, mosquito population monitoring, disease surveillance and public education. The laboratory staff is prepared to conduct disease surveillance in three main ways:
District Laboratory Director Angie Nakano and vector ecologist Tara Roth attended an educational seminar on bed bugs presented by Clark Pest Control on Feb 6. Information was provided on proactive approaches to preventing the spread of bed bugs, prevailing trends in treatment options, and legal issues relating to bed bug infestations, especially in regards to landlord/tenant issues.
