Strangles: What the Research Says
Evidence from 21 peer-reviewed studies
What Professionals Should Know
- •Strangles vaccines should prioritize the 40 core virulence genes for broad protection, but surveillance of geographically variable accessory genes is essential to detect and prevent vaccine escape variants
- •Antimicrobial resistance is not a major concern with S. equi globally, simplifying treatment decisions, though prudent antibiotic stewardship remains important
- •Understanding the specialized roles of mobile genetic elements in virulence gene distribution will inform more targeted vaccine and therapeutic strategies
- •Existing commercial strangles vaccines have documented limitations; this study suggests that PNAG-based approaches alone do not provide adequate field protection even when generating measurable immune responses
- •When managing strangles vaccination programs, practitioners should recognize that in vitro antibody functionality does not guarantee in vivo protection, and contact exposure remains a significant infection risk
- •Future vaccine development should focus on multi-component strategies targeting SEE's immune evasion mechanisms, not single antigens alone
- •Implement strict biosecurity: clean and disinfect shared feed containers and water troughs regularly to reduce transmission in multi-animal facilities
- •Consider isolating affected animals from group housing, as shared overnight accommodation significantly increases strangles spread
- •Be aware that common antibiotics (tetracycline, erythromycin, vancomycin) show high resistance rates in local strains; coordinate with veterinarian on appropriate antimicrobial therapy
- •Strangvac vaccination appears to prevent clinical strangles even when vaccinated horses are exposed to S. equi during outbreak situations on the same farm
- •DIVA-compatible design allows differentiation between vaccinated and naturally infected animals, useful for outbreak management and disease monitoring
- •Consider vaccinating healthy horses early in a strangles outbreak as vaccination during exposure may confer protection without preventing subclinical infection/immunity development
- •Tracheal lavage cytological analysis combined with bacterial culture can help identify beta-hemolytic streptococcal infections in horses with respiratory signs; use this to confirm strangles diagnosis and guide treatment decisions
- •Understanding the inflammatory cell patterns associated with different respiratory symptoms may help differentiate between beta-hemolytic streptococcal infections and other upper respiratory conditions in your practice
- •Early identification of horses with clinical signs (coughing, lymph node swelling, nasal discharge) through these diagnostic methods enables faster isolation protocols and reduces disease spread in riding clubs and boarding facilities
- •Regular cleaning and disinfection of shared feed containers and water troughs is critical to reduce strangles transmission in group settings
- •Isolate affected animals and segregate housing (including separate night turnout areas) to interrupt disease spread, particularly when SeM-positive strains are circulating
- •Be aware that common antibiotics (tetracycline, erythromycin) show high resistance rates—work with your veterinarian to guide treatment decisions based on local resistance patterns
- •Enhanced disinfection protocols for endoscopes and twitches used in strangles diagnosis are necessary to prevent cross-contamination between horses
- •Consider dedicated or single-use equipment when examining suspected strangles carriers to minimize transmission risk
- •Standard disinfection may be insufficient—validate your disinfection procedures specifically for S. equi elimination on these instruments
- •Strangles in donkeys shows age-related susceptibility patterns similar to horses; intensively managed young donkeys represent a high-risk population requiring enhanced biosecurity measures
- •A single novel bacterial strain was responsible for multi-farm transmission, emphasizing the importance of quarantine protocols and movement controls between donkey breeding operations
- •As donkey farming intensifies in Asia, practitioners should remain alert for strangles and implement vaccination and management strategies used in equine herds
- •Pack donkeys and cart horses in this region require species-specific management: focus on back and tail sore prevention in donkeys through improved saddle fit and harness design, and girth sore management in cart horses through better equipment maintenance and hygiene
- •Hoof overgrowth is a critical welfare issue affecting both species (35-62% prevalence) — establish regular farriery services and owner education on hoof care as a priority intervention
- •Epizootic lymphangitis in cart horses (10.7% prevalence) requires targeted disease control and biosecurity measures, while systemic issues (feed, water, housing, training methods causing abnormal behavior) must be addressed through government support and capacity building for local animal health professionals
- •Any qPCR-positive result for S. equi should trigger biosecurity protocols to prevent spread, even if culture is negative, as the organism may still be viable
- •Combining gDNA quantitation with mRNA detection provides better assessment of S. equi viability than culture alone in respiratory secretions
- •Sample handling and processing delays can affect culture results; qPCR methods may be more reliable for confirming strangles in field-collected samples
- •Understanding local SeM types in your region can help trace strangles outbreak sources and epidemiological patterns on your facility
- •Molecular typing of streptococcal isolates provides data to improve outbreak management and biosecurity protocols
- •Regional variation in SeM types may influence vaccination strategy decisions and disease control planning for strangles prevention
- •Establish and implement written antimicrobial protocols in your practice to align prescribing with best evidence, as this was identified as a key decision-making factor by practitioners
- •Consider that bacterial culture results and formal guidelines should drive antimicrobial selection rather than routine empirical prescribing, particularly for conditions like wound infections
- •Advocate for expanded oral antibiotic options and clearer treatment guidelines at industry and regulatory levels to improve antimicrobial stewardship in equine practice
- •Implement strict biosecurity measures including isolation of affected horses, separate equipment, and personnel hygiene to prevent strangles transmission within your facility
- •Request PCR or bacteriological culture testing for suspected cases rather than relying on clinical signs alone, as diagnosis confirmation guides treatment decisions
- •Ensure penicillin remains the antibiotic of choice for treatment; consult with your veterinarian on appropriate dosing and duration for your region's strains
- •Implement rigorous screening protocols for carrier animals during pre-purchase and transit, as asymptomatic horses can transmit strangles across national borders
- •Recognize that strangles has significant international health, welfare and economic costs warranting disease surveillance and control at international level
- •Use genomic analysis tools to track S. equi transmission patterns and identify outbreak sources within and between horse populations
- •Use this consensus guideline to inform your strangles diagnosis and management protocols, especially where research evidence is limited or conflicting
- •The document provides practical recommendations specifically designed for field veterinarians managing strangles cases
- •Current best practice information is now available to standardize approaches across UK equine practice
- •A reliable multiplex PCR diagnostic tool is now available for rapid, sensitive identification of strangles-causing organism with subspecies differentiation in laboratory settings
- •The high sensitivity (detection at 66-132 genomic copies) suggests this test could identify infections at early stages when bacterial load is low
- •The lack of cross-reactivity means this test is specific to Streptococcus equi and won't produce false positives from other common equine pathogens or environmental bacteria
- •Stay informed about emerging vaccine developments for strangles as they represent the most promising approach to disease control
- •Understand that strangles remains a significant challenge in equine practice globally, making vaccination strategies increasingly important
- •Work with your veterinarian to implement evidence-based vaccination protocols as new research advances become available
- •Strangles remains highly prevalent in UK horse populations despite centuries of recognition and >100 years of research; staying current on outbreak protocols is essential for all equine practitioners
- •Understanding the evolutionary history of S. equi is critical for implementing effective biosecurity and outbreak management strategies on farms and in practice
- •This review synthesizes recent advances in pathogen understanding that should inform contemporary approaches to prevention and treatment protocols
- •Negative culture results do not rule out strangles in suspected cases; multiple sampling sites and methods may be necessary for accurate diagnosis
- •Work with your laboratory to understand which sampling sites (nasal swabs, guttural pouch lavage, abscess material) and tests (culture vs. molecular) provide highest diagnostic sensitivity for your practice
- •Consider molecular diagnostic methods alongside traditional culture when strangles is suspected, as they may improve detection rates in difficult cases
- •Practitioners can use SeM typing to determine if strangles cases in their practice are connected to known outbreak strains or represent new introductions
- •Identifying outbreak clusters geographically may help target biosecurity measures and control strategies to high-risk regions
- •This molecular typing method enables better disease surveillance and can support evidence-based decisions about quarantine and movement restrictions during outbreaks
- •Strangles diagnosis and management should incorporate understanding of persistent infections as a source of ongoing transmission on premises
- •New vaccine development based on SeM protein and virulence factor research offers improved preventative options for practitioners to recommend to horse owners
- •Molecular characterization tools are now available to track outbreak strains and understand epidemiological patterns of strangles on individual operations
Key Research Findings
S. equi has a closed pangenome with 1,661 core genes and 982 accessory genes, indicating a highly host-adapted pathogen with limited horizontal gene transfer
40 of 71 identified virulence-associated genes (VAGs) are core genes, while accessory VAGs show significant geographic variation, particularly in nutritional/metabolic and exotoxin genes
No acquired antimicrobial resistance genes were detected except a single qacG gene, indicating minimal AMR acquisition in global S. equi populations
Mobile genetic elements show functional specialization: prophages carry superantigen genes, genomic islands harbor iron acquisition genes, and ICEs carry heme metabolism and streptolysin S-associated genes
Vaccination with poly-N-acetyl glucosamine (PNAG) antigen generated functional antibodies that mediated complement deposition and opsonophagocytic killing of SEE in vitro
Both intramuscular alone and combined intramuscular plus intranasal immunization strategies targeting PNAG failed to protect yearling horses against experimental SEE infection following contact exposure
Current PNAG-based vaccine approach is insufficient for clinical protection and may require additional antigenic components targeting SEE virulence factors
Streptococcus equi was isolated from 31.87% (51/160) of clinically suspected samples, with 31.37% (16/51) carrying the SeM gene
High antimicrobial resistance observed: tetracycline 81.5%, erythromycin 81.5%, and vancomycin 75.5% resistance among isolates
Sharing feed containers increased strangles likelihood 7.59-fold (AOR = 7.59; 95% CI = 1.44–39.93)
Shared water troughs and overnight housing were significant risk factors with 7.74-fold and 5.97-fold increased odds respectively
All 17 vaccinated horses showed increased antibody titres to vaccine components (CCE, Eq85, IdeE) from day 0 to day 28 (p<0.0001)
Zero clinical cases of strangles developed in vaccinated horses despite serological evidence of exposure in 8/17 on day 0 and 9/16 on day 28
Seropositivity in antigen A/C ELISA (infection marker) was detected in 5/7 vaccinated horses at day 489, indicating prior exposure without clinical disease
All three unvaccinated clinical cases tested positive in antigen A/C ELISA on day 28, contrasting with protection in vaccinated cohort
Evidence Base
He Lingyu, Khine Nwai Oo, Song Jeongmin et al. (2025) — Frontiers in veterinary science
Vaccination of yearling horses against poly-N-acetyl glucosamine fails to protect against infection with Streptococcus equi subspecies equi.
Cohen Noah D, Cywes-Bentley Colette, Kahn Susanne M et al. (2020) — PloS one
Streptococcus equi Subspecies equi From Strangles Suspected Equines: Molecular Detection, Antibiogram Profiles and Risk Factors
Bekele Demsew, Desalegn Bereket, Tadesse Belege et al.
Reining in strangles: Absence of disease in horses vaccinated with a DIVA-compatible recombinant fusion protein vaccine, Strangvac, following natural exposure to Streptococcus equi subspecies equi.
Gröndahl Gittan, Righetti Francesco, Aspán Anna et al. (2026) — Equine veterinary journal
Analysis of the presence of Beta-hemolytic streptococci and cytological profiles in horse tracheal lavage samples with diverse clinical respiratory symptoms.
Pınar Orhan, Çelik Doğan Cansu, Aktaran Bala Deniz et al. (2025) — BMC veterinary research
Streptococcus equi subspecies equi from strangles suspected equines: molecular detection, antibiogram profiles and risk factors.
Bekele Demsew, Dessalegn Bereket, Tadesse Belege et al. (2024) — BMC veterinary research
Potential for residual contamination by Streptococcus equi subspp equi of endoscopes and twitches used in diagnosis of carriers of strangles.
Svonni Elin, Andreasson Mikaela, Fernström Lise-Lotte et al. (2020) — Equine veterinary journal
An outbreak of strangles associated with a novel genotype of Streptococcus equi subspecies equi in donkeys in China during 2018.
Dong J, Gao N, Waller A S et al. (2019) — Equine veterinary journal
Health and welfare problems of pack donkeys and cart horses in and around Holeta town, Walmara district, Central Ethiopia
Chala Chaburte, Bojia Endabu, Feleke Getahun et al. (2019) — Journal of Veterinary Medicine and Animal Health
Use of quantitative real-time PCR to determine viability of Streptococcus equi subspecies equi in respiratory secretions from horses with strangles.
Pusterla N, Leutenegger C M, Barnum S M et al. (2018) — Equine veterinary journal
Molecular and sequencing study and identification of novel SeM-type in beta-hemolytic streptococci involving the upper respiratory tract in Iran.
Moghaddam Sina, Lotfollahzadeh Samad, Salehi Taghi Zahraei et al. (2023) — BMC veterinary research
Usage of Antimicrobials in Equine Veterinary Practice in Denmark - A Case-Based Survey.
Jacobsen Alice B J E, Damborg Peter, Hopster-Iversen Charlotte (2023) — Journal of equine veterinary science
Recent updates on strangles caused by Streptococcus equi subspecies equi in Indonesia
Rotinsulu Dordia Anindita (2023) — Current Biomedicine
Globetrotting strangles: the unbridled national and international transmission of <i>Streptococcus equi</i> between horses
(2021) — Equine Veterinary Journal
<i>Streptococcus equi</i>infections: current best practice in the diagnosis and management of ‘strangles’
Rendle David, de Brauwere Nic, Hallowell Gayle et al. (2021) — UK-Vet Equine
Optimization of Conditions for The Multiplex PCR for Diagnostics of Horse Strangles with Subspecies Differentiation of Streptococcus Equi Subsp Equi
Berdimuratova K.T., Makhamed R., Shevtsov A.B. (2020) — Eurasian Journal of Applied Biotechnology
Waller Andrew (2018) — The Veterinary record
Strangles: a pathogenic legacy of the war horse.
Waller Andrew S (2016) — The Veterinary record
Comparison of sampling sites and laboratory diagnostic tests for S. equi subsp. equi in horses from confirmed strangles outbreaks.
Lindahl S, Båverud V, Egenvall A et al. (2013) — Journal of veterinary internal medicine
Molecular characterisation of 'strangles' outbreaks in the UK: the use of M-protein typing of Streptococcus equi ssp. equi.
Ivens P A S, Matthews D, Webb K et al. (2011) — Equine veterinary journal