In 1985, arboviral infections of the central nervous system (CNS) were reported among 90 persons in the United States (Table 1, Figures 1 and 2). A St. Louis encephalitis (SLE) outbreak occurred in Mesa County, Colorado, leading to 17 cases, including one fatality. Four sporadic SLE cases were reported from Texas (one) and California (three). One of the California cases occurred in a Los Angeles resident, where an SLE outbreak occurred in 1984. One case of western equine encephalitis was reported from Texas. Endemic LaCrosse virus transmission in the midwest led to 68 cases of CNS infection. ST. LOUIS ENCEPHALITIS

The SLE outbreak that occurred in Mesa County in western Colorado (Figure 3) included principally residents of Grand Junction, the county's largest town. The age-adjusted attack rate for Grand Junction was 33.5/100,000, compared with 12.2/100,000 for the remainder of the county. Active surveillance failed to disclose cases in neighboring counties. Attack rates were highest among the elderly, but there was no clear increase in risk with advancing age. The age-adjusted attack rate for females was nearly double that for males (26.5/100,000 and 13.7/100,000, respectively; the standard error for the adjusted attack rate for females was 7.7/100,000). One patient, a 73-year-old woman, died. In an ecologic investigation undertaken in late September, fewer than 0.1 Culex tarsalis mosquitoes were caught per trap night. However, cool weather and declining daylight hours mitigated against successful collections. No virus was isolated from 646 pooled arthropods. A serosurvey of Grand Junction residents disclosed inapparent infections among 4% of the city residents, i.e., the outbreak may have led to approximately 1,100 infections. Infection rates for males and females were similar; therefore, increased risk for clinical disease among females could not be attributed to greater exposure.

Elsewhere in the west, sporadic SLE cases were reported from Dawson County, Texas, and from California (three cases). California cases occurred in 17- and 31-year-old males from Riverside County (the latter may have been infected in the Mohave Valley, Arizona) and a 61-year-old Los Angeles woman. Evidence of enzootic SLE transmission was found near the residence of the Los Angeles patient; an SLE virus isolate was recovered from Cx. peus collected in Encino, and a sentinel chicken located near the Sepulveda Reservoir seroconverted to SLE virus.

No human cases were reported in the eastern and central United States for the second consecutive year, and avian surveillance disclosed negligible enzootic transmission except in Florida. OTHER ARBOVIRAL INFECTIONS OF THE CNS

No human eastern equine encephalitis cases were reported. Equine cases occurred principally in coastal southeastern states (Figure 2). Seroconversions in sentinel chickens were observed as far west as Houston, Texas.

Western equine encephalitis was reported in a 27-year-old man from Ellis County, Texas. Equine cases were reported from scattered western states and from Illinois and Indiana, at the eastern-most range of the virus.

LaCrosse virus infections were reported principally from the upper midwest where the disease is endemic. Counties in an endemic focus in southwestern West Virginia reported cases for the third consecutive year. 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: Arboviral infections remain important in the differential diagnosis of CNS infections occurring in the summer and early fall. SLE, the most important cause of epidemic viral encephalitis in the United States, led to 1,815 reported cases in a nationwide outbreak in 1975 (Table 1) (1). More recently, regional outbreaks occurred in Florida (1977) (2), Houston, Texas (1980) (3), and southern California (1984) (4,5).

During the last decade, western equine encephalitis has been sporadic in midwestern and western states. However, extensive outbreaks occurred in the past, leading to over 3,400 cases in 1941 (6). As recently as 1975, 133 cases were reported in an outbreak in the North Red River Valley (7,8). Eastern equine encephalitis is a disease of low frequency (Table 1), occurring principally in Atlantic

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