Encephalitis: 2010 Outbreak in Panama Raises Concern

Larry Hand

August 21, 2013

Although a few cases of eastern equine encephalitis (EEE) in humans are reported each year in North America, the infection has been extremely rare in Latin America until 2010. In a report published online August 21 in the New England Journal of Medicine, researchers report 13 human cases of confirmed EEE, 11 cases of Venezuelan equine encephalitis (VEE), and 1 case of dual infection in the Darién province of Panama between May and August 2010.

Jean-Paul Carrera, from the Department of Research in Virology and Biotechnology, Gorgas Memorial Institute of Health Studies, University of Panama, and colleagues performed antibody assays and conducted tests to detect viral RNA and isolate viruses in serum samples from 190 patients, 99 of whom were suspected or probable cases of infection. Three patients died, 1 of whom had confirmed VEE.

"Until the Darién outbreak, we had become convinced that the virus in South America was fundamentally different in its ability to infect people and cause serious disease," said University of Texas Medical Branch at Galveston professor Scott Weaver, PhD, senior author of the article, in a university news release. "This epidemic broke that dogma's back very quickly."

Unlike North American cases of EEE which are distributed among all age groups, the age distribution of confirmed Panamanian cases was restricted to children younger than 10 years.

Researchers also confirmed 50 cases of horses with EEE and 8 cases of horses with VEE.

The authors report that the reason for the appearance of human EEE and VEE in 2010 is unknown, although they suggest that surveillance in remote indigenous populations for encephalitis has been incomplete and that human expansion into deforested areas may have exposed people to locally circulating virus. In addition, they note, VEE cases may have been misdiagnosed in the past as dengue cases.

The researchers point out that a 1973 EEE outbreak among horses and similar animals had little evidence of human effect. They write, "This difference between the 1973 and 2010 outbreaks suggests a fundamental change in human infectivity since 1973."

In the news release, Dr Weaver said, "We saw only about a one in 10 case-fatality rate in Panama, which is low by U.S. standards. Still, if this virus has changed and become more virulent for people, we need to know, number one, is it going to spread to other parts of Latin America or number two, are other Latin American strains likely to do the same thing?"

This research was supported by the US National Institutes of Health and the Secretaría Nacional de Ciencía, Tecnología e Innovación, Panama. One coauthor reports serving as an unpaid member of the HIV advisory board for Merck Sharp & Dohme. The other authors have disclosed no relevant financial relationships.

N Engl J Med. Published online August 21, 2013.

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