Molecular epidemiology of equine influenza virus subtype H3N8 in Croatia (1968-2021)

: Equine influenza (EI) is one of the most important viral diseases of horses. The first equine influenza virus (EIV) subtype H7N7 was isolated in 1956. It was circulating worldwide until its disappear-ance in the 1980s. In contrast, EIV subtype H3N8, described in 1963, is still circulating in the horse population. Outbreaks of EI have been reported in Croatia since the 60s of the previous century. The first EIV subtype H3N8 was isolated in Croatia in 1968. Since then, the same subtype caused three EI outbreaks in Croatia in 1988, 2004 and 2015, respectively. In July 2021, outbreaks of acute respiratory infection were reported in two stud farms in continental Croatia, with differences in severity of clinical signs and epidemic size. EIV subtype H3N8 was confirmed as a causative agent. Molecular characterization and phylogenetic analysis showed that the virus is a member of Florida sublineage clade 1. It was the first evidence of the Florida sublineage clade 1 EIV circulation in our country because the last two outbreaks, 2004 and 2015, were caused by Florida sublineage clade 2. This study investigated the molecular epidemiology of EIV subtype H3N8 isolated in Croatia from 1968 to 2021. Results confirmed that EIVs subtype H3N8 isolated in Croatia are closely related to other European strains isolated at the time of outbreaks in other European countries. Croatian isolates’ evolution follows global EIV subtype H3N8 evolution, which indicates the sporadical introduction of the EIV in Croatia. In conclusion, molecular epidemiology of EIV subtype H3N8 in Croatia highlights the necessity of immunoprophylaxis with vaccines containing recent strains to protect the horse population.