Veterinarians are well versed in biosecurity practices to minimize risk of disease, but one huge obstacle to overcome is horse owner compliance with routine biosecurity measures.
A recent study reviewed the median attack rate of a simulated influenza infection related to a five-day or 14-day quarantine following return from a two-day equestrian event outside the home farm [Spence, Kelsey L.; O’Sullivan, Terri L.; Poljak, Zvonimir; Greer, Amy L. Estimating the potential for disease spread in horses associated with an equestrian show in Ontario, Canada using an agent-based model. Preventive Veterinary Medicine https://doi.org/10.1016/j.prevetmed.2017.12.013].
In the study, the average length of an outbreak of equine influenza following a 5-day quarantine lasted 8.9 days whereas following a 14-day quarantine, an outbreak lasted 7.75 days. Without a quarantine period, the outbreak persisted nearly 20 days.
Herd immunity through vaccination played a large and effective role in disease prevention. For facilities of 1-5 horses with 75% of the horses immunized prior to the off-site event, the disease attack rate dropped by 23%. Even a 2-day quarantine coupled with vaccination had an effective impact on reducing infection attack rate. It is reported that the probability of a vaccinated horse transmitting influenza virus is half of the risk of an unvaccinated horse passing the virus to others.
The study concurs with the recommendation that “horses should be quarantined for a period of time sufficient to rule out infection after returning home from another location.”
Quarantine for both five and 14 days resulted in the same median attack rate, one that is reduced by 91% from an attack rate on farms with no quarantine.
A five-day quarantine is much more palatable to horse owners and more likely to be implemented than asking them to quarantine show horses for two weeks. A lengthy quarantine period is particularly onerous to implement for horses that come and go frequently from the property for training, clinics and competition.
When a 14-day quarantine is not practical or likely to be carried out correctly, then a farm that practices good immunization protocols might be able to successfully protect the horses from disease by using a 2- to 5-day quarantine period. Management of biosecurity practices is difficult in the best of circumstances; there is greater likelihood of compliance over a shorter number of days.
Based on this information, veterinarians might wish to communicate with clients about the effectiveness in disease reduction by using immunization strategies along with at least a several-day quarantine period following off-site events.