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AAEP 2020: Rapid, Reliable Test for Equine Strangles

A presentation was given by Dr. Nicola Pusterla of UC Davis on the new stall-side equine strangles test at the 2020 AAEP Virtual Convention.
strangles nasal discharge horse nose

The POC S. equi test can support a stall-side diagnosis of strangles in clinically infected horses, allowing rapid implementation of appropriate biosecurity protocols to limit the spread of this highly contagious respiratory pathogen.

A timely diagnosis of strangles to help prevent the spread of disease is limited by the speed with which respiratory secretions collected from a sick horse can be analyzed by a laboratory. Now, however, a point-of-care assay that can be used at the horse’s side yields results in as little as one hour, revolutionizing the management of strangles patients.

Strangles is a common upper respiratory tract infection caused by Streptococcus equi subspecies equi (S. equi) that can have a tremendous clinical impact, especially in young stock. Horses can shed the streptococcal organisms for weeks to even months after recovery from clinical infection.

“This is why an accurate, timely and cost-effective test for strangles is needed—like what we are currently seeing with the COVID-19 drive-through testing,” explained Nicola Pusterla, DVM, PhD, Dipl. ACVIM, AVDC-Equine, of the UC Davis School of Veterinary Medicine.

The current gold standard test for the detection of S. equi is quantitative PCR (qPCR); however, the turnaround time from sample collection to result reporting depends mostly on shipping and laboratory processing time. Delays in turnaround time can negatively impact timely biosecurity protocols.

Recently, a stall-side (point-of-care or POC) PCR assay for S. equi was introduced in the equine market. To determine how this POC PCR assay performed compared to the laboratory gold standard qPCR, Pusterla and colleagues collected 232 swabs from the rostral nasal passages of horses with acute onset of fever and respiratory signs. The results were then compared to those obtained via qPCR.

Key findings were:

· There was 100% agreement between all the analyzed samples except those samples that contained small amounts of S. equi that gave “weak positive” results by qPCR. In that case, agreement was 71%.

· There was no cross-reactivity between S. equi and the closely related S. zooepidemicus (an innocuous commensal bacteria present in the nasal passages of most horses).

· The analytical sensitivity (limit of detection) of the POC PCR assay was 300 times higher than the one for the qPCR gold standard.

“One of the most important take-home messages is that if the POC PCR test is negative but you are strongly suspicious the horse has strangles, repeat the test with a deeper respiratory sample (nasopharyngeal swab/wash or guttural pouch lavage) or a new sample taken 24 hours later,” advised Pusterla.

In conclusion, the POC test for S. equi has good overall agreement with the gold standard of qPCR. Therefore, the POC S. equi test can support a stall-side diagnosis of strangles in clinically infected horses, allowing rapid implementation of appropriate biosecurity protocols to limit the spread of this highly contagious respiratory pathogen.

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