CHAPTER 149 Non-neoplastic Nodular and Proliferative Lesions
The differential diagnosis list for nodular cutaneous lesions in horses is long and includes both neoplastic and non-neoplastic diseases. A survey conducted at Oregon State University revealed that 8.7% of equine pathologic submissions were non-neoplastic nodular and proliferative skin lesions, and at Cornell University this type of lesion comprised approximately 11.8% of equine pathologic submissions.
Table 149-1 Prevalence of Selected Non-neoplastic Nodular and Proliferative Lesions in the Northeastern
and the Northwestern United States

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.
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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