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Strangles

Strangles is a highly contagious bacterial disease primarily affecting the upper respiratory tract of horses, donkeys, and other equids, caused by the gram-positive bacterium Streptococcus equi subsp. equi, and is characterized by fever, mucopurulent nasal discharge, and abscessation of the submandibular and retropharyngeal lymph nodes.^[1]^[2] The disease, also known as equine distemper, has an incubation period of 3–14 days and is most severe in young horses aged 1–5 years, with morbidity rates approaching 100% in susceptible populations but a low mortality rate of 1–10% in uncomplicated cases.^[1]^[3] Transmission occurs through direct contact with infected animals or indirect contact via contaminated equipment, feed, water, or aerosols from nasal discharge, making outbreaks common in densely populated settings like stables or shows; recent trends as of 2025 show increased cases in regions such as the UK and US.^[2]^[3]^[4]^[5] Clinical signs typically begin with a sudden fever of 103–106°F (39.4–41.1°C), followed by depression, anorexia, and serous to purulent nasal discharge that may contain pus-filled "strangles" abscesses rupturing externally.^[1]^[2] Complications, affecting up to 10% of cases, include internal abscessation (bastard strangles) spreading to other organs or immune-mediated purpura hemorrhagica, which can lead to vasculitis and edema.^[1]^[3] Diagnosis is confirmed through bacterial culture or polymerase chain reaction (PCR) testing of nasal swabs, guttural pouch lavages, or abscess aspirates, with PCR offering higher sensitivity for detecting carriers.^[1]^[2] Treatment focuses on supportive care, including rest, hydration, and anti-inflammatory medications, while antibiotics such as procaine penicillin are reserved for severe cases to avoid prolonging the infection and hindering immunity development; surgical drainage of mature abscesses may also be necessary.^[1]^[3] Prevention relies on strict biosecurity measures, such as quarantining new arrivals for 2–3 weeks with daily temperature monitoring, disinfection of premises, and vaccination with intranasal or intramuscular products, though vaccines provide only partial protection and are recommended for high-risk horses; a newer subunit vaccine, Strangvac, available in Europe as of 2025, offers additional options for reducing disease severity.^[2]^[1]^[6] Strangles is a reportable disease in the United States, emphasizing its global significance in equine health management.^[1]

Overview

Definition

Strangles is an acute, highly contagious bacterial disease primarily affecting the upper respiratory tract of equids, characterized by inflammation and abscessation of the lymphoid tissues in the head and neck region.^[1] It is caused by the gram-positive bacterium Streptococcus equi subspecies equi (S. equi), a host-adapted pathogen that leads to high morbidity rates, often approaching 100% in susceptible populations.^[2] The primary hosts are horses, donkeys, and mules within the Equidae family, with the disease manifesting most severely in young animals aged 1–5 years.^[1] Key clinical features include fever, mucopurulent nasal discharge, and swelling of the submandibular and retropharyngeal lymph nodes, which often progress to abscess formation and rupture.^[2] The incubation period typically ranges from 3 to 14 days, after which infected animals shed the bacteria via nasal secretions and pus from ruptured abscesses, facilitating rapid spread within herds.^[7] While mortality is generally low (around 2%), complications can arise in severe cases, underscoring the disease's potential for widespread disruption in equine populations.^[8] Strangles imposes a substantial economic burden on the equine industry due to the costs of diagnosis, treatment, quarantine, and lost productivity during outbreaks.^[8] Individual outbreaks can affect hundreds of horses and exceed £300,000 in management expenses, including veterinary care and biosecurity measures, making it a persistent challenge for horse owners and facilities worldwide.^[8]

History

Strangles, an infectious disease affecting horses, was first described in 1251 by Jordanus Rufus, an Italian veterinary surgeon serving in the court of Emperor Frederick II, who detailed its characteristic fever and lymph node abscessation as a respiratory affliction in equines.^[9] This early account marked one of the initial formal recognitions of the condition in veterinary literature, predating modern microbiology by centuries and highlighting its longstanding presence in equine populations across Europe. Subsequent medieval and early modern texts, such as those from the 17th century, continued to reference similar symptoms, emphasizing the disease's contagious nature and impact on horse health without identifying a specific cause.^[10] The name "strangles" originated from the severe respiratory distress and choking sensation caused by abscesses in the submandibular lymph nodes, which could obstruct the airway and lead to suffocation in affected horses; this terminology reflects the dramatic clinical presentation observed by practitioners.^[11] A pivotal advancement came in 1888 when German veterinarian Friedrich Schütz isolated the causative bacterium, Streptococcus equi subspecies equi, from infected horses, providing the first microbiological confirmation of the pathogen and shifting understanding from empirical observations to bacteriological science.^[12] This discovery facilitated targeted research, including early vaccination attempts in the late 19th and early 20th centuries using live and killed bacterial preparations, though initial efforts yielded inconsistent protection.^[13] Throughout the 19th and early 20th centuries, strangles posed significant challenges during military campaigns, particularly in World War I, where dense concentrations of transport and cavalry horses in Allied and Central Powers armies led to widespread outbreaks, exacerbating losses alongside battle injuries and other infections.^[12] For instance, British veterinary reports from the era documented strangles as a recurrent issue in remount depots and front-line units, contributing to the deaths of millions of equines and straining logistics.^[12] Management practices evolved from rudimentary isolation and lancing of abscesses in the 19th century—based on recognition of contagion through shared stables and equipment—to more systematic quarantine protocols and serological testing by the mid-20th century, informed by bacteriological insights and aimed at curbing transmission in large-scale equine operations.^[14] These developments underscored strangles' enduring role in shaping veterinary strategies for equine infectious diseases. Despite over a century of research and control efforts, strangles persists globally, with a significant rise in UK cases reported in 2025 (over 70 more than in 2024 as of October) and recent genomic studies revealing key transmission drivers to inform better prevention.^[4]^[15]

Etiology and Transmission

Causative Agent

Strangles is caused by Streptococcus equi subsp. equi (S. equi), a Gram-positive, beta-hemolytic coccus classified within Lancefield group C.^[16]^[1] This bacterium is highly host-adapted, acting as an obligate pathogen exclusive to equids such as horses, donkeys, and mules, with no known natural reservoirs in other species.^[1]^[17] The genome of S. equi is a circular chromosome approximately 2.3 Mb in size, encoding around 2,100 protein-coding genes.^[18] It exhibits about 80% sequence identity with the human pathogen Streptococcus pyogenes, reflecting their shared ancestry within the pyogenic group of streptococci, though S. equi has undergone specialization with gene loss and acquisition that restrict it to equine hosts.^[18] A key genomic feature is the sem gene, which encodes the SeM protein, an M-like surface protein that contributes to immune evasion by binding equine fibrinogen and inhibiting phagocytosis.^[19] Virulence in S. equi is driven by several factors that enable adhesion, tissue invasion, and abscess formation. The hyaluronic acid capsule, which is non-antigenic and mimics host extracellular matrix, shields the bacterium from opsonization and phagocytosis, facilitating intracellular survival and persistent infection.^[20]^[21] Streptolysin S, a membrane-damaging toxin, promotes cytotoxicity to host cells, while streptokinase activates plasminogen to plasmin, aiding in fibrinolysis and bacterial dissemination through tissues, which contributes to the characteristic lymph node abscesses.^[20] The SeM protein further supports adhesion to host epithelial cells and evasion of neutrophil killing, enhancing the bacterium's ability to colonize and form suppurative lesions.^[19]^[22] S. equi is differentiated from its close relative Streptococcus equi subsp. zooepidemicus (S. zooepidemicus), which shares over 95% genomic similarity but functions primarily as an opportunistic commensal in the upper respiratory tract of equids and other animals, occasionally causing disease under stress conditions.^[23]^[24] Unlike S. equi, S. zooepidemicus retains broader host range capabilities and can ferment lactose, whereas S. equi has evolved through reductive evolution, losing metabolic genes and gaining equine-specific virulence elements that make it incapable of asymptomatic carriage or zoonotic transmission.^[25]^[18]

Transmission Mechanisms

Strangles, caused by Streptococcus equi subsp. equi, spreads primarily through direct contact between horses, particularly nose-to-nose interactions involving nasal secretions or purulent material from ruptured lymph node abscesses.^[26] Indirect transmission occurs via contaminated fomites such as shared equipment, feed, water troughs, and handler clothing, which can transfer the bacteria to susceptible animals.^[19] The bacterium demonstrates notable environmental persistence on surfaces, facilitating fomite-mediated spread; S. equi can survive for up to 63 days on wood at 2°C or 48 days on glass and wood at 20°C in controlled conditions, with viability extending beyond 30 days in cool, moist indoor environments but declining rapidly under desiccation, heat, or sunlight exposure.^[27] Asymptomatic carriers play a critical role in prolonged transmission, as up to 40% of infected horses develop persistent infections in the guttural pouches, where bacteria form chondroids and shed intermittently into nasal secretions for weeks to years, potentially up to 18 months or longer in rare cases.^[26]^[19] Transmission risk escalates in settings with high-density stabling, commingling of naive and previously exposed horses, and international transport, compounded by an incubation period of 3–14 days that allows silent spread within populations.^[26]

Pathogenesis

Infection Process

Streptococcus equi, the causative agent of strangles, primarily enters the equine host through the nasal mucosa or oral cavity following inhalation or ingestion of contaminated material. Upon entry, the bacteria adhere to epithelial cells in the crypts of the lingual and palatine tonsils, as well as the follicle-associated epithelium of the pharyngeal and tubal tonsils, using surface ligands such as SzPSe, Se73.9, and Se51.9 to facilitate binding.^[28] This initial attachment allows penetration into deeper tonsillar tissues within hours of exposure.^[29] From the tonsillar sites, S. equi rapidly translocates via lymphatic drainage to regional lymph nodes, including the submandibular, retropharyngeal, and mandibular nodes, often within 3 hours post-infection.^[29] In these lymphoid tissues, the bacteria proliferate extracellularly, evading early clearance through virulence factors like the hyaluronic acid capsule and SeM protein, which inhibit phagocytosis by neutrophils attracted via complement-derived chemotactic signals.^[28] This leads to suppurative lymphadenitis, characterized by the formation of neutrophilic abscesses that mature over 3-5 days, encapsulated by a fibrinous wall to contain the infection.^[29] The infection progresses through distinct stages. The incubation period lasts 3-14 days, during which bacterial replication in the pharynx triggers fever onset via pyrogenic exotoxins such as SePE-H and SePE-I.^[28] The acute phase involves abscess maturation in the affected lymph nodes over 7-14 days, with bacterial chains expanding within the purulent core.^[30] Resolution occurs as abscesses rupture 1-4 weeks post-infection, releasing pus laden with viable bacteria into surrounding airways or externally, facilitated by enzymes like streptolysin S and streptokinase that promote tissue breakdown.^[29] S. equi exhibits a strong tropism for mucosal surfaces of the upper respiratory tract and associated lymphoid tissues, particularly the pharyngeal region and draining lymph nodes of the head and neck.^[12] In rare cases, approximately 28% in some outbreaks, metastatic dissemination via hematogenous or further lymphatic spread can occur, leading to "bastard strangles" with abscesses in distant sites such as the abdomen, thorax, or central nervous system.^[28]

Immune Response

The innate immune response to Streptococcus equi infection in horses is characterized by rapid neutrophil recruitment to the tonsils and regional lymph nodes, such as the mandibular and retropharyngeal nodes, leading to heavy infiltration and the formation of purulent abscesses.^[31] This neutrophil influx, triggered shortly after bacterial colonization, contributes to the characteristic lymph node swelling and suppuration observed in strangles.^[32] Concurrently, pro-inflammatory cytokine release, including elevated levels of IL-6, TNF-α, IL-1, and IL-8, drives systemic fever, inflammation, and the acute phase response, exacerbating tissue damage and clinical signs like pyrexia and nasal discharge.^[33]^[34]^[35] The adaptive immune response involves both humoral and cellular components, with a robust antibody-mediated reaction targeting key bacterial antigens. Horses develop systemic IgG and mucosal IgA antibodies primarily against the M-like surface protein SeM, which opsonizes bacteria for phagocytosis and neutralizes superantigen activity, aiding in bacterial clearance from the nasopharynx.^[36]^[13] T-cell responses, including non-specific activation by S. equi superantigens, contribute to lymph node hyperplasia and swelling, while specific T-helper cells support B-cell differentiation and long-term memory formation.^[37] These responses typically peak 2-3 weeks post-infection, coinciding with resolution of mucosal shedding and abscess maturation.^[38] In most cases, recovery from strangles confers protective immunity lasting at least five years in 70-75% of horses, mediated by persistent anti-SeM antibodies and memory T-cells that prevent clinical disease upon re-exposure.^[2] However, this immunity is often incomplete against asymptomatic carriage, allowing persistent nasopharyngeal or guttural pouch colonization in a subset of recovered horses.^[1] Pathological aspects of the immune response can lead to complications, including hypersensitivity reactions upon reinfection or repeated exposure, such as purpura hemorrhagica, an immune-mediated vasculitis causing edema and hemorrhage.^[39] In severe cases, overwhelming infection or dysregulated superantigen-induced T-cell activation may result in immunosuppression, facilitating metastatic abscessation (bastard strangles) in distant lymph nodes or organs.^[40]^[7]

Clinical Features

Signs and Symptoms

Strangles typically manifests after an incubation period of 3 to 14 days following exposure to Streptococcus equi subsp. equi.^[1] The earliest clinical signs include a sudden onset of high fever, often reaching 103° to 106°F (39.4° to 41.1°C), accompanied by depression, anorexia, and a serous nasal discharge that appears within 1 to 3 days of the fever spike.^[1]^[32] These initial symptoms reflect the acute inflammatory response in the upper respiratory tract and may precede more overt signs by 24 to 48 hours.^[1] As the disease progresses, the hallmark features emerge, characterized by bilateral enlargement of the submandibular lymph nodes, which become firm, painful, and abscessed.^[11] Retropharyngeal lymph node swelling often develops concurrently, leading to dysphagia, respiratory stridor, and the characteristic "strangling" appearance due to external edema in the throatlatch region.^[1] The nasal discharge evolves from serous to purulent, typically white or yellow in color, and the submandibular abscesses may rupture externally or drain into the guttural pouches, releasing thick pus and facilitating bacterial shedding.^[41]^[2] In uncomplicated cases, spontaneous resolution occurs within 2 to 3 weeks as the abscesses drain and the horse's immune response clears the infection, though clinical recovery may extend to 3 to 6 weeks in some instances.^[42]^[43] Clinical presentations can vary based on the horse's age and prior immunity. In previously exposed or immune horses, the disease may be mild or subclinical, manifesting only as transient mucoid nasal discharge, mild fever, and minimal lymph node involvement without abscessation.^[1] Conversely, foals and young horses often experience more severe forms, including generalized lymphadenopathy with widespread abscessation beyond the head and neck.^[2]^[44] While most cases resolve without long-term effects, there is a potential for immune-mediated complications such as purpura hemorrhagica in susceptible individuals.^[1]

Complications

Complications of strangles in horses, while uncommon, can significantly worsen outcomes and require intensive veterinary intervention. These secondary conditions arise from metastatic spread of the infection or aberrant immune responses, affecting approximately 20% of cases.^[45] Overall mortality from strangles remains low at less than 10%, but rates can approach 8% in complicated cases, with fatalities often linked to airway obstruction, internal abscessation, or immune-mediated sequelae.^[46]^[47] Bastard strangles, also known as metastatic strangles, involves the dissemination of Streptococcus equi to distant lymph nodes or organs, leading to internal abscess formation in sites such as the lungs, abdomen (e.g., mesentery, liver, spleen, kidney), thorax, or rarely the brain. This condition manifests as pneumonia, peritonitis, or neurological signs like meningitis, occurring in about 2% of infected horses in documented outbreaks.^[48]^[7] Diagnosis typically requires imaging or exploratory procedures, and severe cases may necessitate euthanasia due to organ compromise.^[1] Purpura hemorrhagica is an immune-mediated vasculitis that develops 2-4 weeks after initial infection, triggered by high antibody titers against S. equi surface proteins. It presents with subcutaneous edema in the limbs, head, and trunk; petechial hemorrhages; and potential laminitis or rhabdomyolysis, affecting roughly 6.5% of cases and correlating with anemia in affected horses.^[48]^[7] Skin sloughing may occur in severe instances, and the condition responds to immunosuppressive therapy alongside supportive care.^[45] Guttural pouch empyema results from the rupture of retropharyngeal lymph node abscesses into the guttural pouch, causing chronic accumulation of pus and potential chondroid (mineralized pus concretions) formation. This sequela is observed in some horses post-infection, with studies reporting it in up to 64% of cases examined endoscopically; overall carrier rates are estimated at 10-40%.^[48]^[1] It can lead to dysphagia, respiratory stridor, or secondary mycosis if untreated. Endoscopic evaluation and lavage are essential for detection and management to prevent carrier status.^[7] Less common complications include immune-mediated myocarditis, which may cause cardiac arrhythmias, and septicemia in neonates, leading to rapid systemic failure.^[1] These, along with myositis, contribute to higher mortality in vulnerable populations, though overall fatalities from strangles are rare outside of severe metastatic or immune events.^[7]

Diagnosis

Clinical Assessment

Clinical assessment of strangles begins with a thorough history taking to identify potential risk factors and the timeline of disease onset. Veterinarians inquire about recent exposure to infected horses or premises, including commingling with animals of unknown health status or attendance at events with high horse density, as well as travel history that may have facilitated contact with carriers. The incubation period is typically 3 to 14 days post-exposure, with fever often appearing 24 to 48 hours before other signs, helping to correlate symptoms with recent events.^[7]^[19] During the physical examination, rectal temperature is measured at least twice daily, as pyrexia exceeding 38.5°C is a hallmark early indicator prompting immediate suspicion of strangles. Palpation of the submandibular and retropharyngeal lymph nodes is essential to detect enlargement, heat, pain, or fluctuance indicative of abscessation, with careful attention to avoid rupture that could spread infection. Nasal endoscopy may be performed to visualize the source of any mucopurulent discharge, assessing the pharynx and guttural pouches for inflammation or empyema while minimizing aerosolization of bacteria.^[7]^[19] Clinical severity is often evaluated using scoring indices to guide management decisions, such as quarantine duration. One such system assigns points for fever (rectal temperature >38.2°C: 1 point), leukocytosis (white blood cell count ≥14.0 × 10³/μL: 1 point), nasal discharge severity (0–5 points based on volume and character), lymph node swelling (0–3 points graded by size and firmness), and abscessation or rupture (4 points); a total score of ≥4 classifies the case as clinical strangles and supports prolonged isolation. These indices help prioritize horses for closer monitoring and inform biosecurity measures without relying on laboratory confirmation.^[49] Isolation protocols are initiated immediately upon suspicion to curb transmission, separating potentially affected horses into designated groups: "red" for those showing clinical signs, "amber" for exposed but asymptomatic individuals, and "green" for unaffected horses. Suspect animals are moved to a dedicated area with dedicated equipment, and personnel use personal protective equipment while handling cases last in the daily routine; quarantine typically lasts at least three weeks beyond resolution of signs in the last case.^[7]^[19]

Laboratory Confirmation

Laboratory confirmation of strangles involves microbiological and molecular tests to detect Streptococcus equi subsp. equi (S. equi) in clinical samples, providing definitive evidence beyond clinical suspicion.^[50] These methods are essential for identifying active infections, carriers, and distinguishing S. equi from related streptococci like S. zooepidemicus.^[50] Bacterial culture remains a standard technique for isolating S. equi, using swabs from the nasopharynx, nasal washes, or pus from abscessed lymph nodes. Samples are inoculated onto selective media such as Columbia colistin-nalidixic acid agar with 5% sheep blood, where S. equi forms small (1 mm), translucent colonies exhibiting clear beta-hemolysis after 24-48 hours of incubation at 37°C.^[50]^[51] Antimicrobial sensitivity testing is performed on isolates to guide therapy, though culture sensitivity can be as low as 40% in early or chronic stages due to fastidious growth requirements or low bacterial loads.^[50] Abscess pus yields the highest recovery rates, with results available in 1-2 days.^[50] Polymerase chain reaction (PCR) assays offer rapid, highly sensitive detection of S. equi DNA, particularly useful for early diagnosis and carrier screening, with sensitivities exceeding 95% compared to culture.^[52] Real-time PCR targets specific genes such as eqbE (encoding a putative secreted protein), seeI (superantigen), or SEQ2190, often in multiplex formats including internal controls for validation.^[50] Suitable samples include nasopharyngeal swabs or washes collected 2-3 days post-fever onset, as well as guttural pouch lavages; results are typically available within 24-48 hours and detect both viable and non-viable bacteria, making PCR 3 times more sensitive than culture overall.^[50]^[53] Serological testing via enzyme-linked immunosorbent assay (ELISA) detects antibodies against S. equi antigens, aiding in confirming recent exposure or persistent infection in carriers.^[50] The SeM ELISA measures IgG to the M-like protein SeM, with titers rising 2-3 weeks post-infection, peaking at 5 weeks, and persisting for 6 months or longer; paired serum samples showing fourfold increases indicate active disease.^[50]^[53] Combined ELISAs incorporating SeM and Antigen A (SEQ_2190) enhance specificity by reducing cross-reactivity with S. zooepidemicus.^[50] These tests are particularly valuable for subclinical cases but do not distinguish vaccination from natural infection.^[50] Endoscopic examination of the guttural pouches, combined with lavage, is crucial for detecting carriers with empyema or chondroids, where up to 10% of recovered horses harbor persistent infection.^[50] A flexible endoscope visualizes inflammation, pus, or abscesses, followed by saline lavage through the biopsy channel for subsequent culture or PCR analysis, yielding high detection rates in this reservoir.^[50] In cases of suspected chondroid formation or persistent suppuration, biopsy samples may reveal histopathology consistent with suppurative inflammation, including neutrophilic infiltrates and bacterial colonies.^[50] This approach is recommended for horses with negative nasal PCR but ongoing quarantine concerns.^[50]

Treatment

Supportive Care

Supportive care forms the cornerstone of managing strangles in horses, focusing on alleviating discomfort, supporting natural immune processes, and preventing secondary issues without relying on antimicrobials as the primary intervention. This approach is suitable for most uncomplicated cases, where the disease typically resolves within 2-3 weeks following abscess formation and drainage, allowing the horse's immune system to develop protective responses.^[19]^[1] Strict isolation and hygiene measures are essential to contain the infection and protect herd mates. Affected horses must be quarantined immediately upon detection of fever or clinical signs, with separation into distinct groups for clean, exposed, and sick animals; quarantine should continue until clinical signs have resolved and bacterial clearance is confirmed by negative bacteriologic cultures or PCR testing from nasopharyngeal washes or guttural pouch lavages, typically taking 4-6 weeks but potentially longer to account for prolonged bacterial shedding. Stalls and equipment require daily disinfection using bleach (1:32 dilution of sodium hypochlorite) or equivalent quaternary ammonium compounds after removing organic debris, with facilities rested for at least 2 weeks in dry conditions to ensure bacterial inactivation. Personal protective equipment, including gloves and dedicated clothing, must be worn when handling infected horses, with handlers progressing from clean to sick animals to avoid cross-contamination.^[19]^[7]^[1] Symptom relief emphasizes environmental and topical support to reduce pain and promote recovery. Horses should be housed in a warm, dry, dust-free stall to minimize respiratory irritation and stress. Non-steroidal anti-inflammatory drugs, such as flunixin meglumine (1.1 mg/kg IV or orally once daily) or phenylbutazone (2-4 mg/kg orally once daily), are administered judiciously to control fever, inflammation, and associated discomfort, improving appetite without suppressing abscess development. Hot compresses or poultices applied to submandibular or retropharyngeal lymph nodes for 15-20 minutes several times daily encourage maturation and spontaneous drainage of abscesses. In severe cases with respiratory distress from lymphadenopathy, temporary tracheostomy may be necessary to maintain airway patency.^[1]^[19]^[2] Nutritional support addresses common challenges like dysphagia and reduced intake due to pharyngeal swelling. Soft, moist, highly palatable feeds—such as soaked hay pellets or mashes—should be offered at ground level to facilitate eating, while ensuring constant access to fresh water. In cases of severe dysphagia or prolonged anorexia, nasogastric intubation for fluid and nutrient delivery may be required, with intravenous fluids considered if dehydration develops. Regular monitoring of hydration status, including skin tenting tests and mucous membrane assessment, is critical to detect and address dehydration early, as affected horses are prone to fluid losses from fever and poor intake.^[19]^[1]^[7] Wound management targets post-rupture care to promote healing and prevent contamination. Once abscesses drain, the sites should be gently flushed daily with a 3-5% povidone-iodine solution or dilute chlorhexidine until purulent discharge resolves, typically over 3-5 days, to reduce bacterial load and encourage granulation. Bandaging the affected areas, particularly on the head or neck, helps shield open wounds from fly strikes and environmental contaminants, using absorbent materials changed frequently to maintain cleanliness. Topical ichthammol ointments may be applied prior to bandaging to soften and draw out residual pus if needed.^[1]^[2]^[19]

Antimicrobial Therapy

Antimicrobial therapy for strangles, caused by Streptococcus equi subsp. equi, is reserved for specific clinical scenarios to target the pathogen while minimizing risks to immunity development.^[19] Indications include early infection within 24-48 hours of fever onset to halt bacterial dissemination and shedding, severe cases with high fever (>103°F or 39.4°C), profound malaise, respiratory distress from lymphadenopathy, or complications such as metastatic abscessation and guttural pouch empyema.^[1]^[7] Routine use is not recommended for uncomplicated cases once abscesses form, as it can prolong the disease course by delaying maturation and drainage, thereby impairing the development of protective immunity.^[19] The preferred antimicrobial is procaine penicillin G, administered at 22,000 IU/kg intramuscularly every 12 hours for 7-10 days, due to its efficacy against S. equi and the bacterium's consistent susceptibility to beta-lactams.^[1]^[19] Alternatives include ceftiofur (2.2 mg/kg IV or IM every 12 hours, or extended-release formulations at 6.6 mg/kg IM every 96 hours) for horses intolerant to frequent injections, and trimethoprim-sulfadiazine (30 mg/kg orally every 12 hours) when penicillin is contraindicated, though efficacy can vary.^[19]^[7] Resistance to these agents remains rare, but culture and sensitivity testing should guide selection in suspected refractory cases.^[1] Challenges in therapy arise from S. equi's ability to persist intracellularly within macrophages and form biofilms in guttural pouches, potentially evading antibiotics and leading to carrier states that require prolonged treatment (up to 3-4 weeks) or adjunctive procedures like lavage.^[19] Early or inappropriate antibiotic initiation can heighten the risk of purpura hemorrhagica, an immune-mediated vasculitis, particularly in horses with high serum antibody titers (≥1:3,200), by altering antigen exposure and promoting aberrant immune responses.^[1]^[7] Monitoring involves serial clinical assessments, including fever resolution and lymph node status, alongside bacteriologic cultures or PCR from nasopharyngeal washes or guttural pouch lavages to confirm bacterial clearance.^[19] Therapy should cease once abscesses begin draining to facilitate immunity, with retesting for carriers recommended 3 weeks post-treatment; supportive measures, such as hot packing, may complement therapy during this period.^[1]^[7]

Prevention

Vaccination Strategies

Vaccination remains a key strategy for mitigating strangles in horses, though available vaccines provide only partial protection against Streptococcus equi subsp. equi. In the United States, two primary types are licensed by the USDA: intramuscular (IM) killed vaccines, such as Strepvax II, which use inactivated bacterial extracts including M-protein to stimulate systemic immunity; and intranasal (IN) modified-live vaccines, such as Pinnacle I.N., which employ avirulent strains to induce mucosal immunity at the site of natural infection.^[54]^[55]^[44] Standard protocols for IM vaccines like Strepvax II involve a primary series of three 1 mL doses administered three weeks apart, followed by annual boosters to maintain immunity.^[54] For IN vaccines like Pinnacle I.N., the regimen consists of two doses given two to three weeks apart in healthy horses aged nine months or older, with annual revaccination recommended for at-risk animals.^[55]^[44] Vaccination is risk-based, prioritized for horses in endemic areas or with high exposure potential, and should be avoided during active outbreaks to minimize adverse reactions and complications.^[44] Efficacy varies by vaccine type, with IM killed vaccines reducing clinical disease incidence by approximately 50% after boosting, while IN modified-live vaccines demonstrate higher protection rates against experimental challenges, often exceeding 70% reduction in clinical signs.^[44]^[56] Overall, vaccines achieve 50-80% reduction in disease severity but offer poor prevention of the asymptomatic carrier state, where up to 10% of recovered horses harbor persistent S. equi infections.^[56] Common side effects include local reactions such as injection-site soreness or abscesses for IM vaccines, and transient nasal discharge, mild fever, or submandibular lymphadenopathy lasting 1-7 days for IN vaccines; severe hypersensitivity is rare but requires epinephrine treatment.^[54]^[55]^[44] In Europe, Strangvac, a recombinant subunit vaccine administered intramuscularly, has been available since 2022 and targets proteins to reduce both clinical disease and carrier states. It involves two doses given one month apart for horses at medium to high risk, showing safety and efficacy in reducing strangles signs in field studies as of 2025.^[57]^[6] Ongoing research continues to address gaps in current products, particularly for US-licensed vaccines, with efforts focusing on further improvements in novel subunit formulations to enhance cross-protection, duration of immunity, and elimination of persistent infections.^[44]^[56]

Biosecurity Measures

Biosecurity measures for strangles focus on preventing the introduction and limiting the spread of Streptococcus equi within horse populations through structured protocols that emphasize isolation, hygiene, and surveillance. Quarantine protocols are essential for new arrivals, requiring a minimum isolation period of 3 weeks in a designated area separate from the resident herd to allow for monitoring of clinical signs such as fever or nasal discharge. During this time, daily temperature checks are conducted, and diagnostic testing includes PCR on nasal swabs or nasopharyngeal washes to detect active shedding, alongside serology to assess prior exposure and immune status. This approach ensures that potentially infected horses are identified before integration, reducing the risk of herd-wide outbreaks.^[7]^[1] Facility design plays a critical role in minimizing indirect transmission via fomites and environmental contamination. Stables should feature separate ventilation systems to avoid aerosolized spread between groups, with isolation wards located at least 10-25 meters from main housing to prevent nose-to-nose contact. Footbaths containing disinfectants like 1:10 diluted bleach or peroxygen compounds should be placed at all entrances and exits, particularly during high-risk periods, to decontaminate footwear and reduce mechanical transfer by personnel. Dedicated equipment, such as buckets, grooming tools, and hoses, must be assigned to specific horse groups and disinfected regularly with agents like chlorhexidine or glutaraldehyde to eliminate bacterial persistence on surfaces. Visitor restrictions are strictly enforced during outbreaks, limiting access to essential personnel who must change clothing, use protective gear, and follow unidirectional handling to avoid cross-contamination between clean and affected areas. These measures target the primary transmission routes of direct contact and contaminated fomites.^[58]^[1]^[7] Screening protocols enhance biosecurity by identifying asymptomatic carriers, who can harbor S. equi in the guttural pouches for months. Pre-purchase evaluations routinely include guttural pouch endoscopy to visualize empyema or chondroids, combined with lavage for PCR testing, which offers high sensitivity for detecting low-level shedding. For high-risk populations like show horses, which frequently travel and commingle, routine PCR screening on nasopharyngeal samples or guttural pouch lavages is recommended every 3-6 months or post-event to monitor for subclinical infections. Three consecutive negative PCR tests, spaced 7 days apart, are required to confirm clearance in suspected carriers.^[8]^[1]^[7] Outbreak control relies on rapid implementation of containment strategies to trace and isolate exposed horses. Movement records, including transport logs and event attendance, are reviewed to identify contacts, enabling targeted quarantine of at-risk individuals for up to 6 weeks post-exposure if shedding persists. Affected premises are placed under full quarantine, with twice-daily monitoring and barrier nursing using separate caretakers for sick, exposed, and clean groups. While euthanasia of persistent carriers has been employed in severe outbreaks leading to significant morbidity, standard protocols prioritize isolation and repeated testing over depopulation unless economically or logistically unfeasible. Pastures used by infected horses should rest for 2-6 weeks, depending on environmental conditions, before reuse to allow bacterial die-off.^[7]^[59]^[1]

Prognosis and Epidemiology

Outcomes and Recovery

In uncomplicated cases of strangles, horses typically recover within 3 to 6 weeks, with clinical signs such as fever and lymph node abscessation resolving as the immune response clears the Streptococcus equi infection.^[2]^[1] During this period, bacterial shedding from nasal passages or abscesses usually persists for 2 to 3 weeks after the fever subsides, though supportive care can shorten the acute phase.^[1] Approximately 70% to 75% of surviving horses that are not treated with antibiotics develop protective immunity lasting at least 5 years, reducing the risk of reinfection, while antibiotic use may interfere with this immune development.^[2]^[60] Prognosis is generally favorable for most cases, with overall mortality rates below 10%, though this rises to 40% or higher when complications such as bastard strangles or purpura hemorrhagica occur in about 10% of infections.^[2]^[61] Key factors influencing outcomes include the horse's age, with foals and horses aged 1 to 5 years experiencing more severe disease due to immature or waning immunity, respectively; early supportive intervention, which improves recovery rates by preventing abscess rupture and secondary infections; and the absence of complications, which otherwise prolong convalescence.^[1]^[19] Recurrence is uncommon in non-carrier horses but can occur in up to 25% of cases if prior antibiotic treatment suppresses full immunity development.^[62] Post-recovery, 10% to 40% of infected horses become asymptomatic carriers, harboring S. equi in the guttural pouches for months to years and intermittently shedding the bacteria, which can perpetuate outbreaks.^[1]^[63] Long-term effects primarily involve chronic guttural pouch empyema or chondroid formation in carriers, leading to persistent respiratory obstruction, dysphagia, or recurrent infections that may diminish athletic performance in sport horses by reducing stamina and increasing susceptibility to exercise intolerance.^[1]^[7] Morbidity remains high during outbreaks, affecting nearly 100% of susceptible horses in a herd, but full return to pre-infection health is expected in uncomplicated survivors without carrier status.^[46]

Global Distribution

Strangles, caused by Streptococcus equi subspecies equi, is endemic in horse populations worldwide, with the exception of Iceland, where strict import bans have maintained disease-free status.^[8] Outbreaks are frequently reported in Europe, North America, and Australia, where the disease remains one of the most common infectious conditions in equines.^[8] In Asia, strangles is emerging as a concern, with genetic analyses identifying multiple strains in countries such as Japan, Israel, and the United Arab Emirates.^[8] Key risk factors for strangles outbreaks include intensive equine industries, such as racing and showing events, which facilitate overcrowding and close contact among large horse populations.^[2] International horse trade and movement without adequate screening exacerbate spread, as asymptomatic carriers can transport the bacterium across borders.^[64] The disease exhibits seasonal peaks in spring months, particularly May in the United States, though risk increases during periods of close confinement such as winter stabling; outbreaks increase with rising temperatures from winter to spring.^[65]^[66] Surveillance efforts vary by region but emphasize early detection to curb outbreaks. In the European Union and United Kingdom, voluntary reporting through networks like the Surveillance of Equine Strangles (SES) provides real-time laboratory data, while the United States Department of Agriculture (USDA) monitors strangles as a nationally reportable disease under its Animal and Plant Health Inspection Service (APHIS) guidelines.^[67]^[68] Molecular techniques, including core-genome multilocus sequence typing (cgMLST), have revealed distinct regional strains, such as Bayesian population structure (BAPS) cluster 2 dominating in Europe and BAPS-1 prevalent in North America, aiding in tracing transmission pathways.^[8] As of 2025, strangles outbreaks continue globally, with over 370 laboratory-confirmed cases reported in the UK by October, incidents in the US (e.g., Maryland in August affecting 17 horses), and nine outbreaks in Canada during the first quarter; a July 2025 genomic study by the Royal Veterinary College identified key transmission drivers in UK horses using whole-genome sequencing.^[3]^[69]^[70]^[71]^[15] Control trends show declining incidence in regions with robust vaccination and biosecurity programs; for instance, intranasal vaccines like Pinnacle IN have contributed to reduced outbreak frequency in vaccinated U.S. stables, where annual infection rates can fall below 1% in managed facilities.^[8] Pre-import screening protocols, such as dual-antigen ELISA tests, have similarly lowered risks in high-movement areas like the Middle East.^[8] However, strangles persists at higher levels in developing regions, including parts of Asia and Africa, where limited access to vaccines and biosecurity measures sustains endemic transmission, as evidenced by prevalence rates exceeding 19% in some Egyptian horse populations.^[72]

References

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### Summary of Strangles Infection Process (Streptococcus equi in Horses)
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