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Epidemiologic Notes and Reports St. Louis Encephalitis Outbreak -- Arkansas, 1991

On August 2, 1991, a neurologist in Pine Bluff (Jefferson County) in central Arkansas notified the Arkansas Department of Health of two patients hospitalized with St. Louis encephalitis (SLE). A hospital chart review and heightened surveillance (i.e., notification of physicians and hospital infection-control coordinators in Jefferson and surrounding counties) subsequently identified 24 confirmed or probable cases of SLE. This report summarizes the findings of the ongoing outbreak investigation.

Cases were defined using standard case definitions for public health surveillance (1). Sixteen persons had confirmed SLE (including fever and signs and symptoms of encephalitis or aseptic meningitis and SLE viral-specific IgM in cerebrospinal fluid), and eight persons had probable cases (including these clinical characteristics and viral-specific IgM in serum).

Onset of symptoms for the 24 patients occurred from July 14 through August 17 (Figure 1). All patients resided or worked in Pine Bluff (estimated population: 57,000), and nine lived within a 1 square mile area. Fourteen (58%) patients were female. Eight (33%) cases occurred among persons greater than or equal to 65 years of age (age range: 5 weeks-85 years). All patients were hospitalized; three have residual neurologic defects, and one patient with chronic myelogenous leukemia died. The crude SLE attack rate for persons in Pine Bluff was 39 per 100,000 population. Cases were clustered in low socioeconomic status census tracts.

On August 6, local and state health officials issued recommendations for the public to curtail evening outdoor activities and to apply insect repellent when outdoors. City residents were encouraged to mend screens and to remove containers that collect water. The Pine Bluff/Jefferson County vector-control office has intensified spraying throughout the city to control Culex quinquefasciatus, the suspected mosquito vector. An entomologic survey of Pine Bluff is in progress to measure the distribution and abundance of vector mosquitoes and viral infection rates in vectors. A door-to-door seroepidemiologic survey has been conducted in selected areas to determine the incidence of infection in residents, identify risk factors for infection and illness, and assess behavioral changes in response to the public health messages; analyses of these data are in progress. Reported by: TE Townsend, MD, TP Bishop, MD, BD Higdem, Jefferson Regional Medical Center, Pine Bluff; JP Lofgren, MD, TC McChesney, DVM, State Epidemiologist, Arkansas Dept of Health. Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases; Div of Field Epidemiology, Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: SLE is the leading cause of epidemic viral encephalitis in the United States. Fewer than 1% of infections are clinically apparent. Symptomatic illnesses range in severity from febrile illness and headache to aseptic meningitis or encephalitis. Seven percent of symptomatic cases are fatal (2).

SLE is transmitted in three distinct cycles in the United States (3). Passerine birds (e.g., house sparrows (Passer domesticus)) are the principal vertebrate amplifying host in all locations. However, mosquito vectors differ in each of the three transmission cycles: in the rural West, Cx. tarsalis transmits SLE in an endemic pattern. In northern and southern regions of the central United States, Cx. pipiens and Cx. quinquefasciatus, respectively, are the principal vectors, and in Florida, Cx. nigripalpus is the primary vector.

SLE outbreaks occur at unpredictable intervals in the central United States and Florida. From 1954 through 1977, a series of regional outbreaks occurred at approximately 10-year intervals (1954-1957, 1964-1968, and 1974-1977) (3,4). Since 1977, outbreaks have occurred at irregular intervals--on the Gulf Coast in 1980 and 1986 and in Houston and in Florida in 1990. Although 18 SLE cases occurred in scattered geographic areas of Arkansas in 1975, the outbreak in Pine Bluff in 1991 is the first localized epidemic reported from the state.

The epidemiologic characteristics of the outbreak in Pine Bluff are typical of Cx. quinquefasciatus-borne SLE in the Mississippi River valley. These outbreaks frequently are focused in older neighborhoods where open drainage ditches and peridomestic mosquito breeding sites (e.g., discarded containers) may be prevalent. Open house foundations, which provide mosquito resting sites, and inadequately screened residences without air conditioning are additional risk factors (3-5).

Advanced age is the most clearly defined host factor associated with neuroinvasive SLE. Although SLE attack rates increase with age and mortality is greatest among the elderly, the biologic basis for this increased risk is unknown.

Following the nationwide SLE outbreak in 1975, state and local surveillance systems were established to monitor viral transmission in the enzootic cycle. The premise of these systems is that epidemic transmission can be predicted by identifying viral activity in vector mosquitoes and vertebrate amplifying hosts. The potential utility of this approach was demonstrated in 1986 in Harris County, Texas, and in 1990 in Houston and in Florida (6,7). Outbreaks in these locations were predicted from observations of rising mosquito rates or seroconversions in sentinel chickens.

Through September 1991, surveillance in Mobile, Alabama; Florida; Louisiana; and Memphis has not detected substantial levels of viral transmission and/or outbreaks. The absence of viral transmission in areas of Arkansas other than Pine Bluff and in surrounding states indicates the potential for focal transmission and underscores the need for local programs of surveillance and control (8).


  1. CDC. Case definitions for public health surveillance. MMWR 1990;39(no. RR-13):11-2.

  2. Brinker KR, Monath TP. The acute disease. In: Monath TP, ed. St. Louis encephalitis. Washington, DC: American Public Health Association, 1980:503-34.

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

  4. Tsai TF, Mitchell CJ. St. Louis encephalitis. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Boca Raton, Florida: CRC Press, 1989:113-44.

  5. Tsai TF. Arboviral infections in the United States. Infect Dis Clin N Am 1991;5:73-102.

  6. CDC. St. Louis encephalitis--Baytown and Houston, Texas. MMWR 1986;35:693-5.

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

  8. Smith GC, Francy DB, Campos EG, Katona P, Calisher CH. Correlation between human cases and antibody prevalence in house sparrows during a focal outbreak of St. Louis encephalitis in Mississippi, 1979. Mosquito News 1983;43:322-5.

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