Mosquito-Transmitted Encephaliti-ti-ti-des

A confirmed case of the uncommon LaCrosse encephalitis was reported recently in northern Virginia—a notable story (at least for the local news source), because the disease "typically occurs" further south in the state. The Virginia report and yesterday's news of several cases of West Nile disease in Los Angeles provide an opportunity to review here the mosquito-transmitted encephalitides that occur in the not-always-good-old summertime.

Causing about 70 cases of encephalitis per year in the United States, the LaCrosse virus belongs to the Bunyaviridae family of negative-stranded RNA viruses. The main transmission cycle is between small vertebrate hosts, like squirrels or chipmunks, and the treehole mosquito. LaCrosse encephalitis in humans occurs mainly in Appalachia and the Midwest (the disease was first recognized in 1963 in La Crosse, WI). Although most cases of infection are asymptomatic, the latency from mosquito bite to symptoms (either a flu-like illness or, less commonly, frank encephalitis) ranges from 5 to 15 days. Those who are especially susceptible to disease in endemic areas are persons engaged in outdoor activities and children younger than 16 years of age. The fatality rate, however, is less than 1%.

Both the West Nile and St. Louis viruses belong to the family Flaviviridae, genus Flavivirus—positive-stranded RNA viruses. The transmission cycle for these viruses, like that for most arboviruses, is between wild birds and mosquitoes. In 2003, there were more than 9800 cases of documented West Nile encephalitis in the United States, and the average annual number of cases of St. Louis encephalitis is approximately 110. West Nile encephalitis has spread rapidly throughout the United States, since its recognition in New York in 1999. St. Louis encephalitis occurs most often in the central and eastern parts of the country. The latency from bite to symptoms for both infections ranges from 3 to 15 days; although, the overwhelming majority of infected persons remain asymptomatic. For the less-than-1% of those who do develop encephalitis, the fatality rate is 50% for West Nile (2002-2003 data) and 5%-30% for St. Louis disease. Older individuals are particularly vulnerable.

The Eastern and Western Equine Encephalitis viruses belong to the family Togaviridae, genus Alphavirus— positive single-stranded RNA viruses. The main transmission cycle for these viruses is also between wild birds and mosquitoes, and, as the name implies, horses are common "dead-end" hosts. Infection with the Eastern Equine Encephalitis virus (EEEV) is most common in the eastern part of the United States, with the largest number of encephalitis cases in Florida, Georgia, Massachusetts, and New Jersey. Infection with the Western Equine Encephalitis virus (WEEV) is most prevalent in the plains regions. The average annual number of EEEV and WEEV encephalitis cases in the United States is 5 and 15, respectively. Like persons infected other arboviruses, those with EEEV and WEEV are mostly likely to remain asymptomatic. Symptoms, if manifest, occur approximately 3-10 days after a bite from a transmitting mosquito. Persons older than 50 years of age or younger than 15 years are at greatest risk for developing severe infection with either virus; although, the fatality rate with EEEV (33%) among symptomatic persons is much higher than that with WEEV (3%).

The most common and reportedly efficient way to diagnose infection with these viruses is to assay for neutralizing IgM antibodies in either serum or CSF. Care is purely supportive for symptomatic individuals. There are no vaccines to prevent human disease; although vaccines to prevent EEEV and WEEV in horses are evidently available.

Primary source: CDC

Photo of Culex quinquefasciatus, known to transmit St. Louis encephalitis virus, from Galveston County Mosquito Control.