Eastern equine encephalitis in Latin America

Abstract

Background: The eastern equine encephalitis (EEE) and Venezuelan equine encephalitis (VEE) viruses are pathogens that infect humans and horses in the Americas. Outbreaks of neurologic disease in humans and horses were reported in Panama from May through early August 2010.

Methods: We performed antibody assays and tests to detect viral RNA and isolate the viruses in serum samples from hospitalized patients. Additional cases were identified with enhanced surveillance.

Results: A total of 19 patients were hospitalized for encephalitis. Among them, 7 had confirmed EEE, 3 had VEE, and 1 was infected with both viruses; 3 patients died, 1 of whom had confirmed VEE. The clinical findings for patients with EEE included brain lesions, seizures that evolved to status epilepticus, and neurologic sequelae. An additional 99 suspected or probable cases of alphavirus infection were detected during active surveillance. In total, 13 cases were confirmed as EEE, along with 11 cases of VEE and 1 case of dual infection. A total of 50 cases in horses were confirmed as EEE and 8 as VEE; mixed etiologic factors were associated with 11 cases in horses. Phylogenetic analyses of isolates from 2 cases of equine infection with the EEE virus and 1 case of human infection with the VEE virus indicated that the viruses were of enzootic lineages previously identified in Panama rather than new introductions.

Conclusions: Cases of EEE in humans in Latin America may be the result of ecologic changes that increased human contact with enzootic transmission cycles, genetic changes in EEE viral strains that resulted in increased human virulence, or an altered host range. (Funded by the National Institutes of Health and the Secretaría Nacional de Ciencia, Tecnología e Innovación, Panama.).

Figure 1. Incidence Rates of Eastern Equine Encephalitis (EEE) and Venezuelan Equine Encephalitis (VEE) Virus Infections in Panama in 2010.

Panel A shows a map of Panama, with the areas of interest outlined in red and enlarged in Panels B and C. Panel B shows the incidence rates of EEE on the basis of 14 confirmed cases (13 cases of EEE and 1 case of dual EEE–VEE virus infection). Panel C shows the incidence rates of VEE virus infection on the basis of 12 confirmed cases (11 cases of VEE virus infection and 1 case of dual infection). Incidence rates have been rounded to the nearest 10th and were calculated on the basis of the respective township population available from the 2010–2011 Panamanian census.

Figure 2. Epidemic Curve for Equine and Human Cases Compatible with Infection with the EEE or VEE Virus.

Confirmed cases in humans that were detected during the outbreak and during enhanced surveillance in the provinces of Darién and Panama included 14 cases of infection with the EEE virus (13 cases plus 1 case of dual EEE–VEE virus infection) and 12 cases of infection with the VEE virus (11 cases plus 1 case of dual EEE–VEE virus infection). Confirmed equine cases in the provinces of Panama, Darién, and Colón totaled 69, with neurologic and febrile disease that was consistent with infection with the EEE or VEE virus.

Figure 3. Phylogenetic Trees of Alphaviruses Isolated during the EEE and VEE Outbreaks in Panama in 2010.

Shown are the results of a maximum-likelihood estimation of the phylogenetic trees for 41 strains of the EEE virus (Panel A) and 46 strains of the VEE virus (Panel B), based on the strains isolated in the Panama outbreaks (shown in red). Viral strains are listed according to subtype, country or state of collection, and year of collection. Bootstraps are shown on major branches and were determined in GARLI (Genetic Algorithm for Rapid Likelihood Inference) with 100 replicates. Scale bars show nucleotide sequence divergence.