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Arboviral Disease -- United States, 1994

Arboviruses are mosquitoborne and tickborne agents that persist in nature in complex cycles involving birds and mammals, including humans. Characteristics of arboviral infection include fever, headache, encephalitis, and sometimes death. In 1994, health departments in 20 states reported 100 presumptive or confirmed human cases of arboviral disease * to CDC. Of these, 76 were California (CAL) serogroup encephalitis; 20, St. Louis encephalitis (SLE); two, western equine encephalomyelitis (WEE); one, eastern equine encephalomyelitis (EEE); and one, Powassan encephalitis (POW). This report summarizes information about arboviral disease in the United States during 1994. Powassan Encephalitis

POW was serologically confirmed in a 49-year-old female resident of Massachusetts who had onset of illness May 24. She reported removing an engorged tick from her abdomen approximately 2 weeks before onset of symptoms. She was admitted to the hospital on May 25 with a diagnosis of meningoencephalitis, which progressed during the following 72 hours to encephalitis involving the brain stem and basal ganglia. During hospitalization, the patient was comatose for 3 days and required mechanical ventilation. On June 16, she was discharged to a rehabilitation center and, on July 25, was transferred to a resident health-care facility. On examination in August 1995, she had residual weakness in her right leg requiring a brace. The patient's prolonged convalescence is consistent with that reported for POW encephalitis. California Serogroup Encephalitis

During 1994, a total of 76 human CAL serogroup encephalitis cases were reported from 13 states: West Virginia (32 cases), Ohio (14), Wisconsin (seven), Illinois (six), Minnesota (four), Indiana and North Carolina (three each), Alabama (two), and Iowa, Kentucky, Michigan, Rhode Island, and Virginia (one each). Patients ranged in age from 6 months to 26 years (mean: 7 years). A total of 57 cases (75%) occurred among males. Onsets of illness occurred in May (one case), June (one), July (12), August (35), September (22), and October (five). St. Louis Encephalitis

During 1994, a total of 20 human cases of SLE were reported from five states. Sixteen cases were reported in Louisiana; most (14) occurred in urban New Orleans (Orleans and Jefferson parishes). Three cases (in 44- and 60-year-old men and a 63-year-old woman) were fatal. Patients ranged in age from 12 to 78 years (mean: 46 years). Of the 16 cases, nine (56%) occurred among males. SLE cases also were reported in residents of Riverside County, California; Charlotte County, Florida; Forrest County, Mississippi; and Harris County, Texas (one each). For the 20 total cases, onsets of illness occurred in July (one case), August (nine), September (nine), and October (one). Western and Eastern Equine Encephalomyelitis

During 1994, two human cases of WEE were reported from Goshen County in southeastern Wyoming; the cases occurred in a 40-year-old woman and a 42-year-old man. One human case of EEE in a 67-year-old man was reported from Iberville Parish, Louisiana. Western and Eastern Equine Encephalomyelitis in Animals

Surveillance for arboviral disease includes cases in susceptible animals because, during previous outbreaks, animal cases preceded human cases by 2-3 weeks. During 1994, a total of five WEE cases among horses were reported from three states: Idaho (two cases), Wyoming (two), and Texas (one). WEE was isolated from emus in Boulder County, Colorado (one), and Lancaster County, Nebraska (one), and from a symptomatic pigeon in Stanislaus County, California.

A total of 133 cases of EEE among horses were reported from 11 states: Florida (54 cases), South Carolina (20), North Carolina (15), Michigan (12), Georgia (nine), Alabama and New Jersey (seven each), Indiana and Louisiana (three each), Ohio (two), and Virginia (one). In addition, EEE virus was isolated from other species in five states. In Michigan, virus was isolated from two pheasant flocks. In Florida, EEE virus was isolated from specimens of viscera from a symptomatic duck and from 1-4-week-old piglets during an epizootic in the Florida panhandle in which 50 of 90 piglets observed had objective central nervous system signs; the number of deaths is unknown. In Georgia, EEE virus was recovered from a litter of 3-week-old boxer puppies; three of five puppies in the litter died. EEE cases in emus were reported from New Jersey (10 cases), Florida (three), Georgia (two), and North Carolina (one).

Reported by: State health depts. D Jacoby, MD, Massachusetts General Hospital, Boston. M McGuilf, DVM, Epidemiology Div, J Fontana, MS, B Werner, PhD, Virology Div, Massachusetts Dept of Public Health State Laboratory. L McFarland, DrPH, S Wilson, M Kohn, MD, H Bradford, PhD, Louisiana Dept Health and Hospitals; E Bordes, New Orleans Mosquito Control Board, New Orleans. D Alstad, DVM, National Veterinary Svcs Laboratories, Animal Plant and Health Inspection Svc, US Dept of Agriculture, Ames, Iowa. H Rubin, DVM, Bur of Diagnostic Laboratories, Florida Dept of Agriculture and Consumer Svcs, Kissimmee. S Baldwin, DVM, Veterinary Diagnostic and Investigation Laboratory, Univ of Georgia, Tifton. Epidemiology and Ecology Section, Div of Vector-Borne Infectious Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The findings in this report indicate that CAL serogroup encephalitis remains the most frequently reported arbovirus infection in the United States. Although the number of CAL serogroup encephalitis cases has remained relatively constant since the 1970s and was reported primarily from the Midwest, the number of cases reported from the South has increased. For example, in 1994, Alabama for the first time reported CAL serogroup encephalitis cases, and Kentucky and Virginia -- which previously had reported a total of only six cases since 1964 -- each reported one in 1994.

In general, SLE occurs as periodic focal outbreaks followed by years of sporadic cases. In 1994, a small focal outbreak of SLE occurred in urban New Orleans. Evaluation of case-patients by date of illness onset and location suggests that the earliest cases occurred among persons living within or in proximity to urban public housing projects. Subsequent cases followed a pattern of radial spread from the central urban area, although the small number of cases preclude a definitive analysis. An investigation by New Orleans Mosquito Control Board personnel found large populations of immature and adult Culex pipiens quinquefasciatus mosquitoes under housing units. Leaking sewer lines located in the crawl space beneath these housing units provided an extensive and ideal habitat for the SLE virus vector mosquito.

The POW case in Massachusetts in 1994 was the first reported from that state. Previously, the most recent POW case in the United States occurred in New York in 1978. POW virus is a tickborne flavivirus most closely related to Russian spring summer and Central European encephalitis viruses. Although understanding of the epidemiology of POW virus in the United States is limited, the virus appears to be widely distributed. In North America, Ixodes cookei has been implicated as the principal tick vector, and virus has been recovered from several rodent and carnivore species, including the red squirrel, woodchucks, striped and spotted skunks, foxes, short- and long-tailed weasels, and the white-footed deer mouse. **

Human infections with POW virus occur infrequently, with seroprevalence rates of 0.5%-4.0% in areas where the virus is endemic (1). During 1958-1981, a total of 19 confirmed POW cases among humans were reported in North America, primarily from the northeastern United States and eastern Canada. Since 1981, five additional confirmed cases have been reported from Canada: Quebec (two, one fatal) (H. Artsob, Quebec Laboratory Center for Disease Control, personal communication, 1995); New Brunswick (one) (2); Ontario (one); and Nova Scotia (one) (M. Mahdy, Ontario Ministry of Health Laboratory Services, personal communication, 1995). Based on evaluation of the 24 total POW cases that occurred in North America during 1958-1994, risk for infection may be highest in wooded areas where potential contact with infected rodent or carnivore hosts or tick vectors is greatest. Of the 24 cases, 21 occurred in persons aged less than 20 years. Four of the acute infections were fatal, and two patients died 1 and 3 years after onset as a result of sequelae reported to be directly related to the disease.

Health-care providers should consider arboviruses in the differential diagnosis of aseptic meningitis and encephalitis cases during the summer months. Early identification of arboviral cases is important to implement risk-reduction strategies (i.e., use of vector-control practices, repellents, and changes in human activity patterns). Serum (acute and convalescent) and cerebrospinal fluid samples should be obtained for serologic testing, and cases should be promptly reported to state health departments. New rapid diagnostic techniques, including detection of immunoglobulin M antibody in acute serum or cerebrospinal fluids, have facilitated confirmation of arbovirus infections.


  1. Artsob H. Powassan encephalitis. In: TP Monath, ed. The arboviruses: epidemiology and ecology. Vol IV. Boca Raton, Florida: CRC Press, Inc, 1988:29-49.

  2. Fitch W, Artsob H. Powassan encephalitis in New Brunswick. Can Fam Physician 1990;33:1289-90.

* At CDC, a confirmed case is defined as febrile illness with mild neurologic symptoms, aseptic meningitis, or encephalitis with onset during a period when arbovirus transmission is likely to occur, plus at least one of the following criteria: 1) fourfold or greater rise in serum antibody titer, 2) viral isolation from tissue, blood, or cerebrospinal fluid; or 3) specific immunoglobulin M (IgM) antibody in cerebrospinal fluid. A presumptive case is defined as compatible illness, plus either a stable elevated antibody titer to an arbovirus ( greater than or equal to 320 by hemagglutination inhibition, greater than or equal to 128 by complement fixation, greater than or equal to 256 by immunofluorescent assay, or greater than or equal to 160 by plaque-reduction neutralization test) or specific IgM antibody in serum by enzyme immunoassay. 

** Tamiasciurus hudsonicus, Marmota monax and Mephitis mephitis, Spilogale putorius, Vulpes sp. Urocyon Cinereoargenteus (gray fox), Mustella erminea and Mustella frenata, and Peromyscus maniculatus, respectively.

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