"Equine influenza virus remains the leading cause of viral respiratory disease in the horse," noted Justin D. McCormick, MS, DVM, of Steinbeck Country Equine Clinic in Salinas, Calif., during the 2009 American Association of Equine Practitioners Convention, held Dec. 6-10 in Las Vegas, Nev. "Illness can cause serious setbacks in training, which can lead to financial loss. Respiratory disease can be a serious problem causing distress to the equine athlete and prolonged periods of convalescence correlating to substantial economic loss to the industry."
McCormick discussed the results of a 300-horse study that sought to find the vaccination protocol that best prevents flu in horses--intranasal (IN) or intramuscular (IM), given as a single dose or two doses given 30 days apart (boosted). Researchers also looked at boosting an initial IM dose with an IN dose, and vice versa.
Unsurprisingly, all horses receiving a booster dose of either vaccine had significantly higher levels of protective antibodies two and four months after the first dose. No vaccinated horses, regardless of protocol, had clinical signs of flu severe enough to need treatment during the five-month study.
McCormick noted that the level of antibody response varied with the protocol (IM vs. IN) and the age of the horse, suggesting that a single protocol might not be ideal for all age groups. For example, he reported that older horses had a greater response to the IN vaccine than younger horses. However, if you had to pick one strategy, he said the IM-IM boosted protocol yielded the greatest serologic response across all age groups in this study.
He said he found it interesting that horses receiving any booster after an initial IN vaccine showed a higher response compared to horses receiving a single IM vaccine. "I think everyone involved was surprised by that," he commented.
"We found a significant serological benefit (increased antibody levels in the blood) of getting boosters at 30 days," McCormick concluded. "Vaccinated horses developed fewer clinical signs of disease than unvaccinated ones."
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.