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Epidemiologic Notes and Reports Arboviral Encephalitides -- United States, 1983

Epidemic, hyperendemic, and sporadic transmission of eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis (SLE), and encephalitides caused by California serogroup viruses* (CE) led to 62 confirmed human cases in the United States in 1983 (Figures 1 and 2).

EEE: Ten of the 12 reported human cases and numerous equine and avian cases occurred in recognized endemic and enzootic areas: Massachusetts' Taunton Valley (five human and five equine cases); upstate New York counties near Syracuse (one human and eight equine cases); the Delaware-Maryland-Virginia peninsula (one equine and several hundred pheasant cases from a single premise); Lowndes County, Georgia (single human, equine, and quail cases occurring in the same week); southern Michigan and adjoining northeastern Indiana counties (15 equine cases); Florida (three human cases--one from the panhandle and two from central Florida). Transmission of EEE to horses occurs nearly year-round in Florida, and 55 cases have been reported to date. For the first time, Rhode Island reported human cases of EEE; two cases occurred in conjunction with an epizootic affecting five horses in the state and three in nearby areas of Connecticut. Sporadic and epizootic equine cases occurred elsewhere in the northeast and southeast.

Three deaths occurred (case fatality ratio, 25%)--a 7-year-old boy, a 64-year-old man, and a 66-year-old woman. A 9-month-old infant recovered with profound brain damage, and two other patients recovered with lesser neurologic sequelae.

WEE: In Minnesota, North Dakota, and South Dakota, large vector populations, high mosquito infection rates, and evidence of virus transmission to sentinel chickens and horses had suggested the potential for epidemic WEE this year. Six human cases have been confirmed in 3- and 7-week-old infants, 6-, 10-, and 15-year-old children, and a 22-year-old man. These patients' residences were widely dispersed in the three-state area reflecting high mosquito infection rates and occurrences of equine cases on premises in widespread areas of the states. The 3-week-old infant had significant residual neurologic sequelae; the outcomes were good in the other cases.

A single case of WEE was documented in a man from Hale County, Texas, where WEE has been prevalent in the past. Numerous presumptive and proven WEE cases in horses were reported from the midwest and west.

SLE: Transmission of SLE virus by Culex pipiens, the major epidemic vector of SLE in the central United States, was minimal, and only one confirmed case from Indiana and two suspected cases each from Illinois and Texas were reported.

Flooding of the Colorado River this spring resulted in expansion of mosquito populations and an outbreak of Cx. tarsalis-borne SLE in adjacent California and Arizona counties (Riverside and Imperial Counties, California, and Mohave and Yuma Counties, Arizona). In California, six confirmed cases have been reported in four residents and two visitors of those counties. Two other confirmed cases had no history of travel to the flooded areas. Arizona reported one confirmed and three suspected flood-associated cases and a sporadic case, with onset before the period of flooding.

Using 1980 census data for counties in the flooded area, a crude estimate of the resident population at risk was obtained. For towns and county subdivisions bordering the river, the crude attack rate (counting confirmed and suspected cases) was 5.1/100,000 (8/155,928). In the greater Yuma area, including Bard-Winterhaven, where most of the cases occurred, the attack rate was estimated at 7.2/100,000 (5/70,649).

The outcome was favorable for all but one patient, a 72-year-old man who remains comatose. A dual infection with SLE and echo 11 viruses occurred in a 3-year-old California boy.

CE: Thirty-two cases were confirmed in children who resided in states bordering the Great Lakes. An additional confirmed case was reported from Missouri, and 13 suspected cases await confirmation, including 11 from New York, one from Iowa, and one in a California resident who visited Wisconsin before onset of illness. Reported by LH Lauerman, DVM, Alabama Dept of Agriculture and Industries, WE Birch, MD, State Epidemiologist, Alabama Dept of Public Health; H Webster, MD, W Stromberg, PhD, ME Wright, RL Coppedge, MD, Acting State Epidemiologist, Arizona Dept of Health Svcs; R Emmons, MD, R Murray, MD, R Roberto, MD, J Chin, MD, State Epidemiologist, California State Dept of Health Svcs; JK Emerson, DVM, RS Hopkins, MD, State Epidemiologist, Colorado State Dept of Health; A Main, PhD, R Shope, MD, Yale Arbovirus Research Unit, New Haven, MA Markowski, VD Loverde, MD, State Epidemiologist, Connecticut State Dept of Health Svcs; PS Silverman, DrPH, State Epidemiologist, Delaware Dept of Health and Social Svcs; HL Rubin, DVM, Florida Dept of Agriculture and California State Dept of Health Svcs; JK Emerson, DVM, RS Hopkins, MD, State Epidemiologist, Colorado State Dept of Health; A Main, PhD, R Shope, MD, Yale Arbovirus Research Unit, New Haven, MA Markowski, VD Loverde, MD, State Epidemiologist, Connecticut State Dept of Health Svcs; PS Silverman, DrPH, State Epidemiologist, Delaware Dept of Health and Social Svcs; HL Rubin, DVM, Florida Dept of Agriculture and Consumer Svcs, W Bigler, PhD, JJ Sacks, MD, Acting State Epidemiologist, Florida State Dept of Health and Rehabilitative Svcs; J Cole, DVM, University of Georgia, Tifton, RK Sikes, DVM, State Epidemiologist, Georgia Dept of Human Resources; R Martin, DVM, C Langkop, BJ Francis, MD, State Epidemiologist, Illinois State Dept of Public Health; M Sinsko, PhD, CL Barrett, MD, State Epidemiologist, Indiana State Board of Health; DE Wilcox, MD, State Epidemiolgist, Kansas State Dept of Health and Environment; JC McCammon, Louisville and Jefferson County Dept of Health, MW Hinds, MD, State Epidemiologist, Kentucky State Cabinet for Human Resources; L McFarland, DrPH, CT Caraway, DVM, State Epidemiologist, Louisiana State Dept of Health and Human Resources; CP Lazar, MD, E Israel, MD, State Epidemiologist, Maryland State Dept of Health and Mental Hygiene; V Berardi, H Maxfield, NJ Fiumara, MD, Massachusetts State Dept of Public Health; H McGee, MPH, KR Wilcox, MD, State Epidemiologist, Michigan State Dept of Public Health; MT Osterholm, PhD, AG Dean, MD, State Epidemiologist, Minnesota State Dept of Health; DL Sykes, QA Long, Gulf Coast Mosquito Control Commission, Gulfport, WE Riecken, MD, State Epidemiologist, Mississippi State Board of Health; HD Donnell, MD, State Epidemiologist, Missouri State Dept of Social Svcs; JK Gedrose, State Epidemiologist, Montana State Department of Health and Environmental Sciences; WJ Crans, PhD, New Jersey Agricultural Experiment Station, New Brunswick, WE Parkin, DVM, State Epidemiologist, New Jersey State Dept of Health; J Montes, HF Hull, MD, JM Mann, MD, State Epidemiologist, New Mexico Health and Environment Dept; M Grayson, PhD, R Deibel, MD, DL Morse, MD, R Rothenberg, MD, State Epidemiologist, New York State Dept of Health; JL Pearson, DrPH, Acting State Epidemiologist, North Dakota State Dept of Health; ED Peterson, M Parsons, MS, TJ Halpin, MD, State Epidemiologist, Ohio State Dept of Health; SG Morin, Rhode Island Dept of Environmental Management; R Keenlyside, MD, J Weisfeld, MD, Acting State Epidemiologist, Rhode Island State Dept of Health; KA Senger, State Epidemiologist, South Dakota Dept of Health; RH Hutcheson, Jr, MD, State Epidemiologist, Tennessee State Dept of Public Health; G Hunt, PhD, Harris County Mosquito Control District, RL Johns, PhD, C Reed, MPH, TL Gustafson, MD, Acting State Epidemiologist, Texas State Dept of Health; JM Kobayashi, MD, State Epidemiologist, Washington State Dept of Social and Health Svcs; JP Davis, MD, State Epidemiologist, Wisconsin State Dept of Health and Social Svcs; National Veterinary Svcs Laboratory, US Dept of Agriculture, Ames, Iowa; Div of Vector-Borne Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The occurrence this year of only one confirmed SLE case in the central United States was unexpected. Similarities were noted in climatic conditions this year with those prevailing in previous years, when large Cx. pipiens-borne SLE outbreaks occurred. A mild, wet winter, cool spring, and hot, dry summer are thought to favor overwintering of virus and expansion of vector populations (1). However, this year, only minimal evidence of virus transmission to birds and sentinel chickens was found, and the number of confirmed cases reported to date is unusually low, even for a nonepidemic year.

An outbreak of Cx. tarsalis-borne SLE in the southwest was anticipated from entomologic surveys that disclosed large vector populations in flooded areas of Arizona and California. The attack rate in involved communities was similar to rates observed in Cx. tarsalis-borne outbreaks in California's central valley in the 1950s (1.0-4.7/100,000) (2). The lack of a concommitant WEE outbreak is unexplained.

The age distribution of WEE cases in the upper midwest this year--where infections in infants and children predominated--was typical of WEE outbreaks. Previous investigations in California had

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