Strangles in Horses: What It Is and How to Handle an Outbreak

Reviewed by Dr. Khurrum Shahzad Khosa, DVM

Strangles is one of the most feared infectious diseases in the horse world — and for good reason. Caused by the bacterium Streptococcus equi subspecies equi, it is highly contagious, capable of spreading rapidly through a yard, and can result in severe complications that may be life-threatening. An outbreak can place horses under quarantine for months, disrupt competition schedules, and in some cases result in significant veterinary costs. Yet strangles also carries a degree of stigma that sometimes prevents horse owners from acknowledging and disclosing cases promptly, which paradoxically fuels further spread. Understanding strangles clearly and honestly is the first step in managing it well — both within a yard and within the wider equine community.

What Is Strangles?

Strangles is a bacterial upper respiratory infection caused by Streptococcus equi subspecies equi (often abbreviated to S. equi). The name derives from the dramatic swelling of the lymph nodes around the throat and jaw, which in severe cases can compress the airway sufficiently to cause laboured, noisy breathing — hence "strangles." The condition has been recognised in horses for centuries and remains one of the most commonly diagnosed infectious diseases in equine populations worldwide.

Unlike equine influenza, which is viral, strangles is bacterial. This distinction matters for treatment — antibiotics are potentially effective, though their use in strangles is more nuanced than it might initially appear — and for understanding transmission patterns. S. equi can survive in the environment for extended periods under appropriate conditions, particularly in water and damp material, which contributes to the persistent nature of outbreaks in some yards.

How Strangles Spreads

Strangles spreads through direct contact with infected horses and through indirect contact with contaminated fomites — surfaces, equipment, water, and feed. The bacteria are shed in large quantities in nasal discharge and in the pus from ruptured abscesses. A horse touching noses with an infected individual, drinking from a shared water trough, or being handled by a person who has handled an infected horse without changing clothing or washing hands is at significant risk of infection.

The incubation period ranges from three to fourteen days, which means a horse can enter a yard, appear healthy, and begin shedding bacteria before any clinical signs develop. This is why quarantine of new arrivals is so important as a routine yard-management measure rather than something reserved for outbreak situations.

Crucially, a proportion of horses that recover from strangles become persistent carriers, harbouring the bacteria in their guttural pouches — air-filled extensions of the Eustachian tube located behind the skull — without showing any outward signs of illness. These carrier horses can shed bacteria intermittently for months or years and are the most common source of apparently unexplained new cases on yards. Identifying and treating carriers is one of the most important — and most frequently overlooked — aspects of strangles control.

Clinical Signs

The classic presentation of strangles follows a recognisable progression. The horse typically develops a fever of 39–40°C in the early stages, often accompanied by depression and reduced appetite — signs that can initially be dismissed as mild non-specific illness. Within a day or two, nasal discharge appears, initially clear and watery before becoming thick, purulent, and copious. Bilateral discharge from both nostrils is typical.

Over the following days, swelling of the lymph nodes around the jaw and throat becomes evident — most commonly the submandibular lymph nodes beneath the jaw and the retropharyngeal lymph nodes behind the throat. These swellings enlarge progressively and become hot and painful to the touch. The horse may show difficulty swallowing, stretching its neck, or preferring to eat from the floor. In severe cases, the swelling around the throat can impinge on the airway, causing the noisy inspiratory breathing that gives the disease its name.

The lymph node abscesses eventually soften and rupture, discharging large quantities of thick, creamy pus. Once abscesses have ruptured and the discharge resolves, most horses make a full recovery. The typical clinical course from onset to resolution is two to six weeks in uncomplicated cases.

Complications

While most horses recover from uncomplicated strangles without lasting effects, a significant minority develop complications that are serious and require intensive veterinary management.

Bastard strangles occurs when the bacteria disseminate through the lymphatic system to internal lymph nodes and organs, forming abscesses in the abdomen, chest, liver, spleen, kidneys, and brain. Signs vary depending on the sites affected and may include colic, weight loss, difficulty breathing, and neurological deficits. Diagnosis requires imaging — ultrasound, endoscopy, or CT. Treatment is prolonged and outcomes are variable.

Purpura haemorrhagica is an immune-mediated vasculitis that develops in a small number of horses, typically two to four weeks after the acute infection. The immune system generates antibodies against S. equi M-protein that cross-react with blood vessel walls, causing widespread vascular leakage. Affected horses develop dramatic oedema of the limbs and abdomen, petechial haemorrhages on mucous membranes, and potentially organ failure. Treatment requires aggressive corticosteroid therapy and intensive supportive care. The condition can also be triggered by vaccination in horses with high pre-existing antibody titres.

Guttural pouch empyema is the accumulation of pus within the guttural pouches. If not treated, the pus may inspissate — dry out and harden — forming chondroids that harbour bacteria indefinitely. This is the mechanism by which carrier horses develop and persist. Guttural pouch empyema requires endoscopic diagnosis and treatment.

Diagnosis and Treatment

Clinical diagnosis of strangles is often straightforward in a classic case, but laboratory confirmation is important for accurate identification and to differentiate from other conditions. Bacterial culture of nasal swabs or abscess contents has traditionally been the gold standard, but PCR testing of nasopharyngeal swabs or guttural pouch lavage is now preferred in many practices for its greater sensitivity. Endoscopy of the guttural pouches is essential in any horse suspected of being a carrier or in any horse with a history of strangles that has not been formally cleared.

The role of antibiotics in treating strangles is nuanced. In horses with early-stage disease before abscess formation, penicillin can halt progression. However, once abscesses have formed, antibiotic treatment can prevent them from maturing and rupturing spontaneously — trapping the infection and potentially leading to complications. For most horses with well-developed abscesses, supportive care, hot-packing of the swellings to encourage rupture, and careful monitoring is preferred over antibiotic treatment. Your veterinarian will make individual recommendations based on the stage of disease and the horse's overall condition. For more guidance on managing equine health conditions, visit our comprehensive horse health guide collection.

Quarantine, Biosecurity and Vaccination

Rigorous quarantine is essential when strangles is suspected or confirmed. Affected horses must be isolated from all healthy animals, with dedicated equipment, handlers, and clothing. Disinfection of all potentially contaminated surfaces — stables, water troughs, fencing — should be carried out using disinfectants proven effective against S. equi. The yard should be closed to all new arrivals and departures until the outbreak has fully resolved and all horses have been confirmed clear.

Before lifting quarantine, all horses on the affected yard — not just those that showed clinical signs — should undergo guttural pouch endoscopy and PCR testing to identify carrier animals. At minimum, three consecutive negative weekly tests are required to declare a horse clear of carrier status.

An intranasal modified live strangles vaccine is available and can reduce the severity of disease in vaccinated populations. However, vaccination is not appropriate for horses with recent exposure, those currently showing signs, or those with high antibody titres. Always consult your veterinarian before beginning a vaccination programme on a yard with a strangles history.

Frequently Asked Questions

How long is a horse with strangles contagious?

A horse is contagious from the onset of signs until all abscesses have healed and nasal discharge has fully resolved — typically two to six weeks. However, persistent carriers may shed bacteria for months or years without signs. Formal clearance requires repeated negative guttural pouch testing.

What is bastard strangles and how serious is it?

Bastard strangles refers to internal abscess formation in abdominal, thoracic, or neurological sites. It is a serious and potentially fatal complication requiring intensive investigation and prolonged treatment. Signs vary depending on which organs are affected and may include colic, weight loss, and respiratory or neurological changes.

Should I vaccinate against strangles?

Vaccination is recommended for higher-risk horses, particularly those regularly moved or at yards with a strangles history. The intranasal modified live vaccine reduces severity but does not provide complete protection. It must never be given to recently exposed horses or those with high antibody titres, as this can trigger serious immune complications including purpura haemorrhagica.

What is purpura haemorrhagica and how is it related to strangles?

Purpura haemorrhagica is a serious immune-mediated vasculitis triggered by antibodies against S. equi M-protein reacting with blood vessel walls. It causes dramatic limb and abdominal oedema, petechial haemorrhages, and can be fatal. It typically develops two to four weeks after the acute infection and can also be triggered by vaccination in susceptible horses.

How is a strangles carrier horse identified and treated?

Carriers are identified through guttural pouch endoscopy and PCR or culture of lavage samples. Three consecutive weekly negative tests are needed to confirm clearance. Treatment involves endoscopic removal of chondroids, guttural pouch lavage, and antibiotic therapy. Carriers must remain isolated until fully cleared.

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Disclaimer: This article is for educational purposes only and does not constitute veterinary advice. Always consult a licensed equine veterinarian for diagnosis and treatment.