What Is Strangles in Horses? Symptoms and Treatment

Strangles in horses is one of the most common and contagious infectious diseases affecting horses worldwide – the British Horse Society’s 2021 STEPS strangles guide estimated that more than 600 outbreaks occur in the UK every year. If you keep horses, board at a yard, or attend regular shows and competitions, understanding what strangles is — and how it spreads — could protect your horse and your entire herd. This guide covers everything you need to know about strangles in horses, from the cause and clinical signs to treatment, quarantine, and prevention.

What Is Strangles in Horses?

Strangles in horses (also known as equine distemper) is a highly infectious upper respiratory tract disease caused by the bacterium Streptococcus equi subspecies equi, commonly shortened to S. equi. The disease causes the lymph nodes around the horse’s head and neck to swell, fill with pus, and form abscesses. In severe cases, these swollen lymph nodes press against the horse’s trachea and airway — which is exactly where the name “strangles” comes from.

Strangles has been documented since 1251, making it one of the oldest recorded horse diseases in history. It affects horses, donkeys, and mules of any age, breed, or sex, though younger horses between one and five years old tend to suffer the most severe symptoms. Read our full clinical reference in the Bit & Bridle equine diseases library for the deep-dive medical detail.

What Causes Strangles in Horses?

The bacterium S. equi is a gram-positive, capsulated beta-haemolytic bacterium and is considered an obligate parasite — meaning it can only survive in and infect horses and other equids. It does not infect cattle, dogs, or other livestock under normal circumstances, though rare human cases have been reported, typically in immunocompromised handlers.

Once the bacteria enter the body through the nose or mouth, they travel to the lymph nodes of the upper respiratory tract within hours. There, they multiply rapidly, triggering the characteristic swelling and abscess formation that defines the disease.

How Does Strangles Spread Between Horses?

Strangles is not a true airborne disease, but it spreads with alarming ease. When an infected horse coughs or snorts, it can project infected material several metres. More commonly, however, it spreads through direct contact between horses and indirect contact via contaminated objects, equipment, and people.

Direct Contact

Nose-to-nose contact between horses is one of the most efficient ways strangles moves through a herd. A horse that shares a water trough or greets another horse over a fence can pass on the bacteria within seconds. Morbidity can reach 100% in a naive group that has never been exposed to the disease.

Contaminated Equipment and Surfaces

S. equi can survive for several weeks in water and for more than two weeks in damp conditions like the tread of a boot sole. Shared water buckets, feed troughs, brushes, head collars, tack, and even transport lorries can all carry the bacteria from one yard to another. A groom who handles a sick horse and then walks into another stable without changing gloves can easily transfer the infection.

The Role of Asymptomatic Carriers

One of the biggest challenges with strangles is that up to 10% of naturally infected horses become silent carriers — some studies put the figure higher, towards 40%, depending on case definition. These horses appear completely healthy but continue to harbour live S. equi bacteria in their guttural pouches — the air-filled sacs located at the top of each nasal passage. Carriers shed bacteria intermittently, sometimes for months or even years, creating recurring outbreaks on farms and yards where no new horse has been introduced.

Signs and Symptoms of Strangles in Horses

The incubation period for strangles in horses is typically 3 to 14 days, though it can occasionally take up to 21 or even 28 days for symptoms to appear after exposure. The British Horse Society’s reminder is “hot before snot” – fever is almost always the first sign.

Typical Clinical Signs

Once the incubation period is over, the horse will develop a fever above 38.5°C (101.3°F). Fever does not appear within a day or two of exposure: the BHS puts it at 3 to 21 days after contact, occasionally up to 28, so an exposed horse with a normal temperature after two days is not in the clear. Within 24 to 48 hours of that first fever spike, according to the Merck Veterinary Manual, thick, yellowish-green mucopurulent nasal discharge appears from one or both nostrils. The lymph nodes under the jawline and behind the throatlatch begin to swell noticeably. Horses become lethargic, lose their appetite, and may have difficulty swallowing. Some horses lower their head and neck to reduce pressure on the swollen nodes.

As the abscesses mature — usually within one to four weeks — they rupture and release thick pus. Recovery speeds up considerably once this happens, as the body is actively clearing the bacteria. After rupture, the wound needs to be kept clean by flushing with a diluted povidone-iodine solution.

Atypical (Mild) Strangles

Not every horse will show dramatic symptoms. Older horses with some residual immunity may develop atypical strangles — a milder form with only slight nasal discharge, a brief temperature spike, and a reduced appetite. These horses are still infectious and can pass strangles on to more vulnerable horses in the herd, making them especially dangerous in a mixed yard setting.

Potential Complications of Strangles

Most horses recover from strangles without serious complications. However, studies report that complications develop in roughly 20% of cases, and that is where the deaths are. In the outbreak most often quoted for these figures, on a single US farm, 15 of 74 infected horses (20.3%) developed complications and all six deaths were among those 15 (Sweeney et al., JAVMA 1987). The 8% figure often given for uncomplicated cases is that outbreak’s overall death rate, not the rate in horses without complications.

Bastard Strangles (Metastatic Strangles)

Bastard strangles is a serious complication where the bacteria spread beyond the head and neck lymph nodes and form abscesses in other parts of the body, including the abdomen, lungs, and brain. S. equi is actually the most common cause of brain abscesses in horses, though this remains rare. Horses with weakened immune systems or those in which bacteria rapidly invade the bloodstream are most at risk of developing this form.

Purpura Haemorrhagica

This is an immune-mediated complication where the horse’s own immune system attacks the blood vessels. Signs include severe leg and head swelling, small haemorrhages on the gums, and general inflammation. Purpura haemorrhagica can also be triggered by vaccinating a horse that already has high antibody levels from a recent infection, a risk documented for the SeM-containing vaccines used in North America (ACVIM consensus, Boyle et al. 2018). The UK vaccine, Strangvac, contains no SeM, and its datasheet says its effect on purpura haemorrhagica is not known. With prompt treatment — typically antibiotics and corticosteroids — most horses recover well.

Guttural Pouch Empyema and Chondroids

When the retropharyngeal lymph nodes rupture internally into the guttural pouches rather than externally, pus accumulates there and causes empyema. Over time, this can solidify into hard balls of dried pus called chondroids. These chondroids harbour live bacteria and are the most common cause of long-term carrier status. A horse with chondroids will need endoscopic removal of the material and possible local antibiotic treatment to fully clear the infection.

How Is Strangles Diagnosed?

A vet will often suspect strangles based on clinical signs alone, especially if there is a history of exposure to new horses. Definitive diagnosis, however, requires laboratory confirmation.

Culture and PCR Testing

Bacterial culture from nasal discharge, abscess material, or a guttural pouch wash has traditionally been the gold standard for diagnosing strangles. Culture takes about two to three days. PCR (polymerase chain reaction) testing is faster and more sensitive, detecting bacterial DNA even when bacterial numbers are low, but it cannot distinguish between live and dead bacteria. For this reason, culture and PCR are often used together to give the most accurate picture.

Guttural Pouch Endoscopy

For horses that are recovering or suspected of being carriers, guttural pouch endoscopy is the gold standard test. A small camera is passed into the guttural pouches, allowing the vet to directly examine them for pus or chondroids and take targeted samples. This test is more sensitive than a nasal swab and is the most reliable way to confirm whether a horse has cleared the infection completely.

How Is Strangles Treated?

There is no single cure for strangles, but most horses recover fully with the right supportive care and veterinary guidance.

Supportive Care

The primary treatment for uncomplicated strangles is supportive care. This includes providing a warm, dry, and dust-free stable, offering soft, palatable feed — soaked hay and feed are particularly helpful as swallowing may be uncomfortable — and ensuring the horse stays well hydrated. Anti-inflammatory medications such as flunixin meglumine (sold in the UK as Finadyne; Banamine is the American brand) help manage fever and pain and improve appetite. Applying warm compresses to the swollen lymph nodes daily helps encourage the abscesses to mature and rupture more quickly.

The Antibiotic Question

Antibiotic use in strangles is a nuanced topic that divides opinion among equine vets. In uncomplicated cases, antibiotics are generally not recommended once abscesses have started to form. Treating with penicillin at this stage can interrupt the natural immune response, leaving the horse with reduced long-term protection and making relapse more likely. Horses that recover naturally without antibiotics develop strong immunity that, in about 75% of cases, lasts for five or more years.

Antibiotics are, however, indicated in complicated cases — such as bastard strangles, purpura haemorrhagica, severe respiratory distress, or when a tracheostomy is required. Penicillin is the antibiotic of choice. The UK best-practice panel (Rendle et al., UK-Vet Equine 2021) names doxycycline as an appropriate first-line alternative, particularly where an oral medication is needed; cephalosporins and macrolides, often listed as alternatives in American references, are not among its recommendations. Antibiotic treatment may also be used strategically during an outbreak to prevent horses with an early fever from ever developing full clinical disease.

Managing a Strangles Outbreak on Your Yard

A strangles outbreak is stressful, expensive, and disruptive. The Strangles Awareness Week campaign estimates the average UK outbreak costs a yard between £4,000 and £14,000 in vet bills, lost income from cancelled lessons and lost livery, and weeks of staff hours. Acting quickly is the single most effective thing you can do to limit the spread. Contact your vet at the first sign of illness and implement the following steps without delay.

Isolation and the Traffic Light System

All horse movement on and off the property must stop immediately. Horses should be divided into three groups: infected horses showing clinical signs (red), exposed horses that have had contact with sick animals (amber), and clean horses with no known exposure (green). These groups must not share water, feed, equipment, or caretakers. If separate staff are not available, always handle healthy horses first, then change protective clothing before attending to sick horses. This traffic-light system is the model recommended by the SRUC STEPS (Strategy to Eradicate and Prevent Strangles) guidelines used across the UK.

Temperature Monitoring

Take the rectal temperature of every horse on the yard twice daily. A normal temperature is 37.5 to 38.5°C (99.5 to 101.3°F). Any horse registering above 38.5°C should be immediately isolated and examined by a vet. Infected horses can begin transmitting the bacteria to healthy horses one to two days after their fever starts, so acting on that first temperature spike is critical.

Cleaning and Disinfection

Thorough disinfection is essential throughout an outbreak. Remove all organic material such as manure and soiled bedding first, as it inactivates bleach and iodine-based disinfectants. Use chlorhexidine gluconate or glutaraldehyde-based products on surfaces and equipment. Water troughs are different: empty, clean and dry them before disinfecting, and never add disinfectant to a trough that still holds water, which the UK best-practice panel (Rendle et al., UK-Vet Equine 2021) warns is neither safe nor effective. Manure from infected horses should be composted in an isolated area — not spread on pastures. Rested pastures where sick horses have been kept should be left empty for a minimum of four weeks. S. equi lasts longest in cool, damp conditions, not shortest: a UK study recovered it for up to 34 days from wet surfaces in winter, against up to 9 days in summer (Durham et al., Equine Veterinary Journal 2018).

Clearing Horses from Quarantine

Do not release horses from isolation simply because symptoms have resolved. Horses can continue to shed S. equi bacteria for four to six weeks after clinical recovery. The safest approach is a negative bilateral guttural pouch wash with endoscopy before allowing any horse back into the main herd. Three consecutive negative nasopharyngeal washes at seven-day intervals, still widely quoted from American references as an alternative, is not enough on its own: the UK best-practice panel (Rendle et al., UK-Vet Equine 2021) does not consider three negative nasopharyngeal swabs a satisfactory substitute for a guttural pouch wash when ruling out carriers.

Strangles in the UK: Awareness and Reporting

Strangles is not currently a statutorily notifiable disease in the UK, but it is monitored closely by the equine industry. Reporting a confirmed case helps everyone in the area, and several free resources exist for that purpose.

Surveillance and Reporting

The Equine Infectious Disease Surveillance (EIDS) service at the Animal Health Trust historically ran the national strangles surveillance map; that work continues today at the University of Cambridge, where EIDS runs the Surveillance of Equine Strangles (SES) network, mapping laboratory-confirmed diagnoses reported by UK diagnostic laboratories. The BHS publishes its strangles guidance and the STEPS outbreak booklet; the interactive Strangles Hub is run by Redwings.

Strangles Awareness Week

Strangles Awareness Week (SAW) runs annually in May and is organised by a coalition of UK and Irish equine charities, vet practices and the BEVA. The campaign promotes the “BEST” steps – Boost immunity with strangles vaccination, Engage with trusted information, Separate unfamiliar horses, Temperature check routinely – and produces free downloadable posters and yard plans every year. If you keep horses on a livery yard, ask your yard owner whether they have implemented SAW best practice.

UK Support Charities

Two UK welfare charities have built standout support resources around strangles. Redwings Horse Sanctuary in Norfolk runs a dedicated strangles helpline and publishes detailed downloadable outbreak management plans. The British Horse Society publishes the STEPS outbreak booklet, and its Horse Care and Welfare team gives advice by phone and email.

Prevention and Vaccination

Preventing strangles requires a combination of good biosecurity practices and, in horses at higher risk of exposure, vaccination.

Strangles Vaccines

One strangles vaccine is available in the UK: Strangvac, a protein-based vaccine containing no live bacteria, injected into the muscles of the neck. The BHS STEPS booklet says it reduces the severity of symptoms and the shedding of bacteria and does not cause a false positive on strangles tests, but it does not guarantee complete protection and has not prevented all clinical signs in every case (Horse & Hound, Karen Coumbe MRCVS). Strangvac is not an intranasal vaccine: the live intranasal vaccine, and the intramuscular extract vaccines credited with cutting the clinical attack rate by about 50%, are American products that are not available in the UK.

Vaccination is best suited to horses that travel regularly, attend shows, or live in herds with frequent new arrivals. Only healthy horses should be vaccinated (Strangvac datasheet). During an outbreak, the BHS STEPS booklet says the vaccine can be used for horses in the green or amber groups, although protection can take up to six weeks from the first dose, and horses that have shown clinical signs should not be vaccinated until at least three months after they have recovered. The “never during an outbreak, never within a year of infection” rule often repeated online is American guidance written for the live intranasal and SeM-containing vaccines. Your vet will be able to advise on the most appropriate product and timing for your horse’s individual circumstances.

Everyday Biosecurity Measures

Good biosecurity is the most reliable protection against strangles. Quarantine all new arrivals for 21 days before integrating them into the herd, and take their temperature twice daily during this period. That is the period the BHS STEPS booklet and the UK best-practice panel (Rendle et al., UK-Vet Equine 2021) advocate, with 28 days for horses at high risk of exposure; the panel considers the 14 days many yards use insufficient for strangles. Avoid sharing water buckets, feed equipment, and tack with horses from other yards. At competitions and shows, bring your own water, avoid using communal stabling where possible, and discourage nose-to-nose contact between your horse and unfamiliar horses. Ask about the strangles history of any horse you are considering purchasing and test new arrivals for carrier status if there is any uncertainty.

Recovery and Prognosis

The average course of strangles in an uncomplicated case is around three to four weeks from onset of fever to full recovery. Most horses with uncomplicated infections do very well with supportive care and go on to develop long-lasting immunity. Case fatality rates as high as 8.1% to 9.7% have been reported in large farm outbreaks, but a field study of 108 cases on smaller farms found 0.9% (ACVIM consensus, Boyle et al. 2018), and the risk rises sharply in cases involving bastard strangles or purpura haemorrhagica.

After recovery, horses should not resume work until all draining wounds have fully healed and they have been confirmed clear of the bacteria through appropriate testing. Once your vet has given the all-clear, work can be reintroduced gradually (Horse & Hound, Karen Coumbe MRCVS); until then, keep tack and equipment separate.

When to Call Your Vet

If your horse develops a fever above 38.5°C, shows thick nasal discharge, has swollen lymph nodes under the jaw, or appears dull and off its feed — especially after recent contact with unfamiliar horses — call your vet without delay. Do not wait for a confirmed diagnosis before isolating the animal. Early isolation is one of the single most effective steps you can take to protect the rest of your herd. For related signs to watch for, see our guides on coughing in horses and the wider category of equine respiratory conditions.

Strangles in horses may be one of the oldest equine diseases on record, but with prompt action, sound biosecurity, and good veterinary guidance, it is entirely manageable. Knowing what to look for — and acting quickly when you spot it — makes all the difference.

Frequently Asked Questions