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Current Trends Update: St. Louis Encephalitis -- Florida and Texas, 1990

In July 1990, active surveillance of national arboviral transmission patterns indicated that outbreaks of St. Louis encephalitis (SLE) might occur in Florida and in Houston and Harris County, Texas (1). Subsequently, a cluster of cases was reported from central Florida, and sporadic cases were recognized in Harris County (1,2). This report updates surveillance for SLE in these locations.Florida

As of October 17, 1990, 38 confirmed and 26 presumptive cases* of SLE had been reported to the Florida Department of Health and Rehabilitative Services. Onset of illness occurred from July 28 to October 3, 1990. Two patients have died, one with confirmed and one with presumptive SLE. All persons with confirmed and presumptive cases resided in 15 central and south Florida counties (Figure 1); Indian River County reported 17 confirmed and presumptive cases (27% of all reported cases). Patients ranged in age from 14 to 91 years (mean: 53 years); of the 61 patients for whom sex was known, 33 (54%) were male. The affected counties have maintained programs of larviciding and aerial and ground-based adulticiding for control of Culex nigripalpus, the principal mosquito vector of SLE in Florida. Residents of and visitors to affected counties have been cautioned to continue use of personal protective measures against mosquitoes. In some affected counties, evening recreational activities have been rescheduled to daylight hours.Texas

In 1990, mosquitoes infected with SLE virus were detected in Houston and surrounding Harris County on June 19, almost 1 month earlier than in previous epidemic years and at higher levels than usual (1). In nonepidemic years, surveillance of mosquito vectors and intermediate avian hosts has shown that viral transmission occurs at lower levels or is absent. Active surveillance for possible SLE cases was initiated through weekly contacts with infection-control personnel at all county hospitals. Surveillance was also facilitated by increasing public awareness through mailings and announcements to the local medical community and through the mass media. On September 7, two cases of SLE were reported; since then, 10 additional cases have been confirmed serologically (1). The onset dates of illness of confirmed or presumptive cases ranged from July 20 through September 10.

All 12 patients were residents of Harris County: six cases occurred in residents of Houston; five cases, Baytown; and one, Humble. Patients ranged in age from 17 to 86 years (median: 39 years); 11 patients were hospitalized. Two infected patients died, but the causes of death have not been established.

Mosquito surveillance and control activities have been intensified throughout Harris County, especially in areas reporting human cases and in areas where infected mosquitoes were found. No infected mosquitoes have been detected since September 26. Reported by: KL Hudson, DL Wells, AL Lewis, E Buff, RA Calder, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. RE Barnett, DA Sprenger, PhD, Houston-Harris County Mosquito Control District; J Pappas, VF Flannery, MS, KH Sullivan, PhD, JE Arrandondo, MD, City of Houston Dept of Health and Human Svcs; LJ Kilborn, MPH, MA Canfield, MS, T Hyslop, MD, Harris County Health Dept; KA Hendricks, MD, JP Taylor, MPH, DM Simpson, MD, State Epidemiologist, Texas Dept of Health. Div of Field Svcs, Epidemiology Program Office; Div of Vector-Borne Infectious Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The possibility that SLE outbreaks might occur in central Florida and in Harris County, Texas, was predicted in July (1) when rising seroconversion rates were detected in sentinel chickens in Florida and elevated SLE viral infection rates were detected in vector mosquitoes in Harris County. Although the sensitivity and specificity of these approaches to predicting outbreaks have not been proven rigorously, observations in 1990 and previous experience suggest that measures of viral activity in nature can be used to indicate risk for human disease (1).

In Florida, arboviral surveillance has relied chiefly on monitoring of viral transmission to sentinel chickens (3,4). Seroconversions in chickens in central Florida in June and July 1990 were unprecedented in their early appearance and their proportions, approaching 100% at some sites (1). In previous years, seroconversions in chickens in central Florida did not peak until September and October, and the proportion of infected sentinels never exceeded 25% (3,4).

SLE is transmitted in Florida principally by Cx. nigripalpus, a predominantly exophilic (outdoor biting) mosquito found throughout central and south Florida. Feeding activity is most intense at night, especially at dusk and at dawn. Although vector control is an important means of decreasing transmission of SLE to humans, personal protective measures are also important. These practices include avoiding nighttime outdoor activity in affected counties, especially at dusk and dawn; for persons who cannot avoid outdoor activity during these periods, wearing long-sleeved shirts and long pants of tightly woven material and applying mosquito repellents are recommended.

In Harris County, where a program of mosquito surveillance has been maintained for 24 years, elevated SLE viral infection rates in Cx. quinquefasciatus have been associated geographically and temporally with the occurrence of human cases. In 1986, when 24 cases were reported from Baytown and four cases were reported from Houston, increased mosquito infection rates were observed in both areas in the 2-week period preceding the onset of the first case in the respective areas. Cases occurred only in areas where infected mosquitoes were captured (5; D. Sprenger, Houston-Harris County Mosquito Control District, personal communication, 1990). The geographic specificity of vector surveillance was shown again in 1989 when infected mosquitoes were detected within 1 mile of the residences of each of the four patients (D. Sprenger, personal communication, 1990). In systematic collections elsewhere in the county in both 1986 and 1989, infection rates were either lower or zero (D. Sprenger, personal communication, 1990). Through October 17, 1990, the widespread distribution of infected mosquitoes has correlated with the distribution of human cases in extended areas of the city and county.

In Harris County and throughout the southeastern United States, Cx. quinquefasciatus, the southern house mosquito, is the prinicpal vector of SLE. In contrast to Cx. nigripalpus, which feeds in various outdoor locations, Cx. quinquefasciatus is a highly domesticated species and may feed indoors or outdoors. Risk for acquiring the disease has been epidemiologically associated with inadequately screened residences; conversely, air-conditioned residences, especially residences with central air-conditioning units, were protective in two studies (2,6).

In both central Florida and in Harris County, the risk for further epidemic transmission should decline as the activity of vector mosquitoes diminishes with cooling temperatures.


  1. CDC. Arboviral surveillance--United States, 1990. MMWR 1990;39:593-8.

  2. CDC. Update: arboviral surveillance--Florida, 1990. MMWR 1990;39:650-1.

  3. Day JF. The use of sentinel chickens for arbovirus surveillance in Florida. Journal of the Florida Anti-Mosquito Association 1989;60:56-61.

  4. Monath TP, Tsai TF. St. Louis encephalitis: lessons from the past decade. Am J Trop Med Hyg 1987;37:S40-59.

  5. Tsai TF, Canfield MA, Reed CM, et al. Epidemiologic aspects of a St. Louis encephalitis outbreak in Harris County, Texas, 1986. J Infect Dis 1988;157:351-6.

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

    • Confirmed case: fourfold rise in SLE viral hemagglutination-inhibition (HI) antibody titer in paired serum specimens obtained 2 weeks apart, or presence of IgM antibody in serum or cerebrospinal fluid. Presumptive case: viral HI antibody titer of greater than 1:40 in a single serum specimen.

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