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Description
Cutaneous Habronemiasis is a hypersensitivity reaction to the larvae of the nematode species Habronema and Draschia. The adult nematode lives in the stomach of the horse and produces eggs which are then passed in the faeces and hatch into larvae. The larvae are ingested by the house fly (Musca domestica) or stable fly (Stomoxys calcitrans) in their larval phase, and deposited on equine skin when they feed as adult flies. The condition is characterised by ulcerative, granulomatous skin lesions which do not heal.
Signalment
The condition occurs most commonly in warm wet climates such as the tropics and sub tropics but is also a common skin condition in the UK. It affects moist regions of the body and open wounds; it typically has a seasonal distribution with partial or full resolution during the winter.
Diagnosis
Clinical signs
Lesions most commonly occur on the legs, ventrum, around the eyes, urethral process and prepuce and at the site of pre-existing wounds. The lesions are single or multiple ulcerative nodules with a serosanguinous discharge. There may be excessive granulation tissue and small yellow granules of necrotic debris which surround the larvae.
Pruritus may be mild to severe.
In cases of conjunctival habronemiasis there will be gitty plaques on the conjunctival and palpebral borders, which is painful in severe cases leading to epiphora and blephrospasm. Conjunctival habronemiasis is fairly common and must be differentiated from Squamous cell carcinoma of the third eyelid.
Cytology
Cytology of the exudate or skin scrapings frequently reveal larvae but biopsy is recommended because Habronema larvae commonly invade skin lesions with another underlying cause such as infectious granulomas, sarcoids and ulcerative dermatitis.
Biopsy
Biopsy will reveal nodular/diffuse granulomatous dermatitis with eosinophilic infiltrates and mast cells. Necrotic foci with multiple nemotode larvae, and granulation tissue with neutrophils along the ulcerated surface are also characteristic.
Treatment
Systemic glucocorticoids decrease the inflammatory response of the the hypersensitivity reaction, oral prednisolone is effective at a dose of 1mg/kg SID or BID for 14days and then tapered.
Organophosphate pastes and anti-inflammatory pastes can be applied to the wound until it has healed, or the wound should be bandaged to prevent reinfestation.
Excessive granulation tissue should be removed surgically.
The conjunctival form can be treated with topical ivermectin eye drops with a pH of 7.4.
Ivermectin or appropriate anthelmintics should be used to limit nematode infestation, fly control will also reduce exposure to the larvae.
Prognosis
Recurrence is common. Fly control, meticulus wound management and regular anthelmintic use is needed to prevent or limit the condition.
References
- Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool
- Rose, R. J. and Hodgson, D. R. (2000) Manual of Equine Practice (Second Edition) Sauders.
| Also known as: | Summer sores Granular dermatitis Bursautee |