Strangles (Streptococcus equi)

Strangles is a highly contagious bacterial infection of the upper respiratory tract caused by Streptococcus equi subspecies equi. This disease is characterized by fever, purulent nasal discharge, and abscessation of the lymph nodes of the head and neck, particularly the submandibular and retropharyngeal lymph nodes. The name "strangles" derives from the swollen lymph nodes that can become so enlarged they compromise the airway, creating a sensation of strangling, though actual airway obstruction requiring emergency intervention is relatively uncommon.

Strangles occurs worldwide and is one of the most commonly diagnosed infectious diseases in horses globally. The disease primarily affects young horses between one and five years of age, though horses of any age can become infected if they lack immunity. Facilities with high horse turnover, such as sales barns, training centers, and boarding stables, face increased risk of outbreaks. Once introduced to a susceptible population, strangles spreads rapidly, with attack rates often exceeding 50% in naive herds.

The impact of strangles on equine health and the horse industry is substantial. Individual horses experience significant illness lasting weeks to months, during which time they cannot train or compete. Economic losses result from treatment costs, quarantine requirements, canceled competitions, and lost training days. Outbreak management disrupts normal barn operations and can damage facility reputations. In complicated cases, strangles can cause permanent damage or death, though the overall mortality rate remains low at approximately 1-2% in uncomplicated cases.

The prognosis for horses with uncomplicated strangles is excellent, with approximately 75% of affected horses developing strong, long-lasting immunity following recovery. However, complications such as bastard strangles, purpura hemorrhagica, and guttural pouch empyema can significantly worsen outcomes. Early recognition of strangles signs, appropriate isolation of affected horses, and careful management of the convalescent period are essential for minimizing spread and preventing complications. Understanding this disease is crucial for all horse owners given its highly contagious nature and potential for significant economic and health impacts.

Streptococcus equi subspecies equi is a gram-positive coccal bacterium that has evolved specifically to cause disease in horses, ponies, donkeys, and mules. Unlike many bacteria that exist in the environment, S. equi is an obligate pathogen that survives primarily in equine hosts. The organism possesses several virulence factors that enable it to evade the host immune system, including a hyaluronic acid capsule that protects against phagocytosis and specific surface proteins that inhibit antimicrobial peptides and complement activation.

Transmission of strangles occurs primarily through direct contact with infected horses or their nasal discharge, and through indirect contact with contaminated equipment, water sources, handlers' clothing, or shared implements. The bacteria can survive in water troughs for up to four weeks and on wood surfaces for extended periods under favorable conditions. Fomite transmission via shared tack, grooming equipment, buckets, and even handlers' hands represents a significant route of spread, particularly in facilities with inadequate biosecurity practices.

Carrier horses represent the most important reservoir for maintaining strangles in a population. Approximately 10% of horses recovering from strangles become persistent carriers, harboring the organism in their guttural pouches without showing clinical signs. These carriers can shed bacteria intermittently for months to years, serving as the source for new outbreaks. Some carriers develop inspissated pus accumulations called chondroids within the guttural pouches, which provide protected reservoirs for the organism.

Risk factors for strangles infection include young age with naive immune systems, stress from transport or environmental changes, overcrowding, introduction of new horses without quarantine, poor biosecurity practices, and lack of vaccination history. Facilities that frequently receive horses from multiple sources face higher risk than closed herds. Previous exposure provides protection, but immunity wanes over time, and horses can become susceptible again approximately five years after initial infection if not re-exposed.

The pathophysiology of strangles begins when the organism colonizes the tonsillar tissue and nasopharyngeal mucosa. From this initial site, bacteria invade local lymph nodes, where they multiply and trigger abscess formation. The submandibular lymph nodes are most commonly affected, followed by the retropharyngeal lymph nodes. The host mounts an intense inflammatory response, resulting in fever, depression, and the characteristic swelling that develops over days to weeks. Eventually, abscesses mature and rupture, either externally through the skin or internally into the guttural pouches, releasing large quantities of purulent material containing millions of bacteria.

Early warning signs of strangles often precede the classic presentation by several days and are easily overlooked. Initial symptoms include mild depression, decreased appetite, and low-grade fever that may fluctuate. Horses may appear slightly off or show subtle reluctance to eat hard feed. A clear or slightly cloudy nasal discharge may develop, progressing from thin and watery to thicker consistency. Owners familiar with their horses may notice behavioral changes or reduced enthusiasm for normal activities before more obvious signs emerge.

The classic symptom presentation of strangles includes high fever typically ranging from 103°F to 106°F (39.4°C to 41.1°C), purulent bilateral nasal discharge that becomes thick and copious, and painful swelling of the lymph nodes beneath the jaw (submandibular) and behind the jaw (retropharyngeal). Affected horses often stand with their necks extended and heads lowered to reduce discomfort from swollen lymph nodes. Depression and complete loss of appetite are common as swallowing becomes painful.

Behavioral changes in horses with strangles are pronounced and recognizable. Affected horses become lethargic and withdrawn, showing little interest in their surroundings or herdmates. They may be reluctant to move, especially to lower their heads to eat or drink due to throat pain. Difficulty swallowing may cause drooling or quidding of food. Horses may vocalize differently or refuse to nicker due to laryngeal discomfort. Some horses develop a soft cough, particularly when swallowing.

Physical signs evolve over the course of the disease. Lymph node swelling progresses from firm and painful to fluctuant as abscesses mature over 7-14 days. The submandibular area may become massively enlarged, sometimes described as appearing like a football beneath the jaw. Retropharyngeal lymph node involvement may cause visible swelling behind the jaw or be detected only by internal palpation. Nostrils may become crusted with dried discharge, and the horse's face may become wet and irritated from constant drainage.

Symptom progression in typical cases follows a predictable timeline. Fever develops first, typically 3-14 days after exposure. Nasal discharge appears within days of fever onset and progresses from serous to mucopurulent. Lymph node swelling becomes apparent 1-2 weeks after initial infection and continues expanding until abscess maturation. Abscesses typically rupture spontaneously 2-3 weeks after swelling is first noted, releasing large quantities of creamy, yellow-white pus containing the causative bacteria.

Emergency symptoms requiring immediate veterinary attention include severe respiratory distress with labored breathing, stridor, or cyanosis, which may indicate airway compromise from massive retropharyngeal abscess formation. High fever exceeding 106°F, complete inability to eat or drink, extensive swelling spreading beyond typical lymph node regions, signs of colic, or limb swelling may indicate complications such as metastatic (bastard) strangles or purpura hemorrhagica. Any neurological abnormalities or sudden deterioration in a horse with strangles demands emergency evaluation.

Physical examination of a horse suspected of having strangles focuses on evaluating lymph node size, consistency, and pain response, as well as assessing the character and quantity of nasal discharge. Fever is documented through rectal temperature measurement. The veterinarian will palpate the intermandibular space and retropharyngeal region externally while also potentially performing endoscopic examination to visualize the guttural pouches and pharyngeal area. The classic triad of fever, purulent nasal discharge, and lymph node abscessation strongly suggests strangles.

Laboratory confirmation of strangles involves bacterial culture and/or polymerase chain reaction (PCR) testing. Samples are collected from nasal swabs, nasopharyngeal washes, or aspirates from developing abscesses. Culture remains the gold standard, with typical colonies demonstrating beta-hemolysis on blood agar and characteristic biochemical reactions. PCR testing offers faster results and higher sensitivity, particularly in early disease or in samples from potential carrier horses. The combination of culture and PCR provides optimal diagnostic accuracy.

Advanced diagnostics become important for evaluating complications and identifying carrier horses. Endoscopic examination of the guttural pouches allows direct visualization of empyema, chondroid formation, or other abnormalities indicating carrier status. Guttural pouch lavage with culture and PCR of the fluid provides sensitive detection of S. equi in horses that may be shedding intermittently. Radiography or computed tomography may help evaluate complicated cases with internal abscessation or atypical presentations.

Differential diagnosis for strangles includes other causes of lymph node enlargement and nasal discharge. Pigeon fever (Corynebacterium pseudotuberculosis) can cause similar abscesses but typically involves the pectoral region or ventral abdomen rather than the head. Other respiratory infections including equine influenza, rhinopneumonitis, and equine viral arteritis can cause nasal discharge and fever but typically do not produce the characteristic lymph node abscessation. Dental abscesses, oral foreign bodies, and other upper respiratory conditions must also be distinguished from strangles through appropriate diagnostic testing.

Emergency treatment considerations for strangles focus on managing airway compromise, which represents the most immediately life-threatening complication. Horses with severe retropharyngeal abscess formation causing stridor or respiratory distress may require emergency tracheostomy to establish an alternative airway. Anti-inflammatory medications help reduce swelling while supportive care maintains hydration and nutrition. Emergency drainage of abscesses threatening the airway may be necessary even before complete maturation.

The cornerstone of strangles treatment remains supportive care rather than aggressive antibiotic therapy. Most uncomplicated cases resolve without antibiotics, and premature antibiotic use may actually delay abscess maturation, prolong the disease course, and prevent development of protective immunity. Supportive care includes providing soft, palatable feeds that are easy to swallow, ensuring adequate water intake, maintaining a comfortable environment, and monitoring for complications. Non-steroidal anti-inflammatory drugs such as phenylbutazone or flunixin meglumine reduce fever and pain, improving comfort and encouraging eating and drinking.

Antibiotic therapy remains controversial in strangles management. Penicillin effectively kills S. equi and is indicated in specific situations: very early disease before abscess formation (when rapid intervention may abort infection), severe illness with systemic compromise, and complicated cases including metastatic abscessation. However, routine antibiotic use in uncomplicated strangles is not recommended because it may prolong the carrier state, delay abscess maturation, and prevent development of robust immunity. When antibiotics are used, treatment must continue for adequate duration (typically 10-14 days minimum) to prevent relapse.

Local treatment of abscesses involves encouraging maturation and drainage. Warm compresses applied to developing abscesses several times daily can accelerate maturation. Once abscesses become fluctuant, they may be lanced to establish drainage, though many veterinarians prefer allowing spontaneous rupture to avoid premature opening. After drainage, abscess cavities should be gently flushed with dilute antiseptic solutions to promote complete evacuation and healing. Strict attention to proper disposal of purulent material is essential to prevent environmental contamination.

Management of the convalescent period is critical for preventing spread and identifying carriers. Horses should remain isolated until they have been afebrile for at least 7 days and nasal discharge has completely resolved. Ideally, three negative nasopharyngeal samples or guttural pouch washes, collected weekly, confirm clearance of the organism before returning horses to the general population. Environmental decontamination of contaminated areas, equipment, and water sources helps prevent ongoing transmission.

Treatment decisions balance individual horse welfare with herd health considerations. In outbreak situations, early identification and isolation of affected horses takes priority. Prophylactic antibiotic treatment of exposed but currently healthy horses is generally not recommended due to risk of prolonging the outbreak and creating carrier horses. Close monitoring of in-contact horses with twice-daily temperature checks enables rapid detection and isolation of new cases. The treating veterinarian will develop protocols appropriate for the specific situation, considering factors such as facility layout, horse value and use, and owner resources.

Recovery timelines for uncomplicated strangles typically span 3-6 weeks from initial signs to full resolution. The acute febrile phase lasts approximately 1-2 weeks, during which horses feel most ill. Abscess maturation and drainage occur over the following 1-2 weeks, with gradual improvement in demeanor and appetite as purulent material is evacuated. Complete resolution of nasal discharge and return to normal activity may require an additional 2-4 weeks after abscess drainage. Horses should not return to work until fully recovered and confirmed free of shedding.

Post-treatment care focuses on monitoring for complications and confirming clearance of the organism. Convalescent horses should have abscess sites kept clean and monitored for proper healing. Nutritional support helps recovery, with emphasis on palatable, easy-to-swallow feeds during the healing process. Gradual return to normal diet and activity follows resolution of throat swelling and discomfort. Follow-up veterinary examinations evaluate healing progress and determine when isolation can end.

Prognosis factors for strangles recovery are generally favorable, with approximately 75% of horses developing strong, long-lasting immunity that may persist for five years or longer. Age, overall health status, and promptness of supportive care influence recovery speed. Horses that develop complications such as bastard strangles, purpura hemorrhagica, or guttural pouch empyema have more guarded prognoses requiring extended treatment and carrying risk of permanent sequelae. Carrier horses that fail to clear the organism require specific treatment to resolve guttural pouch infection before they can safely rejoin the herd.

Long-term soundness outlook for horses recovering from uncomplicated strangles is excellent, with virtually all horses returning to their previous level of athletic function. Scarring at abscess drainage sites typically resolves to become cosmetically acceptable. Horses that experienced complications may have longer-term effects depending on the specific complication. Those with resolved purpura hemorrhagica generally recover fully, while horses with internal abscessation may have permanent organ damage. Carrier status must be identified and treated to prevent future outbreaks and allow return to normal life.

Management practices for strangles prevention center on biosecurity protocols that limit introduction and spread of the organism. New horses should be quarantined for a minimum of 3 weeks before introduction to the resident population, with temperature monitoring twice daily and ideally three negative nasopharyngeal samples before release from quarantine. Horses returning from competitions, sales, or other high-risk events should undergo similar quarantine. Facility design that allows effective isolation of new arrivals and sick horses is fundamental to outbreak prevention.

Biosecurity measures extend to equipment, personnel, and environmental management. Dedicated equipment for quarantine horses prevents fomite transmission, and all shared equipment should be thoroughly cleaned and disinfected between horses. Personnel handling quarantined or sick horses should change clothing and footwear, or at minimum cover clothing with disposable gowns or coveralls, before contacting other horses. Water sources should not be shared between groups, and manure management should prevent runoff contamination of pastures used by other horses.

Vaccination against strangles provides partial protection and is appropriate for horses at increased risk of exposure. Available vaccines include killed bacterins, modified-live intranasal preparations, and newer subunit vaccines. Intranasal modified-live vaccines generally provide better protection than injectable products but carry a small risk of causing local reactions or disease in some horses. Vaccination does not guarantee protection and should be viewed as one component of a comprehensive prevention program rather than a substitute for biosecurity.

Environmental management for strangles prevention recognizes that the organism can survive in the environment for variable periods. Water troughs should be cleaned and disinfected regularly, as S. equi can survive in water for up to four weeks. Organic material such as dried discharge should be removed before disinfection, as it protects bacteria from disinfectants. Appropriate disinfectants include chlorhexidine, phenolic compounds, and quaternary ammonium compounds at appropriate concentrations with adequate contact time.

Identification and treatment of carrier horses eliminates the persistent reservoir responsible for most outbreaks. Horses recovering from strangles should undergo guttural pouch examination and sampling before returning to the general population. Identified carriers can be treated through guttural pouch lavage with antibiotics or, in cases with chondroid formation, endoscopic or surgical removal of inspissated material. Screening of newly acquired horses for carrier status through guttural pouch sampling provides additional protection for closed herds.

Daily management during active strangles infection requires strict isolation protocols combined with compassionate supportive care. Affected horses should be housed in a separate barn or at minimum a stall with no direct contact with other horses, with dedicated caretakers or caretaker protocols that prevent cross-contamination. Soft, palatable feeds encourage eating despite throat discomfort, with options including soaked hay cubes, complete feeds moistened with water, or fresh grass when available. Water buckets should be placed at comfortable height and cleaned frequently.

Housing considerations for horses with strangles prioritize isolation, easy observation, and comfort. Stalls should have adequate ventilation without exposing sick horses to drafts or extreme temperatures. Deep, clean bedding provides comfort for horses that may spend increased time lying down when feeling ill. The isolation area should be positioned to allow easy observation while minimizing traffic through the area. Drainage and waste management must prevent contamination of areas used by healthy horses.

Exercise modifications during strangles recovery depend on disease stage and individual condition. Acutely ill horses with fever and developing abscesses require stall rest with only brief hand-walking for stretching if they are willing. As fever resolves and abscesses drain, horses can gradually increase activity. Turnout in isolation paddocks provides mental stimulation during the convalescent period when horses feel better but remain potentially contagious. Full return to exercise awaits complete recovery and confirmation of cleared infection.

Monitoring protocols during the recovery period include twice-daily temperature checks to detect recurrent fever that might indicate complications. Abscess sites should be inspected daily for appropriate healing without secondary infection. Appetite, water consumption, and manure production provide insight into overall recovery progress. Any changes in condition, particularly development of limb swelling, new abscesses in unusual locations, or neurological signs, warrant immediate veterinary evaluation for possible complications.

Quality of life during strangles infection can be maintained through attentive nursing care despite the horse's discomfort. While affected horses feel ill during the acute phase, most maintain reasonable comfort with appropriate supportive care. The self-limiting nature of uncomplicated strangles means that patience and good nursing care are rewarded with full recovery in most cases. Horses that develop serious complications may face more difficult quality of life decisions, but these situations remain uncommon when strangles is recognized and managed appropriately.

Strangles affects all breeds of horses, ponies, donkeys, and mules without significant breed predisposition. The organism Streptococcus equi is an equal-opportunity pathogen that infects any equid lacking immunity regardless of breed, size, or type. Susceptibility depends on immune status rather than genetic factors, with previously unexposed horses of any breed being equally vulnerable to infection when exposed to the organism.

Age and immune status matter more than breed in determining strangles risk. Young horses between ages 1 and 5 years are most commonly affected due to their naive immune systems and frequent exposure to new horses during training, sales, and competition. Older horses that have recovered from previous infection or have been appropriately vaccinated typically have lower susceptibility. However, horses of any age can develop strangles if they lack immunity, including older horses that have never been exposed and whose vaccine-induced immunity has waned.

Breeding recommendations do not apply to strangles as the disease is not hereditary. However, facility management decisions significantly impact outbreak risk. Breeding operations that frequently introduce new horses for breeding purposes face increased risk compared to closed herds. Implementing strict quarantine protocols for all incoming breeding stock, including mares arriving for breeding and newly purchased stallions, is essential for protecting breeding operations from strangles introduction and the significant economic losses that outbreaks cause during the breeding season.

Conditions commonly co-occurring with strangles include secondary complications of the disease itself. Guttural pouch empyema develops when abscesses drain internally, filling the guttural pouches with purulent material that can inspissate into chondroids. Purpura hemorrhagica, an immune-mediated vasculitis, occasionally follows strangles infection, causing severe limb swelling, ventral edema, and potentially life-threatening complications. Bastard strangles refers to metastatic spread of infection to lymph nodes beyond the head and neck, potentially affecting the lungs, liver, spleen, brain, or other organs.

Other upper respiratory infections share some clinical features with strangles and must be differentiated through appropriate testing. Equine influenza causes fever, nasal discharge, and cough but does not produce lymph node abscessation. Equine rhinopneumonitis (herpesvirus) similarly causes respiratory signs without the characteristic strangles presentation. Pigeon fever (Corynebacterium pseudotuberculosis) produces abscesses but typically in the pectoral region or ventral abdomen rather than the lymph nodes of the head. Sinusitis causes nasal discharge but is usually unilateral and does not produce the systemic illness seen with strangles.

Potential complications of strangles extend beyond the immediate respiratory and lymphatic involvement. Aspiration pneumonia can occur if severely affected horses inhale purulent material. Laryngeal hemiplegia (roaring) may develop following retropharyngeal abscess formation that damages the recurrent laryngeal nerve. Myocarditis, though rare, can occur during strangles infection. Guttural pouch mycosis may develop as a separate condition in horses with guttural pouch empyema. Recognition of these potential complications guides monitoring during the recovery period and prompts evaluation of persistent or unusual clinical signs.