Comparison of Immunologic Responses Following Intranasal and Oral Administration of a USDA-Approved, Live-Attenuated Streptococcus equi Vaccine

Although there is a commercially available vaccine for Streptococcus equi subspecies equi licensed for the intranasal (IN) route of administration, some equine practitioners are administering this vaccine orally despite a lack of evidence for its efficacy by this route of administration. The purpose of this study was to compare systemic and local immune responses following IN or oral administration of the United States Department of Agriculture (USDA)-approved, live-attenuated S. equi subspecies equi vaccine (Pinnacle IN; Zoetis, Florham Park, NJ). Eight healthy horses with low S. equi M protein (SeM) titers (<1:1,600) were randomly assigned to an IN or oral two-vaccine series. Streptococcus equi M protein–specific serum immunoglobulins G (IgG) and A (IgA) and nasal secretion IgA were assessed using a commercially available ELISA (SeM ELISA, Equine Diagnostics Solutions, LLC, Lexington, KY) and a novel magnetic microsphere assay utilizing fluorescence. A general linear mixed models approach was used for statistical data analysis. As expected, IN vaccinates showed substantial increases in both serum SeM-specific IgG and IgA levels postvaccination (P = .0006 and P = .007, respectively). Oral vaccinates showed an increase in serum SeM-specific IgG postvaccination (P = .0150), though only one third the magnitude of IN vaccinates. Oral vaccinates showed no evidence of change in SeM-specific IgA postvaccination (P = .15). Results indicate that IN or oral vaccine administration resulted in increased serum SeM-specific IgG, though the magnitude of response differed, where there was a 10-fold increase for IN vaccinates and only a threefold increase for oral vaccinates.