The first reported Florida clade 1 virus in the Nordic countries, isolated from a Swedish outbreak of equine influenza in 2011.

This study explores the first reported occurrence of the Florida Clade 1 virus, a form of Equine Influenza Virus (EIV), in the Nordic countries using samples from a Swedish outbreak in 2011. The researchers assert the importance of incorporating both Clade 1 and Clade 2 of the Florida sublineage in EIV vaccines, in line with ESP and OIE recommendations.

Equine Influenza Virus (EIV) Classification

• The study focuses on the H3N8 strain of EIV, a contagious virus that results in respiratory disease among horses. This virus is known to constantly evolve and change — a process known as antigenic drift.
• Specifically, the researchers examined the HA1 and the NA genes of H3N8 from samples taken during Swedish outbreaks in November and December 2011.
• This research found the presence of both Clade 1 and Clade 2 viruses of the Florida sublineage, a classification further refining the H3N8 strain. This sublineage has been detected in many other countries, but this is the first recorded instance in Sweden.

Origins of the Virus

• The study traced the origins of the Clade 2 index case to Spain, believed to have been transported to Sweden via the Netherlands.
• In contrast, the Clade 1 version was initially found in a Swedish stud farm, indicating a local origin.

Clinical Findings and Implications for Vaccination

• Transmission of the Clade 1 virus occurred largely through unvaccinated young horses between training yards, indicating its high infection rate.
• However, some vaccinated horses showed clinical signs of respiratory disease as well, suggesting the current vaccines’ limited effectiveness against this variant.
• The study did not detect any traces of the virus of the Eurasian lineage during the outbreak.
• These findings highlight the necessity for EIV vaccines to include antigens for both Clade 1 and Clade 2 of the Florida sublineage. This aligns with recommendations from the European Surveillance Programme for Veterinary Antimicrobial Consumption (ESVAC) and the World Organisation for Animal Health (OIE).