Strangles Signs, Risk Factors, and Complications Evaluated

If your horse had strangles, would you be able to tell? He'd probably have those token swollen lymph nodes and maybe a fever, right? It's possible, but researchers recently determined that these signs alone might not be the only ones that should prompt a strangles test. In fact, more than a quarter of the horses in their recent research presented with no signs of disease at all.

At the 2104 American College of Veterinary Internal Medicine Forum, held June 4-7 in Nashville, Tennessee, Ashley Boyle, DVM, Dipl. ACVIM, shared results from a recent review of strangles cases in which her team identified risk factors and complications. Boyle is an assistant professor of medicine in the field service section at the University of Pennsylvania School of Veterinary Medicine's New Bolton Center.

Strangles is a highly contagious and debilitating equine disease caused by the bacterium Streptococcus equi subsp equi.

In their retrospective study Boyle and colleagues reviewed field service medication records from 2005 to 2012 and identified 108 strangles cases and 207 control horses. The team included strangles horses if they had classic clinical signs (including fever, enlarged lymph nodes, and nasal discharge) during an outbreak, a positive S. equi polymerase chain reaction (PCR) test, or a positive aerobic culture. The team included control horses if they had a fever without a strangles diagnosis. Boyle and colleagues evaluated all the horses' signalment (age, breed, sex, etc.), clinical signs, diagnostic testing results, therapies, disease complications, and blood values.

Ultimately, the team determined that, in their study, significant risk factors for a strangles diagnosis included:

  • Being 2 to 7 years old;
  • Having a fever, nasal discharge, and external abscesses (such as enlarged lymph nodes);
  • Hyperfibrinogenemia (excessive fibrinogen in the blood), with levels over 700 milligrams per deciliter; and
  • Anemia (a low red blood cell count).

The team also found that 30% of strangles horses were asymptomatic positives, meaning they weren't showing any outward signs of disease. These horses, Boyle said, were 11% more likely to become carriers (or continue to shed the organism through their respiratory secretions and saliva despite showing no signs of illness for a prolonged period) than horses showing signs of disease at diagnosis.

Additionally, the researchers found a lower prevalence of fever (60%) and external abscesses (22%) in their study compared to previous reports, leading Boyle to note that "fever and abscessation maybe insufficient in disease screening when used alone."

Regarding complications, the team found that horses in their study:

  • Had a higher prevalence of purpura hemorrhagica (a type of immune-mediated inflammation seen in 9% of the horses) than in previous reports, and the only horse in the study that died had developed the condition;
  • Had a higher prevalence of prolonged (more than 29 days) PCR-positive test results; and
  • Had a lower prevalence of metastatic (spreading) abscessation (just 4%) than previously reported.

The researchers ultimately concluded that their results differed from previously published studies.

"Not all strangles cases have the classic clinical signs, and abscessations may not always be in the typical locations," Boyle said. "Any horses with a fever should be examined by your veterinarian for strangles is a possibility.

"Previous exposure and vaccination (which may be unknown to the current owner) can make the clinical signs less obvious," she explained. "The most likely source of infection will be a horse with no external signs, and visual examination via guttural pouch endoscopy and PCR testing are the way to identify these horses."

Boyle said she and colleagues are continuing their research on this topic: "Currently we have a study funded by the Boehringer Ingelheim 2013 Advancement in Equine Research Award and the University of Pennsylvania Raymond Firestone Research Trust Grant comparing different anatomical locations with the horse’s throat to identify strangles carriers and comparing a new type of PCR to currently used PCR methods."

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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