DIAGNOSIS

The primary differential diagnoses for persistent ulcerated lesions such as habronemiasis are squamous cell carcinoma, fibroblastic tumors (especially sarcoid), and melanocytoma. The differential diagnosis for nonulcerated nodular lesions is long and includes cutaneous lymphoma, melanocytic tumors, and benign epithelial tumors as well as a wide array of non-neoplastic disorders including eosinophilic granuloma, fungal granuloma, granuloma secondary to other causes, cyst, sterile panniculitis, and amyloidosis. In some instances, the age of the horse, location of the lesion, external appearance of the lesion, and season of occurrence can aid in diagnosis (Table 149-2). It is vital that this information be included in the history accompanying samples submitted for histologic assessment, as this will greatly aid the pathologist interpreting the sections. Cytologic evaluation of these lesions is not often diagnostic. In almost all cases, final diagnosis will rely on results of histologic evaluation. In instances involving multiple lesions, it is important to excise and submit more than one lesion, as it is possible for a horse to have cutaneous nodular lesions from more than one cause.

Table 149-2 Characteristic Features of Selected Equine Non-neoplastic Nodular and Proliferative Skin Lesions

EXUBERANT GRANULATION TISSUE

Exuberant granulation tissue, so-called proud flesh, has no age, breed, or sex predisposition, and most commonly develops in wounds involving the lower portions of the limbs (Figure 149-1). This site predilection enables a tentative diagnosis to be made on the basis of history and physical examination findings in many instances. Exuberant granulation tissue is defined as cutaneous proliferative growth composed entirely of granulation tissue with no other underlying lesion. In the horse, sarcoid is a primary differential diagnosis. A prior history of a wound in the area is very useful in diagnosis of exuberant granulation tissue but is not available in all instances, and sarcoid can also develop at wound sites. Care must be taken to include deep areas during biopsy of any ulcerated proliferative lesion because this type of lesion, especially sarcoid, can include large zones of granulation tissue beneath the ulcerated surface. Exuberant granulation tissue was the most common non-neoplastic proliferative disorder diagnosed in donkeys in the Oregon State University survey.

Figure 149-1 Exuberant granulation tissue (“proud flesh”) associated with a wound of the lower portion of the limb. (Photograph from the files of Dr. Erwin Pearson, Oregon State University.)

The choice of treatment should take into account the size, location, and duration of the wound. New granulation tissue that does not extend beyond the wound margins can often be managed with judicious use of bandaging. With more extensive granulation tissue, surgical excision is indicated. This helps to decrease inflammation, which contributes significantly to the formation of more granulation tissue. Surgical resection must often be repeated. Ongoing inflammation caused by underlying musculoskeletal trauma must be investigated and controlled. Repair of injuries to deeper structures and cutaneous lesions located over joints may entail the use of casts or splints, but the veterinarian should be aware that most types of bandage can actually promote granulation tissue formation. Short-term topical application of a corticosteroid to reduce inflammation has been recommended but remains controversial. Topically applied antimicrobials, such as 1% silver sulfadiazine or 10% povidone-iodine ointment, are recommended until re-epithelialization has occurred. Modalities that promote inflammation, including cryotherapy and topical caustic agents, are not recommended.

EOSINOPHILIC GRANULOMA

These lesions are considered hypersensitivity reactions and are most common on the neck and dorsal aspect of the trunk. Eosinophilic granulomas are manifest as single to multiple, nonpainful, nonpruritic, firm nodules with normal overlying skin (Figure 149-2

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