Serological investigation of racehorse vaccination against equine influenza in Morocco

International audienceIn order to evaluate the vaccination status against equine influenza (EI) in Moroccan racehorses, a serological investigation was carried out on 509 racehorses using three serological tests: an Enzyme-Linked Immunosorbent Assay (ELISA), the Hemagglutination Inhibition (HI) test and the Single Radial Haemolysis (SRH) assay. The serological analysis showed 56% of seropositivity by ELISA, 67% by HI and 89.4% by SRH (with 69.9% above the clinical protection threshold). Using the Kappa test, the SRH and HI assays showed a strong agreement, the SRH and ELISA assays had a moderate agreement and the HI and ELISA assays showed a poor agreement. Seropositivity was positively correlated with the age of horses and the number of immunisation received. EI vaccines used during the last immunisation before the study had a weak influence on the serological status. This effect was observed when the vaccines Calvenza and Fluvac Innovator® were used, with 94.1% and 100% of seropositivity when measured by HI, and with 100% and 94.7% exceeding the clinical protection threshold when measured by SRH, respectively. No effect was found when other EI vaccines, including Prequenza-Te® (67% coverage (342/509) and Proteqflu-Te® (22% coverage (114/509) were used; with 64% and 67.5% seropositivity (HI) and with 66.4% and 72.8% above the clinical threshold (SRH), respectively. The location and the time since last vaccination have no influence on the serological result. Overall, levels of protective antibody against EI in Moroccan racehorses remain a concern despite mandatory vaccination