Abstract
Every year, several epizooties of equine influenza (EI) are reported worldwide. However, no EI case has been identified in France between 2015 and late 2018, despite an effective field surveillance of the pathogen and the disease. Vaccination against equine influenza virus (EIV) remains to this day one of the most effective methods to prevent or limit EI outbreaks and the lack of detection of the pathogen could be linked to vaccination coverage. The aim of this study was to evaluate EI immunity and vaccine coverage in France through a large-scale serological study. A total of 3004 archived surplus serums from French horses of all ages, breeds and sexes were selected from four different geographical regions and categories (i.e., sanitary check prior to exportation, sale, breeding protocol or illness diagnosis). EIV-specific antibody response was measured by single radial hemolysis (SRH) and an EIV-nucleoprotein (NP) ELISA (used as a DIVA test). Overall immunity coverage against EIV infection (i.e., titers induced by vaccination and/or natural infection above the clinical protection threshold) reached 87.6%. The EIV NP ELISA results showed that 83% of SRH positive serum samples from young horses (≤3 years old) did not have NP antibodies, which indicates that the SRH antibody response was likely induced by EI vaccination alone (the HA recombinant canarypoxvirus-based EI vaccine is mostly used in France) and supports the absence of EIV circulation in French horse populations between 2015 and late 2018, as reported by the French equine infectious diseases surveillance network (RESPE). Results from this study confirm a strong EI immunity in a large cohort of French horses, which provides an explanation to the lack of clinical EI in France in recent years and highlights the success of vaccination against this disease. However, such EI protection has been challenged since late 2018 by the incursion in the EU of a Florida Clade 1 sub-lineage EIV (undetected in France since 2009), which is also reported here.
Highlights
Equine influenza (EI) is one of the most important respiratory diseases of horses
Results for each French region selected in this study follow the same trend with protection rates reaching 84% to 87%
When extrapolated to a country level, they highlight the success of EI vaccination and are concomitant with a lack of detection of the pathogen and an absence of clinical EI in the horse population in France
Summary
Equine influenza (EI) is one of the most important respiratory diseases of horses. Beyond the welfare issue induced by this infectious disease, the potential impact for the equine industry could be Vaccines 2019, 7, 174; doi:10.3390/vaccines7040174 www.mdpi.com/journal/vaccinesVaccines 2019, 7, 174 devastating, as clearly illustrated in 2007 when EI reached for the first time the naïve population of Australia, it infected over 76,000 horses [1]. The efficacy of vaccines against equine influenza virus (EIV) infection has been demonstrated through numerous clinical and field studies [2,3,4,5] but little or no information is available about EI immunity in the field and vaccine coverage in horse populations. Most of the studies that investigate vaccination coverage are based on an epidemiological questionnaire and showed that the influenza vaccination coverage is often less than 60% [6,7,8,9]. As for equine influenza, vaccine coverage is well known to be essential to prevent human influenza, as recently illustrated by Uchida et al [10], who showed that high vaccine coverage was significantly and negatively correlated with the level of influenza epidemic among elementary school units. A postal questionnaire survey of randomly selected horse owners in
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