Efficacy and duration of immunity of a combined equine influenza and equine herpesvirus vaccine against challenge with an American-like equine influenza virus (A/equi-2/Kentucky/95).
Authors: Heldens J G M, Pouwels H G W, van Loon A A W M
Journal: Veterinary journal (London, England : 1997)
Summary
# Editorial Summary Modern equine influenza vaccines are formulated to cover both American and European lineages of A/equi-2, alongside A/equi-1 strains, yet their protective efficacy against contemporary field isolates requires regular validation. Heldens and colleagues (2004) evaluated a combined inactivated influenza–herpesvirus vaccine containing both American and Eurasian A/equi-2 subtypes using a two-part protocol: horses received two doses four weeks apart, with viral challenge performed eight weeks after the first vaccination, whilst a second cohort underwent three vaccinations (at weeks 0, 6, and 32) with serological monitoring at multiple timepoints. Following challenge with the American-like A/equine-2/Kentucky/95 strain, vaccinated horses demonstrated substantially reduced viral shedding and significantly fewer clinical signs (nasal discharge, cough, pyrexia) compared to unvaccinated controls—a critical finding given that virus excretion underpins transmission in mixed populations. Protective antibody titres against all vaccine strains remained above clinically protective thresholds for 26 weeks following the third vaccination, suggesting that triennial boosters may provide robust, sustained immunity. These findings support the utility of regularly updated, dual-lineage vaccines in protecting both individual horses and populations against American-like influenza variants, whilst the duration data provide evidence-based guidance for practitioners establishing vaccination intervals.
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Practical Takeaways
- •This combination vaccine effectively prevents clinical disease and reduces viral shedding when horses are challenged with American-lineage equine influenza, supporting its use in vaccination programmes
- •Protective immunity is established within 4-6 weeks of initial vaccination and booster doses at 6 and 32 weeks maintain protective titres for at least 26 weeks, informing practical revaccination schedules
- •The vaccine's ability to reduce virus excretion is valuable for limiting disease transmission in populations, particularly important in competition and boarding settings
Key Findings
- •Vaccinated horses developed high anti-influenza antibody titres within 4-6 weeks of initial vaccination and remained significantly higher than unvaccinated controls through challenge at 8 weeks
- •Vaccinated horses shed minimal virus after challenge with A/equine-2/Kentucky/95 (H3N8) compared to unvaccinated controls that shed high levels
- •Clinical signs of influenza (nasal discharge, coughing, fever) were substantially reduced in vaccinated versus unvaccinated horses post-challenge
- •Protective antibody titres against all vaccine strains persisted for at least 26 weeks following the third vaccination dose