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Arbovirus Disease -- United States, 1993

During 1993, health departments from 20 states reported 78 cases of arboviral encephalitis to CDC. Of these, 55 (71%) were California encephalitis (CE). This report summarizes information about arboviral encephalitis in the United States during 1993. California Encephalitis

During 1993, a total of 55 human CE cases were reported from 11 states, including Florida (one), Illinois (two), Indiana (two), Iowa (six), Minnesota (three), Mississippi (one fatal), Missouri (one fatal), North Carolina (three), Ohio (six), West Virginia (13), and Wisconsin (17). Patients ranged in age from 5 months to 22 years (mean: 7 years).

The Mississippi case occurred in a 5-year-old child from George County. Immunoglobulin M (IgM) antibody to several California (CAL) serogroup viruses was detected in serum and cerebrospinal fluid (CSF); neutralizing antibody against the same CAL serogroup viruses was detected in the serum but not in the CSF. The Mississippi State Health Department obtained blood samples from three family members and four neighbors of the child. Neutralizing antibody was detected in serum from four persons; in three persons, antibody titers to Jamestown Canyon virus were at least fourfold higher than other CAL serogroup viruses.

The Missouri case occurred in a 4-year-old resident of Stone County; La Crosse virus was recovered from brain tissue. The case in Florida was diagnosed by detection of IgM antibody titers to several CAL serogroup antigens in serum obtained 2 days after onset of illness and by positive IgM titers to multiple CAL group antigens in a convalescent-stage serum sample. Cases from other states were diagnosed as La Crosse virus infections. St. Louis Encephalitis

During 1993, a total of 18 human cases of St. Louis encephalitis (SLE) were reported from five states. Five cases were reported in the Texas gulf coast area in two foci: two cases occurred in contiguous Galveston and Brazoria counties, and three occurred in Nueces County. North of the coastal area, single cases were reported from Harris County (Houston) and Denton County, north of Dallas-Fort Worth. Patients ranged in age from 19 to 84 years (mean: 40 years).

In Florida, two cases were reported from Lee County and three from contiguous Collier County. Patients ranged in age from 21 to 54 years. Three cases were reported in California, including two from San Bernardino County and one from Imperial County. Patients ranged in age from 19 to 64 years. In Colorado, one case was reported in an 11-year-old child in Arapahoe County. In Illinois, one case (in a person aged 33 years) was reported from Whiteside County and one from Cook County (in a person aged 69 years). Eastern Equine Encephalomyelitis

During 1993, a total of five human cases of eastern equine encephalomyelitis (EEE) were reported from four states. Two cases -- including one fatal -- occurred in Michigan in persons aged 73 and 60 years. One case each was reported in George County, Mississippi (in a person aged 14 years), and Holmes County, Florida (in a person aged 23 years). A fatal case in Rhode Island occurred in a 14-year-old exposed during a recreational outing. Blood samples were obtained from 16 other participants in the outing; EEE IgM antibody was detected in one person who reported no recent illness. Western Equine Encephalomyelitis and EEE in Domestic Animals During 1993, 13 western equine encephalomyelitis (WEE) cases among horses were reported from 10 states: Arizona (three), Nebraska (two), and one each in California, Colorado, Idaho, North Dakota, Oklahoma, Oregon, Utah, and Wisconsin. WEE virus was isolated from a turkey in Merced County and from emus in Placer and Yuba counties, California, and from an emu in Arkansas.

A total of 88 EEE cases among horses were reported from 10 states: Florida (67 cases); South Carolina (four cases); three cases each from Georgia, Maryland, Michigan, and Virginia; Mississippi (two cases); and one case each from Indiana, Louisiana, and North Carolina. EEE was isolated from other species in Georgia (dog, chukar, quail, and emu), Maryland (pheasant), Mississippi (emu), North Carolina (emu), and South Carolina (emu). Enzootic Arbovirus Activity

During 1993, enzootic arbovirus activity was reported from 15 states, including CE in Arizona, New York, and Texas; SLE in Arizona, California, Texas, Illinois, and Ohio; EEE in Alabama, Delaware, Massachusetts, Maryland, New Jersey, New York, and Rhode Island; and WEE in Arizona, California, Colorado, South Dakota, and Utah. Documented arbovirus activity was high in the Sacramento Valley in California based on isolates of WEE virus from Culex tarsalis mosquitoes and serologic conversions in sentinel chicken flocks; however, no cases were reported in humans or domestic animals.

Reported by: K Reilly, DVM, Veterinary Public Health, R Emmons, MD, Viral and Rickettsial Laboratory; GW Rutherford III, MD, State Epidemiologist, California Dept of Health Svcs. M Currier, MD, J Goddard, PhD, E White, FE Thompson, MD, State Epidemiologist, Mississippi State Dept of Health. HD Donnell, Jr, MD, State Epidemiologist, Missouri Dept of Health. A Gettman, PhD, Rhode Island Dept of Environmental Management; M Rittman, U Bandy, MD, BT Matyas, MD, State Epidemiologist, Rhode Island Dept of Health. D Alstad, DVM, National Veterinary Svcs Laboratory, Animal Plant Health Inspection Svc, US Dept of Agriculture, Ames, Iowa. Participating state epidemiologists, veterinarians, and vector-control coordinators. Arbovirus Diseases Br and Medical Entomology/Ecology Br, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: In 1993, 19 states conducted arbovirus surveillance using virus isolation or antigen detection in mosquitoes and/or serologic assays in wild or sentinel birds. In collaboration with CDC, in 1993, six additional states initiated surveillance in response to the potential for increased mosquitoborne disease associated with flooding in the midwestern United States. These surveillance programs provide current information on arbovirus transmission and assist in characterizing levels of enzootic/epizootic activity associated with the risk of human disease.

The CE case in southeastern Mississippi in 1993 was the first reported from the state since 1967. In Missouri, where the other fatal CE case occurred, 12 CE cases have been reported since data collection began in 1963. The CE case in Florida was the first reported from that state since 1963 (1). Previous serosurveys have documented widespread infection with CAL serogroup viruses in the United States. However, because of the limited availability of serologic testing for infection with multiple CAL serogroup viruses, illness associated with CAL serogroup viruses may be underdiagnosed.

Although the pathogenic arboviruses in the continental United States usually cause sporadic disease in humans and/or domestic animals, they can cause severe epidemics and epizootics. Surveillance and identification of early seasonal enzootic transmission is important in detecting and controlling arbovirus outbreaks and reducing the risk for human disease through vector control and modification of human activity patterns.

Health-care providers should consider arboviruses in the differential diagnosis of all cases of aseptic meningitis and viral encephalitis, obtain appropriate specimens for serologic testing, and promptly report cases to state health departments. New rapid diagnostic assays that detect both virus-infected mosquitoes and human antiviral IgM antibodies have facilitated confirmation of virus activity.


  1. Quick DT, Smith AG, Lewis AG, Sather GE, Hammon WM. California encephalitis virus infection: a case report. Am J Trop Med Hyg 1965;14:456.

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