Equine Purpura Hemorrhagica Study Guide

Overview and Clinical Importance

Purpura hemorrhagica (PH) is an acute, non-contagious, immune-mediated aseptic necrotizing vasculitis in horses. It is characterized by widespread edema, petechial and ecchymotic hemorrhages of the mucous membranes, and subcutaneous tissue swelling. PH is most commonly associated with Streptococcus equi subspecies equi (strangles) infection but can occur following other bacterial or viral infections, vaccination, or idiopathically. Understanding this condition is essential for NAVLE success as it represents a classic example of Type III hypersensitivity in veterinary medicine.

Etiology and Associated Conditions

Purpura hemorrhagica can develop secondary to various infectious agents and antigenic stimuli. The most common associations include:

Pathophysiology

Type III Hypersensitivity Mechanism

Purpura hemorrhagica is a classic example of a Type III hypersensitivity reaction (immune complex-mediated). The pathogenesis involves the following sequence:

• Initial Sensitization: Horse is exposed to S. equi M-protein (SeM) or similar antigen through infection or vaccination
• Antibody Production: Immune system produces antibodies (primarily IgA, also IgG) against the streptococcal M-like protein
• Re-exposure: Upon second encounter with antigen (2-4 weeks later), antigen-antibody immune complexes form
• Immune Complex Deposition: These complexes deposit in small blood vessel walls (particularly capillaries and venules)
• Complement Activation: Immune complexes activate the complement cascade
• Inflammatory Response: Complement activation attracts neutrophils and basophils, which release vasoactive amines and lysosomal enzymes
• Vascular Damage: Endothelial cell damage leads to increased vascular permeability, vasculitis, hemorrhage, and edema

Key Immunological Features

• IgA predominance: Immune complexes containing IgA and S. equi M-protein (SeM) are found in affected vessel walls
• Leukocytoclastic vasculitis: Characteristic histopathologic finding showing neutrophil infiltration and nuclear debris in vessel walls
• Cross-reactivity: Antibodies against S. equi M-protein may cross-react with proteins on endothelial cells

Clinical Signs and Physical Examination

Signalment and History

• Age: Can occur at any age; reported range 6 months to 19 years (mean 8.4 years)
• Breed: No specific breed predisposition
• History: Recent respiratory infection (2-4 weeks prior), strangles exposure, or recent vaccination
• Onset: Clinical signs typically appear within 2-4 weeks following infection (up to 3 months reported)

Classic Clinical Presentation

Complications and Severe Manifestations

Infarctive Purpura Hemorrhagica

A severe form characterized by infarction of multiple tissues including skeletal muscle, GI tract, pancreas, and lungs. Clinical signs include unrelenting colic-like pain and focal muscle swelling. This form has a high mortality rate (4 of 5 horses euthanized in one study).

Multi-organ Involvement:

• Gastrointestinal: Colic, GI hemorrhage, small intestinal intussusception (rare)
• Respiratory: Dyspnea from head swelling, lung involvement
• Renal: Glomerulonephritis (immune complex deposition)
• Muscular: Myositis, muscle infarction
• Secondary complications: Laminitis, thrombophlebitis, cellulitis, DIC (rare)

Exam Focus: When you see a question about a horse with marked limb and ventral edema plus petechiae on mucous membranes 2-3 weeks after a respiratory infection - think PURPURA HEMORRHAGICA first!

Diagnosis

Diagnostic Approach

Diagnosis is typically based on clinical signs, history of recent respiratory infection or vaccination, and supporting laboratory findings.

Differential Diagnoses

• Equine viral arteritis (EVA)
• African horse sickness (in endemic areas)
• Disseminated intravascular coagulation (DIC)
• Immune-mediated thrombocytopenia
• Vasculitis from other causes (drug reactions, photosensitization)
• Hendra virus (Australia)

Treatment

Primary Therapy: Corticosteroids

Immunosuppression with corticosteroids is the cornerstone of PH treatment.

Antibiotic Therapy

Indications:

• Active bacterial infection (residual S. equi nidus)
• High risk of secondary infection due to immunosuppression
• Skin sloughing with risk of cellulitis

Supportive Care

• NSAIDs: Phenylbutazone or flunixin meglumine for fever and pain relief
• Hydrotherapy: Cold water hosing of edematous limbs to reduce swelling
• Leg bandaging: Support wraps to reduce edema and prevent skin sloughing
• Fluid therapy: IV fluids for dehydration; monitor for protein loss
• Exercise: Mild hand-walking or paddock turnout to promote lymphatic drainage
• DMSO: IV DMSO (1 g/kg as 10% solution) as free-radical scavenger (anecdotal)
• Gastroprotectants: Consider with prolonged corticosteroid use

Prognosis

With early, aggressive treatment, prognosis is generally fair to good. One retrospective study reported 92% survival with appropriate therapy. However, prognosis worsens significantly with severe or infarctive forms, delayed treatment, or multi-organ involvement.

Prevention

Strangles Management

• Proper management during strangles outbreaks (isolation, biosecurity)
• Isolate infected horses for at least 1 month following infection
• Monitor exposed horses with twice-daily temperature checks
• Disinfection of fomites and good hygiene practices

Vaccination Considerations

CRITICAL: Horses with high SeM antibody titers are at INCREASED risk of developing PH following vaccination. Pre-vaccination titer testing is recommended for horses with recent strangles exposure.

PH = "PURPLE HORSE"

• P - Post-streptococcal (2-4 weeks after strangles)
• U - Urticarial-like edema (well-demarcated)
• R - Red spots (petechiae/ecchymoses on mucous membranes)
• P - Painful swelling of limbs, head, ventrum
• L - Leukocytoclastic vasculitis (histopathology)
• E - Elevated SeM titers (greater than 1:12,800)
• HORSE - High-dose steroids (dexamethasone 0.1-0.2 mg/kg)

"Type THREE = IgA TREES"

Remember that PH is a Type III hypersensitivity with IgA-containing immune complexes depositing in vessel walls like branches of a tree.

SeM Titer Rule of 3s:

• 1:3,200 = Do NOT vaccinate (3)
• 1:12,800 = Supports PH diagnosis (12 = 3 x 4)
• 2-4 weeks = Time from infection to PH onset