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.
The prevalence of non-neoplastic and proliferative lesions that are caused by infectious agents can vary according to geographic location, with variations related to climate, presence of infectious agents, and husbandry. However, comparison of survey information from Cornell University in the northeastern United States and from Oregon State University in the northwestern United States suggests that differences between these two regions are not dramatic (Table 149-1). Nodular non-neoplastic and proliferative lesions appear to be much less common in the skin of donkeys and mules than in horses. Only six cases of non-neoplastic lesions (four donkeys with exuberant granulation tissue, eosinophilic granuloma in a mule, and congenital papilloma in a mule) were identified in a study of 40 nodular and proliferative skin lesions in donkeys and mules at Oregon State University.
The most common non-neoplastic and proliferative skin lesion in horses studied at Oregon State University and in southeastern Queensland, Australia, was exuberant granulation tissue. Data on the prevalence of exuberant granulation tissue at Cornell University was not available. Pooling of data from Cornell University, Oregon State University, and southeastern Queensland, Australia, indicates that the other most common non-neoplastic and proliferative lesions in the skin of horses are eosinophilic granuloma, cysts, mucocutaneous habronemiasis, fungal granuloma, other granulomas, bacterial pseudomycetoma, pseudolymphoma, and calcinosis circumscripta. At Oregon State University, exuberant granulation tissue, eosinophilic granuloma, fungal granuloma, cysts, and habronemiasis constituted 91% of all non-neoplastic nodular and proliferative lesions identified and 5.85% of total equine pathology accessions. A previously undescribed nodular lesion within pinnal cartilage (nodular auricular chondropathy) has been seen in several horses at Oregon State University and will be briefly described.
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.
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.
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.
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