Description of the outbreak of equine influenza (H3N8) in the United Kingdom in 2003, during which recently vaccinated horses in Newmarket developed respiratory disease.
Authors: Newton J R, Daly J M, Spencer L, Mumford J A
Journal: The Veterinary record
Summary
# Editorial Summary: Equine Influenza H3N8 Outbreak, UK 2003 Between March and May 2003, a widespread outbreak of equine influenza H3N8 affected at least 12 locations across the UK, with the most severe impact concentrated in 21 Newmarket thoroughbred training yards housing over 1,300 racehorses, where clinical signs of cough and nasal discharge persisted for nine weeks. Notably, many infected horses had received vaccination within the preceding three months using a bivalent vaccine covering both European (A/eq/Newmarket/2/93) and American (A/eq/Newmarket/1/93) lineages, marking the first documented UK isolation of the American Kentucky/5/02 sublineage. Antigenic and genetic sequencing revealed close homology between the outbreak strain and the American vaccine strain, with vaccinated horses demonstrating adequate single radial haemolysis antibody titres, yet vaccine protection remained incomplete. Interestingly, two-year-old vaccinated horses experienced significantly fewer infections than older vaccinated animals despite equivalent antibody levels, suggesting the vaccine failure reflected qualitatively compromised cellular immunity rather than insufficient antibody titre alone. These findings underscore the importance of understanding that adequate serological responses do not guarantee clinical protection, and that age-related factors in immune maturation warrant consideration when evaluating vaccination efficacy in working horses and designing future vaccine protocols.
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Practical Takeaways
- •Vaccination alone does not guarantee protection against equine influenza; recently vaccinated horses can still develop clinical disease, requiring additional biosecurity measures during outbreaks
- •Monitor recently vaccinated horses for respiratory signs and isolate suspected cases, as vaccine-strain mismatch with emerging virus lineages can occur without obvious antigenic drift
- •Age-related differences in infection susceptibility exist among vaccinated horses that are not explained by antibody titre alone, suggesting vaccine-induced immunity quality may vary by age at vaccination
Key Findings
- •Equine influenza H3N8 affected 21 thoroughbred training yards in Newmarket with over 1,300 racehorses between March and May 2003, causing respiratory disease in both vaccinated and unvaccinated horses
- •The outbreak strain was closely related to American virus lineage Kentucky/5/02, the first isolation of this sublineage in the UK
- •Recently vaccinated horses developed clinical disease despite adequate single radial haemolysis antibody levels and no significant antigenic differences between vaccine strain A/eq/Newmarket/1/93 and infecting virus
- •Fewer two-year-old horses became infected than older animals despite similar high antibody levels, suggesting qualitative rather than quantitative immune deficiency from vaccination