strangles in horses

An infectious respiratory disease caused by Streptococcus equi subspecies equi bacteria, strangles in horses spreads with ease. Thus, it’s important that veterinarians diagnose it quickly and accurately to prevent outbreaks. They have two methods of doing so: culture and the more reliable qPCR test, which detects bacterial and viral DNA.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, professor of medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine, reviewed these methods and their efficacy at the 2018 American Association of Equine Practitioners Convention, held Dec. 1-5 in San Francisco, California.

In recent years, he said, qPCR has supplanted traditional S. equi culture as the gold-standard diagnostic test for strangles in horses. While culture is convenient and cost-effective, it’s time-consuming and can produce false negatives. The qPCR test is much faster, with better sensitivity and specificity (ability to accurately identify results). The results, however, can be difficult to interpret, said Pusterla, as veterinarians often can’t determine whether it has detected a dead—indicating past exposure or infection—or live organism.

“This is important because we need to engage in appropriate biosecurity, as well as know when we can take (an affected) horse out of isolation,” said Pusterla.

So he conducted a study of 30 horses with suspected S. equi infection to determine how to best interpret qPCR results. Pusterla collected 85 biological samples via nasopharyngeal swabs and guttural pouch (an internal sac that represents an outgrowth of the eustachian tube and acts to equalize pressure within the ear) lavage. He used three molecular methods to determine organism viability: absolute quantitation of the target gene (basically, determining its numbers), detection of messenger RNA, and absolute quantitation after enrichment (incubation in a culture broth to grow bacteria).

Based on his results, Pusterla said any of these qPCR approaches could be effective in diagnosing true cases of strangles in horses, but if absolute quantitation produces positives, consider those S. equi organisms to be viable.

This can help the veterinarian determine whether a horse has an active infection and take the appropriate biosecurity steps to prevent disease spread, he said. It can also confirm whether it’s safe to remove a horse from quarantine.