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Equine influenza A virus (EIV) is a highly infectious respiratory pathogen of horses (Hannant and Mumford 1996, Palese and Shaw 2007). The illness is characterised by an abrupt onset of fever, depression, coughing and nasal discharge, and is often complicated by secondary bacterial infections that can lead to pneumonia and death. Two subtypes of EIV, H3N8 and H7N7, have been isolated. The H7N7 subtype was first isolated from a horse in Czechoslovakia in 1956 (Prague/56), and the H3N8 subtype was first isolated from a horse in Miami in 1963 (Sovinova and others 1958, Waddell and others 1963). The last confirmed outbreak of H7N7 occurred in 1979, and this subtype is now considered to be either extinct or circulating at low levels in a few geographical areas (Ismail and others 1990, Webster 1993, Singh 1994, Madic and others 1996, van Maanen and Cullinane 2002). The H3N8 subtype is a common cause of disease in horses worldwide, particularly in areas where vaccination is not routinely performed (Paillot and others 2006).
Phylogenetically, the H3N8 subtype can be separated into five distinct clades, denoted as predivergence, Eurasian, American (Kentucky), Florida clade 1 and Florida clade 2 (Bryant and others 2009). The predivergence clade is composed of isolates from the 1960s to 1980s that are now extinct in nature. The Eurasian and American lineages emerged in the 1980s and continue to circulate (Daly and others 1996). These lineages were initially named on the basis of the geographical locations from which they were isolated, although strains in the American lineage have since been isolated in Europe. Evolution of the American lineage has resulted in the emergence of American (Kentucky), Florida clade 1 and Florida clade 2 (Bryant and others 2009).
There is little information on the seroprevalence and geographical distribution of EIV in Mexico. Previous studies of the prevalence and clinical manifestations associated with influenza virus infections in Mexico have generally focused on human beings and birds (Ayora- Talavera and others 2005, Cabello and others 2006, Kuri-Morales and others 2006, Villarreal 2006, Senne 2007). The overall aim of the present study was to estimate the seroprevalence of EIV in two geographically distinct areas of Mexico, Nuevo Leon State in north-east Mexico and Guerrero State in the south of the country.
A total of 242 horses at 10 study sites (114 horses from three sites in Nuevo Leon State and 128 horses from seven sites in Guerrero State) were sampled between September 2007 and May 2008 (Table 1). All of the study sites were on privately owned ranches or farms. According to the owners, none of the horses had a history of travel and none had been vaccinated against EIV. All the horses appeared healthy at the time of sampling.