Mosquito Transmitted Disease Surveillance Program

Background
Mosquito-borne diseases that are of concern in Ventura County are St. Louis Encephalitis (SLE), Western Equine Encephalitis (WEE), West Nile Virus (WNV) and Malaria. Encephalitis is a viral disease that affects the central nervous system and causes an inflammation of the brain. The different encephalitis strains are caused by different viruses but are all transmitted by the bite of a mosquito. Malaria is caused by the organism, Plasmodia, and is characterized by periodic attacks of chills and fever, and can persist in individuals without treatment. Malaria is also transmitted by the bite of a mosquito.

The Ventura County Arboviral Surveillance Program was initiated in 1985 after an unprecedented 27 cases of SLE occurred in four Southern California counties during 1984. An extensive survey performed in Ventura County showed that all encephalitis vector mosquitoes, especially Culex tarsalis, were found Countywide but no evidence of SLE or WEE was documented at that time. Information from this survey also provided locations and population densities of Anopheles mosquitoes which are the vector for malaria. Click here for a list of the mosquito species found in Ventura County, their population density, and the diseases which can be transmitted by each species. If you would like to see a map of the locations of Malaria Mosquito (Anopheles) sources click map on right.

Based on this information, the Board of Supervisors established an Arboviral Surveillance and Countywide mosquito abatement program which has continued to the present time. For more information on mosquito control, click here.

Encephalitis Surveillance
The primary objective of the Arboviral Surveillance Program is to prevent the transmission of encephalitis virus to humans. The surveillance program in Ventura County is multifaceted and includes:

1. Mosquito population and species monitoring through the placement of 19 mosquito light traps in representative areas of the County.
2. Adult mosquito monitoring and testing for SLE, WNV and WEE virus.
3. Serological monitoring of sentinel chicken flocks in four representative areas of the County.
4. Participation in the California Department of Health Service's wild bird surveillance program.

Dead birds are collected and submitted for testing for WNV. To report a dead bird, call toll free: 1-877-WNV-BIRD. To report a dead bird using an online form, click here. If you would like to see a map of mosquito light trap and chicken flock locations, click map on right.

In 1992, 2 chickens from the Hill Canyon, Thousand Oaks sentinel flock tested positive for SLE. At this same time, the only known confirmed human case of SLE in Ventura County was diagnosed in a woman who lived in Oxnard. As of March 3, 2004, adult mosquitoes have tested positive for WNV. For more information on WNV, click here.
The following actions will be taken in response to a positive finding of encephalitis virus:

1. Notify the County Health Officer.
2. Notify all local and State agencies that could be involved in any follow up effort.
3. Notify the media via a press release.
4. Check mosquito light trap data from the area to determine population density, and species of mosquitoes.
5. Set out live traps for mosquitoes.
6. Survey all mosquito breeding sources in the area.
7. Investigate area for unknown sources of mosquito breeding.
8. Depending on these findings, send adult mosquitoes collected in the area to the State for virus testing.

Mosquito Species Of Ventura County

Population Density*

* DEFINITIONS:

                    High - This species is found throughout Ventura County,

sometimes in high numbers.

                    Medium - This species is only found in specific areas.

                    Rare - This species is seldom found in Ventura County.