Humoral immune responses and adverse effects of inactivated West Nile virus and tetanus toxoid vaccines administered via the subcutaneous or intramuscular route in horses

IntroductionMyosin heavy chain myopathy (MYHM) is a genetic disorder in Quarter Horse-related breeds that can result in non-exertional rhabdomyolysis and immune-mediated myositis. Infectious diseases and muscle damage, particularly from intramuscular (IM) vaccination, have been implicated as potential triggers for this condition. However, the subcutaneous (SC) injection route has not been investigated as an alternative vaccination method for MYHM-affected horses. The hypothesis was that there would be no difference in humoral immune responses or injection site reactions in healthy horses receiving killed West Nile virus (WNV) and tetanus toxoid vaccines via SC and IM routes. The primary objective was to determine serum antibody titers and the adverse effects elicited by both vaccination routes.MethodsA randomized, single-blind, two-arm, two-period study compared the responses in horses receiving WNV and tetanus vaccines via IM or SC routes. A total of 17 adult mares, all of which were negative for the MYH1 mutation, from a university research and teaching herd were included in the study. Serum antibody levels were assessed on the day of each vaccination. Antibody levels were also assessed on days 28 and 56 after WNV vaccine and on day 28 after tetanus toxoid administration. Vaccine site reactions were assessed daily for 72 h after vaccination. Antibody levels and injection site reactions were analyzed using two-way repeated measures ANOVA, rank sum tests, and Fisher’s exact test, with statistical significance at a p-value of <0.05.ResultsSerum neutralization antibody titers increased after WNV vaccination regardless of the route of vaccine administration (p = 0.007), with no differences observed between routes of administration. Injection site swelling was greater after SC vaccination than after IM injection route for the WNV vaccine (p = 0.003). Baseline tetanus titers were high in almost all horses, and no change in titer was observed following vaccination. Injection site swelling was greater after SC tetanus vaccination than after IM vaccination (p = 0.002).DiscussionSubcutaneous vaccination was associated with significant local reactions. An accurate assessment of humoral responses was challenging as the horses included in the study had high baseline titers from previous vaccinations. As a result, drawing definitive conclusions regarding the equivalence of responses to SC versus IM vaccination is difficult. Therefore, the results regarding this comparison must be considered inconclusive.