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Scientific-Technical Services

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The Scientific-Technical Services staff is part of a comprehensive approach to protecting public health. Their surveillance program utilizes a variety of field and laboratory techniques to monitor mosquito-borne diseases, as well as population densities of mosquitoesblack flies and midges.

Disease surveillance efforts detect West Nile virus (WNV), St. Louis encephalitis (SLE) and Western Equine encephalomyelitis (WEE) in mosquitoes, as well as antibodies for infections in wild birds and sentinel chickens. Mosquito and sentinel chicken blood samples are shipped to the UC Davis Center for Vectorborne Diseases (CVEC) for analysis.

The department is headed by a Scientific-Technical Services Director and staffed by three full-time Vector Ecologists, three Assistant Vector Ecologist and two Field Assistants.

Disease Surveillance

The disease surveillance program serves as an early warning system in the detection of mosquito-borne viruses that can infect people and animals. Monitoring diseases allows the District to be more efficient and timely conducting control efforts and outreach to affected communities.

The District’s response level is based on a set of guidelines provided by the California Department of Public Health (PDF), reflecting the level virus transmission risk to humans.

Disease Surveillance Testing

Testing for the presence of virus/pathogens in mosquitoes, sentinel chickens, and dead birds helps identify the disease transmission risk before human cases occur.

Mosquito Testing

  • Disease surveillance is conducted to detect West Nile virus (WNV), St. Louis encephalitis (SLE) and Western equine encephalomyelitis (WEE) within District boundaries. Initial surveillance begins by setting mosquito traps to collect adult mosquitoes. Only females are submitted for virus testing since they require a blood meal to produce eggs, making them an integral part of the disease transmission cycle. Ten to 50 live anesthetized female mosquitoes are deposited into vials to comprise a mosquito “pool” or sample. These “pools” are stored at minus 70 degrees Celsius and shipped to the UC Davis Center for Vectorborne Diseases (CVEC) for testing.

Sentinel Chicken Testing

  • Due to a regional quarantine on chickens, our sentinel chicken program is suspended until further notice. 
  • Vector Ecologists maintain seven chicken flocks throughout the District. The sentinel chicken flocks serve as confirmation of active virus transmission to people and animals. Blood samples are taken bi-weekly from the sentinel chickens and analyzed by the state health department’s Viral and Rickettsial Disease Lab to determine the presence of antibodies. The presence of antibodies indicate that mosquitoes in the area have been transmitting viruses that impact public health.
  • Sentinel chickens provide the early warning system that allows the District to effectively communicate potential threats to the public. It also facilitates an unbiased assessment of the severity of a disease outbreak due to the very consistent yearly efforts.

Wild Bird Testing

  • Eight wild bird traps are maintained within District boundaries. Many wild bird species, such as sparrows and finches, are reservoir hosts for the same mosquito-borne viruses that can infect people. For this reason, they play an important role in the District’s disease surveillance program.
  • Blood samples from captured free-ranging wild birds are taken bi-weekly to obtain sera samples and analyzed by the UC Davis Center for Vectorborne Diseases (CVEC) for the presence of WNV, SLE and WEE antibodies. The amount of antibodies in birds is important. The more antibodies present in birds, the less likely the diseases can spread. This is because their immune systems is more likely to fight off an infection. Birds are captured, bled and release unharmed.

 Dead Bird Surveillance Program

  • Wild birds are the animals from which mosquitoes primarily acquire West Nile virus (WNV). Although many birds that are infected with West Nile Virus will not appear ill, WNV can cause serious illness and death in some bird species. The most severe illnesses are seen among the corvids (including crows, jays, ravens, and magpies). Dead American crows are visible due to their size, which makes them the most commonly reported bird infected with WNV.
  • If you find a bird that has recently died, particularly a crow or other corvid, please call the California Department of Health Services hotline at 1-877-WNV-BIRD. 
  • Not all birds will qualify for testing; however, all reports of dead birds will be recorded. To move or discard a carcass, wear rubber or latex gloves, or simply use a plastic bag turned inside-out over your hand and invert the bag to surround the bird. Seal and discard in the trash, if the bird will not be picked-up for testing.

Surveillance Tools

EVS/CO2 Baited Traps

  • The encephalitis vector surveillance (EVS) traps use dry ice (CO2) as a bait to attract host-seeking female mosquitoes. CO2 attracts a wide variety of mosquito species and EVS traps are used mainly to determine the presence of species variety and abundance.
  • Since these mosquitoes are seeking a blood meal they are less likely to be infected than the mosquitoes seeking a place to deposit eggs (see Reiter-Gravid Trap). During mosquito season, traps are set overnight at fixed locations throughout the District to capture mosquitoes.

Reiter Traps

  • The Reiter-Gravid trap uses a hay and yeast infusion to attract gravid (pregnant) mosquitoes, which have already obtained a blood meal. This makes the Reiter-Gravid trap an important tool for collecting mosquitoes for virus testing because the blood-fed mosquitoes are more likely to contain virus.
  • Unfortunately, due to the very smelly nature of the yeast infusion, almost exclusively Culex quinquefasciatus (southern house mosquito) are attracted to these traps. The southern house mosquito plays an important role in the transmission of WNV to the human population. Like EVS traps (above), these traps are placed late in the afternoon and collected the following morning.


  • Ovitraps are set throughout the District’s jurisdiction to monitor for populations of invasive day-biting mosquitoes. The ovitrap is designed and used to target mosquitoes that prefer to lay their eggs on the interior walls of containers, such as Aedes albopictus (Asian tiger mosquito) and Aedes aegypti (yellow fever mosquito).
  • The ovitrap simulates a container, such as a plant saucer or bucket. It has a special cardboard-lined wall which stimulates egg-laying and is filled with water. A mosquito attracted to this trap will lay her eggs on the artificial lining of the cup. A vector ecologist will retrieve the lining and inspect the material for eggs.