West Nile Virus in Horses, Guatemala

To the Editor: West Nile virus (WNV, Flaviviridae: Flavivirus) is emerging as a public health and veterinary concern.Since its introduction into North America in 1999, it has spread rapidly, reaching the Caribbean Basin in 2001, Mexico in 2002, El Salvador in 2003, and Colombia in 2004 (1).However, reports of equine illness and deaths in Latin America are inconclusive.With the exception of viral isolates from a dead bird, a human, and a mosquito pool in Mexico (2,3), all reports of WNV presence in Latin America have relied on serologic evidence.WNV is a member of the Japanese encephalitis serocomplex, which in the Western Hemisphere includes St. Louis encephalitis virus (SLEV) (4).Serologic investigations for WNV in Latin America must use highly specific assays to differentiate WNV infection from potentially cross-reactive viruses such as SLEV or possibly additional unknown viruses.In particular, SLEV is of concern since it was previously isolated from Guatemalan mosquitoes (5).