Strangles in horses: signs, diagnosis and prevention
Strangles in horses: signs, diagnosis and prevention
A clinical guide from Glasgow Equine Hospital & Practice, with Vikki Scott (BVetMed MSc DipACVIM MRCVS), an RCVS Recognised Specialist in Equine Internal Medicine at Glasgow Equine Hospital & Practice, University of Glasgow.
Summary
Strangles, caused by the bacterium Streptococcus equi subspecies equi, is the most commonly diagnosed infectious disease of horses worldwide. The Surveillance of Equine Strangles (SES) scheme recorded 477 lab-confirmed cases in the UK in 2025 — a 77% increase on 2024. The disease is manageable when recognised early, but it does not always present in the way owners expect. This guide explains how strangles presents clinically, why the guttural pouches matter, how diagnosis is performed, and what owners and yard managers can do to reduce risk.
Key facts
- Cause: Streptococcus equi subspecies equi (a bacterium)
- Most common signs: fever, thick nasal discharge, swollen or abscessing lymph nodes under the jaw
- Atypical signs: fever alone, mild nasal discharge alone, mild loss of appetite — many cases do not show the full picture
- Incubation period: typically 3–21 days, occasionally up to 28 days
- Normal adult horse temperature at rest: approximately 37.0–38.0°C
- UK case data 2025 (SES scheme): 477 lab-confirmed cases, a 77% increase on 2024
- Carrier state: an estimated 10% of recovered horses retain S. equi in their guttural pouches and can shed intermittently; some studies report figures up to 20%
- Diagnosis: PCR and bacterial culture from nasopharyngeal swabs, guttural pouch lavage, or pus from abscesses; serology has a role in screening
- Prevention framework: the Strangles Awareness Week BEST acronym — Boost immunity with vaccination, Engage with trusted information, Separate unfamiliar horses, Temperature check routinely
What is strangles?
Strangles is a contagious upper respiratory tract infection of horses, ponies and donkeys, caused by the bacterium Streptococcus equi subspecies equi. It spreads horse-to-horse through direct contact and indirectly via shared equipment, water troughs, handlers’ hands and clothing, and contaminated environments. The bacterium can survive for several weeks in water and on damp surfaces.
It is one of the oldest recognised infectious diseases of the horse, and it remains the most commonly diagnosed equine infectious disease in the UK and internationally. The British Horse Society estimates around 600 UK outbreaks per year. Despite its long history, strangles continues to circulate widely — partly because of the silent carrier state described later in this article.
What are the clinical signs of strangles?
The textbook description of strangles is a young horse with a high fever, thick nasal discharge, and visibly enlarged or abscessing lymph nodes under the jaw. That picture is real, and we do see it. But it is only part of what strangles can look like.
One thing every horse owner should know about strangles — Vikki Scott
Speaking on this point, Vikki Scott explains:
“One of the most important things to know about strangles is although the most common clinical signs will be nasal discharge and fever and enlarged lymph nodes, many cases of strangles don’t present with all of those signs. So sometimes just the horse with fever or the horse just with nasal discharge might have strangles too. So it’s important to know the different clinical signs that we can see in presentation.”
In clinical practice, atypical presentations are common. A horse may show only:
- A raised temperature, with no other obvious sign
- Discharge from a single nostril
- Mild loss of appetite or a single off-colour day
- Subtle changes in demeanour or willingness to work
The phrase frequently used in equine practice is “hot before snot” — fever often precedes nasal discharge by 24–48 hours, and is one of the earliest reliable signals that something has changed. This is why routine temperature monitoring is recommended on yards where new horses are arriving or where cases have been reported in the area.
A small proportion of infected horses show no clinical signs at all, but can still shed the bacterium and infect others. This sub-clinical pattern is one of the reasons strangles can move through a yard before anyone has identified the first case.
What are guttural pouches and why do they matter?
The guttural pouches are central to understanding why strangles persists in the horse population.
What are guttural pouches? — Vikki Scott
In her own words:
“Humans do not have guttural pouches. They are anatomical structures unique to the horse that connect the middle ear to the pharynx and they are air-filled sacs that contain a number of really important anatomical structures, such as cranial nerves.”
The guttural pouches are paired air-filled diverticula of the auditory (Eustachian) tubes, located at the back of the head behind the pharynx. They are unique to equids and a small number of other species, and they sit in close anatomical relationship with major blood vessels — including the internal carotid artery — and with several cranial nerves.
In strangles, the guttural pouches matter for two reasons:
- They are a primary site of infection. Lymph nodes adjacent to the guttural pouches can rupture into the pouches during the course of disease, leading to accumulation of pus (empyema) and, sometimes, the formation of dried, hardened concretions called chondroids.
- They are the principal site of long-term carriage. After clinical recovery, an estimated 10% of horses continue to harbour S. equi within their guttural pouches, with some studies reporting figures up to 20%. Carriage may persist for months or years. These animals — silent carriers — appear entirely healthy. They do not have a fever, do not show discharge, and behave normally. But they can shed bacteria intermittently and act as a source of new outbreaks.
This is the part of strangles that most surprises owners. A horse that has visibly recovered is not necessarily clear of infection. Without testing, there is no way to know.
How is strangles diagnosed?
Diagnosis of strangles depends on the stage of disease and the question being asked. The two questions are usually:
- Is this current illness strangles? — typically answered by PCR and culture of nasopharyngeal swabs or pus from a draining abscess.
- Is this recovered horse genuinely free of infection? — typically answered by guttural pouch lavage with PCR and culture, sometimes preceded by serology as a screening tool.
For the second question — clearance testing — the gold-standard sample is taken directly from the guttural pouches under endoscopic guidance.
How is a guttural pouch sample taken? — Vikki Scott
Vikki Scott describes the procedure:
“We use an endoscope and we pass it up the nasal passages into the upper airway just towards the back of the head in the horse and we can take a sample and then submit it to the laboratory for testing.”
In practical terms, an endoscope is passed gently through one nostril and guided into the opening of the guttural pouch. A small volume of sterile saline is flushed into the pouch and recovered, and the lavage fluid is submitted for PCR and bacterial culture. The procedure is well-tolerated by most horses with appropriate handling and sedation, and is performed routinely in equine practice — both on the yard and in a hospital setting.
A negative PCR result on a guttural pouch lavage gives a much higher level of confidence in clearance than a negative nasopharyngeal swab alone. For horses returning to a shared yard after suspected or confirmed strangles, or for horses with an unknown history, this is the test most likely to give a meaningful answer.
At Glasgow Equine Hospital & Practice, guttural pouch endoscopy is available as part of our integrated first-opinion and hospital service at the Weipers Centre, with a clear route from ambulatory assessment into the hospital where further investigation is needed.
How can I prevent strangles on my yard?
Prevention is built around four practical actions, summarised by the Strangles Awareness Week campaign as BEST:
B — Boost immunity with vaccination
A licensed strangles vaccine is available in the UK. It is one tool among several, and it does not replace quarantine or biosecurity. Vaccination decisions are made horse-by-horse and yard-by-yard in conversation with your vet.
E — Engage with trusted information
The volume of online comment about strangles is high, and accuracy varies. The principal sources of reliable, current information in the UK are listed in the Further Resources section below.
S — Separate unfamiliar horses
A quarantine period — typically three weeks — for any new horse arriving on a yard, ideally combined with a screening blood test before the horse mixes with the resident herd, is one of the single most effective measures available. Practical biosecurity within quarantine includes:
- Separate water buckets, feed bowls and tools for new arrivals
- No shared tack, headcollars or grooming kit between groups
- Hand washing between groups
- Boots and outerwear changed when moving between groups, where possible
T — Temperature check routinely
Knowing each horse’s normal resting temperature, and checking it during periods of higher risk, is the most reliable way to pick up strangles early — often before nasal discharge appears.
When should I call my vet?
Contact your vet promptly if your horse shows any of the following, particularly if there has been recent contact with other horses, new arrivals on the yard, or a strangles case reported in your area:
- A temperature above 38.5°C, or any significant change from the horse’s normal resting temperature
- Thick or discoloured nasal discharge from one or both nostrils
- Swelling, pain, or visible abscessation in the lymph nodes under the jaw
- Difficulty swallowing, reduced appetite, or noisy breathing
- A horse that is simply “not right” following recent travel, competition, or new contact
Do not wait for confirmation before isolating the affected horse. Prompt isolation, combined with stopping all horse movement on and off the yard, prevents wider spread while diagnostic testing is carried out.
Frequently asked questions about strangles
Is strangles contagious to humans?
No. Streptococcus equi subspecies equi is a horse pathogen. It does not cause disease in humans under normal circumstances. People can, however, mechanically carry the bacterium between horses on their hands, clothing, and equipment, which is why hand hygiene and protective clothing matter during an outbreak.
Can my horse get strangles more than once?
Yes. Many horses develop a degree of immunity following natural infection, but that immunity is not lifelong, not absolute, and varies considerably between individuals. Reinfection can occur — in some cases within months of the original episode, in other cases years later, and in some horses not at all. Previous infection should not be treated as a guarantee of future protection.
How long does strangles immunity last?
This is less certain than is sometimes suggested. The widely cited figure is that approximately 75% of horses develop convalescent immunity following natural uncomplicated infection — a figure that comes from the ACVIM Consensus Statement on strangles. What is much less certain is how long that immunity lasts.
Published reports vary considerably. Some studies and reviews describe immunity lasting several years; others describe horses becoming susceptible to reinfection within months. The phrase “long-term immunity” appears throughout the veterinary literature, but in practical terms “long-term” may mean only a matter of months in some individual horses.
The honest summary is that immunity to strangles after natural infection is variable, individual, and not lifelong. It should not be assumed that a horse which has previously had strangles is protected against future infection.
How long is a horse infectious?
A horse with active strangles can shed bacteria from the point at which clinical signs begin and for several weeks after apparent recovery. A subset of horses — silent carriers — can shed intermittently for months or years without ever showing clinical signs again. Clearance testing is the only reliable way to confirm that a recovered horse is no longer infectious.
What is the incubation period for strangles?
Typically 3 to 21 days from exposure, with occasional cases developing signs up to 28 days later.
Does my horse need to be euthanised if it has strangles?
No. The vast majority of horses recover from strangles with appropriate veterinary care and yard-level management. A small number of cases develop serious complications — including bastard strangles (abscesses in distant organs) or purpura haemorrhagica (an immune-mediated vasculitis) — which require more intensive treatment. Euthanasia is rare and is reserved for severe complications or welfare grounds.
Should I vaccinate against strangles?
Vaccination is one tool in a wider strategy. Whether it is appropriate for your horse and your yard depends on the level of risk, the frequency of new arrivals, the nature of the yard’s activities, and the wishes of the owner. This is a conversation worth having with your vet rather than a decision made in isolation.
How is strangles treated?
Most cases are managed with supportive care: anti-inflammatories, soft palatable food, and warm compresses to encourage abscess maturation and drainage. Antibiotics are not given routinely, as they can interfere with the development of natural immunity and may complicate the resolution of abscesses. Antibiotic use in strangles is reserved for specific clinical indications and is decided case-by-case.
How do I know my horse is clear of strangles after recovery?
The most reliable approach is endoscopic guttural pouch lavage with PCR and bacterial culture. Serology (a blood test) is sometimes used as an initial screening step, particularly for horses with no known history. Negative results on a nasopharyngeal swab alone are not sufficient to confirm clearance.
About the expert
Vikki Scott holds the qualifications BVetMed MSc DipACVIM MRCVS and is an RCVS Recognised Specialist in Equine Internal Medicine. A graduate of the Royal Veterinary College, she completed a neonatal intensive care fellowship at the University of Pennsylvania’s New Bolton Center, an internship at Rossdales LLP in Newmarket, and a residency in equine internal medicine at The Ohio State University, becoming a Diplomate of the American College of Veterinary Internal Medicine in 2014. She is currently an Equine Internal Medicine Clinician at Glasgow Equine Hospital & Practice, University of Glasgow.
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Article by James Risk BVM&S MRCVS
Head of Glasgow Equine Practice and Deputy Director of the equine division, James works in the first opinion equine practice.
About Glasgow Equine Hospital & Practice
Glasgow Equine Hospital & Practice is part of the University of Glasgow School of Veterinary Medicine, based at the Weipers Centre. The service combines a first-opinion ambulatory practice covering central Scotland with a referral hospital offering internal medicine, surgery, diagnostic imaging, reproduction, and isolation facilities. Ambulatory and hospital teams work as one integrated service, providing a clear route from field assessment into hospital investigation when needed.
If you have questions about your horse, or would like to talk through strangles testing, vaccination, or quarantine planning for your yard, please contact us.
Further resources
This article was prepared for Strangles Awareness Week 2026 (4–10 May). It is intended as general information and does not replace direct veterinary advice for an individual horse. For clinical concerns, please contact your vet.
