Equine strangles is caused by the bacterium Streptococcus equi subsp. equi (S. equi). Strangles is a highly transmissible upper respiratory disease in horses. For that reason, it has important economic impacts on the equine industry. 

What is Equine Strangles

Strangles in equids usually presents with an acute onset of fever and respiratory signs. Those can be accompanied by a mucopurulent (white/yellow) nasal discharge. 

Unfortunately, strangles cases are underreported in North America. The Equine Disease Communication Center (EDCC) confirmed 139 cases of strangles in 2023. That included 42 cases confirmed in Michigan and 39 cases confirmed in Florida. (Editor’s note: Strangles is not a reportable disease in every U.S. state. Many suspect cases are not included in this count. You can go to the EDCC website and search for individual diseases by date or state.) 

Study of Equine Strangles

The aim of a study presented at the 2023 American Association of Equine Practitioners’ Convention was to provide results of a field study. The study describe epidemiological aspects, including vaccine status, of horses with acute onset of fever and respiratory signs tested by qPCR for S. equi. 

The presentation was titled, “Voluntary Biosurveillance of Streptococcus equi subsp. equi in Nasal Secretions of 9,409 Equids with Upper Airway Infection in the United States.”  

Survey for Strep Equi in Horses 

Researchers collected biological samples and epidemiological information of diseased horses via a voluntary biosurveillance program from 2008-2020. They gathered information about every index case’s demographics, location, season, clinical signs, and vaccination status. Nasal secretions were tested via qPCR for S. equi and common respiratory viruses (equine influenza virus, equine herpesvirus-1 and -4, and equine rhinitis A/B virus).  

Two hundred and sixty-one equine veterinary practices across the U.S. participated in the study. 

Equine Strangles Research Results 

Here are some of the key findings from this research on equine strangles.

  • Of the 9,409 horses in the study, 715 tested qPCR-positive for S. equi. Of those 715 horses, 32% had coinfections with respiratory viruses.  
  • The median age for the S. equi qPCR-positive horses was 8 (plus or minus 4 years). The researchers noted that older horses presenting during winter and spring were more likely to test positive for S. equi. 
  • Horses vaccinated against S. equi appeared to be protected. 
  • Horses used for competition and ranch/farm use were the animals most likely to test positive for S. equi.  
  • During winter and spring, a greater number of horses tested qPCR-positive for S. equi than in summer and fall.  
  • There was no statistical difference between sex, history of transport, or location for horses testing positive and negative for S. equi.  
  • The most common clinical signs reported were nasal discharge (89%), fever (83%), and lethargy (75%).  

Authors for this study were: Camilo Jaramillo-Morales, DVM, DACVIM, MS*; Kaitlyn James, Epi., MS; Samantha Barnum, MS; Wendy Vaala, VMD, DACVIM; Duane E. Chappell, DVM; Chrissie Schneider, DVM, DABVP (Equine); Bryant Craig, DVM; Fairfield Bain, DVM, MBA, DACVIM, DACVP, DACVECC; Craig Barnett, DVM; Earl Gaughan, DVM, DACVS; and Nicola Pusterla, DVM, PhD, DACVIM, DAVDC-Equine. 

  • Kimberly S. Brown
    Kimberly S. Brown is an award-winning writer and publisher. She is the Editorial Director for My Senior Horse. Brown spent 10 years at Equine Network, parent company of My Senior Horse. Prior to that she worked for three years in equine nutrition after she retired from nearly 30 years working at The Blood-Horse. Brown spent the last 15 years of her time at that organization creating and developing The Horse and TheHorse.com.
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