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.

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.