An outbreak of equine influenza virus in vaccinated horses in Italy is due to an H3N8 strain closely related to recent North American representatives of the Florida sub-lineage

Abstract

In December 2005, equine influenza virus infection was confirmed as the cause of clinical respiratory disease in vaccinated horses in Apulia, Italy. The infected horses had been vaccinated with a vaccine that contained strains representatives from both the European (A/eq/Suffolk/89) and American (A/eq/Newmarket/1/93) H3N8 influenza virus lineages, and the H7N7 strain A/eq/Praga/56. Genetic characterization of the hemagglutinin (HA) and neuraminidase (NA) genes of the virus from the outbreak, indicated that the isolate (A/eq/Bari/2005) was an H3N8 strain closely related to recent representatives (Kentucky/5/02-like) of the American sub-lineage Florida, that was introduced in Italy through movement of infected horses from a large outbreak described in 2003 in United Kingdom. Strain A/eq/Bari/2005 displayed 9 amino acid changes in the HA1 subunit protein with respect to the reference American strain A/eq/Newmarket/1/93 contained in the vaccine. Four changes were localized in the antigenic regions C-D and likely accounted for the vaccine failure.