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

1. VIRAL ENCEPHALITIS

An arbovirus (arthropod-borne virus) natural cycle includes periods in two hosts: a blood feeding arthropod and a population of wild vertebrates. The virus is picked up during a blood meal. It multiplies and eventually lodges in the arthropod's salivary glands, ready to be injected during any subsequent feeding. The arthropod can remain infective for life, without being harmed. There are no preventive vaccines for man, so adequate vector control is the only front-line defense against these viruses.

Public health workers in California keep watch for three especially dangerous arboviruses: Western Equine Encephalitis (WEE) and Saint Louis Encephalitis (SLE), and the rapidly spreading West Nile virus (WNV). Typical vertebrate hosts are wild birds, like sparrows, finches, jays, robins, doves and pigeons. Their seasonal north-south migrations mean that surveillance needs to be maintained throughout the state.

The most important insect vector in California is the encephalitis mosquito, Culex tarsalis, common in both Marin and Sonoma Counties. Other Culex species can also serve as vectors. Encephalitis outbreaks in wild vertebrates occur during the months when mosquito density is highest, and sometimes they develop very quickly. Nevertheless, the environmental factors (like seasonal weather patterns, temperature and rainfall) that might help predict increased human exposure and risk of infection still need to be studied.

Occasional human infections with WEE have occurred in both Marin and Sonoma, and a least 13 cases have been found in horses in Sonoma County. Man and horses are accidental, dead end hosts for the virus. Transmission does not occur directly from person to person, and is unlikely from man or horses to mosquitoes because virus levels in circulating blood remain low.

From 1945 to 1987, there were 1,307 human deaths in California due to arbovirus infection. About 3 in every 100 such infections prove fatal, while others cause long-term disability. But for every recognized case, many more go undiscovered. Surveys in areas like Kern County, where high frequency of exposure would be expected, turned up many residents with antibodies against WEE and SLE arboviruses, but with no apparent symptoms of disease.

When symptoms do appear they can be severe, including fever, headache, and central nervous system disease with encephalitis (inflammation of the brain). Horses have a much greater exposure to migratory birds and mosquitoes. Equine cases tend to be severe, with between 25% and 30% fatalities. Recent experience in Sonoma county suggests that imported Australian emus may be extremely susceptable to the virus.

Several California vector control districts and the State Department of Health Services put out sentinel chicken flocks each spring to alert them of arbovirus presence in wild birds and transmission by local mosquitoes. This is how they discovered that WEE was active in Sonoma County during 1992, 1993, 1994 and 1997, and in neighboring Napa and Solano counties in 1996.