Streptococcus equi equi is a gram-positive bacteria that induces infection and contagious disease in the upper respiratory tract of horses, donkeys, andmules. Common clinical responses may include isolating any horse with signs of strangles to prevent population outbreaks. Current diagnostic testing includes detection of shedding via bacterial culture and more sensitively with molecular PCR confirmation.

The Fluxergy S. equi subsp. equi assay can detect a conserved region of the S. equi subsp. equi genome that is highly unique to the specific streptococcus subtype.

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Horses with subclinical Strangles infections can transmit the disease to other horses.¹⁰ Strains^5,6 and variations within its genetic structure^7,8, coupled with its ability to reside in a subclinical state, have likely contributed to its ability to survive and avoid detection. Over time, strains have developed resistance to antimicrobials⁹, threatening to limit effective treatment options.

Seasonality—perhaps relative to environmental temperatures and humidity—may play a role, as researchers identified a higher likelihood of Strangles infection in the spring than in the summer. ³ “Cases with anemia were more likely to develop purpura hemorrhagica than were cases without anemia. No risk factors were identified for the development of guttural pouch empyema or metastatic abscesses.” ³ Older horses^4,5 have also been identified to be at greater risk. One epidemiological study found “horses used for competition and ranch/farm use were more likely to test qPCR-positive for S. equi (p = 0.006),” while “[S. equi] vaccination was associated with a lower frequency of S. equi qPCR-positive status.” ⁴

Yes, S. equi equi seems to favor wet, cooler conditions and can be viable for up to 34 days. In warm, dry conditions, viability is restricted to two days.¹ Nylon halters have also been identified as potential fomites if not washed in hot water and thoroughly disinfected.²

1.   Durham AE, Hall YS, Kulp L, Underwood C. A study of the environmental survival of Streptococcus equi subspecies equi. Equine Vet J. 2018 Nov;50(6):861-864. doi: 10.1111/evj.12840. Epub 2018 May 7. PMID: 29654609.

2.    Ryden A, Fernström LL, Svonni E, Riihimäki M. Effectiveness of Cleaning and Sanitation of Stable Environment and Riding Equipment Following Contamination With Streptococcus equi Subsp. equi. J Equine Vet Sci. 2023 Feb;121:104204. doi: 10.1016/j.jevs.2022.104204. Epub 2022 Dec 29. PMID: 36586522.

3.    Duffee LR, Stefanovski D, Boston RC, Boyle AG. Predictor variables for and complications associated with Streptococcus equi subsp equi infection in horses. J Am Vet Med Assoc. 2015 Nov 15;247(10):1161-8. doi: 10.2460/javma.247.10.1161. PMID: 26517620.

4.    Jaramillo-Morales C, James K, Barnum S, Vaala W, Chappell DE, Schneider C, Craig B, Bain F, Barnett DC, Gaughan E, Pusterla N. Voluntary Biosurveillance of Streptococcus equi Subsp. equi in Nasal Secretions of 9409 Equids with Upper Airway Infection in the USA. Vet Sci. 2023 Jan 20;10(2):78. doi: 10.3390/vetsci10020078. PMID: 36851382; PMCID: PMC9962190.

5.    Tartor YH, El-Naenaeey EY, Gharieb NM, Ali WS, Ammar AM. Novel Streptococcus equi strains causing strangles outbreaks in Arabian horses in Egypt. Transbound Emerg Dis. 2020 Nov;67(6):2455-2466. doi: 10.1111/tbed.13584. Epub 2020 May 10. PMID: 32304282.

6.    Mitchell C, Steward KF, Charbonneau ARL, Walsh S, Wilson H, Timoney JF, Wernery U, Joseph M, Craig D, van Maanen K, Hoogkamer-van Gennep A, Leon A, Witkowski L, Rzewuska M, Stefańska I, Żychska M, van Loon G, Cursons R, Patty O, Acke E, Gilkerson JR, El-Hage C, Allen J, Bannai H, Kinoshita Y, Niwa H, Becú T, Pringle J, Guss B, Böse R, Abbott Y, Katz L, Leggett B, Buckley TC, Blum SE, Cruz López F, Fernández Ros A, Marotti Campi MC, Preziuso S, Robinson C, Newton JR, Schofield E, Brooke B, Boursnell M, de Brauwere N, Kirton R, Barton CK, Abudahab K, Taylor B, Yeats CA, Goater R, Aanensen DM, Harris SR, Parkhill J, Holden MTG, Waller AS. Globetrotting strangles: the unbridled national and international transmission of Streptococcus equi between horses. Microb Genom. 2021 Mar;7(3):mgen000528. doi: 10.1099/mgen.0.000528. Epub 2021 Mar 8. PMID: 33684029; PMCID: PMC8190609.

7.    Chanter N, Talbot NC, Newton JR, Hewson D, Verheyen K. Streptococcus equi with truncated M-proteins isolated from outwardly healthy horses. Microbiology (Reading). 2000 Jun;146 ( Pt 6):1361-1369. doi: 10.1099/00221287-146-6-1361. PMID: 10846214.

8.    Riihimäki M, Aspán A, Ljung H, Pringle J. Long term dynamics of a Streptococcus equi ssp equi outbreak, assessed by qPCR and culture and seM sequencing in silent carriers of strangles. Vet Microbiol. 2018 Sep;223:107-112. doi: 10.1016/j.vetmic.2018.07.016. Epub 2018 Jul 27. PMID: 30173735.

9.    Arafa AA, Hedia RH, Ata NS, Ibrahim ES. Vancomycin resistant Streptococcus equi subsp. equi isolated from equines suffering from respiratory manifestation in Egypt. Vet World. 2021 Jul;14(7):1808-1814. doi: 10.14202/vetworld.2021.1808-1814. Epub 2021 Jul 14. PMID: 34475702; PMCID: PMC8404119.

10.    Pringle J, Venner M, Tscheschlok L, Waller AS, Riihimäki M. Markers of long term silent carriers of Streptococcus equi ssp. equi in horses. J Vet Intern Med. 2020 Nov;34(6):2751-2757. doi: 10.1111/jvim.15939. Epub 2020 Oct 19. PMID: 33074578; PMCID: PMC7694814.