49. Clinical Characteristics of the 2019 Eastern Equine Encephalitis Virus Outbreak in Michigan

BACKGROUND: Eastern Equine Encephalitis Virus (EEEV) is a mosquito-borne alphavirus responsible for unpredictable outbreaks of severe neurologic disease in humans. While the vast majority of human EEEV infections are either asymptomatic or clinically nonspecific, a minority of patients develops neuroinvasive disease (EEE), which is a devastating illness with a mortality of at least 30%. No treatments are known to be effective. EEEV infection is relatively rare in the United States, with an annual average nationwide incidence of 7 cases between 2009 and 2018. However, 2019 was an exceptionally active year for human EEEV disease, yielding 38 nationwide confirmed cases, including 10 in Michigan, comprising the state’s largest outbreak to date. METHODS: EEE cases were identified by a regional network of physicians. Cases were defined by presentation with clinical symptoms of encephalitis, and by identification of EEEV IgM antibodies or RNA in cerebrospinal fluid (CSF), or EEEV-specific IgM in serum as confirmed by plaque reduction neutralization test. Radiographic images were evaluated and clinical data abstracted through chart review and clinical follow-up where possible. RESULTS: Records from 7 patients were identified and reviewed. The median age was 64, with a male predominance, and all presented in August. Notably, commercial arboviral CSF serology was uniformly negative on the initial CSF sample, and diagnosis was not made until a mean of 23 days (range: 12–38 days) after presentation. Testing in public health laboratories yielded the diagnosis in 5 out of 7 cases. Imaging findings were heterogeneous, but most patients exhibited abnormal findings in the thalamus and/or basal ganglia, and one patient displayed prominent pons and midbrain abnormalities. 4 patients died, while 2 patients survived with severe neurologic sequelae, and 1 patient recovered without sequelae. One patient underwent a limited postmortem examination, which revealed diffuse meningoencephalitis and focal vascular necrosis. CONCLUSION: EEE is a frequently fatal condition whose diagnosis is often delayed, and for which no effective treatments are known. Improved diagnostics are needed to facilitate further clinical studies of EEE and encourage the development of potential therapies. DISCLOSURES: All Authors: No reported disclosures