Skip Navigation LinksSkip Navigation Links
Centers for Disease Control and Prevention
Safer Healthier People
Blue White
Blue White
bottom curve
CDC Home Search Health Topics A-Z spacer spacer
Blue curve MMWR spacer

Arboviral Infections of the Central Nervous System -- United States, 1996-1997

Arboviruses include mosquitoborne and tickborne agents that persist in nature in complex cycles involving birds or mammals, including humans. Arboviral infection can cause fever, headache, meningitis, encephalitis, and sometimes death. During 1996-1997, health departments in 19 states reported to CDC 286 confirmed or probable * cases (eight fatal) of arboviral encephalitis in humans (132 cases in 1996 and 154 provisionally in 1997). Surveillance programs in 18 states detected enzootic arboviral activity in mosquito or sentinel or wild bird populations, and cases of arboviral disease were recognized among horses or emus in 24 states. This report summarizes information about arboviral infection of the central nervous system in the United States during 1996-1997.

La Crosse Encephalitis

During 1996-1997, a total of 252 La Crosse encephalitis (LAC) cases (103 confirmed and 149 probable; one fatal) were reported from 12 states. Patients ranged in age from 5 months to 78 years (mean: 9 years), and 95% of cases occurred in persons aged less than 18 years; 153 (61%) cases occurred in males, 209 (83%) in whites, and seven (3%) in persons of races other than white; in 36 (14%) cases, race was unspecified. Dates of onset of illness ranged from late June to early November. West Virginia reported 139 cases (55% of the national total), an average of 3.8 per 100,000 population per year (Table_1). Among persons aged less than 18 years, who accounted for 133 (96%) of the total number of cases in West Virginia, the incidence was 15.8 per year. A fatal case occurred in a 19-month-old child in Minnesota who became ill in early August 1997 and died in November.

St. Louis Encephalitis

During 1996-1997, a total of 15 St. Louis encephalitis (SLE) cases (14 confirmed and one probable; two fatal) were reported from six states (Table_1). Patients ranged in age from 6 months to 83 years (mean: 54 years). Ten (67%) cases occurred in females; 12 (80%), in whites; and two (13%), in blacks; in one (7%) case, race was unspecified. Dates of onset of illness ranged from July 21 to late October. During 1997, Florida reported nine cases from seven central or southern counties. Enzootic SLE virus activity in sentinel chickens was detected several weeks before the first human case was diagnosed, prompting state and local public health authorities to issue public health alerts and intensify mosquito-control measures.

Eastern Equine Encephalomyelitis

During 1996-1997, a total of 19 eastern equine encephalomyelitis (EEE) cases (all confirmed; five fatal) were reported from eight states (Table_1). Patients ranged in age from 10 months to 81 years (mean: 35 years); 10 (53%) cases occurred in males; 15 (79%), in whites; and three (16%), in blacks; in one (5%) case, race was unspecified. In all but one case, dates of onset of illness ranged from early July to mid-November. The exception was a 58-year-old man from southwestern Alabama who became ill with EEE on January 8, 1996, and died in early February. The most likely location of the patient's exposure to EEE virus was in a neighboring county at a quail farm near a hardwood swamp.

Enzootic and Epizootic Arbovirus Activity

During 1996-1997, a total of 23 states conducted surveillance for SLE, EEE, and/or western equine encephalomyelitis (WEE) virus activity using virus isolation or antigen detection in captured mosquitoes, virus-specific antibody assays in sentinel or wild birds, or a combination of methods. Enzootic arboviral activity was reported from 18 states (Figure_1).

Although arboviral disease cases among horses or other animals are not officially reported to CDC, some state health departments attempt to track such cases because cases of EEE and WEE in horses may indicate incipient human cases. During 1996-1997, a total of 274 cases of arboviral encephalitis in horses (151 cases in 1996 and 123 in 1997) were reported to public health authorities in 21 states (Table_2). In addition, epizootics or sporadic clinical cases of hemorrhagic enterocolitis associated with infection with EEE virus (Alabama, Arkansas, Connecticut, Florida, Georgia, Louisiana, Maryland, Mississippi, North Carolina, Rhode Island, Texas, Virginia, and Wisconsin) or central nervous system disease cases associated with infection with WEE virus (California) were detected on emu farms in 14 states.

Reported by: S Wiersma, MD, RS Hopkins, MD, State Epidemiologist, Florida Dept of Health. LE Haddy, MS, State Epidemiologist, West Virginia Dept of Health and Human Resources. 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: During 1996-1997, LAC encephalitis remained the most frequently reported arboviral disease in the United States. The fatal case of LAC encephalitis reported from Minnesota during 1997 emphasizes that severe cases occur and can result in transient or permanent neurologic sequelae or death (2). The incidence, public health impact, and other aspects of this endemic disease are poorly understood and require further study. Ongoing CDC-supported studies and active surveillance efforts are expected to provide better estimates of the incidence of LAC encephalitis in West Virginia and other states (e.g., North Carolina) outside the north-central region, which has been most closely associated with LAC virus transmission. In addition, these studies will provide a better understanding of geographic and ecologic factors associated with LAC virus transmission (e.g., the relative contribution of treeholes and artificial containers such as used tires in the production of Aedes triseriatus, the eastern treehole mosquito and primary vector of LAC virus) (2).

SLE virus remains the most important cause of epidemic encephalitis in North America (3). Surveillance for early seasonal enzootic SLE virus transmission continues to be important in detecting and controlling outbreaks and reducing human risk through vector control and modification of human activity patterns (4). For example, in August 1997, following the detection of epizootic SLE virus activity in central Florida through active surveillance, public health authorities issued health alerts advising the public to 1) minimize outdoor activities during evening and nighttime hours (the peak biting period for Culex nigripalpus, the primary vector of SLE virus in Florida) (5), 2) ensure proper screening of residential doors and windows, 3) wear long-sleeved shirts and long pants when involved in nighttime outdoor activities, and 4) use DEET-containing repellents in an approved manner. These alerts may have prevented a more extensive outbreak by prompting residents to change their evening activity patterns (S. Wiersma, M.D., Florida Department of Health, personal communication, 1998) and may in part explain the marked difference in the severity of the Florida SLE epidemics of 1997 (nine cases, one death) and 1990 (223 cases, 11 deaths).

EEE is the most severe of the arboviral encephalitides, with an overall case-fatality rate of approximately 35% (6). The fatal EEE case in an Alabama resident during 1996 was unusual because it was the first human EEE case reported from Alabama since 1965, and the onset of illness was in January. In most regions of the United States where EEE virus is enzootic, transmission to humans usually occurs during May-October (7). This case illustrates that year-round EEE virus transmission can occur near the Gulf Coast.

The emu is an imported species of large, flightless bird farmed for meat and other products nationwide. Emus are highly susceptible to EEE virus infection, which typically results in acute hemorrhagic enterocolitis and death (8). High-titered viremias develop in infected emus; therefore, emus may contribute to EEE virus amplification in the peridomestic environment, placing humans at increased risk. In addition, EEE virus can be isolated from the bloody feces of infected emus, and emu-to-emu transmission of EEE virus has been documented experimentally (CDC, unpublished data, 1998). No human infections with EEE virus have been associated with raising or handling emus. In the eastern United States, vaccination of emus with EEE virus vaccines approved for use in horses is a common practice that can protect them against an otherwise lethal challenge dose of EEE virus under experimental conditions (CDC, unpublished data, 1998).

No human cases of WEE have been reported since 1994, and only three cases have been reported during the 1990s. Reasons for the decrease in cases may include underrecognition and underdiagnosis of cases.

Health-care providers should consider arboviral infections in the differential diagnosis of all cases of aseptic meningitis and viral encephalitis, obtain appropriate specimens for laboratory testing, and promptly report cases to state health departments. Reasons for making a specific etiologic diagnosis in such cases include 1) ruling out diseases for which specific therapy is available, 2) better informing patients and their families about prognosis, and 3) alerting public health authorities to take appropriate control measures. Human disease risk can be effectively reduced with active environmental surveillance systems and appropriate mosquito-control measures, and by providing timely information to the public.


  1. CDC. Case definitions for infectious conditions under public health surveillance. MMWR 1997;46(no. RR-10):12-3.

  2. Grimstad PR. California serogroup virus disease. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Vol 2. Boca Raton, Florida: CRC Press, 1988:99-136.

  3. Monath TP. Epidemiology. In: Monath TP, ed. St. Louis encephalitis. Washington, DC: American Public Health Association, 1980:239-312.

  4. CDC. Guidelines for arbovirus surveillance in the United States. Fort Collins, Colorado: US Department of Health and Human Services, Public Health Service, CDC, 1993.

  5. Mitchell CJ, Francy DB, Monath TP. Arthropod vectors. In: Monath TP, ed. St. Louis encephalitis. Washington, DC: American Public Health Association, 1980:313-79.

  6. Deresiewicz RL, Thaler SJ, Hsu L, Zamani AA. Clinical and neuroradiographic manifestations of eastern equine encephalitis. N Engl J Med 1997;26:1867-74.

  7. Morris CD. Eastern equine encephalomyelitis. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Vol 3. Boca Raton, Florida: CRC Press, 1988:1-20.

  8. Tully TN Jr, Shane SM, Poston RP, et al. Eastern equine encephalitis in a flock of emus (Dromaius novaehollandiae). Avian Diseases 1992;36: 808-12.

* For national surveillance, a confirmed case is defined as febrile illness associated with neurologic manifestations ranging from headache to aseptic meningitis or encephalitis with onset during a period when arbovirus transmission is likely to occur, plus at least one of the following criteria: 1) fourfold or greater serial change in serum antibody titer; 2) isolation of virus from, or demonstration of viral antigen or genomic sequences in, tissue, blood, cerebrospinal fluid, or other body fluid; or 3) demonstration of specific immunoglobulin M (IgM) antibody in serum or cerebrospinal fluid by IgM-capture enzyme immunoassay with confirmation by demonstration of IgG antibodies by another serologic assay (e.g., neutralization or hemagglutination inhibition). A probable case is defined as compatible illness occurring during a period when arbovirus transmission is likely, plus an elevated but stable (twofold or less serial change) antibody titer to an arbovirus (e.g., greater than or equal to 320 by hemagglutination inhibition, greater than or equal to 128 by complement fixation, greater than or equal to 256 by immunofluorescence, greater than or equal to 160 by neutralization, or greater than or equal to 400 by IgM-capture enzyme immunoassay) (1).

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Number and annual rate* of reported human cases of La Crosse encephalitis
(LAC), eastern equine encephalomyelitis (EEE), and St. Louis encephalitis (SLE), by
state -- United States, 1996-1997
                                           LAC                      EEE                       SLE
                                 ----------------------   -----------------------   -----------------------
                      1996            No.                      No.                       No.
                   population    ------------    Annual   ------------     Annual   -----------      Annual
State             (thousands)+   1996    1997     rate    1996    1997      rate    1996   1997       rate
Alabama                  4,273      0       0       --       1       0      0.01       0      1       0.01
California              31,878      0       0       --       0       0        --       0      1       0.002
Florida                 14,400      0       0       --       1       3      0.01       0      9       0.03
Georgia                  7,353      0       0       --       0       3      0.02       0      0         --
Illinois                11,847     13       3     0.07       0       0        --       0      0         --
Indiana                  5,841      3       1     0.03       0       0        --       0      0         --
Kentucky                 3,884      0       3     0.04       0       0        --       0      0         --
Louisiana                4,351      1       0     0.01       1       4      0.06       0      0         --
Massachusetts            6,092      0       0       --       0       1      0.01       0      0         --
Michigan                 9,594      0       0       --       0       1      0.01       0      0         --
Minnesota                4,658      7       5     0.13       0       0        --       0      0         --
North Carolina           7,323      3       4     0.05       2       0      0.01       0      0         --
Ohio                    11,173     20      13     0.15       0       0        --       0      0         --
South Carolina           3,699      0       0       --       0       2      0.03       0      0         --
Tennessee                5,320      1      10     0.10       0       0        --       0      0         --
Texas                   19,128      1       1     0.01       0       0        --       2      0       0.01
Virginia                 6,675      2       6     0.06       0       0        --       0      1       0.01
West Virginia            1,826     66      73     3.81       0       0        --       0      1       0.03
Wisconsin                5,160      8       8     0.16       0       0        --       0      0         --

Total                             125     127                5      14                 2     13
* Per 100,000 population.
+ U.S. Department of Commerce, Economics and Statistics Administration, Bureau of the Census.

Return to top.


Return to top.

Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 2. Number of eastern (EEE) and western (WEE) equine encephalomyelitis cases
among horses, by state -- United States, 1996-1997
                        EEE             WEE
                   -------------   ------------
State              1996     1997   1996    1997
Alabama               9       13      0       0
Arkansas              0        1      0       0
California            0        0      1       4
Colorado              0        0      3       0
Connecticut           0        0      0       0
Florida              69       42      0       0
Georgia               5       11      0       0
Indiana               0        1      0       0
Kentucky              1        1      0       0
Louisiana             0       17      0       0
Maryland              0        0      0       0
Minnesota             2        0      1       0
Mississippi          29        4      0       0
Nebraska              0        0      1       0
New Hampshire         3        0      0       0
North Carolina       10        3      0       0
North Dakota          0        0      0       3
Ohio                  0        1      0       0
Rhode Island          0        1      0       0
South Carolina        6        1      0       0
Tennessee             0        3      0       0
Texas                 6       15      0       2
Virginia              5        0      0       0
Wisconsin             0        0      0       0

Total               145      114      6       9

Return to top.

Disclaimer   All MMWR HTML versions of articles are electronic conversions from ASCII text into HTML. This conversion may have resulted in character translation or format errors in the HTML version. Users should not rely on this HTML document, but are referred to the electronic PDF version and/or the original MMWR paper copy for the official text, figures, and tables. An original paper copy of this issue can be obtained from the Superintendent of Documents, U.S. Government Printing Office (GPO), Washington, DC 20402-9371; telephone: (202) 512-1800. Contact GPO for current prices.

**Questions or messages regarding errors in formatting should be addressed to

Page converted: 10/05/98


Safer, Healthier People

Morbidity and Mortality Weekly Report
Centers for Disease Control and Prevention
1600 Clifton Rd, MailStop E-90, Atlanta, GA 30333, U.S.A


Department of Health
and Human Services

This page last reviewed 5/2/01