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

Abstract

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. Aged horses will have a lesser increase in serum antibody response to vaccination. Thirty-four aged healthy horses (> or = 20 years) and 29 younger adult horses (4-12 years) of various breeds. 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. 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. 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.