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

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

Zuzanna Marta Pietras
Master of Science (MS), Washington State University
2026
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Zuzanna Pietras - Final Thesis
Embargoed Access, Embargo ends: 10/16/2027

Abstract

Introduction: Myosin heavy chain myopathy (MYHM) is a genetic disorder of quarter horse-related breeds 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. Subcutaneous (SC) injection has not been investigated as a possible alternative vaccination route for MYHM affected horses. The hypothesis was that there is 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. Objectives were to determine serum antibody titers and adverse effects elicited by SC and IM vaccination. Methods: A randomized, single-blinded crossover study compared responses in horses receiving WNV and tetanus vaccines via IM or SC routes. Seventeen adult mares negative for the MYH1 mutation from a university research herd were included in the study. Serum antibody titers were assessed on day of each vaccination. Titers were also assessed on days 28 and 56 after WNV vaccine and on day 28 after tetanus toxoid administration. Vaccine site reactions were assessed on days daily for 72 hours after vaccination. Antibody titers and injection site reactions were compared by two-way repeated measures ANOVA and rank sum testing with significance at p < 0.05. Results: Serum antibody titers increased after WNV vaccination regardless of route of vaccine administration (p = 0.007), with no differences observed between routes of administration. Injection site swelling was larger after SC vaccination than after IM injection for WNV vaccine (p = 0.03). Baseline tetanus titers were high in almost all horses and no change in titer was observed with vaccination. Injection site swelling was larger after SC tetanus vaccination than after IM vaccination (p = 0.002). Discussion: Subcutaneous 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.

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