Laboratory Report - 2005
PHONE
800-231-3236 (toll free)
707-285-2200 (office)
707-285-2210 (fax)
ADDRESS
595 Helman Lane
Cotati, California
94931-9736
HOURS
Monday through Friday
7:00am to 3:30pm
Closed
11:30am - 12:30pm
Ron Keith, Asst. Manager, Vector Ecologist
The arrival of West Nile Virus increased the demands on our lab staff. In addition to identifying larval and adult mosquito samples and sentinel chicken surveillance, we tested substantial numbers of mosquito pools and dead birds, both in-house and in collaboration with the California disease surveillance program. The lab also collected ticks in China Camp and Annadel State Parks to test for Borrelia, the bacteria that cause Lyme disease.
We worked closely with the field staff, concentrating our trapping efforts in areas of possible high mosquito production, like the Laguna de Santa Rosa, the weed-overgrown flood control channels in Rohnert Park and Cotati, Remillard Park near Larkspur (a restored cattail wetland) and selected locations in newly annexed regions of Marin and Sonoma counties.
Around mid-summer, we were pleased to launch our RT-OCR (Real Time - Polymerase Chain Reaction) program, in collaboration with Dr. Judy Sakanari of Sonoma State University. We are using this "gold-standard" of diagnostic technology to test dead birds and mosquito pools for West Nile virus. The method seems to give quick, reliable results in bird species like corvids and raptors that respond to infection by generating very high levels of virus and antibodies, but it appears less helpful for most other birds that die before producing high virus or antibody levels.
The lab processed 705 dead birds, with 92 proving positive for West Nile virus. Since we know that the virus is widely spread in both Sonoma and Marin counties, dead bird testing will be done on a more targeted basis in 2006.
In the future, we will survey for WNV much like we do for Western Equine and St. Louis encephalitis viruses. Both are endemic and can reoccur under the right environmental conditions.
We processed 954 mosquito pools (groups of 50 or fewer) in an attempt to locate live West Nile virus, but there were no positive returns. We also submitted 1,232 blood samples from our 77 sentinel chickens to state labs for testing, again with no positives.
Two West Nile disease cases in mammals included a man from Petaluma who was most likely infected in the Sacramento area where he had traveled prior to becoming symptomatic, and a horse from Sonoma that had been held in quarantine in Davis before being transported to Sonoma.
We continue a broad survey to identify important mosquito breeding sites and the most common mosquito species in the newly annexed areas of Sonoma and Marin counties. Laboratory staff took part in the District’s outreach programs to help inform the newly served communities about the need to recognize and report mosquito producing sources so the options for control can be assessed.
The District has held firm on continued surveillance and control of known vectors in situations where disease transmission to the public could easily occur, specifically Culex erythrothorax, C. pipiens and C. tarsalis, the primary carriers of encephalitis viruses. Culex stigmatosoma was similarly targeted; it is an exceptionally efficient vector of WNV in bird populations and widely distributed in both counties.
Mosquito population surveillance (by Piper Kimball). During 2005, the Laguna de Santa Rosa’s main channel and the adjacent flood control channels were subject to heightened surveillance. Both had become massively overgrown with invasive Ludwigia (water primrose), making normal larval control difficult or impossible. Baited traps set along the main channel collected Culex pipiens (29%) C. tarsalis (25%) and C. stigmatosoma (14%), all three efficient encephalitis virus vectors. Culex pipiens in particular was much more common than ever before.
Traps set along the overgrown flood control channels captured much higher concentrations of C. pipiens, C. tarsalis and C. stigmatosoma than in previous years. Because of the dense water primrose growth, difficulty of aerial larvicide application, and high concentration of adult mosquitoes, the District was forced to use adulticide applications near the flood channels to help lower the numbers of adult mosquitoes.
Arbovirus Surveillance and Testing (by Sarah Klobas). 1. Dead birds. WNV surveillance procedures during 2005 included submitting 569 dead birds to the California Animal Health and Food Safety Laboratory at UC Davis. Some birds were also tested in-house using the RAMP system, designed to look for evidence of infection in corvids (crows and jays). Thirty-one of the 200 corvids tested positive using the District’s RAMP setup, compared to 48 using the UC Davis RT-PCR system, suggesting an in-house RAMP reliability of 65%.
Of the 569 birds tested by Real Time-PCR at UC Davis, 92 were positive for West Nile virus. Fourteen of these positive birds were from Marin County and 78 from Sonoma County. Ninety-four species of birds were processed at the District during 2005, with 33 different species testing positive for West Nile virus.
The District is now collaborating with Dr. Sakanari’s laboratory at Sonoma State University using the RT-PCR system. The method has reliably identified WNV infections in corvids, but it is less sensitive for non-corvid species.
2. Sentinel Chickens. As in previous years, the District maintained seven chicken flocks of 11 birds each, to help rapidly detect local transmission of West Nile virus, Western Equine (WEE) and St. Louis encephalitis virus (SLE). Three flocks were located in Marin County and four in Sonoma County. No virus infections were detected in the sentinel chickens during 2005.
3. Mosquito Pools. A total of 954 pools (316 from Marin County and 638 from Sonoma County) were tested for WNV using the in-house RAMP system. No positive mosquito pools were found in either county during 2005.
Mosquitofish Program (by Teresa Thomas). The District supplies mosquitofish to help reduce mosquito larvae production in ponds and other enclosed breeding sites. Most mosquitofish are distributed between March and November, with peak distributions usually in May and June.
In 2005, the fish were distributed in three ways: residents picked them up at the District office, field technicians delivered them to residents, or they were delivered and maintained for resident pickup at specified nurseries in both counties. Since 2002, the Nursery Program has been supplemental, and has lowered the amount of fish-related service requests for the technicians. This, in turn, allowed the technicians more time for surveillance and control.
During 2005, the District distributed 127 pounds of fish, with 55% going to the nurseries.The technicians, however, delivered 27% more than the year before, and they received their fish earlier than the nurseries. This was part of an effort to help the District thoroughly inspect the newly annexed areas for mosquito breeding sources. Many of the fish-related service requests were assigned to the technicians instead of referring the residents to a participating nursery. This gave the technicians an opportunity to look for additional breeding habitats in the area. To help cover the added work load, the District hired five more technicians to conduct field surveillance and distribute mosquitofish.
There has been a substantial increase in fish requests from the public in recent years. This is probably due to the heightened awareness of West Nile virus, plus the addition of approximately 1,200 new square miles of service area.
Tick Collection and Tick-borne Disease Surveillance (by Kristen Holt). The western deer tick, Ixodes pacificus, is the most important carrier of Lyme disease and other tick-borne disease agents in northern California. During 2005, we collected a total of 577 deer ticks from China Camp State Park near San Rafael, to test for the Lyme disease agent. Twenty ticks (3.5%) were carrying the Borrelia bacterium. The rate was higher in female (4.9%) than male ticks.
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