NAVLE Hemic and Lymphatic

Equine Ulcerative Lymphangitis Study Guide

📅 March 28, 2026 ⏱ 25 min read

Overview and Clinical Importance

Ulcerative lymphangitis is a chronic, suppurative infection of the cutaneous lymphatic vessels in horses, primarily caused by Corynebacterium pseudotuberculosis (biovar equi). This condition is one of three clinical manifestations of C. pseudotuberculosis infection in horses, alongside external abscesses ("pigeon fever") and internal abscesses. Although ulcerative lymphangitis represents only approximately 1-2% of C. pseudotuberculosis infections, it is clinically significant due to its potential for chronic progression, residual limb damage, and significant treatment challenges.

The disease occurs worldwide but is most prevalent in the western and southwestern United States, where environmental conditions favor bacterial survival. Peak incidence occurs during summer and fall months when fly populations are highest, as insect vectors play a crucial role in disease transmission. Understanding ulcerative lymphangitis is essential for the NAVLE, as questions frequently address differential diagnosis, treatment protocols, and the distinction between sporadic, ulcerative, and epizootic forms of lymphangitis.

High-YieldRemember that C. pseudotuberculosis causes THREE distinct syndromes in horses: external abscesses (91% of cases), internal abscesses (8%), and ulcerative lymphangitis (1%). Board questions may test your ability to differentiate these presentations.

Characteristic	Details
Gram Stain	Gram-positive
Morphology	Pleomorphic rod (club-shaped)
Oxygen Requirement	Facultative anaerobe
Biovar	Equi (nitrate-positive) - affects horses and cattle; Ovis (nitrate-negative) - affects sheep and goats
Cell Wall Feature	High lipid content (corynomycolic acid) - facilitates intracellular survival
Virulence Factor	Phospholipase D exotoxin - damages cell membranes, promotes vascular permeability
Environmental Survival	Can survive in soil, feces, hay, and shavings for extended periods (months)

Anatomy of the Equine Lymphatic System

The equine lymphatic system is remarkably extensive, containing approximately 8,000 lymph nodes compared to only 600 in humans. This system functions as a secondary circulatory system, running parallel to the cardiovascular system. Unlike the blood circulatory system, the lymphatic system has no central pump and relies on skeletal muscle contraction, breathing movements, and intrinsic vessel contractility to propel lymph fluid.

Key Anatomical Points

Lymph vessels: Thin-walled vessels with smooth muscle lining that can contract to propel lymph; contain one-way valves to prevent backflow
Lymph nodes: Filter lymph fluid, removing pathogens and debris; become enlarged during infection
Thoracic ducts: Final collection point where filtered lymph returns to the bloodstream via subclavian veins
Distal limb anatomy: No skeletal muscle below the knee/hock means the lymphatic system in these areas is particularly dependent on movement for drainage

High-YieldThe absence of skeletal muscle below the knee/hock makes horses particularly vulnerable to lymphatic dysfunction in the distal limbs. This explains why ulcerative lymphangitis most commonly affects the lower limbs and why movement is essential for treatment.

Sign	Description
Limb Swelling	Painful edematous swelling, often affecting entire limb; initially pitting, progresses to firm; "tree-trunk leg" or "big-leg" appearance
Nodules	Multiple subcutaneous nodules along lymphatic chains; firm initially, become fluctuant before rupturing
Lymphatic Cording	Thickened, cord-like lymphatic vessels palpable under the skin; pathognomonic finding
Ulcers	Form when nodules rupture; discharge thick, tan/cream, blood-tinged, ODORLESS exudate (greenish exudate suggests mixed infection)
Lameness	Variable severity; may be severe enough to mimic fracture; often non-weight-bearing initially
Serum Oozing	Clear to yellowish lymph fluid leaks through skin due to pressure; may dry as crusty scabs
Systemic Signs	Fever (often 104-106°F/40-41°C), depression, anorexia, weight loss in severe cases
Location	Most commonly distal hindlimbs (fetlock to hock); can extend proximally; usually unilateral initially

Etiology

Primary Causative Agent

Corynebacterium pseudotuberculosis biovar equi is the primary causative agent of ulcerative lymphangitis. This organism is a gram-positive, pleomorphic, rod-shaped, facultatively anaerobic, intracellular bacterium with a high lipid content in its cell wall (particularly corynomycolic acid), which facilitates survival within macrophages and contributes to chronic infection.

Organism Characteristics

Other Causative Organisms

While C. pseudotuberculosis is the primary agent, other organisms can cause or contribute to ulcerative lymphangitis:

Staphylococcus aureus - common skin commensal that can opportunistically infect wounds
Streptococcus spp. - particularly S. equi subspecies zooepidemicus
Rhodococcus equi - another intracellular pathogen
Pseudomonas aeruginosa - associated with greenish discharge
Mixed bacterial infections are common

Test	Method/Findings	Clinical Significance
Bacterial Culture	Aspirate from intact abscesses or swab of exudate; aerobic culture on blood agar; pinpoint whitish colonies with weak hemolysis at 24-48 hours	GOLD STANDARD for definitive diagnosis; may be negative due to intracellular location or prior antibiotic use
SHI Test	Synergistic Hemolysis Inhibition test; detects IgG antibodies to phospholipase D exotoxin; titers 1:4 to 1:4096	Titers greater than or equal to 1:512 suggest active infection; greater than or equal to 1:128 indicates exposure; most useful for internal abscesses
CBC	Inflammatory leukogram: leukocytosis, neutrophilia, hyperfibrinogenemia; anemia in chronic cases	Supports diagnosis of infection; helps monitor treatment response
Chemistry	Hyperglobulinemia, elevated SAA (serum amyloid A)	Indicates chronic inflammation; SAA useful for monitoring response
Cytology	Impression smears or aspirates; Gram stain shows gram-positive pleomorphic rods; pyogranulomatous inflammation	Quick presumptive diagnosis; organism may be difficult to find
Skin Biopsy	Superficial to deep perivascular dermatitis; pyogranulomatous or suppurative inflammation; edema and fibrosis	Helps rule out differentials; tissue can be cultured
Ultrasound	Defines abscess boundaries; identifies deep abscesses; guides aspiration/drainage	Essential for treatment planning; rules out tendinitis/fracture differentials

Pathophysiology

Route of Infection

The organism enters the body through skin wounds, which may be very minor (abrasions, insect bites, injection sites). Transmission occurs via:

Insect vectors: House flies (Musca domestica), stable flies (Stomoxys calcitrans), and horn flies (Haematobia irritans) act as mechanical vectors
Direct contact: Contaminated soil, fomites (tack, grooming equipment, bedding)
Wound contamination: Entry through pre-existing wounds in conditions of poor hygiene

Disease Progression

Initial Invasion: Bacteria enter through skin wounds and establish infection in the subcutaneous tissue and lymphatic vessels
Intracellular Survival: C. pseudotuberculosis survives and replicates within macrophages due to its high lipid cell wall content
Lymphatic Spread: Organism spreads along lymphatic vessels, causing inflammation and obstruction
Nodule Formation: Pyogranulomatous nodules develop along lymphatic chains, creating characteristic "corded" appearance
Ulceration: Nodules rupture and form ulcers that discharge thick, tan, blood-tinged, odorless exudate
Lymphatic Obstruction: Chronic inflammation leads to lymphatic vessel damage, fibrosis, and impaired drainage causing persistent limb edema

NAVLE TipThe intracellular location of C. pseudotuberculosis is crucial for understanding treatment challenges. Standard antibiotics may show in vitro susceptibility but fail in vivo because they cannot penetrate macrophages. This is why rifampin (excellent intracellular penetration) is often added to treatment protocols.

Condition	Key Features	Differentiating Points
Sporadic Lymphangitis	Acute onset limb swelling; often after period of inactivity; "Monday morning leg"	NO nodules or ulcers; resolves with exercise; culture often negative; no lymphatic cording
Cellulitis	Acute diffuse soft tissue swelling; extreme pain; often following wound	Lymphatic vessels NOT typically damaged; no nodule formation; responds to standard antibiotics
Epizootic Lymphangitis	Fungal disease (Histoplasma farciminosum); NOTIFIABLE; affects neck, limbs, chest	Geographic (Africa, Middle East, Asia); honey-like discharge; fungal culture positive; NOT present in US/UK
Glanders (Farcy)	Burkholderia mallei; NOTIFIABLE; nodules along lymphatics; nasal involvement common	EXOTIC to US/UK; respiratory signs; honey-like exudate; Mallein test positive; REPORTABLE
Sporotrichosis	Sporothrix schenckii fungus; nodules along lymphatics; chronic ulceration	Non-contagious; cigar-shaped yeast on cytology; responds to iodides; PAS/GMS staining helpful
Strangles	Streptococcus equi subsp. equi; lymph node abscessation of head/neck	Affects submandibular/retropharyngeal nodes; NOT limbs; respiratory signs; highly contagious
Pythiosis	Pythium insidiosum (oomycete); "kunkers" in wounds; granulation tissue	Associated with standing water; characteristic yellow-gray granules; pruritic; Southeast US

Clinical Signs

Ulcerative lymphangitis has a variable and often insidious onset. The disease typically affects the distal limbs, most commonly the hindlimbs, and usually involves only one limb initially.

Classic Clinical Presentation

High-YieldThe exudate in ulcerative lymphangitis is characteristically ODORLESS and tan/cream colored. A greenish, malodorous discharge suggests mixed infection with Pseudomonas or other bacteria. This is an important differentiating feature on board exams.

Drug	Dose	Route/Frequency	Notes
Rifampin	2.5-5 mg/kg	PO q12h	EXCELLENT intracellular penetration; ALWAYS combine with another antibiotic (resistance develops rapidly if used alone); orange urine normal
TMS	30 mg/kg (combined)	PO q12h	Good first-line oral option; combine with rifampin for best results; achieves intracellular concentrations
Penicillin G (Procaine)	20,000-22,000 IU/kg	IM q12h	Good initial therapy; may switch to oral maintenance; C. pseudotuberculosis typically susceptible
Ceftiofur	2.5-5 mg/kg	IV/IM q12h	Often combined with rifampin for initial IV therapy; switch to oral after improvement
Doxycycline	10 mg/kg	PO q12h	Alternative oral option; good tissue penetration; MIC90 less than or equal to 2 mcg/mL
Enrofloxacin	7.5 mg/kg	PO q24h	Good intracellular penetration; excellent in vitro activity (MIC90 less than or equal to 0.25 mcg/mL)

Diagnosis

Clinical Diagnosis

Diagnosis is based on clinical signs, history, and geographic location. Classic findings include the combination of limb swelling, lymphatic cording, nodule formation along lymphatic chains, and ulceration with characteristic discharge.

Laboratory Diagnostics

NAVLE TipThe SHI (Synergistic Hemolysis Inhibition) test is most useful for diagnosing INTERNAL C. pseudotuberculosis abscesses, not external abscesses or ulcerative lymphangitis. For ulcerative lymphangitis, culture remains the gold standard, but may be negative in up to 40% of cases due to the intracellular nature of the organism. Never rely on serology alone!

Differential Diagnosis

It is critical to differentiate ulcerative lymphangitis from other causes of limb swelling and lymphangitis. The differential diagnosis includes:

High-YieldGLANDERS and EPIZOOTIC LYMPHANGITIS are NOTIFIABLE/REPORTABLE diseases! Glanders is EXOTIC to the United States and UK. If you suspect either condition, immediately contact state/federal animal health officials. Treatment of glanders is contraindicated as it creates carrier animals.

Treatment

Treatment of ulcerative lymphangitis requires early, aggressive, and prolonged therapy to prevent chronic complications. Unlike external C. pseudotuberculosis abscesses where antibiotics may be contraindicated, ulcerative lymphangitis ALWAYS requires antimicrobial treatment.

Antimicrobial Therapy

Treatment Protocol

Initial Phase: IV antibiotics (ceftiofur or penicillin G) combined with rifampin (PO) until lameness and swelling improve
Maintenance Phase: Oral antibiotics (TMS + rifampin or doxycycline + rifampin) to prevent relapse
Duration: Minimum 30 days; often 6-12 weeks; median duration 36 days in studies
Monitoring: Serial CBC, fibrinogen, and SAA to monitor response; continue until inflammation resolves

Supportive Care

NSAIDs: Flunixin meglumine (1.1 mg/kg IV/PO q12-24h) or phenylbutazone (2.2-4.4 mg/kg PO q12h) for pain and inflammation
Corticosteroids: Short-term dexamethasone may be used in severe cases to reduce swelling; use with caution (immunosuppression, laminitis risk)
Hydrotherapy: Cold hosing or equine spa to reduce swelling and provide pain relief
Bandaging: Thick padded bandages (cotton wool/wadding) with compression; may push swelling proximally if applied incorrectly
Exercise: Controlled walking essential to promote lymphatic drainage; hand walking or turnout in small paddock
Wound Care: Daily antiseptic scrubbing with iodine-based shampoo; lance and flush abscesses; hot packing immature abscesses
Iodide Therapy: Sodium iodide IV or potassium iodide PO historically used; may help with dermatitis component

High-YieldRIFAMPIN must NEVER be used as monotherapy because resistance develops rapidly. Always combine with another antibiotic (TMS, penicillin, ceftiofur, or doxycycline). The rationale for rifampin is its excellent intracellular penetration, which is essential because C. pseudotuberculosis survives within macrophages.

Prognosis

The prognosis for ulcerative lymphangitis is GUARDED, depending on the promptness and aggressiveness of treatment.

Favorable factors: Early recognition, prompt aggressive treatment, single limb involvement, young horses
Unfavorable factors: Delayed treatment, chronic disease, multiple limb involvement, concurrent internal abscesses
Complications: Permanent lymphatic damage, chronic limb swelling ("filled legs"), fibrosis, recurrence predisposition
Mortality: Low for ulcerative lymphangitis alone (less than 5%); internal abscesses have 30-40% mortality

Horses that recover often have some residual lymphatic damage and may develop "stocking up" (passive edema) when stabled. These horses are predisposed to recurrence and may never return to their original limb contour.

Prevention and Control

Fly control: Comprehensive fly management including insect growth regulators, fly sheets, fly spray, manure management
Wound care: Prompt treatment of all wounds, no matter how minor; clean with antiseptic and apply barrier protection
Hygiene: Clean stabling, avoid wet/muddy conditions, regular grooming, clean tack and equipment
Isolation: Separate infected horses; abscess drainage is a major source of environmental contamination
Fomite control: Do not share grooming equipment, tack, or buckets between horses
Vaccination: A conditionally licensed bacterin/toxoid is available in the US; efficacy data limited
Exercise: Regular daily movement promotes healthy lymphatic function

Memory Aids

"CORD" Mnemonic for Clinical Signs

C - Cording of lymphatic vessels (pathognomonic)

O - Odorless exudate (tan/cream colored)

R - Rupturing nodules along lymphatic chains

D - Distal limb affected (usually hindlimb)

"PIGEON" Mnemonic for C. pseudotuberculosis Syndromes

P - Pectoral abscesses (external - 91% of cases)

I - Internal abscesses (8% - liver, spleen, kidney)

G - Gram-positive intracellular rod

E - Equi biovar (nitrate-positive)

O - One percent = ulcerative lymphangitis (rare form)

N - Need rifampin (intracellular penetration)

NAVLE TipRemember "RIF-NEVER-ALONE" - Rifampin should NEVER be used as monotherapy due to rapid resistance development. Always combine with another antibiotic such as TMS, penicillin, or ceftiofur for treating C. pseudotuberculosis infections.

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