Decreased Clinical Severity of Strangles in Weanlings Associated with Restricted Seroconversion to Optimized Streptococcus equi ssp equi Assays.

This research investigated why weanling horses exhibited unexpectedly mild symptoms during an outbreak of strangles, a disease caused by the bacterium Streptococcus equi ssp. equi. The study found that infected horses did not display distinct strangles symptoms but had reduced seroconversion to a certain antigen, suggesting this could play a role in lessening the disease’s severity.

Objective and Methodology

• The research aimed to study the clinical, serologic, and microbiologic reasons for a mild severity of strangles in a group of immunologically naïve weanling horses during a natural outbreak.
• The researchers took 112 warmblood weanling horses and conducted a prospective longitudinal observational study throughout the outbreak, starting from the index case and proceeding until the disease was resolved.
• At the peak of the outbreak, all the horses were examined through deep nasal swabs for the presence of S. equi. This was confirmed by the culture method and quantitative Polymerase Chain Reaction (PCR).
• The horses were also examined clinically and serologically, using an optimized Enzyme-Linked Immunosorbent Assay (ELISA) to determine their immune response and seroconversion.
• Analysis of data was done using descriptive statistics and the Wilcoxon signed-rank test.

Key Findings

• The results showed that 53% of the cohort showed signs of the disease, with 9 of 14 horses being culture positive and 26 of 53 horses being qPCR positive for S. equi without showing signs typical of strangles.
• By the end of the outbreak, 91 out of 112 horses had seroconverted to Antigen A —they had developed an immune response against this antigen— as determined by the ELISA.
• Interestingly, there was minimal seroconversion to antigen C, another important part of the SeM protein of S. equi.
• The sequenced isolates had no changes in the SeM protein. However, they did find a 61 base pair deletion in the gene SEQ_0402.

Conclusions

• The results led to the conclusion that the absence of clinical signs alone in naïve horses might not conclusively signify the end of a strangles infection and thus, releasing them from quarantine based solely on this criterion may not be sufficient.
• The restricted seroconversion to antigen C was thought to potentially associate with the decreased clinical severity of the disease seen in the cohort. Further studies are needed to confirm this relationship.
• The researchers also believe the deletion in the gene SEQ_0402 might impact the virulence of S. equi, a hypothesis that warrants additional investigation.