Equine Pharyngitis Study Guide

Overview and Clinical Importance

Pharyngitis refers to inflammation of the pharynx, a critical component of the equine upper respiratory tract. In horses, the pharynx is uniquely organized with complete separation into nasopharynx and oropharynx, unlike most other domestic species. This anatomical arrangement has significant clinical implications for the diagnosis and management of pharyngeal diseases.

Pharyngeal disorders represent a significant cause of poor performance, abnormal respiratory noise, and dysphagia in horses. The most commonly encountered pharyngeal condition is pharyngeal lymphoid hyperplasia (PLH), which is particularly prevalent in young horses and often associated with viral respiratory infections. Additionally, severe pharyngitis can occur secondary to bacterial infections such as strangles (Streptococcus equi) and viral infections including equine herpesvirus (EHV) and equine influenza virus (EIV).

Anatomy of the Equine Pharynx

The equine pharynx serves as the crossroads of the respiratory and digestive systems. Unlike other species, horses have a complete separation of the nasopharynx and oropharynx due to the unique positioning of the soft palate. The soft palate merges into the walls of the pharynx, creating an interlocking junction with the epiglottis that maintains this separation except during swallowing.

Key Anatomical Structures

• Nasopharynx: Dorsal to the soft palate, contains pharyngeal lymphoid tissue (tonsils)
• Oropharynx: Ventral to the soft palate, connects oral cavity to esophagus
• Guttural Pouches: Paired diverticulae of the auditory tubes opening into the lateral nasopharynx
• Soft Palate: Separates nasopharynx from oropharynx; interlocks with epiglottis
• Dorsal Pharyngeal Recess: Site of lymphoid follicle accumulation

Pharyngeal Lymphoid Hyperplasia (PLH)

Definition and Pathophysiology

Pharyngeal lymphoid hyperplasia (PLH) is characterized by the proliferation of lymphoid follicles on the dorsal and lateral walls of the pharynx, extending to the openings of the guttural pouches and onto the soft palate. Unlike humans, horses do not have discrete tonsillar masses; instead, they have diffuse mucosal-associated lymphoid tissue (MALT) spread throughout the pharyngeal mucosa.

The equine pharyngeal tonsil consists of lymphoid follicles containing B-cells surrounded by interfollicular T-cells. Exposure to environmental antigens, allergens, bacteria, viruses, and irritant particles stimulates the local lymphoid tissue, leading to mucus production and local immunoglobulin synthesis.

Age Distribution and Epidemiology

PLH is most prevalent in young horses aged 1-5 years, with 2-year-old Thoroughbred racehorses showing the highest prevalence and severity. Studies have demonstrated:

• Virtually all young horses develop some degree of PLH during their training years
• Grade 3-4 PLH in 37% of 2-year-old Thoroughbreds, decreasing to nearly 0% in horses greater than 6 years
• Self-resolution without treatment is typically seen with increasing age
• The condition represents a normal immunologic response in most cases

PLH Grading System

Clinical Signs of PLH

Most horses with PLH are asymptomatic, and the condition is often identified incidentally during endoscopic examination for other reasons. When clinical signs are present, they may include:

• Reduced appetite or dysphagia (rare)
• Frequent swallowing
• Abnormal respiratory sounds during exercise
• Exercise intolerance
• Nasal discharge (if concurrent respiratory infection)
• Cough

Infectious Causes of Pharyngitis

Viral Pharyngitis

Equine Herpesvirus (EHV-1 and EHV-4)

Equine herpesviruses are ubiquitous worldwide and are among the most important causes of respiratory disease in horses. EHV-4 is primarily associated with respiratory disease, while EHV-1 can cause respiratory disease, abortion, neonatal death, and neurologic disease (equine herpesvirus myeloencephalopathy, EHM).

Equine Influenza Virus (EIV)

Equine influenza is a highly contagious acute respiratory disease caused by influenza A virus subtypes H3N8. It is characterized by rapid spread among susceptible horses and can infect entire barns within days.

Key Clinical Features:

• Incubation period: 1-3 days (very short)
• Fever: Up to 106°F (41.1°C), lasting 1-5 days
• Cough: Harsh, dry, nonproductive; develops early and may persist for weeks
• Nasal discharge: Initially serous, may become mucopurulent with secondary bacterial infection
• Lethargy, anorexia, muscle soreness

Bacterial Pharyngitis: Strangles (Streptococcus equi)

Strangles is a highly contagious upper respiratory disease caused by Streptococcus equi subspecies equi. It is characterized by pharyngitis followed by abscess formation in the submandibular and retropharyngeal lymph nodes. The name 'strangles' derives from the severe pharyngeal obstruction that can occur when enlarged lymph nodes compress the upper airway.

Pathogenesis

S. equi enters through the mouth or nose and attaches to cells in the crypts of the lingual and palatine tonsils. Bacterial enzymes and toxins cause rhinitis, pharyngitis, and fever. The organism then spreads to regional lymph nodes, causing lymphadenitis and abscess formation. Lymph node rupture releases creamy-yellow pus either externally (submandibular) or into the guttural pouch (retropharyngeal).

Clinical Signs

• Incubation: 3-14 days
• Initial fever: Often exceeding 103°F (39.4°C), precedes other signs by 24-48 hours
• Nasal discharge: Initially serous, progresses to thick, mucopurulent, yellow-white
• Lymphadenopathy: Submandibular and retropharyngeal lymph nodes become swollen, hot, and painful
• Pharyngitis with narrowed pharynx on endoscopy
• Extended head and neck position (due to pharyngeal pain)
• Dysphagia, anorexia, depression
• Respiratory distress (in severe cases requiring tracheostomy)

Complications of Strangles

Diagnostic Approach

Physical Examination

A thorough physical examination should include:

• Rectal temperature (normal: 99-101.5°F or 37.2-38.6°C)
• Assessment of nasal discharge (character, volume, unilateral vs bilateral)
• Palpation of submandibular and retropharyngeal lymph nodes
• Thoracic auscultation
• External observation of head position and neck extension

Endoscopy

Upper airway endoscopy is the definitive diagnostic tool for evaluating pharyngeal disorders. It allows direct visualization of:

• Pharyngeal mucosa and lymphoid follicles (PLH grading)
• Guttural pouch openings
• Soft palate position and function
• Laryngeal function
• Mucus accumulation in nasopharynx and trachea
• Pharyngeal narrowing due to retropharyngeal lymphadenopathy

Laboratory Diagnostics

Treatment Strategies

PLH Treatment

Treatment is rarely necessary for PLH, as it typically represents a normal immunologic response in young horses that resolves spontaneously with age. However, for horses demonstrating pharyngeal pain or Grade 3-4 PLH with clinical signs:

Viral Pharyngitis Treatment

Treatment for viral pharyngitis (EHV, EIV) is primarily supportive:

• Rest: Critical for recovery; 1 week per day of fever minimum
• NSAIDs: For fever and inflammation (flunixin, phenylbutazone)
• Fluid therapy: IV or oral fluids for dehydrated horses
• Isolation: Quarantine affected horses; handle last with dedicated equipment
• Environmental management: Good ventilation, dust reduction

Strangles Treatment

Antibiotic use in strangles is CONTROVERSIAL. The decision depends on the clinical stage:

• Uncomplicated strangles: Supportive care; hot-packing abscesses to encourage drainage; NSAIDs for pain and fever
• Early febrile stage (before abscess formation): Antibiotics (penicillin) may abort infection but can prolong outbreak
• Established abscesses: Allow abscesses to mature and drain; antibiotics may delay resolution
• Complications (bastard strangles, dyspnea): Aggressive antibiotic therapy indicated; tracheostomy if needed

Prevention and Biosecurity

Vaccination

Biosecurity Measures

• Quarantine: New arrivals quarantined 2-4 weeks; monitor temperature twice daily
• Isolation: Sick horses handled last with dedicated equipment and clothing
• Disinfection: Clean and disinfect shared surfaces; S. equi survives 1-2 days dry, up to 34 days wet/cold
• No shared equipment: Use dedicated water buckets, feed containers, grooming supplies

Prognosis

• PLH: Excellent; self-limiting in most cases
• Viral pharyngitis (EHV, EIV): Good with adequate rest; recovery 2-3 weeks for uncomplicated cases
• Strangles (uncomplicated): Good; 3-6 weeks recovery; 70-75% develop lasting immunity
• Strangles (complicated): Guarded to poor; mortality up to 40% with metastatic disease