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Best Test to Diagnose Strangles in Horses Identified

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PCR is a molecular diagnostic test to find DNA within a sample through amplification that uses heat. LAMP is a type of DNA amplification that does not use heat, but rather travels in a loop.

Boyle is an internationally recognized expert on Strangles who has been conducting research on the disease for more than 10 years. Her work is focused on ways to improve the sensitivity of diagnostic tests to detect the bacteria that cause this highly contagious, upper respiratory disease.

Strangles is caused by Streptococcus equi bacteria, related to the bacteria that cause strep throat in humans. Symptoms include nasal discharge, lethargy, and very high fever (over 103 degrees) in horses. The lymph nodes can swell to the point of cutting off the airway, thus the name Strangles.

If the infected lymph nodes break open in the throat, the puss can collect in the guttural pouch, an air-filled structure within the horse's head that arises from the Eustachian tube on each side, Boyle said. Once in the guttural pouch, the bacteria can be very difficult to eradicate.

Typically it takes two months to clear up an outbreak, Boyle said, and it can easily last for four months.

Boyle’s goal is to significantly reduce the time, effort, and expense required to test horses for Strangles. Through her research, she is working toward a highly accurate stall-side test.

The current recommendation, now 10 years old, is to get a wash of the throat and obtain negative tests three times, each a week apart, to safely declare a horse clear of the infection.

Until a barn is cleared, horses suspected of infection must be quarantined, and biosecurity protocols should be followed to prevent the spread of the disease, which can come at great expense and inconvenience to horse owners.

“We are working to avoid going to farms three separate times in three separate weeks,” said Boyle, who is an assistant professor and board-certified clinician in Internal Medicine.

It is important to determine which horses are carriers, which horses are not carriers, and which horses have come in contact with the bacteria and are immune, but have become carriers, Boyle said. “Our recommendation is to go into the guttural pouch for the sample to determine if they are carriers.”

Boyle presented the abstract of her research earlier this year, titled “Determining Optimal Sampling Site for Streptococcus equi Subspecies Equine Carriers Using a Loop-Mediated Isothermal PCR Assay,” at the Havemeyer Foundation workshop on streptococcal disease in Denmark, and at the American College of Veterinary Internal Medicine Conference in Indianapolis.

Boyle worked with a team of veterinarians, including Dr. Shelley Rankin, Associate Professor of Microbiology at Penn Vet.

The team tested the hypothesis that samples obtained from the equine guttural pouch would be more sensitive than samples obtained from the nasopharynx to identify carriers of Streptococcus equi. In addition, they hypothesized that a PCR test using LAMP technology would be more sensitive than a real-time PCR test.

A total of 123 samples were collected from 40 horses. The horses were sampled in three different ways: nasopharyngeal flocked nylon swab (41 samples), nasopharyngeal wash (38 samples), and endoscopically guided guttural pouch lavage (44 samples).

The LAMP test – which does not require heat cycling and is successfully used in human medicine – was performed on each. The guttural pouch lavage samples were tested using three different technologies: LAMP, realtime PCR, and the traditional culture.

“Guttural pouch lavage was the best sample to detect carriers,” the report stated.