Outbreak of equine influenza among Thoroughbred horses in Maroñas, Montevideo (Uruguay) during March and April 2012

Abstract

Equine Influenza (EI) is regarded as the most economically important respiratory disease of horses. The aim of this work is to report the epidemiological picture along with the phylogenetic pattern of the virus detected during an EI outbreak occurred among Thoroughbred horses housed in Maronas racing and training facilities, in Montevideo, Uruguay, during March and April 2012. Even though vaccination is not mandatory in Uruguay, racetrack internal regulations require a vaccination certificate for racing; therefore, all the affected horses had been vaccinated at least once in their lifetimes. The first suspicious cases were observed around March 10th. The disease affected approximately 40% of a total population of 1700 horses, and was more severe in the two to three year-old horses. The duration of pyrexia and other clinical signs was variable amongst individuals, yet it was, in average, seven days. Some horses were more severely affected, and two deaths occurred presumably due to pulmonary complications. On March 28th the Animal Health Authority intervened, and carried out a clinical surveillance (to determine the scope of the outbreak), sampling of acutely affected horses and laboratory diagnosis. Equine influenza virus was detected by real time PCR in eight out of ten nasopharyngeal swabs obtained. All the horses had haemagglutination inhibition antibodies at the time of sampling, and the titres ranged from 8 to 256. On March 30th, the World Organisation for Animal Health (OIE) was notified. The outbreak resulted in the withdrawal of several horses from races, temporary cancellation of race meetings, and the ban on the movement of Thoroughbred horses both nationally and internationally. Massive booster vaccination of the horse population at risk was made compulsory. No new cases were observed by April 20th. The origin of the virus has not been precisely identified; nevertheless, the press had reported a similar respiratory syndrome in Porto Alegre and Curitiba (Brazil) racetracks, and horses from these regions had been moved to Maronas for racing purposes. Phylogenetic analysis of the haemagglutinin (HA) gene revealed this EIV is closely related to Clade 1 of the Florida sublineage within the American lineage. HA amino acid alignment shows the Uruguay virus is identical to those identified in Kentucky and New York in 2011, and differs from those causing the outbreak in Japan and Australia in 2007, which have serine (S) instead of proline (P) in the position 176 and lysine (K) instead of glutamine (Q) in the position 203. Recent outbreaks of Clade 1 EIV in the United States and Uruguay indicate its predominance in America. Nowadays, EIV inactivated vaccines currently marketed in Uruguay are under revision and updating following the OIE recommendations.