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.
During February, 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. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum.
The biannual Vertebrate Pest Control Conference was held February 26 – March 1 in Rohnert Park. This conference, hosted by the University of California Division of Agriculture and Natural Resources, covers a variety of topics in pest control, such as commensal and field rodents, vertebrate control materials, bird management and wildlife disease. The first day of the conference was a field trip focusing on human-wildlife conflicts in Sonoma County. Most of the District staff had the opportunity to attend at least part of the conference.
On January 29, Vector Ecologist Dr. Tara Roth gave a presentation at the annual conference of the Mosquito and Vector Control Association of California in Monterey, CA. Tara received her PhD in August of 2016 from University of California, Davis, where she researched the vector-borne disease tularemia. Surveillance for tularemia in San Mateo County will be beginning soon through cooperation with the California Department of Public Health.
During January, 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. Ixodes pacificus ticks will be tested for the presence of bacteria Borrelia burgdorferi, Borrelia miyamotoi and Anaplasma phagocytophilum. Parks that were surveyed in January were Thornewood Open Space Preserve near Portola Valley and Memorial County Park near Pescadero.
Winter is adult Ixodes pacificus (western black-legged tick) season in California. Laboratory staff have begun surveillance of this tick and the disease-causing bacteria they carry in parks and open space throughout San Mateo County. Ticks are collected by dragging a 1 meter square white flannel flag over the vegetation alongside trails.
During the cooler months, when there are fewer mosquitoes flying around and not as many service requests, District staff take the opportunity to plan and prepare for the upcoming West Nile virus season. One change this year will be a discontinuation of the sentinel chicken coop at the Jasper Ridge Biological Preserve in Woodside. After looking at West Nile activity throughout San Mateo over the last decade, District staff decided that the Jasper Ridge coop location was not the best use of surveillance resources.
On December 2, San Mateo Mosquito and Vector Control District laboratory staff helped lead a field trip at Wunderlich County Park for Stanford students learning about tick borne diseases. The class was entitled Parasites and Pestilence and was led by professor and District Trustee Dr. Scott Smith. Students were given tick flags and instructed on basic tick biology as well as how to collect ticks and sort by sex and species.
The West Nile Virus season has wrapped up for the year, with the final chicken bleeding on October 30. Although one bird tested positive in January, the District did not detect any evidence of West Nile Virus (WNV), Saint Louis encephalitis, or Western Equine encephalitis in San Mateo County during the active summer season. The surveillance efforts are summarized below:
This summer the laboratory has been assisted by Enger German, who conducted most of the surveillance for invasive Aedes mosquitoes this season. Enger has a master’s degree in Entomology, and experience in insect monitoring, especially in Florida. The District hired Enger for the season using grant funding from Public Health Foundations Enterprises, Inc. on behalf of the California Department of Public Health.