The Effect of Age on Serum Antibody Titers after Rabies and Influenza Vaccination in Healthy Horses

Background: The proportion of geriatric horses within the equine population has increased in the past decade, but there is limited information on the immune function of these animals. Hypothesis: Aged horses will have a lesser increase in serum antibody response to vaccination. Animals: Thirty-four aged healthy horses (≥ 20 years) and 29 younger adult horses (4–12 years) of various breeds. Methods: All horses were vaccinated with vaccines of killed rabies and influenza virus. Horses in each age group were allocated to receive either rabies or influenza booster vaccine 4 weeks after the initial vaccination. Serum samples were taken at 0, 4, 8, and 24 weeks. Rabies serum neutralization titers and equine influenza virus specific antibody sub-isotypes (IgGa, IgGb, IgG(T), and IgA) as well as single radial hemolysis (SRH) titers were determined. Results: Rabies antibody titers were similar in the 2 age groups at all sampling times. Aged horses had higher IgGa and IgGb influenza antibody titers before vaccination than younger horses but similar titers after vaccination (P= .004 and P= .0027, respectively). Younger horses had significantly greater increases in titer than aged horses at all sampling times for IgGa (P= .001) and at 8 and 24 weeks for IgGb (P= .041 and .01, respectively). There was no detectable serum IgG(T) at any time point. A significant booster vaccine effect was seen for both antirabies and anti-influenza titers. Anti-influenza titer before vaccination also had a significant effect on subsequent antibody response. Conclusions and Clinical Importance: Healthy aged horses generated a primary immune response to a killed rabies vaccine similar to that of younger adult horses. Aged horses had a significantly reduced anamnestic response to influenza vaccine.