JPC SYSTEMIC PATHOLOGY
Signalment (JPC # 1957368): Adult horse, age and breed unspecified
HISTORY: Horse with pruritic nodules ventral to medial canthus of each eye
MICROSCOPIC DESCRIPTION: Haired skin and subcutis: Markedly infiltrating the dermis and subcutis and extending into the panniculus carnosus, there are multifocal to coalescing eosinophilic granulomas up to 2 mm in diameter which contain rare cross-sections of nematode larvae. The granulomas frequently contain a core of eosinophilic necrotic debris mixed with many degenerate eosinophils surrounded by epithelioid macrophages and occasional multinucleated giant cells (foreign body type), further bounded by a layer of fibroblasts and fibrosis. Rarely, granulomas contain up to 60 um diameter nematode larvae with a smooth, 1-2 um thick cuticle, coelomyarian-polymyarian musculature, prominent lateral chords, and a digestive tract lined by cuboidal, uninucleate cells. Diffusely, in the surrounding subcutis and dermis, there are numerous eosinophils with fewer lymphocytes and plasma cells, admixed with variably sized small caliber blood vessels that are lined by reactive endothelial cells and surrounded by hypertrophied fibroblasts (granulation tissue), all of which separates and surrounds collagen bundles, muscle fibers and adnexa. Numerous collagen fibers are shrunken, hypereosinophilic, fragmented or hyalinized and occasionally surrounded by degranulated eosinophils (flame figures). Multifocally myocytes are atrophied, have vacuolated sarcoplasm (degeneration) or have hypereosinophilic sarcoplasm with pyknosis and loss of cross striations (necrosis). The epidermis is mildly hyperplastic and there is a focally extensive ulcer with replacement by many degenerate inflammatory cells, fibrin, edema and necrotic debris. .
MORPHOLOGIC DIAGNOSIS: Haired skin and subcutis: Eosinophilic granulomas, multiple, with severe diffuse eosinophilic dermatitis and panniculitis, ulceration, collagen flame figures, and rare spirurid nematode larvae, breed unspecified, equine.
ETIOLOGY: Habronema muscae, Habronema majus, Draschia megastoma
ETIOLOGIC DIAGNOSIS: Cutaneous habronemiasis
CONDITION: Summer sores
CONDITION SYNONYMS: Granular dermatitis, bursatti, esponja
- Habronemiasis is a common cause of ulcerative cutaneous and ocular granulomas in horses around the world
- No apparent age or gender predilection
- One report in 2003 states that Arabians are overrepresented and thoroughbreds are underrepresented; this may be due to the attraction of flies to light hair coat colors
- Usually seen in the summer and early fall when flies are active; often regresses partially or totally during the winter
- Habronema muscae, Habronema majus (formerly microstoma), and Draschia megastoma (formerly H. megastoma) are members of the order Spirurida, suborder Spirurina, and superfamily Habronematoidea
- Adult nematodes inhabit the stomach of horses
- Habronema muscae, majus– attach to the fundic mucosa near the margo plicatus
- Draschia megastoma – found in nodules (brood pouches) adjacent to the margo plicatus
- Nematode eggs and larvae (L1) passed in feces > ingested by fly maggots (Musca domestica for megastoma and H. muscae, Stomoxys calcitrans for H. majus)> mature to L3 in fly, migrate to fly labium > deposited onto horse through proboscis of host fly (in moist or traumatized areas where flies feed and can penetrate intact skin)
- Where nematode larvae end up:
- If deposited near the horse’s mouth > swallowed > stomach > complete life cycle
- If deposited on the nose > may migrate to lungs
- If deposited on other areas of body > migrate locally within the skin and cause granulomas
- Exact pathogenesis unknown
- Thought to involve a hypersensitivity reaction to dead or dying larvae since it is seasonal, sporadic and often recurs in the same horses every summer
TYPICAL CLINICAL FINDINGS
- The location of the lesions is in the moist exposed areas of the body that attract flies
- Most common sites are the medial canthus of the eye, the glans penis and prepuce, and any cutaneous wound
- Lesions are also common on the distal extremities because this is a common site for lacerations
- Lesions are consistently pruritic and pruritus ranges from mild to severe; during early infection leads to abrasion and formation of granulation tissue
- Persistent conjunctivitis with ulcerated, proliferative nodules (usually not exceeding 2 cm; located on the third eyelid and near the medial canthus); profuse lacrimation; photophobia; chemosis; corneal abrasion and eyelid inflammation
- Lesions 2 to 3 cm below the medial canthus are characteristic of lacrimal duct involvement
- Lung: Pulmonary habronemiasis uncommon; usually asymptomatic
- Stomach: Gastric habronemiasis usually asymptomatic
TYPICAL GROSS FINDINGS
- Ulcerated, tumorous masses of red-brown granulation tissue
- Lesions may be single or multiple, 5 to 15 cm in diameter and 0.5 to 1.5 cm in depth
- Onset: Rapid development of papules, or failure of wound to heal with development of exuberant granulation tissue
- Ulceration, exudation, intermittent hemorrhage
- On cut section – multifocal, small (1 to 5 mm), caseous to gritty, yellow to white, non-branching granules (necrotic, caseous or calcified material surrounding nematode larvae)
- Lungs: Multiple granulomas with central caseous necrosis in interstitial and peribronchial areas
TYPICAL MICROSCOPIC FINDINGS
- Nodular to diffuse eosinophilic dermatitis +/- eosinophilic granulomas
- Multifocal areas of necrosis +/– degenerating nematode larvae in center
- Epithelioid macrophages, multinucleated giant cells and degenerative eosinophils often surround degenerating larvae which may be mineralized
- Fibrous connective tissue diffusely infiltrated with large numbers of eosinophils with fewer mast cells, lymphocytes and plasma cells
- Larvae may be rare or absent in many caseous foci
- Larvae have smooth, 1-2 um thick cuticle, coelomyarian-polymyarian musculature, prominent lateral chords, and a digestive tract lined by cuboidal, uninucleate cells
ADDITIONAL DIAGNOSTIC TESTS
- History, clinical signs, location of lesions, presence of yellowish granules
- Biopsy: Method of choice to confirm diagnosis
- Habronemiasis may be superimposed on other conditions
For gross findings:
- Granulomas: Bacterial, fungal
- Pythiosis: Gomori’s methenamine silver stain demonstrates causative agent; kunkers are branching unlike granules seen with habronemiasis
- Botryomycosis: Staphylococcus aureus; chronic granulomas - neck and pectoral region
- Exuberant granulation tissue
- Squamous cell carcinoma
- Equine sarcoid
For microscopic lesions (eosinophilic nodular dermatitis):
- Pythiosis (Pythium insidiosum): Pale 3-8 micron irregular branching hyphae with rare septae and non-parallel walls surrounded by eosinophilic and granulomatous inflammation
- Equine nodular collagenolytic granuloma (nodular necrobiosis) - withers, back, and lateral neck; normal overlying hair coat and skin; eosinophilic granulomas
- Mast cell tumors: Nodules and/or sheets of well-differentiated mast cells often surrounding eosinophilic granulomas; can be difficult to distinguish as they may be primarily composed of eosinophils with few mast cells
- Cutaneous habronemiasis occurs in donkeys, mules, zebras, and there is one documented case in a dog, and one in a dromedary camel
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