October 2019



Signalment (JPC # 1957368): 11 year old Friesian mare


HISTORY: Mass removed from right front limb.


MICROSCOPIC DESCRIPTION: Haired skin, right front limb: Markedly infiltrating the dermis and widely separating and surrounding remaining adnexa are multifocal to coalescing eosinophilic granulomas up to 1 mm in diameter which occasionally are centered on cross-sections of degenerate nematode larvae. The granulomas often 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. Occasionally, granulomas are centered on a larval nematode up to 60 um in diameter with a 1-2 um thick cuticle with regularly spaced ridges, coelomyarian-polymyarian musculature, and a digestive tract. Diffusely the surrounding dermis is infiltrated by numerous eosinophils and fewer lymphocytes and plasma cells, admixed with variably sized small caliber blood vessels that are lined by reactive endothelial cells and surrounded by perpendicularly oriented, hypertrophied fibroblasts (granulation tissue), all of which separate and surround collagen bundles, muscle fibers, and adnexa. Numerous collagen fibers are shrunken, hypereosinophilic, fragmented, or hyalinized and occasionally surrounded by degranulated eosinophils (flame figures). The epidermis is extensively ulcerated with replacement by a serocellular crust composed of many degenerate inflammatory cells, fibrin, edema, and necrotic debris with superficial mixed bacterial colonies and plant material.


MORPHOLOGIC DIAGNOSIS: Haired skin, right front limb: Eosinophilic granulomas, multifocal to coalescing, marked, with intralesional nematode larvae and extensive epidermal ulceration, granulation tissue, and serocellular crust with superficial mixed bacterial colonies, Friesian, equine.


ETIOLOGY: Habronema muscae, Habronema majus, or Draschia megastoma


ETIOLOGIC DIAGNOSIS: Cutaneous habronemiasis


CONDITION: Summer sores


CONDITION SYNONYMS: Granular dermatitis, bursatti, esponja



·      Habronemiasis is a common cause of ulcerative cutaneous and conjunctival granulomas in horses around the world

·      Common disease of horses, donkeys, and mules

·      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 H. microstoma), and Draschia megastoma are members of the order Spirurida, suborder Spirurina, and superfamily Habronematoidea



·      Adult nematodes inhabit the stomach of horses

·      Habronema muscae, H. majus– attach to the fundic mucosa near the margo plicatus

·      Draschia megastoma (D-P08) – found in nodules (brood pouches) adjacent to the margo plicatus

·      Life cycle: Nematode eggs and larvae (L1) passed in feces > ingested by fly maggots (Musca domestica for D. 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) > subsequent life cycle/symptoms depend on where larvae are deposited:

·      If deposited near the horse’s mouth > swallowed > to 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

·      Cutaneous and ocular lesions are due to hypersensitivity reaction to larvae within skin/mucous membranes



·      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; pruritus ranges from mild to severe; during early infection, pruritis leads to self-trauma/abrasion > formation of granulation tissue

·      Eyes:

·      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



·      Ulcerated, tumorous masses of red-brown granulation tissue with exudation and intermittent hemorrhage

·      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

·      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



·      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 are only found in 50% of cases

·      Larvae have smooth, 1-2 um thick cuticle, coelomyarian-polymyarian musculature, prominent lateral chords, and a digestive tract lined by cuboidal, uninucleate cells



·      History, clinical signs, location of lesions, presence of yellowish granules

·      Biopsy: Method of choice to confirm diagnosis



·      Habronemiasis may be superimposed on other conditions, especially those causing ulcers


For gross findings:

·      Granulomas: Bacterial, fungal

·      Pythiosis (I-F02): Gomori’s methenamine silver stain demonstrates causative agent; kunkers are branching unlike granules seen with habronemiasis

·      Botryomycosis (I-B02): Staphylococcus aureus; chronic granulomas - neck and pectoral region

·      Exuberant granulation tissue (“proud flesh”)

·      Squamous cell carcinoma (I-N04B)

·      Equine sarcoid (I-N17)


For microscopic lesions (eosinophilic nodular dermatitis):

·      Pythiosis (I-F02, Pythium insidiosum): Pale, 3-8 micron, irregular, branching hyphae with rare septa and non-parallel walls surrounded by eosinophilic and granulomatous inflammation

·      Eosinophilic granuloma (I-M11C)- withers, back, and lateral neck; normal overlying hair coat and skin; eosinophilic granulomas

·      Mast cell tumors (I-N21C): 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



·      There is one documented case in a dog, and one in a dromedary camel (Mauldin, Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals 2016)


1.    Bowman DD. Georgis’ Parasitology for Veterinarians. 10th ed. St. Louis, MO: Elsevier Saunders; 2014: 211-212.

2.    Foster RA. Male reproductive system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 1217-1218.

3.    Hargis AM, Myers S. The integument. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 1120.

4.    Labelle P. The eye. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017: 1310.

5.    Mauldin EA, Peters-Kennedy J. Integumentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 560-1, 685-686.

6.    Uzal FA, Plattner BL, Hostetter JM. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016: 209-210.

7.    Wilcock BP, Njaa BL. Special Senses. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 1. 6th ed. St. Louis, MO: Elsevier; 2016: 425-6.

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