Testing for strangles (Streptococcus equi)

Tim Pasma

Animal Health Laboratory, University of Guelph, Guelph, ON.

AHL Newsletter 2024;28(2):23.

Strangles, caused by Streptococcus equi subspecies equi, can be detected by PCR or bacterial culture at the AHL.

 The PCR test is highly sensitive, can detect small numbers of bacteria, and is useful for detecting carrier horses.  However, the PCR cannot differentiate between bacteria that are alive or are dead/fragmented, and therefore, a positive PCR result may not indicate an active infection.

A bacterial culture is less sensitive and has a longer turnaround time.  It can miss horses that are in the early stage of infection, or those carrying a low bacterial load.  A culture is also required for antimicrobial sensitivity testing.

 It is important to understand that no single test will detect all positive animals.  Sensitivity of testing for strangles can be increased by:

• sampling by guttural pouch lavage and/or nasopharyngeal lavage;
• sampling 48 hours or more after the onset of fever;
• testing at regular intervals if you suspect a horse has been exposed or infected;
• submitting samples from as many animals as possible in an outbreak situation;
• using an endoscope to assist with collection of the sample and visualization of the affected passages.

 Bacterial culture detects viable S. equi, while PCR will detect DNA from viable or non-viable bacteria.  A recent study by Weese et al found that a qPCR Ct of 34.2 was a reasonable breakpoint for the likelihood of the presence of culturable S. equi.

 PCR sequence typing will differentiate between vaccine and field strains, and this test is available at the AHL for a strong positive qPCR result (Ct < 30) and pure bacterial culture results.

 If you obtain a Strep. equi test result that is not expected based upon clinical findings and history, please contact the AHL to review the result and discuss follow-up options.   AHL

 Reference

1. Weese JS, Saab M, Moore A, Cai H, McClure JT. Relationship between quantitative real-time PCR cycle threshold and culture for detection of Streptococcus equi subspecies equi. Can Vet J 2023;64:549-552.