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Epidemiologic Notes and Reports St. Louis Encephalitis -- Baytown and Houston, Texas

In the summer of 1986, Harris County (Baytown and Houston), Texas, experienced its largest outbreak of St. Louis encephalitis (SLE) since 1980. As of October 7, 25 confirmed cases and one presumptive case had been reported; four patients died. The focus of virus activity was Baytown, a town of 56,910 located 30 miles east of Houston. Twenty-one cases occurred in the Baytown area; 19 of the patients resided within the city limits. Four of the patients with confirmed disease resided in Houston, and the patient with the presumptive case was a Pasadena, Texas, resident who had been exposed to infection in Houston.

Before any cases were recognized, routine mosquito surveillance had led the Harris County Mosquito Control District to anticipate the outbreak. Vector mosquitoes collected in Baytown on July 15 and in the weeks that followed had had unusually high SLE virus infection rates (minimum infection rate 20 infected mosquitoes/1,000 tested), suggesting the likelihood of subsequent human disease. By August, the medical community in Harris County had been alerted to the possibility of a large SLE outbreak, and concentrated efforts to destroy adult mosquitoes had begun. Mosquito surveillance was intensified in Houston, but substantial numbers of infected mosquitoes were not detected there until August and September.

A countywide epidemic of enteroviral meningitis that began in March and continued through July made initial detection of the outbreak difficult. However, in September, a search of all Baytown hospital records disclosed only one case of SLE that had not already been recognized by physicians or infection-control nurses.

The 26 patients became ill in the period July 28-September 16 (Figure 1). Patients who lived in or were exposed to infection in Houston became ill notably later than those in Baytown. Although patients ranged from 10 to 84 years of age, 11 of them--including all four who died--were greater than or equal to 55 years of age. The age- and sex-specific attack rates for Baytown (Table 1) showed a sharp increase in risk with advancing age. The attack rate for Baytown was 33.4/100,000 population, but cases were clustered principally in old, impoverished neighborhoods in the center of town. Ten of the 19 Baytown patients lived in the city's three poorest census tracts (attack rate, 85.2/100,000).

A case-control study of risk factors for infection was conducted. Controls were patients who were initially believed to have SLE but whose serologic test results did not indicate SLE infection. Preliminary results indicated that the risk of acquiring SLE was associated with a) inadequate screening on dwellings (odds ratio (OR)=6.0), b) lack of air-conditioning (OR greater than or equal to 10.0), and c) sitting outside the residence (OR=5.6). A trend toward risk was associated with window air-conditioning as opposed to central air-conditioning and with smoking cigarettes (p=0.07, Fisher's exact test). Patients and controls did not spend a significantly different number of hours outdoors or in such activities as gardening, walking outdoors, or watching television indoors. Reported by MA Canfield, MS, VL Flannery, MS, T Hyslop, MD, MPH, CY Svrcek, MSN, Harris County Health Dept, DA Sprenger, PhD, RE Bartnett, Harris County Mosquito Control District, HW Brister, CA Riser, MD, Baytown Health Dept, CR Craig, Gulf Coast Hospital, BM Conrad, San Jacinto Methodist Hospital, ME Lewis, Humana Hospital, Baytown, KH Sullivan, PhD, GR Reeve, PhD, LM Little, PhD, J Haughton, MD, MPH, City of Houston Dept of Health and Human Svcs, CM Reed, MPH, CE Alexander, MD, MPH, State Epidemiologist, Bureau of Epidemiology, Texas Dept of Health; Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Harris County has been the focus of numerous SLE epidemics. In 1964, an outbreak centered in Houston involved over 1,000 cases (1,2). Since that time, there have been frequent outbreaks of SLE in Houston and other areas of Harris County, but the cluster of cases this summer was the first reported for Baytown.

The epidemiologic characteristics of this outbreak followed a typical pattern. Attack rates rose with advancing age, and the risks for illness and mortality were highest for the elderly (1,3,4). The attack rate in Baytown--33.4/100,000 population--was typical for urban-centered SLE epidemics (3,4) and similar to the incidence in the 1964 Houston epidemic (19.5/100,000) (1). The highest attack rates in Baytown were associated with old, impoverished neighborhoods in the center of the city. This pattern was also similar to that for the 1964 Houston outbreak, i.e., the epicenter of epidemic activity was downtown Houston, and risk declined in direct proportion to distance from the center of the city (1).

The association between risk and dwellings poorly sealed against mosquitoes as well as the absence of an association between risk and the number of hours spent outdoors suggests that exposure may have occurred indoors or in a peridomestic setting outside. The present case-control study confirms an earlier report from serologic surveys that risk of infection is associated with inadequate screening or the absence of air-conditioning (5). The reasons underlying the association between cigarette smoking and risk of acquiring SLE are unknown. Entomologic descriptions of the peridomestic habits of the epidemic vector, Culex quinquefasciatus, have led to its common name, "southern house mosquito" (6). The case-control study provides epidemiologic evidence that epidemic SLE virus infection may be transmitted indoors.

When outbreaks of SLE occur, the usual public health advisory to seek protection indoors assumes that houses are adequately sealed against mosquitoes. An advisory to repair window screens--and help for persons who need assistance in doing so--may be a more effective preventive measure in impoverished neighborhoods where the risk of SLE infection appears highest.

The 1986 outbreak of SLE in Baytown and Houston provides a clear example of the value of mosquito surveillance in anticipating and limiting human infection. Results of mosquito surveillance corresponded temporally and geographically to the occurrence of human cases. Cases and virus isolates from mosquitoes in Houston were reported relatively late in comparison with the reporting of cases and virus activity in Baytown. Mosquito surveillance also accurately indicated that a large outbreak would occur in Baytown but not elsewhere in the county. Present methods of detecting viruses in mosquitoes through cell culture or mouse innoculation require 1 to 2 weeks. An antigen detection immunoassay that identifies SLE virus antigen in 1 day is being evaluated in several mosquito control districts and should improve the timeliness of surveillance.

Although the outbreak in Baytown was the largest cluster of SLE cases this year, sporadic SLE virus activity was widespread in the United States. The Baytown outbreak was part of more diffuse activity on the Gulf Coast. To the south, a case was reported from Matagorda County, Texas. To the east, four cases were reported in Port Arthur, Texas, and one case each was reported in Lake Charles and in Baton Rouge, Louisiana. In the Upper Midwest, cases were reported in Fargo, North Dakota, and Scotts Bluff, Nebraska. (The Scotts Bluff patient was visiting the area.) In July, two cases were reported from Los Angeles, California.

Major urban-centered SLE outbreaks have recurred in a 10-year cycle. The last nationwide epidemic of SLE occurred in 1976, when more than 2,000 cases were reported (3). Small premonitory outbreaks often have foreshadowed larger epidemics the following year (3). The 1986 SLE virus activity indicates both the possibility of more widespread transmission next year and the need for careful surveillance.


  1. Luby JP, Miller G, Gardner P, et al. The epidemiology of St. Louis encephalitis in Houston, Texas, 1964. Am J Epidemiol 1967;86:584-97.

  2. Cooperative Study Group. Epidemic St. Louis encephalitis in Houston, 1964. JAMA 1965;193:139-46.

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

  4. Luby JP. St. Louis encephalitis. Epidemiologic Reviews 1979;1:55-73.

  5. Henderson BE, Pigford CA, Work T, et al. Serologic survey for St. Louis encephalitis and other group B arbovirus antibodies in residents of Houston, Texas. Am J Epidemiol 1970;91:87-98.

  6. Horsfall WR. Mosquitoes: their bionomics and relation to disease. New York: Ronald Press Co., 1955:574.

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