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Current Trends

Nonpolio Enterovirus Surveillance -
United States, January-October 1984

Through October 1984, 896 nonpolio enterovirus (NPEV) isolates have been reported in the United States through CDC's enterovirus surveillance system. This is substantially lower than the 1,415 isolates reported for the same period in 1983. The five most frequently reported NPEV types this year have been Echovirus 9 (170/896; 19.0%), Echovirus 30 (91/896; 10.2%), Coxsackievirus B5 (89/896; 9.9%), Echovirus 11 (72/896; 8.0%), and Coxsackievirus A9 (69/896; 7.7%). The five most common NPEV types in each of the U.S. regions are listed in Table 6. Reported by Respiratory and Enterovirus Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. Editorial Note: Earlier this year, it was predicted that five NPEV types (Echoviruses 9 and 7 and Coxsackieviruses A9, B4, and B5) would be frequently isolated in the United States based on early isolation data from three U.S. regions (1). To date, these types make up 421 (47.0%) of 896 of all reported NPEV isolates, and all five types have been among the 10 most commonly reported types in the United States this year. In years (such as 1984) when fewer than 1,800 enteroviruses are reported, a retrospective CDC study demonstrated that fewer than half of all NPEV reported are predictable; in years with 1,800 or more enteroviruses reported, over 69% of reported NPEV can be predicted. Reference 1. CDC. Enterovirus surveillance - United States, 1984. MMWR 1984;33:388.

TABLE 6. Five most commonly isolated nonpolio enterovirus types, by region – United States, January-October 1984

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abetic Nursing-Home Residents - Colorado," the p value at the end of the fourth sentence in the fourth paragraph on page 622 should be: (p > 0.05). Also, on page 623, the last word on the second line of the title for Figure 1 should be: preintervention.

The Morbidity and Mortality Weekly Report is prepared by the Centers for Disease Control, Atlanta, Georgia, and available on a paid subscription basis from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402, (202) 783-3238.

The data in this report are provisional, based on weekly reports to CDC by state health departments. The reporting week concludes at close of business on Friday; compiled data on a national basis are officially released to the public on the succeeding Friday.

The editor welcomes accounts of interesting cases, outbreaks, environmental hazards, or other public health problems of current interest to health officials. Such reports and any other mat. ters pertaining to editorial or other textual considerations should be addressed to: ATTN: Editor, Morbidity and Mortality Weekly Report, Centers for Disease Control, Atlanta, Georgia 30333. Director, Centers for Disease Control

Editor James O. Mason, M.D., Dr.P.H.

Michael B. Gregg. M.D. Director, Epidemiology Program Office

Assistant Editor Carl W. Tyler, Jr., M.D.

Karen L. Foster, M.A.

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In early October 1984, a private physician notified the Long Beach City Health Department that a local private medical laboratory had confirmed the diagnosis of St. Louis encephalitis (SLE) in a city resident with encephalitis. Subsequently, the health department learned that the laboratory also had serologic evidence of recent SLE infection in several other patients with central nervous system infections. As of October 22, SLE had been confirmed in 11 persons- seven from Los Angeles County, three from Orange County, and one from Riverside County. Dates of onset ranged from August 2 to October 5 (Figure 1). There were no deaths. All but two of the patients were 50 years of age or over. Investigations suggest that all patients were infected locally, because none had recently traveled far from home. At least five additional suspected cases are under investigation.

Routine surveillance of arboviral activity in 1984 had been unremarkable until seroconversions to SLE virus were noted in sentinel chickens from Harbor City (Los Angeles County) and Irvine (Orange County) bled on August 30. SLE virus was isolated from pools of Culex tarsalis mosquitoes collected from the Harbor City site on September 13 and September 18. More seroconversions were found in bleedings of the Harbor City flock on September 21 and the Irvine flock on October 11. These seroconversions probably reflected viral transmission in the previous 2 weeks. With the onset of cooler weather, by mid-October, mosquito populations had decreased below levels normally associated with risk of transmission to humans (1). Reported by Local mosquito control agencies, 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 Sucs, Orange County Health Care Agency, County of Riverside, California State Dept of Health Svcs; Div of Vector-borne Viral Diseases, Center for Infectious Diseases, CDC. Editorial Note: From 1945 to 1959, major combined outbreaks of western equine encephalitis (WEE) and SLE occurred in California, principally in the Sacramento and San Joaquin Valleys, although 10% of cases were reported from Imperial and Riverside Counties (2). In 1952, when the largest arbovirus encephalitis outbreak in the state's history occurred, no human cases occurred in Los Angeles or Orange Counties (2). The epidemiology of WEE and SLE in the central valley was characterized by endemic transmission, resulting in increased immunity in the population with increasing length of residence (3). Consequently, most clinical infections occurred among children and young adults (3). In contrast to this pattern, in east-central and Atlantic states, where SLE transmission occurs intermittently and the population is largely susceptible, major, often urban-centered outbreaks occur, affecting principally the elderly, who are biologically more susceptible (4).


St. Louis Encephalitis Continued

Most of the persons in the current outbreak were over 50 years of age, indicating that I larger numbers of milder clinical cases in younger age groups may not have been recognized. The crude attack rate to date for Long Beach, where cases clustered, was 3/100,000 population among persons 60 years of age or older. Although this is a relatively low attack rate, compared with previously described urban SLE outbreaks (3), case finding in Los Angeles has been passive thus far. The age distribution in this outbreak suggests that endemic SLE-virus

1 transmission has not previously occurred in the area and that the underlying level of immunity in the population may be low.

In urban SLE outbreaks in the east, Cx. p. pipiens and Cx. p. quinquefasciatus are the principal vectors. Cx. tarsalis is the vector of SLE and WEE in the rural west; however, investigations have indicated a potential role for Cx. p. quinquefasciatus in SLE transmission in Imperial County, California (5), and, in 1966, in Tucson, Arizona (6). In Dallas, Texas, an outbreak in 1966 was attributed to introduction of SLE virus from the rural Cx. tarsalis cycle to urban Cx.

1 p. quinquefasciatus (7,8). The identity of the vector species in this outbreak was not deter

1 mined. Comprehension of the vector ecology and epidemiology of SLE in Los Angeles will be

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Seroconversions to SLE virus in sentinel chickens, Long Beach, weeks 34 and 37; Irvine, weeks 34 and 41.

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