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Arboviral Infections of the Central Nervous System -- United States, 1986

In 1986, 115 arboviral infections of the central nervous system (CNS) were reported to CDC (Figures 1, 2). Infections from California serogroup viruses, occurring chiefly in endemic areas of the upper Midwest, accounted for 64 cases. Hyperendemic and epidemic transmission of St. Louis encephalitis (SLE) virus in Texas and Louisiana communities on the Gulf of Mexico accounted for 37 cases, including a focal outbreak in Harris County, Texas (1). Six other SLE cases occurred sporadically in the Midwest and in southern California, and seven western equine encephalitis (WEE) cases were reported from endemic areas of the Texas panhandle, northern Colorado, and California. One case of eastern equine encephalitis (EEE) was reported from Florida.

Sporadic WEE cases in 44 horses were reported from western states; 94 EEE cases in equines were reported from eastern, principally coastal states (Figure 1). St. Louis Encephalitis

An outbreak in Harris County, Texas, accounted for 28 cases, five of them fatal. The outbreak was centered in Baytown, where 23 cases occurred (attack rate for Baytown=37/100,000). In five cases, patients had been exposed in Houston. The attack rate rose concurrently with age, and all five of the persons who died were greater than55 years old. A case-control study disclosed that risk of acquiring SLE was associated with residences poorly sealed against mosquitoes, indicating that exposure may have occurred indoors. Nine additional SLE cases, two of them fatal, were reported from other coastal cities in Texas and Louisiana (Figure 3).

California Serogroup Viruses

LaCrosse virus was the presumed cause of 62 reported CNS infections, and cross-neutralization tests showed that Jamestown Canyon virus caused two cases. Of the persons with LaCrosse virus infections, 46 (74%) were males, 44 of whom were less than18 years of age, and the others were girls, all less than18 years of age. The incidence of CNS infections from LaCrosse virus for the resident population less than18 years in Iowa, Ohio, Wisconsin, Minnesota, Indiana, and Illinois was 0.51/100,000 (range=0.87 (Iowa) to 0.03/100,00 (Illinois)). The two persons with Jamestown Canyon virus infections were a 39-year-old man and a 10-year-old girl, both Indiana residents.

Western Equine Encephalitis

Four of the seven persons with WEE were infants less than 6 months of age. Two infants presented with clinical manifestations of aseptic meningitis or encephalitis, initially suggesting an enteroviral infection. Another infant was believed to have a poliovirus infection resulting from immunization. The other patients were a 16-year-old girl and a 37-year-old woman. Reported by: E Hughes, Mobile County Health Dept; L Lauerman, DVM, Alabama State Dept of Agriculture and Industries; WE Birch, DVM, State Epidemiologist, Alabama State Dept of Public Health. J Doll, PhD, M Wright, R Cheshier, PhD. W Stromberg, PhD, GG Caldwell, MD, State Epidemiologist, Arizona Dept of Health Svcs. TC McChesney, DVM, State Epidemiologist, Arkansas Div of Health. Microbiology Reference Laboratory, Long Beach; Long Beach City Health Dept; Arbovirus Research Unit, School of Public Health, University of California, Berkeley; Epidemiology, Laboratory, and Vector Control Svcs, County of Los Angeles Dept of Health Svcs; Orange County Health Care Agency; County of Riverside; R Emmons, MD, Viral and Rickettsial Disease Laboratory Section, R Murray, PhD, R Roberto, MD, Infectious Disease Section, J Chin, MD, State Epidemiologist, California Dept of Health Svcs. J Emerson, DVM, SW Ferguson, PhD, State Epidemiologist, Colorado Dept of Health. A Main, PhD, R Shope, MD, Yale Arbovirus Research Unit, New Haven; D Mayo, MA Markowski, JL Hadler, MD State Epidemiologist, Connecticut State Dept of Health Svcs. M Verma, PhD, J Jean, PhD, PR Silverman, DrPH, State Epidemiologist, Delaware Dept of Health and Social Svcs. MP Hunt, J Gamble, East Volusia County, Mosquito Abatement District, Daytona Beach; HL Rubin, DVM, State of Florida Dept of Agriculture and Consumer Svcs; L McCaig, S Lieb, MPH, W Bigler, PhD, FM Wellings, PhD, EC Prather, MD, State Epidemiologist, Florida Dept of Health and Rehabilitative Svcs. J Cole, DVM, University of Georgia, Tifton; RK Sykes, DVM, State Epidemiologist, Georgia Dept of Human Resources. W Turnock, MD, Chicago Dept of Health; HJ Dominick, C Langkop, BJ Francis, MD, State Epidemiologist, Illinois Dept of Public Health. MJ Sinsko, PhD, CL Barrett, MD, State Epidemiologist, Indiana State Board of Health. NS Swack, PhD, LA Wintermeyer, MD, State Epidemiologist, Iowa Dept of Health. J Pearson, DVM, US Dept of Agriculture, Ames, Iowa. R French, MD, Acting State Epidemiologist, Kansas State Dept of Health and Environment. JC McCammon, MD, Louisville and Jefferson County Dept of Health; MW Hinds, MD, State Epidemiologist, Kentucky Dept of Health Svcs. HB Bradford, Jr, PhD, L MacFarland, DrPH, Acting State Epidemiologist, Louisiana Dept of Health and Human Resources. T Scott, PhD, University of Maryland, College Park; G Stern, DVM, Maryland Dept of Agriculture; C Lazar, MD, M Josephs, PhD, E Israel, MD, State Epidemiologist, Maryland State Dept of Health and Mental Hygiene. V Berardi, H Maxfield, GF Grady, MD, State Epidemiologist, The State Laboratory Institute, Massachusetts Dept of Public Health. H McGee, MPH, KR Wilcox, Jr, MD, State Epidemiologist, Michigan Dept of Public Health. TF Smith, PhD, Mayo Clinic, Rochester; L Boyd, PhD, J Korlath, MPH, MT Osterholm, PhD, State Epidemiologist, Minnesota Dept of Health. DL Sykes, QA Long, Gulf Coast Mosquito Control Commission, Gulfport; FE Thompson, MD, State Epidemiologist, Mississippi State Dept of Health. J Goins, PhD, HD Donnell, Jr, MD, State Epidemiologist, Missouri Div of Health. KL Quickenden, PhD, JK Gedrose, State Epidemiologist, Montana State Dept of Health and Environmental Sciences. PA Stoesz, MD, State Epidemiologist, Nebraska State Dept of Health. W Crans, PhD, New Jersey Agricultural Experiment Station, New Brunswick; WE Parkin, DVM, State Epidemiologist, New Jersey State Dept of Health. P Hayes, HF Hull, MD, State Epidemiologist, New Mexico Health and Environment Dept. D White, PhD, M Grayson, PhD, R Deibel, MD, DL Morse, MD, State Epidemiologist, Bureau of Communicable Disease Control, Center for Laboratories and Research, New York State Dept of Health. N Newton, PhD, Vector Control Br, Environmental Health Section, Div of Health Svcs, F Crout, PhD, JN MacCormick, MD, State Epidemiologist, North Carolina Div of Health Svcs. K Tardif, JL Pearson, DrPH, State Epidemiologist, North Dakota State Dept of Health. E Peterson, M Parsons, MS, TJ Halpin, MD, State Epidemiologist, Vector-Borne Disease Unit, Ohio Dept of Health. EJ Witte, VMD, State Epidemiologist, Pennsylvania State Dept of Health. J Cookman, S Morin, Dept of Environmental Management, RA Keenlyside, MBBS, State Epidemiologist, Rhode Island Dept of Health. KA Senger, State Epidemiologist, South Dakota State Dept of Health. JG Hamm, JR Oates, SJ Jones, WP Kelly, Memphis-Shelby County Health Dept; RH Hutcheson, Jr, MD, State Epidemiologist, Tennessee State Dept of Health and Environment. D Sprenger, PhD, Harris County Mosquito Control District, Houston; B Elliot, PhD, RL Johns, PhD, C Reed, MPH, CE Alexander, MD, State Epidemiologist, Texas Dept of Health. BT Haslam, CR Nichols, MPA, State Epidemiologist, Utah Dept of Health. S Jenkins, MD, M Cader, MD, GR Miller, Jr, MD, State Epidemiologist, Virginia State Dept of Health. JM Kobayashi, MD, State Epidemiologist, Washington Dept of Social and Health Svcs. W Schell, JP Davis, MD, State Epidemiologist, Wisconsin State Dept of Health and Social Svcs. Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: St. Louis encephalitis is the principal cause of epidemic encephalitis in the United States, leading to periodic widespread outbreaks in the Mississippi Valley (2,3). The cluster of cases occurring on the Gulf Coast in 1986 was remindful of events in 1980, when outbreaks in New Orleans, the Houston Standard Metropolitan Statistical Area, and the coastal counties of Victoria and Nueces, Texas, accounted for 78 cases (Texas Department of Health, unpublished data). In 1986, the case-fatality ratio in Houston and other Gulf Coast cities where cases were not actively sought was 3/14 (21 %). From 1971 through 1982, in the United States, where there also was no active case finding, the case-fatality ratio for SLE was 6.5%, suggesting that in the 1986 outbreak only the most severe cases were reported and that more extensive transmission escaped detection. Since 1971, when county-based surveillance data became available, Harris County has reported the most SLE cases of any county in the United States (CDC, unpublished data).

LaCrosse virus infections are endemic in the eastern United States and are most prevalent in the upper Midwest (3). Cases occur principally in children. Boys, who account for two-thirds of cases, are at highest risk because of their greater outdoor exposure. In the decade between 1977 and 1986, the average number of cases per year reported to CDC was 83. In the upper Midwest states mentioned above, the average annual incidence for this period (based on reported cases) was 0.7/100,000 population less than18 years. However, LaCrosse virus infections may be significantly underreported. A population-based study of Olmsted County, Minnesota, showed that LaCross virus was the most frequently diagnosed cause of viral encephalitis in the county from 1950 through 1981 (4). Extrapolations from this report indicate that the annual incidence for LaCrosse encephalitis in the county may have been 3/100,000. Although LaCrosse encephalitis is rarely fatal (less than 5% of cases), residual convulsive disorders occur in 5%-15% of patients (3). Viewed from this perspective, LaCrosse virus infections are an important public health concern as a cause of CNS morbidity among children in endemic areas.

Jamestown Canyon virus infections recently have been prevalent in the Midwest, with a point seroprevalence rate of 28% in one study of Michigan residents (5). Aseptic meningitis, which occurs principally in adults, is the most common clinical presentation (6). Aedes albopictus, an Asian mosquito recently introduced into the United States (7), has been shown experimentally to transmit both LaCrosse and Jamestown Canyon viruses (8). No naturally infected mosquitoes or instances of human infections with California viruses have been attributed to this vector. However, the spread of this mosquito to the midwestern states, where it may reproduce and enter the maintenance cycle of these viruses, has raised concern that an increase in California serogroup CNS infections may result.

Although a major WEE epidemic occurred in the United States as recently as 1975, most WEE cases occur sporadically in the western United States (3). The age-specific incidence is highest in children less than5 years of age; 78% of the persons affected are males, who are more likely than females to encounter the vector mosquito, Culex tarsalis, in rural agricultural areas.

EEE is a rare disease in the United States, with fewer than 20 cases reported in most years. However, in the last decade, 30% of the cases have been fatal.


  1. CDC. St Louis encephalitis -- Baytown and Houston, Texas. MMWR 1986;35.

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

  3. Tsai TF, Monath TP. Viral diseases in North America transmitted by arthropods or from vertebrate reservoirs. In: Feigin R, Cherry J, eds. Textbook of pediatric infectious diseases, vol II. 2nd ed. Philadelphia. Saunders Co, 1987:1417-56.

  4. Beghi E, Nicolosi A, Kurland LT, Mulder DW, Hauser WA, Shuster L. Encephalitis and aseptic meningitis, Olmsted County, Minnesota, 1950-1981 -- I: epidemiology. Ann Neurol 1984;16: 283-94.

  5. Grimstad PR, Calisher CH, Harroff RN, Wentworth BB. Jamestown Canyon virus (California serogroup) is the etiologic agent of widespread infection in Michigan humans. Am J Trop Med Hyg 1986;35:376-86.

  6. Srihongse S, Grayson MA, Diebel R. California serogroup viruses in New York State: the role of subtypes in human infections. Am J Trop Med 1984;33:1218-27.

  7. CDC. Update: Aedes albopictus infestation -- United States. MMWR 1986;35:649-51.

  8. Shroyer, DA. Aedes albopictus and arboviruses: a concise review of the literature. J Am Mosq Control Assoc 1986;2:424-8.

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