Mosquito Borne Disease
WEE & SLE Encephalitis


Prior to the introduction of West Nile virus, Western Equine Encephalitis (WEE) was most prominent in San Joaquin County and still is considered endemic to the area. In addition, St. Louis Encephalitis (SLE) is also of public health significance. Below is general information on WEE; information on SLE can be found at the CDC link.

* "The clinical term “encephalitis” means inflammation of the brain. When the spinal cord also is involved, the correct term is “encephalomyelitis”. Encephalitis may be caused by a variety of disease agents including fungi, bacteria and viruses. When a central nervous system (CNS) disease cannot be diagnosed by culturing or directly examining spinal fluid, the condition is called “aseptic” and is usually of viral origin. Some of the aseptic CNS cases are caused by viruses that are transmitted biologically by mosquitoes. These arthropod-borne viruses are called “arboviruses”. As in most viral infections, such as the common cold, there currently is no known cure for arboviral infections and health workers can provide only supportive care. In humans, clinical manifestations of arboviral infections vary greatly. Most infected persons show no disease symptoms. However, severe cases can exhibit fever, headache, malaise, convulsions and coma. Death can occur rapidly in two to four days or after prolonged illness. Recovery can be spontaneous and complete or the patient may exhibit sequelae including mental retardation, seizures or spasticity.

In 1930, a major outbreak of encephalomyelitis occurred in horses in the San Joaquin Valley. The disease disappeared in the fall, but reoccurred during the summer of 1931. In subsequent years, the disease was detected during the summer months throughout California. In 1930, researchers from the University of California isolated the pathogen from the brain of a horse. The virus and its disease subsequently was named “western equine encephalomyelitis”, abbreviated WEE. By 1938, a vaccine had been developed to protect horses from the disease. The first isolation of WEE virus from a human was reported in 1938 from a fatal case involving a child.

During the period of 1945 through 1987, 751 cases of WEE were reported in California with half occurring during the epidemic year of 1952. Most of the 1952 cases were from the San Joaquin Valley coincidental with extraordinary flooding along the Kern River and the Tehachapi earthquake. Both catastrophic events left many residents homeless and exposed to mosquito bites. Human cases rarely have been detected in recent years, although WEE virus continues to be transmitted actively among mosquitoes and birds.

WEE virus primarily is a health problem in rural agricultural areas and small towns. In spring and early summer, WEE virus is amplified among passeriform (perching) birds including red wing and tricolor blackbirds which inhabit riparian (river) and marsh habitats. The principal vector, Cx. tarsalis feeds almost entirely on birds during this period. When weather conditions favor an increase in Cx. tarsalis abundance, WEE virus may become amplified sufficiently to spread to peridomestic environments where house sparrows and house finches, as well as galliforms (mostly chickens), become involved. During mid to late summer, Cx. tarsalis expands its host range and blood feeds more frequently on mammals, including rabbits, equines and humans. Rabbits and some equines develop sufficiently elevated viremia (quantity of virus in peripheral blood) to infect Ae. melanimon mosquitoes which feed mostly on mammals and are more susceptible to infection than Cx. tarsalis.

A secondary cycle involving Ae. melanimon and rabbits develops in the Central Valley where Ae. melanimon may be responsible for some of the transmission of WEE virus to humans and equines. Recent evidence indicates that WEE virus also may be transmitted vertically within Ae. dorsalis populations on coastal marshes, but the significance of these findings will require further research."

* From: The Biology and Control of Mosquitoes in California. 1996. Mosquito and Vector Control Association of California. Elk Grove, CA., CDPH