A topic presented by Tom Divers, DVM, ACVIM, ACVEUV, of Cornell University, at the 2020 NAEP (Northeast American Association Equine Practitioners) Symposium looked at the efficacy of co-administration of multivalent Potomac horse fever (PHF) vaccine with rabies compared to using two monovalent vaccines at separate sites [McKenzie, H.C.; Funk, R.A., et al. Immunogenicity of Potomac horse fever vaccine when simultaneously co-administered with rabies vaccine in a multivalent vaccine or as two monovalent vaccines at separate sites. Equine Veterinary Journal Mar 2019].
The study administered multivalent inactivated vaccine in 46 horses at a single site and the monovalent inactivated vaccines at separate sites in 45 horses. Both vaccination protocols developed poor immunogenicity, and only one-third actually increased titers to what is considered a protective level.
The results were stated simply: By one- and two-months post-vaccination, the monoclonal PHF vaccine increased immunogenicity and IFA titers more than the multivalent PHF-rabies combination vaccine, but neither developed a sufficient serological response. Results were comparable in both groups by three months post-vaccination.
Marginal efficacy from the multivalent product could be due to: a) interference by the rabies antigens or adjuvants; b) the multivalent vaccine may not have a high enough antigenic load; c) the horses developed deficient immune responses; d) immunoglobulin G antibodies are poorly protective against intra-cellular pathogens; and/or e) there could be variations in PHF strains.
When using PHF vaccine in endemic areas, despite relatively poor immunogenicity results, the authors recommend using the monovalent form to maximize the immunologic response as much as possible. The actual protective effects of PHF vaccine were not evaluated in this study, only the serologic response.
The authors of this open-access article concluded: “The multivalent PHF/Rabies vaccine exhibited lower immunogenicity as compared to the monovalent PHF vaccine co-administered with a separate rabies vaccine.”
Editor’s note: It’s believed that mayflies, caddis flies and other aquatic insects carry the Neorickettsia risticii (NR) organism that causes Potomac horse fever.