Proline-glutamic acid-proline-lysine repetition peptide as an antigen for the serological diagnosis of strangles.
Authors: Hobo S, Niwa H, Anzai T
Journal: The Veterinary record
Summary
# Editorial Summary Hobo and colleagues evaluated a synthetic peptide antigen based on the PEPK (proline-glutamic acid-proline-lysine) repeat motif, derived from *Streptococcus equi* surface proteins, to determine whether it could reliably distinguish strangles infection from cross-reactive antibody responses in a large sample of 3,176 equine sera. Using serological testing, the researchers found that whilst multiple PEPK repetitions enhanced detection of antibodies in horses infected with *S. equi* subspecies equi, increasing the number of repeats unfortunately also increased false positivity with sera from horses infected with the closely related *S. equi* subspecies zooepidemicus, creating a specificity problem. In experimentally infected horses, antibody reactivity to the five-repeat PEPK peptide emerged within one week of inoculation and remained elevated for at least four weeks, whilst sera from recovered strangles cases and healthy controls showed appropriately low optical density values. For equine practitioners, these findings suggest that whilst PEPK peptide antigens show promise for serological strangles diagnosis—particularly their relatively rapid and sustained antibody response—careful optimisation of repeat number remains essential to avoid misdiagnosis of *S. zooepidemicus* infections, which can occur clinically as respiratory disease in performance horses and may require different management strategies.
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Practical Takeaways
- •The PEPK peptide antigen can be used for serological diagnosis of strangles with detectable antibodies appearing within one week of infection
- •Clinicians should be aware that higher numbers of PEPK repetitions may reduce diagnostic specificity due to cross-reactivity with S. zooepidemicus; five repetitions appears to be an optimal balance
- •This serological test may help identify horses recovering from strangles and distinguish active S. equi infection from S. zooepidemicus exposure or healthy status
Key Findings
- •PEPK repetition peptide antigen shows high reactivity with sera from horses infected with S. equi subspecies equi, with reactivity increasing with multiple PEPK repetitions
- •Increased PEPK repetitions also increase cross-reactivity with S. zooepidemicus-infected sera, limiting diagnostic specificity
- •PEPK antigen with five repetitions shows detectable reactivity one week post-inoculation in experimentally infected horses, continuing to increase over four weeks
- •Sera from horses recovered from strangles and experimentally infected horses show high OD values, while healthy horses and those recovered from S. zooepidemicus show comparatively low OD values