Eastern equine encephalitis incubation time periods of 5 and 8 days.

Abstract

To the Editors: Despite outbreaks of eastern equine encephalitis in humans,1–3 and sporadic cases reported, an actual incubation period for eastern equine encephalitis in humans has not previously been determined. For the purpose of the control of communicable diseases, a handbook has stated that an estimated incubation period in humans of arthropod-borne viral encephalitides, in general, is usually 5–15 days.4 For didactic purpose, a teaching hospital has stated that the incubation period in humans generally exceeds 1 week.5 In mid-August, patient 1, male neonate, 4 weeks of age, had symptoms of fever, lassitude and a focal seizure beginning 1 day before admission to hospital. Serum eastern equine encephalitis virus antibody titer by hemagglutination inhibition was 1:320 on the fifth day in hospital, 1:1280 on the eighth day in hospital and 1:5120 on the twelfth day in hospital.6 The certificate of death stated eastern equine encephalitis. This patient met Centers for Disease Control criteria for a confirmed case by having a 4-fold or greater rise in a hemagglutination inhibition titer.6 This patient had numerous mosquito bites during a family picnic on a single day, 5 days before onset of the first symptom.6 In late-August, patient 2, male child, 7 years of age, had symptoms of fever, headache, lassitude and a generalized seizure beginning 7 days prior to admission to hospital. Serum eastern equine encephalitis virus antibody titer by hemagglutination inhibition was 1:40 and by neutralization was 1:80 on hospital day 2. This patient died on hospital day 2, so a serological test was obtained on only a single occasion, and the presence or absence of a 4-fold rise in titer could not be determined. The certificate of death recorded eastern equine encephalitis. This patient met Centers for Disease Control criteria for a probable case,7 by having hemagglutination inhibition antibody and neutralizing antibody in serum. On a single night, 8 days before the onset symptoms this patient had camped in a residential backyard, near a wooded wetland where eastern equine encephalitis virus was circulating.8 Clinical estimates of the incubation time periods for eastern equine encephalitis were 5 and 8 days, in 1 neonate and 1 child, respectively. In these 2 cases, the eastern equine encephalitis was established based on clinical features compatible with diagnosis and serologic results, meeting the standard for epidemiologic and sufficient for clinical purposes.6,7 Western equine encephalitis was excluded on the basis of serology6,7; and has not been reported in New York state. Herpes simplex encephalitis was excluded on the basis of serology6,7; was not likely as it often involves initially a unilateral temporal lobe; and has no seasonality or geographic specificity. Although autopsies were not performed, there are no published pathognomonic autopsy features of eastern equine encephalitis. It seems clinically reasonable that with these 2 patients, the infections were acquired at the time they were outdoors in proximity to wooded wetland with mosquitoes circulating eastern equine encephalitis.8 Among all of the known and reported cases of eastern equine encephalitis in this geographic region, all had a history of several hours of outdoor activity near a wooded wetland, during the 5-week transmission season. Here, there has been no known case of eastern equine encephalitis in which there was a history of only being indoors or near a home remote from a wooded wetland. We think that the probability of being fed on by an infected mosquito indoors or outdoors remote from a wooded wetland would be lower than the probability outdoors near a wooded wetland. The certainty of a prospective controlled study of exposure of volunteers to infected mosquitoes, as was performed with yellow fever virus and Aedes aegypti, cannot be obtained at the present time. As the geographic range increases for eastern equine encephalitis,9 this information on the incubation time period may be of use to clinicians and epidemiologists. James A. Sherwood, MD Central New York Regional Office Department of Health of the State of New York JoAnne Oliver, PhD Vector Surveillance Unit Bureau of Communicable Diseases Division of Epidemiology Department of Health of the State of New York Syracuse, New York