Updated guidance published on strangles

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Strangles, a highly contagious upper respiratory tract disease caused by Streptococcus equi subsp equi, remains one of the most common diseases in horses. As of 2017, strangles is a reportable disease in the United States and many other countries.

The American College of Veterinary Internal Medicine published an updated consensus statement on strangles Feb. 9 in the Journal of Veterinary Internal Medicine, available at https://jav.ma/Stranglesdoc.

Dr. Ashley Boyle (center) of the University of Pennsylvania School of Veterinary Medicine took the lead in writing a new consensus statement on treating, controlling, and preventing the equine infectious disease strangles. (Courtesy of PennVet)

​The document gives valuable updates from the original one published in 2005, reflecting current published knowledge and opinion about clinical signs, pathogenesis, epidemiology, treatment, complications, and control of strangles. Dr. Ashley Boyle, an associate professor of medicine at the University of Pennsylvania School of Veterinary Medicine, co-authored the report with Dr. John Timoney of the Gluck Equine Research Center at the University of Kentucky, Drs. Richard Newton and Andrew Waller of Animal Health Trust in the U.K., Dr. Melissa T. Hines of the University of Tennessee, and Dr. Ben Buchanan of Brazos Valley Equine Hospital in Texas.

One of the key areas where the guidance has shifted in the new statement is on determining when an animal is free of disease. This is a critical component of strangles control, as 10 to 40 percent of animals that have had the disease can remain carriers, able to pass on disease to other animals even when they appear healthy.

The previous consensus statement indicated a throat wash should be tested three times to be sure the horse was free of disease.

"But it was hard to get owners and clients to do that three times," Dr. Boyle said in a March 13 PennVet press release. "And multiple studies have shown that those three may not even be good enough."

In the new statement, Dr. Boyle and colleagues instead recommend "guttural pouch sampling," a technique that tests the fluid that sits in an area between a horse's ear and throat, akin to the human eustachian tube, in combination with using an endoscope to visually assess the area for unruptured abscesses.

"We're trying to encourage more primary care veterinarians to do this, even though it's time consuming, because this is how the disease perpetuates itself," Dr. Boyle said in the release. "In the end it is easier, faster, and more effective than doing the three throat washes. That's one of our big take-home messages."

Another change is a shift away from considering bacterial culture to be the gold standard of diagnosis.

"We no longer consider this the gold standard," Dr. Boyle said in the release, as horses with low concentrations of bacteria may still be carriers but not test positive. The authors see DNA amplification technologies such as polymerase chain reaction along with visualization of the guttural pouch as replacing the culture standard. "There is also a lot of work being done by my colleagues in Europe looking at genomic sequencing. At some point in the future, we may be able to use this technology to trace from where an outbreak came."

Overall, the publication lays out best practices for quarantine and examination to prevent the spread of disease, and biosecurity protocols to reduce transmission in facilities where infected horses have been housed. It spells out treatment protocols, urging judicious use of antimicrobials, and explains how to recognize, evaluate the risk for, and treat one of the more serious complications of strangles, an autoimmune reaction known as purpura hemorrhagica, which can be fatal. It also explores the pros and cons of strangles vaccines and the use of various blood tests that can measure previous exposure to disease and determine when it is safe to give vaccines.