E20 - Effects of Intramuscular Versus Subcutaneous West Nile Virus and Tetanus Toxoid Vaccination in Horses

Abstract:

Background: Myosin heavy chain myopathy (MYHM1) is a disorder of Quarter Horses that can result in non-exertional rhabdomyolysis and immune-mediated myositis. Infectious diseases and muscle damage, including intramuscular (IM) vaccination, have been implicated as possible triggers, complicating disease prevention in affected horses. Subcutaneous (SC) vaccination has not been investigated as a possible alternative for affected horses.
Hypothesis/

Objectives: Objectives were to determine serum antibody titers and adverse effects elicited by SC and IM vaccination. The hypothesis was that there is no difference in serum antibody titers or injection site reactions in healthy horses receiving tetanus toxoid and killed West Nile virus (WNV) vaccines via SC and IM routes.
Animals: Seventeen adult mares negative for the MYHM1 mutation from a university research herd.

Methods: A randomized, single-blinded crossover study compared responses in horses receiving tetanus and WNV vaccines via IM or SC routes. Serum antibody titers were assessed at days 0 and 28. Vaccine site reactions were assessed on days 1 to 3. Antibody titers and injection site reactions were compared by two-way repeated measures ANOVA.

Results: Serum antibody responses increased after WNV vaccination regardless of route of vaccine administration (p = 0.004) with no differences in titers observed between routes of administration. Injection site swelling was greater after SC vaccination than after IM injection (p = 0.002).
Conclusions and Clinical Importance: SC vaccination was associated with significant local reactions. Accurate assessment of humoral response was challenging as the horses included in the study had high baseline titers from previous vaccinations.