Strangles · The evidence
Strangles: what every horse owner should know
Equine8 · updated
This page summarises veterinary research and guidance on strangles for horse owners. It is not veterinary advice: call your vet if you suspect strangles. Nothing here is a claim about any EQ8 product, and the organisations and authors cited do not endorse EQ8.
At a glance
What strangles is
Strangles is a highly contagious infection of the upper airway and its lymph nodes, caused by the bacterium Streptococcus equi subsp. equi (S. equi). It infects only horses, donkeys and other equids.[5][2] Its name comes from the swollen lymph nodes and throat, which can narrow the airway.[2]
Where it came from. S. equi evolved from an ancestral strain of S. zooepidemicus, a bacterium many horses carry harmlessly, and the two share more than 98% of their DNA.[6] In becoming a specialist of horses it lost or inactivated many genes.[7] Genome studies date the common ancestor of today’s strains to the late 19th or early 20th century, when a single lineage replaced the rest.[8]
How it beats the immune system. A thick hyaluronan capsule and a surface protein called SeM stop white blood cells from killing the bacterium.[2] It also carries superantigens that overstimulate the immune response, and an iron-scavenging system called equibactin.[7]
How common it is
Strangles is one of the most frequently diagnosed infectious diseases of horses around the world.[3] About 600 outbreaks are estimated to occur in the UK each year.[9]
Confirmed UK cases rose 77% in 2025
- 217
- 270
- 477
Laboratory-confirmed cases
- Netherlands: S. equi caused 64% of 364 confirmed equine disease outbreaks reported between 2019 and 2023.[11]
- United States: 601 cases were reported from 31 states between 2018 and 2022.[12]
- United Arab Emirates: genome sequencing found four different strains entering one yard in seven months. The UAE introduced compulsory blood-test screening of imported horses.[3]
- Globally: strains move between countries with horses; outbreaks in Japan were traced to a horse imported from the United States.[3]
Reporting. Strangles is not legally notifiable in the UK, though racehorse trainers must report it to the British Horseracing Authority.[13] It is not on the list of notifiable equine diseases in UAE federal law.[14]
How serious. In groups with no immunity, up to every horse can be affected.[15] Deaths were 0.9% on farms with fewer than 50 horses, but reached 8.1–9.7% in large farm outbreaks.[2]
Signs and course
Fever and dullness are usually the first signs, 3–14 days after exposure, and come before any nasal discharge.[2] Temperatures of 39.4–41.1 °C are typical.[15] The British Horse Society sums it up as “hot before snot”.[16]
A horse can spread strangles for weeks after it looks better
PhaseAcute illnessHidden risk
fever 3–14 days after exposureNasal shedding
starts 2–3 days after fever, lasts 2–3 weeksAbscesses burst
7 days to 4 weeks after infectionStill infectious
at least 6 weeks after discharge driesSilent carriers (~10%)
months to years
Days after exposure (approximate)
- Thick nasal discharge, a sore throat and difficulty swallowing.[2][15]
- Swollen lymph nodes under the jaw and behind the throat, which abscess and usually burst between 7 days and 4 weeks after infection.[2]
- Noisy or difficult breathing if swelling narrows the airway.[2]
- About 75% of horses not treated with antibiotics develop long-term immunity; 20–25% can catch it again within months.[2]
Complications
Complications may affect up to 20% of cases; on one farm, 15 of 74 affected horses (20.3%) developed them.[2]
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| Complication | How often | What happens |
|---|---|---|
| Spread to other lymph nodes (“bastard strangles”) | Usually low; 7 of 25 cases (28%) in one two-farm study[2] | Abscesses in the chest or abdomen; can be fatal |
| Purpura haemorrhagica | 4 of 74 cases (about 5%) in one report; 3 of 22 affected horses died[2] | Immune complexes inflame blood vessels; limb swelling and bleeding spots[2] |
| Muscle disease | Three types described[2] | Muscle infarction, acute muscle breakdown, or immune-mediated muscle wasting |
| Guttural pouch infection | About 10% of horses in outbreaks[2] | Pus thickens into hard “chondroids” that hide bacteria; source of carriers[2] |
| Other | Rare[2] | Heart muscle or valve inflammation, eye infections, loss of milk in mares |
Silent carriers: why strangles keeps coming back
About one horse in ten fails to clear the infection from its guttural pouches, air sacs behind the throat, and looks completely healthy while shedding bacteria on and off.[2] Around 75% of outbreaks produce at least one carrier.[4]
- In one long outbreak, carriers stayed infected for 7 to 39 months.[17]
- Genome studies found UK carriers that had lived on their yards for up to 8 and 15 years.[8]
- Carriers looked no different from their herd-mates, and 3 of 12 culture-positive carriers had negative blood tests.[18]
- Carriers are rarely found by routine pre-purchase or transport checks, which is how strains travel between countries.[3]
How it spreads
Strangles spreads by direct contact between horses, and indirectly through shared housing, water, feed and feeding utensils, and the clothing and equipment of people who handle horses.[2]
- The bacterium lasts 4–6 weeks in water.[2] In one study, horses that shared troughs had higher odds of strangles (adjusted odds ratio 7.74).[19]
- It survived up to 34 days in a wet bucket in winter, but only 1–3 days outdoors in summer sun.[20][21]
- Polyester halters stayed contaminated after a 40 °C wash; a 60 °C wash cleared them.[22]
- Visitors’ sleeves picked up related streptococci on 13 of 27 stable visits, and the strangles bacterium survived at least 24 hours on cotton and polyester.[23]
Testing
Culture can miss more than half of infected horses, and PCR is about three times more sensitive.[2] Your vet chooses the test and the sample; the evidence below explains why the choice matters.
PCR on several samples found 95% of infected horses; culture found 63%
Infected horses detected
Guttural pouch samples found far more carriers
Samples positive by PCR in recovered horses
Blood tests. A blood test combining two antigens (A and C) detects exposure with 93.3% sensitivity and 99.3% specificity.[26] It is used to screen new arrivals and to decide whether vaccination is safe, but a negative result does not rule out a carrier.[13][18]
Treatment, led by your vet
Treatment is a veterinary decision. The bacterium remains consistently susceptible to penicillin, the drug of choice, but when to use antibiotics is debated.[2][13]
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| Situation | What the guidelines say |
|---|---|
| Most cases | Rest in a dry, warm stable; no treatment beyond good nursing[2] |
| Very early, fever only | Penicillin given before abscesses form may cure the horse and prevent abscesses[2] |
| Abscesses already present | Antibiotics are generally ineffective and may prolong the illness; let abscesses mature and drain[2] |
| Welfare cases | Antibiotics if the horse cannot swallow, struggles to breathe or has a persistent fever[4] |
| Supportive care | Soft, moist feed and water within easy reach; anti-inflammatories; hot packs; lancing of mature abscesses[2] |
| Carriers | Guttural pouch flushing, removal of chondroids and antibiotic gel placed in the pouch[2] |
| Exposed but healthy horses | No preventive antibiotics[2] |
Vaccination
There is no expert consensus on vaccines: they reduce illness but do not give complete protection.[2] In the UK, a subunit vaccine (Strangvac) was authorised in 2021. It is licensed for horses from 5 months as two injections 4 weeks apart, with immunity from 2 weeks after the second dose and a licensed duration of 2 months.[27][28] In July 2026 the maker applied to extend this to 12 months, citing a 2026 analysis of seven trials that estimated about 80% protection a year after the second dose.[29][30]
A third dose protected 15 of 16 ponies in a challenge trial
Ponies protected from strangles after deliberate infection
- This vaccine does not interfere with blood tests or cause strangles.[13][31]
- Common side effects are a temperature rise of up to 2.6 °C and swelling of about 5 cm at the injection site, lasting up to 5 days. Vaccinated horses can still become infected.[28]
- For vaccines that contain the SeM protein, horses that had strangles in the past year, or have high SeM antibody levels (1:3,200 or above), should not be vaccinated because of the risk of purpura.[2] Strangvac is made from other proteins of the bacterium; follow its datasheet.[27][28]
Prevention and biosecurity
New arrivals. Isolate every new horse for at least 3 weeks, ideally 3–4, with daily temperatures and screening, including a blood test on arrival and after 3 weeks.[2][13] UK experts advise 21 days for all arrivals and 28 days for high-risk horses, with temperatures twice daily and immediate isolation of any horse at 38.5 °C or above.[4]
Don’t wait for a diagnosis. Isolate first, then test.[16] The British Horse Society uses a traffic-light system:
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| Group | Who | What to do |
|---|---|---|
| Red | Suspected or confirmed cases[16] | Strict isolation with a 5 m exclusion zone, dedicated staff and red-labelled kit[16] |
| Amber | Possible contact in the past 3 weeks[16] | Separate buckets, no nose-to-nose contact, temperatures twice daily; act on any rise above 38.5 °C[16] |
| Green | No contact for at least 3 weeks[16] | Keep apart from red and amber; temperatures twice daily[2] |
Ending an outbreak. Keep the yard closed until at least 3 weeks after the last case recovers, and test recovered horses before they mix or leave.[2][13] The sources differ on the best clearance test:
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| Source | When | How |
|---|---|---|
| ACVIM consensus[2] | No sooner than 3 weeks after signs resolve | One guttural pouch lavage tested by PCR is more efficient and sensitive than three nasopharyngeal washes over three weeks |
| HBLB Codes of Practice[13] | At least 4 weeks after the last signs | Three nasopharyngeal swabs a week apart detect more than 90% of carriers; guttural pouch endoscopy recommended |
| UK expert roundtable[4] | About 4 weeks after the last case | Guttural pouch lavage PCR is the only reliable proof of freedom from infection |
Tell your farrier, vet, coach, transporters and any venue you used in the past 3 weeks.[16]
Cleaning up after strangles
Routine cleaning and disinfection removed the strangles bacterium from concrete, wood, plastic and leather in a 2023 study; polyester halters needed a 60 °C wash.[22] Remove all organic material first, because chlorine products and quaternary ammonium compounds do not work in its presence.[2]
Where hypochlorous fits. As a final step on cleaned surfaces. In the 2023 study, plastic carrying a clinical strain was cleared after detergent and a disinfectant that its maker describes as hypochlorous-based, and in another study sodium hypochlorite removed bacterial DNA from twitches as well as live bacteria.[22][32][33] No published study has yet tested hypochlorous on its own against the strangles bacterium, and that study gave no strength or contact time.[22]
Read the stable hygiene guide: the full clean-first routine.
What owners think
A UK survey of about 2,000 owners found strong demand for screening, but little screening in practice.[34]
Four in five owners want screening; few yards do it
Share of respondents
Redwings has run Strangles Awareness Week each May since 2020.[1]
What the evidence does not yet show
- How much environmental survival matters in real outbreaks is unknown; most transmission is from horses.[4]
- No single test finds every carrier; blood tests miss some, and sampling the guttural pouch needs an endoscope.[18][25]
- The licensed duration of vaccine protection is short (2 months), although an extension to 12 months has been applied for.[28][29]
- No published study has tested hypochlorous against the strangles bacterium.
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