Published June, 2010

We've all heard the statistics about an aging America. The elderly represent the fastest growing-proportion of the U.S. population. In recent years horses have experienced a similar population shift.

A large portion of the equine population (about 15%) is composed of horses older than 20 and, even at this age, many remain actively involved in equestrian sports, reproduction, or as companions. Thus, further understanding of geriatric horses' immune systems has become increasingly important to preserve good health and quality of life through their golden years.

A goal of the immunology research program at the University of Kentucky Gluck Equine Research Center is to understand how age affects the geriatric horse's immune response to vaccination, particularly the equine influenza vaccine.

Previous studies have shown the immune response to inactivated equine influenza virus vaccines decreases with age; however, it remains unknown whether aging has an impact on vaccine efficacy. Because alternative vaccination approaches may prove more efficacious in aged horses, a vaccine/challenge study was recently performed at the Gluck Center using a canarypox recombinant virus-vectored vaccine. This live vaccine has limited replication and therefore mimics that of a natural infection, which is expected to induce a more significant and complete stimulation of the immune response compared to killed vaccines.

A recent study performed at the Gluck Center by David Horohov, PhD, William Robert Mills chair and professor; Tom Chambers, PhD, professor of veterinary virology; and Amanda Adams, PhD, postdoctoral scholar, addressed whether old horses respond to vaccination and if vaccination provides protection. The study involved 15 horses 20 years of age and older. While the vaccination history of these animals was unknown, they had probably been exposed to the vaccine and/or the equine influenza virus because all had low-level hemagglutinin inhibition (HI) titers, a measure of antibody production, at the start of the trial. Seven of the horses were vaccinated (one dose) and eight were not vaccinated.

The study also involved 13 young horses (6 months to 1 year old) that had no serological evidence of previous exposure to the virus (naive animals). Seven of the young horses received the same vaccine as the elderly horses (two doses, as recommended by the manufacturer) and six were left unvaccinated. The vaccine was effective at inducing both humoral (in the blood) and cell-mediated immune responses in naive horses. By contrast, the antibody response of old horses was not as great as that of the younger animals. All of the older horses had evidence of virus-specific cell-mediated immunity prior to vaccination, and this was enhanced only slightly by the vaccine.

To determine vaccine efficacy, all horses were exposed to aerosolized equine influenza virus for 45 minutes 14 days after the last vaccination, and were then monitored for clinical signs of infection (nasal discharge, rectal temperature, etc.). In the naive unvaccinated group of young horses, a temperature peak typically occurred two days after challenge and persisted for five to six days. This also correlated with increased transcription of pro-inflammatory cytokines in their peripheral blood cells. These animals also shed virus in their upper respiratory tract for an average of four and a half days.

In the unvaccinated older horses, a febrile peak was also observed post-challenge, although this was not as pronounced as in the young horses. This also correlated with increased pro-inflammatory cytokine expression. The unvaccinated older horses also shed the virus, but for a shorter time than the naive young horses.

This study showed that aged horses are susceptible to infection with equine influenza virus. The vaccine was effective at protecting both the young and old horses upon challenge, reducing both clinical signs and virus shedding. Hence, vaccination of aged horses with the recombinant vaccine was effective.

Currently, the immunology group is evaluating the immune response of old horses to inactivated equine influenza vaccines compared to new generation, live-vectored vaccines. The group is also interested in determining how other factors such as nutritional status and underlying conditions (e.g., equine Cushing's disease) might affect the immune response of geriatric horses to vaccination. The goal of this research is to identify optimal methods for protecting aged horses from infectious disease.

Amanda A. Adams, PhD, is a post-doctoral scholar at the Gluck Equine Research Center.