Arboviral encephalitis

Alan C Jackson MD (

Dr. Jackson of Lake of the Woods District Hospital has no relevant financial relationships to disclose.

John E Greenlee MD, editor. (Dr. Greenlee of the University of Utah School of Medicine has no relevant financial relationships to disclose.)
Originally released May 26, 1994; last updated July 27, 2020; expires July 27, 2023


Arboviruses are members of a group of animal viruses that multiply in an arthropod (mosquitoes, ticks, and sandflies) and are transmitted to a vertebrate host, including humans. Some arboviruses may cause encephalomyelitis in a minority of infected humans, whereas a mild systemic illness associated with fever is a more common manifestation of infection. West Nile virus infection is currently the largest problem in the United States. In this update, the author reviews the clinical features, pathogenesis, epidemiology, diagnosis, and management of arboviral encephalitis.

Key points


• Most infections caused by arboviruses are asymptomatic or only cause mild systemic disease.


• Vaccines are available for the prevention of some of the arboviruses that cause encephalitis; otherwise, preventative measures should be focused on prevention of mosquito and tick bites.


• Laboratory diagnosis of arboviral encephalitis is based on antibody testing on sera and CSF, and on detection of nucleic acids in CSF.


• Therapy of arboviral encephalitis is symptomatic except in clinical trials.

Historical note and terminology

Arboviruses (an acronym for arthropod-borne viruses) are members of a group of animal viruses that multiply in an arthropod and are transmitted to a vertebrate host (Centers for Disease Control and Prevention 1985). In the 1950s the Rockefeller Foundation established a network of laboratories around the world, which facilitated the identification of arboviral diseases. Some of the arboviruses were given names of the places that they were first isolated (eg, St. Louis encephalitis virus, Japanese encephalitis virus, and California encephalitis virus). These names do not imply that the infections are a problem in these locations at the present time. In fact, only a single case of encephalitis due to California encephalitis virus has been reported in California during the past 50 years (Eldridge et al 2001).

Many of the arboviruses were isolated in the 1930s, although their diseases may have been recognized much earlier. Immature rodents were commonly used for viral isolation. Later, cell culture was also used. Epizootics of eastern equine encephalitis in horses occurred as far back as 1831 (Hanson 1957). There were epidemics of Japanese encephalitis in Japan as far back as 1870, and an epidemic in 1924 caused 3797 deaths (Burke and Leake 1988). No epidemics have occurred in Japan since the 1960s. An epidemic of western equine encephalitis in 1941 resulted in more than 3300 human cases (Reisen and Monath 1989). In 1971, there was a large epidemic of Venezuelan equine encephalitis in Mexico and the United States (Ehrenkranz and Ventura 1974), and a large epidemic of St. Louis encephalitis occurred in the United States and Canada in 1975 (Luby 1979). The emergence and re-emergence of epidemic arboviral disease has been a public health problem in past years (Shope 1994) and will continue to be a problem in the future. The incursion of West Nile virus infection into North America in 1999 was an important event, and it is expected that variable numbers of human cases will occur on this continent for many decades or centuries in the future.

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