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Current Trends Arboviral Surveillance -- United States, 1990

Through August 27, 1990, surveillance of mosquito vectors of St. Louis encephalitis (SLE) and eastern equine encephalitis (EEE) has detected unusually early and high levels of viral transmission in several states, indicating a potential risk for epidemic transmission. This report summarizes arboviral surveillance activities in Texas, Florida, Massachusetts, New Jersey, and New York. In addition, the report summarizes cases of confirmed or possible arboviral infections in persons in Texas, Florida, North Carolina, and South Carolina, and equine cases in Georgia and Maryland. St. Louis Encephalitis

Texas. In the city of Houston and Harris County, the number and distribution of and SLE viral infection rates for Culex quinquefasciatus mosquitoes are monitored throughout the year. During the summer transmission season, greater than 300 mosquito pools from various sampling points in the county are tested for SLE virus. Mosquito surveillance is coupled with programs of routine mosquito control and emergency measures directed at areas where viral transmission is detected.

On June 19 (approximately 1 month earlier than in previous epidemic years), SLE virus was recovered from collections in a northeastern Houston neighborhood. In succeeding weeks, greater than 239 suspected SLE viral isolates were recovered from widely separated areas of the county and city--but particularly in the Denver Harbor, Houston Heights, and Fifth Ward sections of central Houston. Six isolates were recovered from Baytown, the site of an SLE outbreak in 1986 (1). In central areas of Houston, minimum infection rates (MIRs) in Cx. quinquefasciatus have averaged 5 per 1000 mosquitoes (2). However, in an intensively studied square-mile area in the northeastern quadrant of the city, MIRs in this species were as high as 25 per 1000 mosquitoes captured in gravid traps between August 3 and August 10. Surveillance of wild birds in this site indicated a point seroprevalence of 60%.

After greater than 40 SLE viral isolates had been identified in mosquitoes, the potential for an outbreak was publicized in a series of news conferences and announcements in early July. Active surveillance by telephone and by mail was instituted to identify patients with central nervous system (CNS) infection in all county hospitals. Two cases were serologically diagnosed by the Houston City Health Department and confirmed by CDC. The cases were in a young woman from northeastern Houston and an elderly woman from Baytown; dates of onset of illness were July 20 and July 21, respectively. Both patients died; however, the causes of death have not been determined.

Mosquito-control measures have been intensified at sites where viral isolates were recovered. In areas of the city where Cx. quinquefasciatus use storm sewers as resting sites, pyrethrins administered as thermal fogs into the sewers are the principal means of control.

Florida. Florida maintains a program of SLE and EEE surveillance by monitoring seroconversions in sentinel chickens in 14 counties. In early June, seroconversions to SLE virus were noted in flocks in several central and eastern Florida counties. In July and August, increasing seroconversion rates were noted, including 100% of chickens in Indian River County and 22%-33% of chickens in Lee, Manatee, and Orange county flocks. Surveillance was then intensified, and weekly blood samples of flocks in these counties detected rising seroconversion rates: for example, in Lee and Orange counties, 50% and 80% of chickens, respectively, seroconverted during the week of August 13. Hospital-based surveillance for encephalitis cases was initiated in the affected counties. After seroconversions to SLE virus were noted in early July, ground-based ultralow volume (ULV) adulticiding and larviciding were intensified. Five confirmed cases and one presumptive case subsequently were identified in encephalitis patients from Indian River, Lake, and Highlands counties. The dates of onset of illness in these cases ranged from July 28 to August 17. Eastern Equine Encephalitis

Massachusetts. Since 1957, adult mosquitoes in freshwater swamps of central southeastern Massachusetts (excluding Cape Cod, Martha's Vineyard, and Nantucket) have been monitored in a standardized surveillance program for EEE. In 1990, EEE virus was recovered earlier and in greater numbers than at any time previously in these areas.

In early June, mosquito surveillance that used unbaited miniature light traps was initiated in Bristol, Plymouth, and other counties. The first EEE viral isolate was from a known enzootic site in southeastern Massachusetts and was obtained from a collection on June 20, nearly 1 month earlier than in previous years. The virus was recovered from a pool of Culiseta melanura mosquitoes, the species selectively favored by trap design and placement, and the species believed to be the primary enzootic vector of viral transmission and amplification in Massachusetts. Isolations of EEE have increased progressively during the summer and, in collections through August 8, 597 pools of Cs. melanura (24,836 mosquitoes tested in pools of less than or equal to 50) have yielded 49 EEE isolates, representing a crude MIR of approximately 2 per 1000. One site has yielded 27 isolates from 5080 mosquitos, an MIR of more than 5 per 1000. In addition, one EEE isolate was recovered from 176 pools (9484 mosquitos) of Coquillitidea perturbans, an epizootic vector that transmits the virus from the enzootic cycle to horses and humans. No isolates have been recovered from Aedes vexans or Ae. canadensis, probably the most important epizootic vectors in Massachusetts; however, the populations of these species do not usually peak until later in the season, and the risk for epizootic transmission may rise as these vector species increase in number.

The risk of EEE viral transmission in southeastern Massachusetts in 1990 was anticipated from observations of rainfall patterns and the relative abeyance of EEE virus during 5 preceding years. Historically, EEE activity has occurred in the second of two consecutive seasons of excessive rainfall, as occurred in 1989 and 1990. Preseason warnings in April 1990 to local mosquito-control districts and health departments were followed by public warnings in July, when EEE viral isolations in Cs. melanura exceeded the historical warning threshold of 1 per 1000 mosquitoes. During August, 18 equine deaths clinically compatible with EEE were reported; EEE virus was recovered from the only well-preserved brain submitted, and two of five horses tested serologically were positive. In early August, a contingency plan was initiated for wide-scale aerial ULV application of malathion over Bristol and Plymouth counties. Shortly after the decision to schedule the ULV application for August 27-29, serologic tests of a comatose 7-year-old Plymouth County resident indicated that he had EEE. His onset of fever was August 16, and EEE antibody titers on specimens from August 23 and 27 were less than 10 and greater than 40, respectively.

New York. Surveillance of mosquito vectors and avian hosts of EEE virus is conducted in four counties near Syracuse (Madison, Oneida, Onondaga, and Oswego). Since 1971, outbreaks in these counties have resulted in two human deaths and dozens of equine fatalities. As of August 24, arboviral isolation attempts were completed on 1477 pools (110,900 adult female mosquitoes) collected from May 23 to August 16. EEE virus was detected in 17 pools of Cs. melanura, two pools of Cs. morsitans, and two pools of Ae. canadensis mosquitoes captured in Oswego County from July 23 to August 16. In addition, EEE virus was recovered from two pools of Cs. melanura collected in Madison County and Onondaga County from July 30 to August 16; this species is the primary enzootic mosquito vector of EEE among wild avians in this region.

A total of 627 samples (brain, cerebrospinal fluid, and blood clots) from vertebrates in five upstate counties also was tested for virus. None of five equine samples from Cayuga, Otsego, and Monroe counties or 79 avian specimens from Onondaga County yielded an isolate. However, 15 of 343 wild avians (36 species) captured in Oswego County were viremic. EEE virus was recovered from blood clots in 15 of 384 passerine birds sampled from July 16 to August 7. EEE was confirmed in two unvaccinated horses from Oswego County by isolation of the virus from brain tissue. Onset of clinical signs of CNS infection were noted on August 15 and August 19, respectively.

Other than the enzootic focus near Syracuse, serologic surveillance has not detected evidence of EEE transmission in Cayuga or other counties. In June, 18% of 96 wild avians in Oswego County had low-titered HI antibody (1:20-1:80) to EEE virus, indicating previous infection. In contrast, 2 weeks after the first viremic birds were detected, 28 (39%) of 72 avians blood sampled between July 29 and August 1 were seropositive for EEE virus, and 18 (64%) of these exhibited HI titers greater than or equal to 1:160, suggesting a recent infection.

Because this high level of viral activity indicated a potential for epidemic/epizootic transmission, on August 2-3 and August 3, respectively, the Cicero Swamp in northern Onondaga County and Toad Harbor Swamp in southern Oswego County were treated with aerial applications of an insecticide to reduce vector populations. Insecticide applications were repeated in late August in response to viral isolations from mosquitoes captured inside and near previously treated areas.

New Jersey. At coastal and inland locations, EEE viral transmission is surveyed through collections of Cs. melanura and epizootic vectors, including Cq. perturbans and Ae. sollicitans. In early August, a dramatic surge in abundance of Cs. melanura was observed in all sites; however, this surge is typical of the seasonal dynamics of the mosquito in New Jersey. Since July, EEE viral isolates have been recovered in all monitoring sites; MIRs have ranged from 3.8 to 7.8 per 1000 Cs. melanura. These rates are high for July for the areas under surveillance and indicate a risk for epizootic transmission; however, one presumptive equine case has been the only evidence of epizootic transmission.

The risk for epizootic transmission also is surveyed by monitoring the age structure of Ae. sollicitans, the principal epizootic vector in coastal areas. When landing collections exceed 10 parous females per minute, indicating a relative abundance of mosquitoes that have previously fed (possibly on viremic birds), adulticiding is intensified. This ongoing program of mosquito control maintains young populations of Ae. sollicitans, which lowers the risk of epizootic transmission from this species.

Other States. In July, a fatal case of EEE with onset of illness on June 1 was reported in a woman from South Carolina; three equine cases were also reported from this state. From July 13 to August 2, three equine cases of EEE were reported from coastal counties of North Carolina, and a presumptive human case of EEE with onset of illness on August 1 was reported from Orange County, an inland area where EEE rarely occurs. In April and June, three equine cases were confirmed from southeastern Georgia; in July, two equine cases were reported from Maryland's eastern shore. Reported by: RE Bartnett, DA Sprenger, PhD, Houston-Harris County Mosquito Control District; J Pappas; VF Flannery, MS, KH Sullivan, PhD, JE Arradondo, 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, DM Simpson, MD, State Epidemiologist, Texas Dept of Health. DL Wells, MD, RS Hopkins, MD, RA Calder, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. RJ Timperi, MPH, BG Werner, PhD, P Etkind, MS, A deMaria, PhD, GF Grady, MD, State Epidemiologist, Massachusetts Dept of Public Health. DJ White, PhD, MB Grayson, PhD, DL Morse, MD, State Epidemiologist, New York State Dept of Health. WJ Crans, PhD, Rutgers Univ; KC Spitalny, MD, State Epidemiologist, New Jersey State Dept of Health. MA Greco, DVM, JK Grigor, DVM, E Israel, MD, State Epidemiologist, Maryland State Dept of Health and Mental Hygiene. WB Gamble, MD, South Carolina Dept of Health. JR Cole, DVM, Univ of Georgia, Athens; RK Sikes, DVM, State Epidemiologist, Georgia Dept of Human Resources. JN MacCormack, MD, State Epidemiologist, North Carolina Dept of Human Resources. Div of Vector-Borne Infectious Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: SLE is the leading cause of epidemic viral encephalitis in the United States (3-5). Large outbreaks have occurred periodically in areas of the Gulf Coast and the Mississippi and Ohio valleys. The last major outbreak, in 1975, resulted in nearly 3000 reported cases. In response to that outbreak, many state and local health and mosquito-control agencies established programs of avian and mosquito surveillance to monitor SLE viral transmission in its natural cycle and conditions favoring epidemic transmission.

Because of the rare occurrence of outbreaks, rigorous evaluation of the sensitivity, specificity, and cost-benefit of avian and mosquito surveillance has been difficult. However, in certain instances, human cases have been preceded by a high prevalence of vector mosquitoes, rising infection rates in vectors, and increasing seroprevalence in wild or sentinel avians (6,7).

SLE viral activity in Houston-Harris County during 1990 has been unusual because the first viral isolates were discovered remarkably early and because of widespread distribution of viral isolates in the county. The highest infection rates, however, remain within the city's central area, consistent with a previously observed gradient of more intense viral activity at the city's center (8). The markedly elevated MIR in northeastern Houston suggests that a risk for epidemic transmission exists in this area. Because enzootic SLE viral transmission in Houston usually does not wane until early October, the risk for human disease potentially will continue or rise throughout this period.

In Florida, the importance of sentinel flock seroconversions as an indicator of epidemic risk has been ambiguous. From 1982 to 1986, up to 20% of sentinel chickens seroconverted each year even though no human cases occurred. However, during that period, October was the peak month of viral transmission to chickens. In 1990, seroconversion rates have been remarkably higher and have occurred 2 months earlier than usual (7). These findings suggest that viral transmission in the enzootic cycle could build to higher than usual levels in Florida during the fall months, with a concomitant increase in risk for transmission to humans.

EEE is a rare disease: in most years, fewer than five cases are reported nationwide. The magnitude of EEE outbreaks generally is small; however, during epidemic years, the 30% case-fatality rate associated with the illness underscores the severity of this public health problem (9-11). EEE cases are usually sporadic; viral transmission is localized to specific and relatively constant enzootic foci, related to freshwater swampy habitats that support breeding of Cs. melanura, the principal enzootic vector of EEE virus (12). Southeastern Massachusetts, the four-county area of New York state described in this report, and coastal locations in New Jersey and mid-Atlantic and southeastern states have long been recognized as areas of enzootic EEE. In these locations, individual mosquito species vary in their importance as epizootic vectors for equine and human transmission.

The physical, biologic, and ecologic factors associated with epizootic transmission are complex, but the abundance of EEE virus circulating in the enzootic cycle and various characteristics of the epizootic vectors are important determinants of risk. During 1990, the early appearance of EEE virus in Cs. melanura in Massachusetts and New York, as well as the recovery of numerous viral isolates, has indicated a potential for epizootic transmission and triggered intensified programs of mosquito control and public warnings. In New Jersey, an ongoing program of adulticiding is linked to the maturity of Ae. sollicitans populations, and emergency measures have not been considered necessary (13).

This report illustrates the use of surveillance data on arboviral transmission patterns in nature to guide public health interventions before human infections occur. The approach to surveillance of arboviral diseases is unique in this respect, as are the opportunities to prevent human illness by monitoring and controlling vector mosquitoes. Additional correlations of ecologic and epidemiologic data are needed to assess the predictive value of these indices in forecasting arboviral epidemics.


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of a St. Louis encephalitis outbreak in Harris County, Texas, 1986. J Infect Dis 1988;157:351-6. 2. Chiang CL, Reeves WC. Statistical estimation of virus infection rates in mosquito vector populations. Am J Hyg 1962;75:377-91.

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7. Monath TP, Tsai TF. St. Louis encephalitis: lessons from the last decade. Am J Trop Med Hyg 1987;37(suppl):40S-59S.

8. Luby JP, Miller G, Gardner P, et al. The epidemiology of St. Louis encephalitis in Houston, Texas 1984. Am J Epidemiol 1967;86:584-97.

9. Przelomski MM, O'Rourke E, Grady GF, Berardi VP, Markley HG. Eastern equine encephalitis in Massachusetts: a report of 16 cases: 1970-1984. Neurology 1988;38:736-9. 10. Bigler WJ, Lassing EB, Buff EE, et al. Endemic eastern equine encephalomyelitis in Florida: a twenty year analysis: 1955-1974. Am J Trop Med Hyg 1976;25:884-90. 11. Tsai TF, Monath TP. Viral diseases in North America transmitted by arthropods or from vertebrate reservoirs. In: Feigin RD, Cherry JD, eds. Textbook of pediatric infectious diseases. Philadelphia: WB Saunders, 1982:1417-56. 12. Morris CD. Eastern equine encephalomyelitis. In: Monath TP, ed. The arboviruses: epidemiology and ecology. Boca Raton, Florida: CRC Press, 1988:1-20. 13. Crans WJ. The status of Aedes sollicitans as a vector of eastern equine encephalitis in New Jersey. Mosquito News 1977;37:85-9.

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