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Letter
West Nile Virus in Horses, Guatemala
Maria Eugenia Morales-Betoulle,* Herber Morales,† Bradley J. Blitvich,‡ Ann
M. Powers,§ E. Ann Davis,¶ Robert Klein,* and Celia Cordón-Rosales*
*Universidad del Valle de Guatemala, Guatemala City, Guatemala; †Ministry
of Agriculture and Livestock, Guatemala City, Guatemala; ‡Colorado State University,
Fort Collins, Colorado, USA; §Centers for Disease Control and Prevention,
Fort Collins, Colorado, USA; and ¶US Department of Agriculture-Animal
and Plant Health Inspection Service, Guatemala City, Guatemala
Suggested
citation for this article
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).
Alerted by the findings of WNV transmission in the region (1), we
collected serum samples from horses from 19 departments of Guatemala from
September 2003 to March 2004, to initially estimate the extent of WNV spread
and its potential public health risk. Because no animals exhibited signs of
neurologic illness at the time of the survey, only healthy horses were sampled.
Before 2005, equine WNV vaccines were prohibited and unavailable in Guatemala
(Unidad de Normas y Regulaciones, Ministerio de Agricultura Ganadería y Alimentación,
Guatemala, pers. comm.); as such, cross-reactivity due to prior vaccination
is highly unlikely. Samples were initially tested for WNV-reactive antibodies
by using an epitope-blocking enzyme-linked immunosorbent assay (blocking ELISA)
(6). The ability of the test sera to block the binding of the monoclonal
antibodies to WNV antigen was compared to the blocking ability of control
horse serum without antibody to WNV. Data were expressed as relative percentages
and inhibition values >30% were considered to indicate the presence
of viral antibodies.
A subset of positive samples was further confirmed by plaque-reduction neutralization
test (7). Of 352 samples, 149 (42.3%) tested positive with the 3.1112G
WNV-specific monoclonal antibody. Of 70 blocking ELISA–positive samples, the
neutralization tests indicated the infecting agent was WNV, SLEV, and undifferentiated
flavivirus in 9, 33, and 21 samples, respectively. Titers were expressed as
the reciprocal of serum dilutions yielding >90% reduction in the
number of plaques in a plaque-reduction neutralization test (PRNT90).
PRNT90 titers of horses seropositive for WNV ranged from 80 to
320. PRNT90 titers of horses seropositive for SLEV ranged from
40 to 2,560. For the differential diagnosis of samples with neutralizing antibody
titers against both WNV and SLEV in this test, a >4-fold titer difference
was used to identify the etiologic agent. The undifferentiated flavivirus-reactive
specimens had <4-fold difference in cross-neutralization titers. Likely
possibilities for the inability to distinguish the infecting virus include
previous infection with these or other flaviviruses (previously described
or unknown) resulting in elevated cross-reactive titers. The remaining 10%
of specimens that tested negative by PRNT probably represent nonneutralizing
antibodies in the serum or false positivity in the blocking ELISA.
Our serologic results provide indirect evidence of past transmission of WNV,
SLEV, and possibly other flaviviruses to horses in Guatemala. Although no
confirmed cases of WNV-attributed disease have been reported in Central America
to date, flavivirus transmission appears to be widely distributed in Guatemala
(Figure). Efforts are under way to confirm WNV transmission by viral isolation
and to evaluate the impact of WNV on human, horse, and wildlife populations.
More information is needed to establish the public health threat of WNV and
other zoonotic flaviviruses in the region.
Acknowledgments
We thank animal owners for permitting sampling, numerous veterinarians of
the "Ministerio de Agricultura Ganadería y Alimentación," Alejandra Castillo
for technical assistance, and Nicholas Komar for helpful discussions and reviewing
the manuscript.
This work was supported in part by Division of Vector-Borne Infectious Diseases,
Centers for Disease Control and Prevention, "Organismo Internacional Regional
de Sanidad Agropecuaria" and by grant U50 CCU820510 from the Centers for Disease
Control and Prevention.
References
- Komar N, Clark G. West
Nile virus activity in Latin America and the Caribbean. Rev Panam Salud
Publica. 2006;19:112–7.
- Estrada-Franco JG, Navarro-Lopez R, Beasley DW, Coffey L, Carrara AS,
Travassos da Rosa A, et al. West
Nile virus in Mexico: evidence of widespread circulation since July 2002. Emerg Infect Dis. 2003;9:1604–7.
- Elizondo-Quiroga D, Davis TC, Fernandaz-Salas I, Escobar-Lopez R, Olmos
DV, Gastalum LCS, et al. West
Nile virus isolation in human and mosquitoes, Mexico. Emerg Infect Dis.
2005;11:1449–52.
- Calisher CH, Karabatsos N, Dalrymple JM, Shope RE, Porterfield JS, Westaway
EG, et al. Antigenic
relationships between flaviviruses as determined by cross-neutralization
tests with polyclonal antisera. J Gen Virol. 1989;70:37–43.
- Cupp EW, Scherer WF, Lok JB, Brenner RJ, Dziem GM, Ordonez JV. Entomological
studies at an enzootic Venezuelan equine encephalitis virus focus in Guatemala,
1977–1980. Am J Trop Med Hyg. 1986;35:851–9.
- Blitvich BJ, Bowen RA, Marlenee NL, Hall RA, Bunning ML, Beaty BJ. Epitope-blocking
enzyme-linked immunosorbent assays for detection of West Nile virus antibodies
in domestic mammals. J Clin Microbiol. 2003;41:2676–9.
- Beaty BJ, Calisher CH, Shope RE. Arboviruses. In: Lennette EH, Lennette DA,
Lennette ET, editors. Diagnostic procedures for viral, rickettsial, and chlamydial
infections. 7th ed. Washington: American Public Health Association; 1995.
p. 189–212.
Suggested citation
for this article:
Morales-Betoulle ME, Morales
H, Blitvich BJ, Powers AM, Davis EA, Klein R, et al. West Nile virus in horses,
Guatemala [letter]. Emerg Infect Dis [serial on the Internet]. 2006 Jun [date
cited]. Available from http://www.cdc.gov/ncidod/EID/vol12no06/05-1615.htm
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