Strangles, caused by Streptococcus equi subsp. equi (S. equi), is a highly contagious respiratory disease that causes substantial economic losses to the horse industry worldwide. Serological diagnosis currently relies mainly on the imported IDvet^® dual-antigen sandwich ELISA kit, which shows good diagnostic performance but is expensive and lacks a domestic alternative in China, thereby limiting its routine use in primary veterinary settings. The aim of this study was to develop a domestic indirect ELISA based on novel antigen targets and an untagged antigen purification strategy. Using comparative bioinformatics analyses, two conserved fragments with species-level specificity for S. equi, designated EQ8 (150 aa) and SclF (44 aa), were identified and fused for recombinant expression. The fusion protein was purified by GST affinity chromatography, cleaved with PreScission protease, and subjected to secondary purification to obtain an untagged EQ8–SclF protein. Western blotting and ELISA demonstrated that the purified protein retained good immunoreactivity and that the GST tag did not cause detectable cross-reactivity, with all OD₄₅₀ values obtained using GST-coated plates remaining below 0.09, well under the cut-off value of 0.115. Checkerboard titration established the optimal assay conditions as an antigen coating concentration of 1 μg/mL and a serum dilution of 1:100. Using the IDvet kit as the reference method, 157 clinical serum samples (67 positive and 90 negative) were tested in parallel. The newly developed assay showed a relative sensitivity of 89.55% (60/67), a relative specificity of 94.44% (85/90), an overall agreement rate of 92.36% (145/157), and a kappa value of 0.84, indicating excellent agreement between the two methods. Good inter-assay reproducibility was observed, with coefficients of variation all below 9.29%. In conclusion, an indirect ELISA based on an untagged EQ8–SclF fusion protein was preliminarily established for the serological detection of strangles. This assay showed diagnostic performance highly consistent with that of the imported commercial kit while offering potential advantages in terms of lower cost, simpler operation, and elimination of tag-associated interference. It therefore shows potential as a domestic alternative for the serological diagnosis of strangles.

As a naturally self-assembling protein nanocarrier, ferritin enables multivalent antigen display and functions as an intrinsic adjuvant to enhance vaccine-induced immune responses. Streptococcus equi subsp. equi (S. equi) is the causative agent of equine strangles, an acute and highly contagious respiratory disease responsible for substantial economic losses worldwide. However, currently available vaccines often show suboptimal immunogenicity and limited protective efficacy. In this study, we developed a recombinant equine ferritin (rHF)-based nanoparticle vaccine, rSE5Mix, presenting five core protective antigens (EQ8, EQ5, CNE, IdeE, EAG). The fusion proteins efficiently assembled into uniform nanoparticles. Immunization of BALB/c mice elicited rapid, high-titer antigen-specific IgG antibodies and a balanced Th1/Th2 immune response without additional adjuvants. Following lethal challenge with S. equi, rSE5Mix-immunized mice showed 100% survival and markedly milder clinical signs. Histopathological analysis demonstrated significantly alleviated organ damage, and bacterial loads in major tissues were reduced to nearly undetectable levels. Importantly, compared with the octavalent tandem vaccine rSE8, rSE5Mix induced faster elevation of partial antigen-specific antibodies, especially for EQ8 and CNE. Their antibody titers were comparable at later stages. This study developed a safe and effective ferritin nanoparticle vaccine candidate against equine strangles and verified that equine ferritin is a promising candidate delivery platform for veterinary bacterial vaccines.