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Arboviral Diseases -- United States, 1992

During 1992, health departments from 23 states reported to CDC 45 cases of arboviral encephalitis in humans and 97 in horses. An additional four states reported detection of arboviral activity in bird and mosquito populations. Unlike 1990 and 1991, when three St. Louis encephalitis (SLE) epidemics and an eastern equine encephalitis (EEE) epizootic occurred, during 1992, no focal outbreaks of arboviral disease were reported. This report summarizes information regarding arboviral encephalitis in the United States during 1992.

SLE. During 1992, 14 sporadic SLE cases occurred in Texas (12 cases) and California (two) (1) -- a substantial decrease from 1990 and 1991 (247 and 78, respectively), when SLE cases were at their highest level since 1976.

LaCrosse encephalitis (LAC). During 1992, 29 cases of LAC encephalitis were reported from Illinois (seven cases), Ohio (six), West Virginia (six), Wisconsin (four), Minnesota (three), and North Carolina (three). This is the lowest number of LAC cases reported since surveillance began in 1964.

EEE and Western equine encephalitis (WEE). During 1992, Florida and Massachusetts each reported one case of EEE. Because of isolation of EEE virus from Aedes albopictus during 1991 in Florida, human case surveillance was intensified at five regional medical centers. From May through September 1992, 357 cerebrospinal fluid samples were collected from persons with symptoms suggestive of meningitis or encephalitis. None had EEE-specific immunoglobin M antibody. In 1992, 88 cases of EEE in horses were reported from Florida (54 cases), Georgia (nine), Virginia (nine), Mississippi (four), South Carolina (four), North Carolina (three), Texas (two), Arkansas (one), Kentucky (one), and Michigan (one). Although no cases of WEE were reported in humans, nine cases of WEE in horses were reported during 1992: Idaho (two cases), Missouri (two), Oklahoma (two), Colorado (one), South Dakota (one), and Utah (one).

Enzootic arbovirus activity. In 1992, 28 states conducted arboviral surveillance using virus isolation or antigen detection in captured mosquitoes or viral-specific antibody assays in sentinel or wild birds. Enzootic arboviral activity was reported from 16 states: EEE (Delaware, Florida, Georgia, Massachusetts, Michigan, New Jersey, North Carolina, Ohio, and South Carolina), SLE (Arizona, California, Illinois, Michigan, and Texas), WEE (Arizona, California, Colorado, Nevada, and Utah), and LAC (Illinois).

Reported by: WG Hlady, MD, Florida Dept of Health and Rehabilitative Svcs. Participating state epidemiologists, veterinarians, and vector-control coordinators. Arbovirus Diseases Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: An increased number of EEE cases had been anticipated in 1992 for two reasons: 1) in 1991, EEE virus had been isolated from Aedes albopictus (2), a more anthropophilic mosquito vector; and 2) in 1991, an EEE epizootic occurred in the Southeast (1). Although arboviral infections are often underreported, the results of intensified surveillance in Florida suggest that human EEE infection did not increase in 1992.

The last nationwide arboviral epidemic (1975 and 1976) resulted in 2194 cases of SLE in 35 states and was preceded by a modest increase in human SLE cases in 1974. Because early recognition of arboviral activity allows for early institution of preventive measures, surveillance of virus activity in mosquito, avian, equine, and human populations has been emphasized.

During 1990 and 1991, moderate increases in arboviral encephalitis cases were noted with outbreaks in Arkansas, Florida, and Texas (3,4). Despite changes in the arboviral surveillance system to encourage a greater number of states to report regularly, only 45 cases of human arboviral encephalitis were reported -- the lowest number of cases reported since the early 1960s. Most arboviral encephalitis cases were reported from midwestern states. Serosurveys indicate that arboviral infections have a wide geographic distribution in the United States, and that cases are often underreported.

Because early identification of cases is important in reducing the risk for arboviral disease through vector-control practices and changes in human activity patterns, health-care providers should consider arboviruses in the differential diagnosis of viral meningoencephalitis, obtain appropriate specimens for serologic testing, and promptly report cases to state health departments.


  1. CDC. Eastern equine encephalitis -- Florida, eastern United States, 1991. MMWR 1991;40:533-5.

  2. Mitchell CJ, Niebylski ML, Smith GC, et al. Isolation of eastern equine encephalitis virus from Aedes albopictus in Florida. Science 1992; 257:526-7.

  3. CDC. Update: St. Louis encephalitis -- Florida and Texas, 1990. MMWR 1990;39:756-9.

  4. CDC. St. Louis encephalitis outbreak -- Arkansas, 1991. MMWR 1991;40:605-7.

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