As we all found out last year, equine influenza (EI) is a highly contagious respiratory disease. Mandatory vaccination for Thoroughbred racehorses has been implemented in the UK and Ireland for nearly 40 years. Vaccination stimulates a protective immune response in individual horses and protects equine populations at herd level. It has been suggested that greater than 70% of a population must be fully vaccinated in order to prevent EI epidemics1. Although vaccination plays a vital role, it should not be relied on as the sole preventive strategy. The degree to which vaccination can reduce transmission depends on several factors. This includes the antigenic relatedness of the vaccine strain and the field virus, an individual's immune response to vaccination, time since last vaccination and the network and frequency of contacts.
An investigation into an EI outbreak which occurred among vaccinated horses in four racing yards (two National Hunt, one Flat, one mixed National Hunt racing/breeding yard) in Ireland within a 4‐week period has recently been published2. Epidemiological and vaccination data along with repeat clinical samples were collected from 118 horses on the four premises in order to identify the source of infection and monitor virus spread among a vaccinated population. Up‐to‐date vaccination records were available for between 40% and 87% of horses on affected yards. Partial vaccine compliance and the mixing of racing and other Thoroughbred populations (breeding/pretraining) with inadequate vaccination histories contributed to disease spread as did the failure to implement appropriate biosecurity measures following the introduction of new arrivals and the return of horses from equestrian events. The index case(s) on all premises was vaccinated in accordance with Jockey Club rules. Vaccine breakdown was observed across all products in 27/80 horses (33.8%) with an up‐to‐date vaccination record. Eighteen of the 27 (66.7%) horses had not received a booster vaccination within the previous 6 months and 10 (37%) horses were due annual booster vaccination at the time of developing clinical signs.
The authors concluded that annual booster vaccination should not be relied on as the sole preventative measure against EI. The findings of this study suggest that increasing the frequency of booster vaccinations may be beneficial particularly in young horses and that synchronised scheduling of vaccination regimes across racing yards may contribute to high‐risk periods for EI virus (EIV) transmission.
1. Baker DJ. Rationale for the use of influenza vaccines in horses and the importance of antigenic drift. Equine Vet J. 1986;18:93–6.
2. Gildea, S, Lyons, P, Lyons, R, Gahan, J, Garvey, M, Cullinane, A. Annual booster vaccination and the risk of equine influenza to Thoroughbred racehorses. Equine Vet J. 2020; 52: 509– 515. Available HERE