113 Leptospirosis vaccination and titer concentration testing in equine

Abstract

Vaccines can help protect animals from diseases. Standard practice includes administering an initial vaccination followed by a booster a few weeks later, and then annual boosters thereafter. However, boosters may not be needed each year if immunity lasts longer. Titer concentration, which evaluates antibodies in blood, may be helpful in determining immune response and estimating protected status after vaccination. Leptospirosis is a bacterial disease that can cause blindness and abortion in horses. There is one USDA approved vaccine available for Leptospirosis pomona (L. pomona), the serovar associated with most clinical cases in North America. However, there is limited research on how the immune system in horses responds to the vaccine, and what titer concentrations may result from vaccination. Thirty-seven university horses previously unvaccinated for L. pomona and with no medical history of uveitis or abortion were selected. Blood was drawn and evaluated for baseline leptospirosis titers. Horses were then divided into TRT (vaccinated, n = 20; all geldings; average age 16 yr) or CON (not vaccinated, n = 17; 8 mares, 9 geldings; average age 17 yr) groups, and TRT horses were vaccinated. Blood samples were again collected from all 37 horses 14 d post-vaccination. The TRT group received boosters 3 wk after the first vaccination, and blood samples were again collected from all 37 horses 14 d post-booster. Data were evaluated using 2-samplet-Tests assuming unequal variances. Baseline results showed a higher seroprevalence of L. pomona as compared with previously published research (n = 4 of 37, 10.8%, versus 4% and 1%). Titer concentration in positive horses ranged from 1:100 to 1:3200. Management aspects could have influenced this, as the university horses were more concentrated geographically than populations previously surveyed. There was no difference between TRT and CON at baseline (P = 0.313). There was a difference over time for TRT horses (P < 0.001), but no change in CON. As expected, TRT horses had higher titers at 14 d post-vaccination (CON versus TRT mean concentration 429 and 6221, respectively, P < 0.005) and 14 d post-booster (CON versus TRT mean concentration 224 and 3900 respectively, P < 0.001). Titer concentrations in TRT horses ranged from 1:200 to 1:25,600 post-vaccination and from 1:200 to 1:6400 post-booster. There was no difference in TRT titers between post-vaccination and post-booster (mean concentration 6221 and 3900, respectively, P = 0.158). Results showed that vaccination did promote immune response, but the wide range of titer concentrations suggest that a single assessment of titer may not be appropriate for assessing protected status.