26.3 Acellular diseases of the nervous system  (Page 2/13)

Western equine encephalitis (WEE) is caused by western equine encephalitis virus (WEEV). WEEV is usually transmitted to horses and humans by the Culex tarsalis mosquitoes and, in the past decade, has caused very few cases of encephalitis in humans in the United States. In humans, WEE symptoms are less severe than EEE and include fever, chills, and vomiting, with a mortality rate of 3–4%. Like EEEV, birds are the natural reservoir for WEEV. Periodically, for indeterminate reasons, epidemics in human cases have occurred in North America in the past. The largest on record was in 1941, with more than 3400 cases. US Centers for Disease Control and Prevention, “Western Equine Encephalitis—United States and Canada, 1987,” Morbidity and Mortality Weekly Report 36, no. 39 (1987): 655.

St. Louis encephalitis (SLE) , caused by St. Louis encephalitis virus (SLEV), is a rare form of encephalitis with symptoms occurring in fewer than 1% of infected patients. The natural reservoirs for SLEV are birds. SLEV is most often found in the Ohio-Mississippi River basin of the central United States and was named after a severe outbreak in Missouri in 1934. The worst outbreak of St. Louis encephalitis occurred in 1975, with over 2000 cases reported. US Centers for Disease Control and Prevention, “Saint Louis encephalitis, Epidemiology&Geographic Distribution,” Accessed June 30, 2016. http://www.cdc.gov/sle/technical/epi.html. Humans become infected when bitten by C. tarsalis , C. quinquefasciatus , or C. pipiens mosquitoes carrying SLEV. Most patients are asymptomatic, but in a small number of individuals, symptoms range from mild flu-like syndromes to fatal encephalitis. The overall mortality rate for symptomatic patients is 5–15%. US Centers for Disease Control and Prevention, “Saint Louis encephalitis, Symptoms and Treatment,” Accessed June 30, 2016. http://www.cdc.gov/sle/technical/symptoms.html.

Japanese encephalitis , caused by Japanese encephalitis virus (JEV), is the leading cause of vaccine-preventable encephalitis in humans and is endemic to some of the most populous countries in the world, including China, India, Japan, and all of Southeast Asia. JEV is transmitted to humans by Culex mosquitoes, usually the species C. tritaeniorhynchus . The biological reservoirs for JEV include pigs and wading birds. Most patients with JEV infections are asymptomatic, with symptoms occurring in fewer than 1% of infected individuals. However, about 25% of those who do develop encephalitis die, and among those who recover, 30–50% have psychiatric, neurologic, or cognitive impairment. US Centers for Disease Control and Prevention, “Japanese Encephalitis, Symptoms and Treatment,” Accessed June 30, 2016. http://www.cdc.gov/japaneseencephalitis/symptoms/index.html. Fortunately, there is an effective vaccine that can prevent infection with JEV. The CDC recommends this vaccine for travelers who expect to spend more than one month in endemic areas.