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Current Trends Arboviral Infections of the Central Nervous System -- United States, 1984

In 1984, arboviral infections of the central nervous system (CNS) occurred in 109 persons (Figures 2 and 3). An outbreak of 26 St. Louis encephalitis (SLE) cases in southern California was the first urban-centered SLE outbreak in the western United States. Elsewhere, few SLE cases were reported, and enzootic SLE activity was minimal. Two sporadic western equine encephalitis (WEE) cases were reported from South Dakota. Five eastern equine encephalitis (EEE) cases occurred in recognized endemic foci in the eastern United States. CNS infections from LaCrosse virus were reported in record numbers from Indiana (15 cases) but occurred in usual numbers elsewhere in the upper Midwest. ST. LOUIS ENCEPHALITIS

Twenty-six confirmed or presumptive SLE cases were reported in California in 1984, the largest annual reported number since 1959, when 40 cases were reported (1). The outbreak was focused in the greater Los Angeles area, which previously had reported only one SLE case between 1945 and 1982. The crude attack rates and standardized morbidity ratios (SMRs) were highest in Riverside County and lowest in San Diego County (Table 2); these extremes in counties distant from the epicenter in greater Los Angeles are unexplained, although surveillance artifact may have contributed to the low attack rate in San Diego. The high attack rate in the Riverside County population may have been related to more intense exposure to vectors in a rural transmission cycle (see below). Attack rates rose with age but declined in the elderly (65 years of age or older) (Table 2). The only fatality occurred in a 62-year-old woman. An analysis of attack rate by sex and county of residence (Table 3) showed a gradient of increasing attack rate for males from Los Angeles County eastward through Orange County to Riverside County. No significant trend was observed in attack rates for females. Cases in Riverside County occurred earlier than those in other counties. However, the index patient was a Los Angeles city resident.

In Arizona, four human cases were reported in August and September. Two cases were reported from Maricopa County, and one each, from Navajo and Pinal Counties. All four cases occurred in females; one was a 25-year-old woman, and the others were children.

In Texas, Colorado, and Florida, sporadic SLE cases occurred in the absence of notable enzootic activity. A case of SLE occurred in a 20-year-old man from New Providence, Bahamas, who became ill 10 days after arriving in Tampa, Florida. The interval between his arrival in Florida and onset of illness was consistent with exposure in either Nassau or Florida. No coincident SLE activity was observed in sentinel birds in the Tampa area or elsewhere in Florida.

In Ohio, Indiana, Illinois, Kentucky, Tennessee, and Iowa, no human SLE cases were reported. Remarkably little evidence of SLE virus infections in wild birds was adduced in any of the surveillance schemes in place in these states. Overall, less than 0.1% of all birds in these surveillance efforts had SLE virus antibody. EASTERN EQUINE ENCEPHALITIS

In Massachusetts, two human and three equine cases were reported. The putative but unproven site of exposure for one patient, a boy from Farmington, was southern coastal New Jersey. Infections in a 63-year-old woman from Foxboro and three equine cases from Taunton, Brockton, and Middleboro were presumably acquired in eastern Massachusetts counties where EEE occurs perennially.

In Maryland, no human cases were reported. However, a dramatic epornitic occurred at the Patuxent Wildlife Research Station, where whooping cranes (Grus americana) and sandhill cranes (G. canadensis) are bred to produce eggs that are placed in the wild for rearing. Only 150 whooping cranes are extant. Of 39 whooping cranes in the colony, precipitous deaths occurred in seven between September 17 and November 4. Four were found dead in their pens, and three were moribund and died within hours of discovery. EEE was confirmed as the cause of death by the U.S. Department of Agriculture Laboratory at Ames, Iowa, which isolated the virus from liver and spleen of the dead cranes. Pathologic examination at the National Zoo in Washington, D.C., disclosed widespread visceral involvement with diffuse and focal hepatic, renal, and splenic necrosis, necrotizing enterocolitis, and orchitis or oophoritis. Histopathologic findings of encephalitis were absent. One of 17 whooping crane sera collected and stored frozen since 1974 had EEE antibody, indicating that enzootic infections had occurred in previous years. However, no clusters of unexplained deaths had been noted before.

In New Jersey, a record number of encephalitis cases in horses was reported in 1984. Nineteen confirmed or presumptive cases were reported, chiefly in southern counties where EEE occurs perennially. A single human EEE case (nonfatal) was reported in a 6-year-old boy from Dividing Creek, in southern Cumberland County.

In Florida, two human EEE cases occurred in 1984--in a 55-year-old Polk County man and a 52-year-old Jacksonville woman. Eighty-one confirmed or presumptive equine cases were reported, chiefly in northeast counties, where a hyperenzootic focus exists. WESTERN EQUINE ENCEPHALITIS

Nationwide, little epizootic activity was observed (Figure 2). The only human WEE cases in the United States were reported in two South Dakota men. Since 1964, North Dakota and South Dakota have had the highest WEE incidence, with crude average annual rates of 0.598/100,000 and 0.293/100,000, respectively. In 1984, evidence of Culex tarsalis activity and WEE virus transmission were unremarkable in the two states. Two equine cases were reported from North Dakota, and one equine case was reported from South Dakota. No WEE virus was isolated from Cx. tarsalis collected in South Dakota or Minnesota. The low minimum infection rate of Cx. tarsalis collected in North Dakota, 0.48/1,000, was consistent with the observed low level of epizootic activity. VIRUSES OF THE CALIFORNIA SEROGROUP

In the United States, LaCrosse virus is the principal agent of morbidity from infections by California serogroup viruses. From 1963 to 1983, the states with the highest annual incidence of reported CNS infections from LaCrosse virus have been in the upper midwest (in declining order): Wisconsin--0.434/100,000/year (average crude rate); Minnesota--0.308; Ohio--0.236; Iowa--0.144; and Indiana--0.083.

In 1984, 15 cases were reported from Indiana (0.282/100,000), the largest annual number of cases notified since 1982, when 12 were reported. Seven cases were reported from Iowa (0.244/100,000), chiefly from central and eastern counties. Twenty-one confirmed and five presumptive cases of CNS infection from LaCrosse virus were reported from Ohio (0.195/100,000). Two cases were reported from Illinois, an unusually low number. These changes probably reflect surveillance artifact, e.g., recent increased interest in Jamestown Canyon virus infections in Indiana (2).

In Wisconsin, 11 cases were reported from counties in recognized endemic areas, principally in southwestern counties. In LaCrosse County, where intensive control activities have been focused, a gradual decline in reported cases began in 1980, and no cases have been reported in the last 3 years. However, a secular decline in reports of LaCrosse CNS infections has also been observed in surrounding counties, where control programs have not been as aggressive.

In New York, two confirmed cases of LaCrosse encephalitis and six other suspected Jamestown Canyon or LaCrosse virus infections were reported. South Carolina reported its first LaCrosse case in the last 20 years. Oklahoma reported its first case of CNS infection from LaCrosse virus in 1984. North Carolina reported three cases, all in western counties near Asheville. 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, N Petersen, SM, State Epidemiologist, Arizona Dept of Health Svcs; TC McChesney, DVM, Acting State Epidemiologist, Arkansas Div of Health Maintenance; 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, S 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, P Silverman, DrPH, State Epidemiologist, Delaware State Dept of Health and Social Svcs; R Montali, DVM, National Zoo, ME Levy, MD, Administrator, District of Columbia, Preventive Health Svcs

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