Immunostimulating effect of inactivated <i>Parapoxvirus ovis</i> on the serological response to equine influenza booster vaccination

<p dir="ltr"><b>Background:</b> Equine influenza virus (EIV) is responsible for recurring outbreaks that are detrimental to the equine industry. Vaccination is key for prevention, but the effectiveness and duration of protection provided by existing vaccines is insufficient.</p><p dir="ltr"><b>Objectives:</b> To improve vaccine efficacy, the benefit of immune stimulation with inactivated Parapoxvirus ovis (iPPVO¹) on the antibody response induced by a vaccine boost against EIV was evaluated.</p><p dir="ltr"><b>Study design:</b> Experimental vaccination challenge.</p><p dir="ltr"><b>Methods:</b> Twenty healthy mares, breed Selle-français, Anglo-Arabian, trotter and draught horses between 11 and 25 years of age were enrolled in the study. The horses were stratified into three different age groups (i.e., 11–14, 15–18 and 19–25 years) and then randomly allocated to a control group that received the semi-annual EI booster (SAB) vaccination or the iPPVO group that received the SAB vaccination plus iPPVO injections. Antibody levels were measured with the single radial haemolysis (SRH) assay at 1, 3 and 6 months post-vaccination.</p><p dir="ltr"><b>Results:</b> Results revealed that horses that received iPPVO had higher antibody levels than the control group injected with the EI vaccine alone. Although the vaccine used contains only a clade 1 and European lineage strain, the increase in protective antibodies was also observed against a clade 2 strain.</p><p dir="ltr"><b>Main limitations:</b> It should be noted that in this experimental protocol, iPPVO was not mixed and co-injected with the vaccine, but administered near the injection site of the vaccine.</p><p dir="ltr"><b>Conclusions:</b> Immune stimulation with iPPVO, a substance already marketed as an immunostimulant, could be used to improve vaccination protocols in horses and potentially other species.</p>