NAVLE Integumentary

Equine Cutaneous Habronemiasis – NAVLE Study Guide

📅 March 28, 2026 ⏱ 25 min read

Overview and Clinical Importance

Cutaneous habronemiasis (also known as "summer sores," "bursatti," "swamp cancer," or "granular dermatitis") is a seasonally-occurring parasitic skin disease of equids caused by aberrant larval migration of spirurid stomach worms in the genera Habronema and Draschia. The condition represents a hypersensitivity reaction to larvae deposited by flies on wounds or moist body surfaces. Understanding this disease is essential for the NAVLE as it tests knowledge of parasitology, dermatology, and treatment protocols.

This condition is distributed worldwide, predominantly in tropical and subtropical regions, but also occurs in temperate climates during warm months. It causes significant economic losses in sport and working horses due to aesthetic concerns, performance impairment, and prolonged treatment requirements.

Species	Fly Vector	Adult Location	Key Features
Habronema muscae	Musca domestica (house fly)	Gastric mucosa surface	Most common species; yellow-orange color; no pharyngeal teeth
Habronema microstoma	Stomoxys calcitrans (stable fly)	Gastric mucosa surface	Whitish color; has pharyngeal teeth; also called H. majus
Draschia megastoma	Musca domestica (house fly)	Fibrous nodules in gastric submucosa	Can cause granulomatous gastritis; may perforate stomach wall

Etiology

Causative Agents

Three species of spirurid nematodes (Superfamily Habronematoidea) cause habronemiasis in equids:

High-YieldRemember the fly-parasite associations: House fly (Musca domestica) transmits H. muscae and D. megastoma; Stable fly (Stomoxys calcitrans) transmits H. microstoma. The mnemonic "House = Muscae" helps remember this key association.

Life Cycle

Understanding the life cycle is critical for both diagnosis and prevention. The life cycle involves an indirect transmission pattern requiring muscid flies as intermediate hosts.

Normal Life Cycle (Gastric Form)

Adult worms reside in gastric mucosa - Adults live in the stomach wall (glandular mucosa near margo plicatus) where they mate
Egg production and passage - Females produce elongated, thin-shelled, larvated eggs (40-80 μm) that pass in feces
Intermediate host ingestion - Fly maggots breeding in manure ingest eggs/L1 larvae
Larval development in fly - Larvae develop synchronously with fly development; L3 (infective) larvae migrate to fly mouthparts as fly matures (approximately 2 weeks)
Transmission to horse - Adult flies deposit L3 larvae around horse's lips while feeding; larvae are swallowed
Gastric maturation - Larvae mature to adults in stomach (prepatent period approximately 2 months)

Aberrant Life Cycle (Cutaneous Form)

When flies deposit L3 larvae on wounds, moist areas (eyes, genitalia, nostrils), or damaged skin instead of the lips, larvae cannot complete their life cycle. These aberrant larvae migrate into tissues, causing an intense hypersensitivity reaction characterized by granulomatous inflammation. Larvae survive less than one month in cutaneous tissues but trigger persistent inflammatory responses even after death.

NAVLE TipThe NAVLE frequently tests understanding that cutaneous habronemiasis is a DEAD-END for the parasite - larvae cannot mature to adults outside the stomach. This explains why the condition is self-limiting in winter when fly activity ceases.

Factor	Clinical Significance
Seasonality	Peak incidence in spring/summer during fly activity; lesions may regress in winter but recur annually
Breed Predisposition	Arabians overrepresented; gray horses and those with diluted coat colors (palomino, buckskin, dun) more susceptible; Thoroughbreds underrepresented
Individual Susceptibility	Some horses develop annual recurrence while others on same premises never affected; suggests genetic/immunologic predisposition
Pre-existing Wounds	Any wound, especially on distal limbs, attracts flies and provides entry point for larvae
Moist Areas	Medial canthus, genitalia, and areas with excessive secretions attract flies seeking moisture

Clinical Presentation

Predisposing Factors

Clinical Forms and Lesion Characteristics

Cutaneous Habronemiasis (Summer Sores)

Most common locations: Distal limbs (especially fetlocks), ventral abdomen, prepuce/penis, commissures of lips, and any wound site

Lesion characteristics:

Rapidly growing, proliferative, ulcerative granulomatous masses
Exuberant granulation tissue resembling "proud flesh"
Pathognomonic finding: Yellow, gritty, rice-grain sized calcified granules ("sulfur granules") within lesion - these are dead/calcified larvae
Serosanguineous to hemorrhagic discharge
Variable pruritus (mild to severe)
Non-healing wounds that fail to respond to standard wound care

Ocular/Periocular Habronemiasis

Most common location: Medial canthus of the eye (attracted by lacrimal secretions)

Clinical signs:

Conjunctivitis and blepharitis with granulomatous nodules
Epiphora (excessive tearing)
Blepharospasm and photophobia
Chemosis (conjunctival edema)
Mucopurulent discharge
Secondary corneal abrasions common

Genital Habronemiasis

Affected structures: Urethral process, glans penis, prepuce (males); vaginal fornix (females)

Clinical signs:

Thick, firm, irregularly shaped masses ("Kunkurs")
Dysuria and frequent urination
Urine spraying (especially if urethra affected)
Lesions bleed readily on manipulation due to highly vascular granulation tissue

Seasonal (warm months)

Ulcerative granulomas

Moist areas affected

Medial canthus involvement

Exuberant granulation tissue

Recurs annually in susceptible horses

Sulfur granules pathognomonic

Ocular and genital predilection

Resolution in winter

Eosinophilic dermatitis on histopathology

Method	Findings	Limitations
Impression Smears/Cytology	High numbers of eosinophils; may detect sulfur granules or motile larvae (3mm x 60μm with spiny tail)	Larvae may be absent or necrotic; eosinophilia is non-specific
Deep Scrapings	May reveal larvae within tissue, especially from granule areas	Larvae only survive less than 1 month in skin; often negative despite active disease
Histopathology (Gold Standard)	Eosinophilic dermatitis; coagulative necrosis; palisading granulomas; mast cell infiltration; cross-sections of larvae (50μm diameter with spiny cuticle)	Larvae seen in only 44% of biopsies; must biopsy deep enough to rule out underlying neoplasia
PCR (Molecular)	Detects Habronema DNA with 97% sensitivity and 100% specificity; can identify species	Not widely commercially available; research tool

Diagnosis

Clinical Diagnosis

Presumptive diagnosis is often made based on characteristic clinical appearance combined with: seasonal occurrence during fly season, typical lesion locations, presence of sulfur granules, and failure to heal with standard wound management.

Diagnostic Methods

Key Histopathologic Findings

Nodular to diffuse granulomatous dermatitis
Dense eosinophilic infiltration - hallmark finding
Coagulative necrosis with "flame figures" (hypereosinophilic collagen fibers)
Mast cell and macrophage infiltration
Palisading granuloma formation around necrotic foci
Larvae characteristics when visible: 50μm diameter, external cuticle with spines, coelomyarian musculature, muscular esophagus

High-YieldDead larvae cause MORE inflammation and calcification than living parasites. This explains why lesions may initially worsen after anthelmintic treatment as larvae die and trigger intensified immune response.

Differential Diagnosis

CRITICAL: Several conditions mimic habronemiasis and must be differentiated, especially since habronemiasis can occur concurrently with other conditions including sarcoids and squamous cell carcinoma.

NAVLE TipWhen you see a non-healing granulomatous wound in the southern US, ALWAYS consider pythiosis in addition to habronemiasis. Key differentiator: Pythiosis kunkers are LARGER (coral-like masses) and the prognosis is much WORSE. Habronemiasis sulfur granules are rice-grain sized and prognosis is generally GOOD.

Condition	Key Differentiating Features	Diagnostic Approach
Pythiosis (Swamp Cancer)	LARGER kunkers (coral-like masses); more invasive; occurs in horses exposed to stagnant water; POOR PROGNOSIS; more common in Gulf Coast states	GMS stain for fungal hyphae; ELISA serology; culture; histology shows eosinophilic granulomas with broad, rarely septate hyphae
Equine Sarcoid	Non-seasonal; multiple forms (verrucous, nodular, occult, fibroblastic); associated with BPV-1/2; fibroblastic form most similar to habronemiasis	Biopsy (may worsen sarcoid); histology shows fibroblast proliferation without eosinophils; no sulfur granules
Squamous Cell Carcinoma	Non-seasonal; older horses; affects mucocutaneous junctions (eye, genitalia); unpigmented skin predisposed; may have concurrent habronemiasis	Deep biopsy essential; histology shows neoplastic epithelial cells, keratin pearls, invasion
Exuberant Granulation Tissue (Proud Flesh)	History of wound; no sulfur granules; not seasonal; responds to debridement and pressure bandaging	Clinical history and response to standard wound care; biopsy if non-responsive
Onchocerciasis	Ventral midline dermatitis pattern; alopecia, scaling, depigmentation; caused by Onchocerca cervicalis microfilariae	Skin snip biopsy; response to ivermectin (may temporarily worsen)

Treatment

Treatment of cutaneous habronemiasis requires a multimodal approach targeting: parasite elimination, inflammation control, wound management, and prevention of reinfection.

Treatment Protocol

Treatment Goals

Eliminate adults in stomach - reduces environmental contamination
Kill larvae in lesions - anthelmintics target larvae systemically and topically
Control inflammation - corticosteroids address hypersensitivity reaction
Debulk lesion - surgical removal of excessive granulation tissue
Prevent reinfection - fly control and wound protection

High-YieldLesions may TEMPORARILY WORSEN after ivermectin treatment due to inflammatory reaction against dying larvae. This is expected and patients should be pre-treated or concurrently treated with corticosteroids to manage this reaction.

Prognosis

The prognosis for cutaneous habronemiasis is generally GOOD with appropriate treatment. Key prognostic factors include:

Lesion location: Genital lesions are most difficult to treat and may require surgical intervention
Lesion size: Smaller lesions respond better to medical management
Chronicity: Chronic lesions may require more aggressive treatment
Recurrence: Susceptible horses often develop lesions annually; owners should be counseled about preventive measures
Healing time: Most lesions heal within a few weeks with appropriate treatment

Treatment Component	Medications/Methods	Notes
Systemic Anthelmintics	Ivermectin 200 μg/kg PO once (may repeat in 2 weeks); OR Moxidectin 400 μg/kg PO once	Kills adults in stomach (reduces transmission); may temporarily WORSEN lesions as larvae die
Systemic Corticosteroids	Prednisolone 0.5-1 mg/kg PO daily for 10-14 days, then taper over 2 weeks	Controls hypersensitivity reaction; reduces inflammation from dying larvae; monitor for laminitis
Topical Treatment	Compounded mixture of: Corticosteroid + DMSO + Ivermectin/moxidectin ± antibiotic; Apply daily	DMSO enhances penetration; must keep wound covered to prevent fly access
Surgical Debridement	Remove granulation tissue and debulk lesion; may require general anesthesia for large lesions	Indicated for large or refractory lesions; genital lesions may require amputation of urethral process
Ocular Treatment	Echothiophate 0.03% drops BID (larvicidal); Dexamethasone/antibiotic ophthalmic ointment; Subconjunctival steroids for severe cases	Granulomas may require surgical excision/curettage

Prevention and Control

Prevention focuses on breaking the life cycle at multiple points:

Fly Control (Most Important)

Regular removal and composting of manure away from horses
Environmental insecticides and barn spray systems
Fly predators for biological control
Fly masks (especially for horses with periocular involvement)
Fly sheets and leg wraps
Topical fly repellents on horses

Wound Management

Prompt treatment and covering of all wounds
Apply fly-repellent wound dressings
Keep wounds covered with bandages when possible

Anthelmintic Programs

Regular deworming with ivermectin or moxidectin kills adult stomach worms
Consider additional treatments during fly season for susceptible horses
Reduces larval contamination of environment through feces

NAVLE TipThe key to preventing habronemiasis recurrence is FLY CONTROL. The mnemonic "FLIES" helps remember: Fly masks/sheets, Larvicides in manure, Insecticides in environment, Early wound treatment, Strategic deworming.

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Equine Cutaneous Habronemiasis &ndash; NAVLE Study Guide