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.

The research in this paper investigates the effectiveness of a vaccine for Potomac Horse Fever (PHF) in comparison to a multivalent PHF/Rabies vaccine. The study found that the single disease vaccine was more effective in stimulating an immune response in the horses tested.

Introduction

This research primarily centers around Potomac Horse Fever (PHF), a disease in horses caused by the bacterium Neorickettsia risticii. The disease, first identified forty years ago in Maryland, USA, is known to be prevalent in many regions across North America, and has also been documented in South America and Europe. While monovalent PHF vaccines are commercially available, their effectiveness in providing consistent clinical protection remains inconsistent and under question.

Objective and Methodology

• The primary goal of this study was to assess and compare the immunogenicity – the ability to provoke an immune response – of a monovalent PHF vaccine and a combined multivalent PHF/Rabies vaccine in horses.
• The research team used a randomized parallel group trial approach, with 91 client or University-owned horses participating in the study, which was also randomized.
• The monovalent PHF vaccines were given to 45 horses at separate sites and another batch of 46 horses received the combined multivalent vaccine at a single site.
• The horse’s serum PHF IFA (Immunofluorescence Assay) titres – a measure of the immune response – were recorded twice before vaccination and then at 1, 2, and 3 months post-vaccination.

Results

• The results of the study showed that both vaccine protocols – monovalent and multivalent – demonstrated low immunogenicity, with only about a third of the animals showing seroconversion, or an increased titre of more than 400 over the baseline, at any point post-vaccination.
• Interestingly, the monovalent PHF vaccine proved significantly more effective at eliciting an immune response – as measured by the number of horses showing seroconversion – compared to the multivalent vaccine, particularly at one and two months post-vaccination.
• The trend was also visible in terms of median IFA titres, with the monovalent PHF vaccine generating significantly higher titres compared to the multivalent one at one and two months post-vaccination.
• However, by the third month post-vaccination, there was no significant difference observed between the two groups in terms of either seroconversion rate or median IFA titers.

Conclusions

• It’s important to clarify that the researchers did not directly examine the implication of these findings on actual protective immunity against PHF, but used the serologic response to vaccination as an indirect indicator of immunity.
• Nonetheless, the key finding of this study indicates that a multivalent PHF/Rabies vaccine displayed lower immunogenicity as compared to when a single-disease PHF vaccine was co-administered with a separate Rabies vaccine. This outcome prompts several potential new avenues for further research into improving PHF vaccine effectiveness.