The virus that causes western equine encephalitis has a complex life cycle involving birds and a specific type of mosquito, Culex tarsalis , that is common in farming areas and around irrigated fields. Humans, horses, and other mammals are not an important part of the life cycle of the virus. In rare cases, however, people who live in or visit an area where the virus lives can be infected by the bite of an infected mosquito. Horses are common in these regions and can also be infected. After infection, the virus invades the central nervous system, including the spinal cord and brain.
Infection can cause a range of illnesses, from no symptoms to fatal disease.
People with mild illness often have only a headache and sometimes fever. People with more severe disease can have sudden high fever, headache, drowsiness, irritability, nausea, and vomiting, followed by confusion, weakness, and coma. Young infants often suffer seizures. Symptoms usually appear in 5 to 10 days after the bite of an infected mosquito. Diagnosis is based on tests of blood or spinal fluid.