Long term silent carriers of Streptococcus equi ssp. equi following strangles; carrier detection related to sampling site of collection and culture versus qPCR.

This research investigates the occurrence of long-term silent carriers of Streptococcus equi ssp. equi (S. equi), a bacterium causing strangles in horses, after outbreaks and the efficiency of different detection methods. The results showed that a significant number of horses continued to carry the bacteria even after recovery, sometimes for several months, and qPCR was more sensitive in detecting these silent carriers compared to culture methods.

Introduction and Methods

• The study stemmed from the fact that S. equi, the bacteria that causes strangles in horses, can stay in seemingly healthy carriers for months or even years following an outbreak.
• Two outbreaks of strangles, one amongst 98 yearling warmblood horses (outbreak A) and the other amongst 38 mature Icelandic horses (outbreak B) were studied longitudinally to assess the carrier state of the bacteria and the best methods to detect its presence.
• The horses, all of which had fully recovered, were sampled at least 6 months after the index cases using nasal swabs, nasopharyngeal lavage, and guttural pouch visualisation and lavages.
• Both culture and qPCR methods were used to detect S. equi in the samples.

Results

• According to culture methods, the carrier rates were found to be 3% in outbreak A and 13% in outbreak B.
• The qPCR method, however, displayed significantly higher carrier rates – 15% in outbreak A and 37% in outbreak B.
• All culture positive samples also tested positive by qPCR, but not necessarily vice versa, indicating that qPCR was a more sensitive method of detection.
• Even a horse that tested negative by culture but positive by qPCR later tested positive by culture, suggesting that qPCR can detect silent carriers that might be missed by culture methods.
• These findings support the requirement of not relying solely on sampling and appearance of guttural pouch for identifying silent carriers.

Conclusion

• The results from this research imply that there may be a significant number of silent carriers of S. equi following a strangles outbreak.
• The use of qPCR method for detection of S. equi in suspected carriers is recommended, due to its higher sensitivity compared to culture methods.
• Even horses that test negative by culture but positive by qPCR should be suspected of carrying live bacteria, as culture methods may not be able to detect all silent carriers.