. /.  WORLD TRADE  
ORGANIZATION   
G/SPS/GEN/1124 
19 October 2011  
 
(11-5174) 
Committee on Sanitary and Phytosanitary Measures Original:   Spanish 
  
 
REPORT ON THE EPIDEMIOLOGICAL ANALYSIS OF OUTBREAKS OF 
VENEZUELAN EQUINE ENCEPHALITIS (ENZOOTIC IE STRAIN) 
 
Communication from Mexico 
 
 
 The following communication, received on 18 October 2011, is being circulated at the request of the delegation of Mexico. 
 
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 1. Following two outbreaks of Venezuelan Equine En cephalitis (enzootic IE strain) last July and 
September in the states of Tabasco and Veracruz, the National Agriculture and Food Health, Safety 
and Quality Service (SENASICA) carried out an epid emiological analysis in order to provide further 
details on the outbreaks and on the closure of th ese cases to the World Organisation for Animal 
Health (OIE).  The analysis revealed the following: 
2. According to the epidemiological inves tigations conducted to date, there is no 
epidemiological relationship between the two case s, nor have there been any further cases. 
3. Mexico has been free from Venezuelan Equi ne Encephalitis (VEE) caused by epizootic 
strains since 1972.  SENASICA has maintained its passive epidemiological surveillance through the 
notification of suspect neurological cases in which antibodies against the virus can be detected in 
percentages that are insignificant from a statistical and epidemiological point of view, but the virus 
has not been isolated in the population at risk. 
4. In the recently notified outbreaks, the enzoo tic virus was isolated, but its dissemination 
capacity within the sample population of horses or in  the surrounding areas could not be established.  
This means that we can assume that although the enz ootic variant of the virus could affect an equid, 
owing to its low virulence and pathogenicity it  would not multiply or be transmitted through the 
infected equid to the rest of the exposed populat ion.  Consequently, the outbreak is basically 
self-limiting in the infected equids.  Moreover, the preventive use of vaccinations against both 
enzootic and epizootic viruses means that the serol ogical diagnosis is not conclusive, so that the 
isolation and genetic characterizati on of the virus is what counts. 
5. In the two VEE cases identified in July 2011 (Tab asco and Veracruz), Mexico concluded that 
the histopathological results and the lesions  observed corresponded to discrete localized 
leukoencephalomalacia with gliosi s and moderate multifocal haemo rrhaging, and no lesions were 
observed that would suggest a viral process.  At the same time, the viral isolation for VEE was 
negative. 
6. It should be stressed that Mexico maintains c ontinuous passive epidemiological surveillance, 
with daily work plans involving vis its to contact points in the livest ock sector to encourage reporting, 
for the purposes of early detection of any health pr oblems that pose a risk for production, productivity G/SPS/GEN/1124 
Page 2  
 
  and public health.  At the same time, Mexico ma intains a vector control programme in places where 
Dengue has previously been present, including the states of Tabasco and Veracruz.  This control 
programme is intensified during the rainy season, which is when Culicoides are most common in 
sub-tropical and tropical areas, which contributes indirectly to reducing the risk of VEE. 
7. In view of the above considerations and in order to speed up the return or exportation of 
equids participating in the XVI Pan American game s to be held in Guadalajara, Jalisco, from 14 to 
30 October 2011, Mexico proposes that Members: 
 Maintain the recognition of Mexico as free from epizootic Venezuelan equine 
encephalomyelitis;  or 
 recognize the closure of VEE cases in accord ance with the decisions notified to the 
OIE on 29 August and 13 September 2011;  or 
 accept the regional nature of the outbreaks,  recognizing the state of Jalisco as free 
from the disease. 
8. This communication is being made for reasons of transparency under Article 7 of the 
Agreement, and is without prejudice to Mexico's rights under that Agreement. 
 
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