Serological investigation of racehorse vaccination against equine influenza in Morocco.

This research investigates the effectiveness of vaccinations against equine influenza (a highly contagious disease among horses) in racehorses in Morocco. The study found high levels of seropositivity (presence of antibodies) in the horses tested, showing a good response to the vaccination, but results varied depending on the vaccine used and the number of immunizations given.

Methodology

• The researchers undertook a serological investigation of 509 racehorses in Morocco. This means they studied blood samples to detect the presence of antibodies against equine influenza, which are produced when the body fights off a disease.
• The presence of vaccines-related EI antibodies was detected using three different serological tests: the Enzyme-Linked Immunosorbent Assay (ELISA), the Hemagglutination Inhibition (HI) test and the Single Radial Haemolysis (SRH) assay.

Findings

• Overall, 56% of horses tested showed seropositivity according to the ELISA test, 67% according to the HI test, and 89.4% according to the SRH assay. Among those detected by SRH, 69.9% had antibody levels above the clinical protection threshold, suggesting they had significant immunity to equine influenza.
• The SRH and HI assays agreed strongly with each other, the SRH and ELISA assays had moderate agreement, while the HI and ELISA assays did not agree well. Thus, different testing methods can yield different results.
• Seropositivity was positively correlated with the age of horses and the number of immunizations they had received. Older horses and those with more vaccinations were more likely to show seropositivity.

Influence of the type of EI Vaccines

• The vaccines used last before the study seemed to affect the serological status of the horses. When the Calvenza and Fluvac Innovator® vaccines were used, high levels of seropositivity were found: 94.1% and 100% respectively for the HI test, and 100% and 94.7% (exceeding the clinical protection threshold) respectively for the SRH test.
• No such effect was found when other vaccines—Prequenza-Te® and Proteqflu-Te®—were used, despite their extensive coverage (67% and 22% respectively). Their seropositivity rates were lower: 64% and 67.5% for the HI test, and 66.4% and 72.8% (exceeding the clinical protection threshold) for the SRH test.

Influence of Location and Time Since Last Vaccination

• Neither the geographical location of the horses, nor the time since their last vaccination, affected the serological results.

Conclusion

• Despite mandatory vaccinations against equine influenza in Morocco, the protective antibodies’ level remains a concern. Despite high rates of seropositivity, the level of protection may not be sufficient in many horses, depending on the type of vaccine and the number of immunizations.