In addition, Bain says he treats several patients each year with granulosa cell tumors.
Confluent infiltrative melanomas on the tail and perineum are one of the most common presentations of this neoplastic disease
in pale-coated horses.
Squamous cell carcinomas
The type and number of equine oncology cases seen at the University of Florida (UF) College of Veterinary Medicine's Alec
P. and Louise H. Courtelis Equine Hospital vary significantly and can be categorized based on location of the tumor, says
Michael Porter, DVM, Ph.D. Dipl. ACVIM. By far, the most common category is cutaneous tumors, including squamous cell carcinomas
(SCC), sarcoids, melanomas and papillomas. Typically, melanomas and papillomas do not present a problem to the equine patient
and are merely noted on physical exam. However, on occasion, geriatric horses with clinical signs ranging from ataxia to recurrent
colic that present with cutaneous melanomas might require a more extensive exam to determine the extent of melanoma formation.
As a referral institution, the cases presenting for primary evaluation of cutaneous tumors at UF typically have had some prior
surgical/medical intervention by the referring veterinarian without complete resolution of tumor growth. The most common include
SCC of the penis and the third eyelid.
Microscopic appearance of melanoma, showing variably pigmented polygonal cells.
"These conditions often require aggressive surgical debulking, such as penile amputation and complete eye removal," Porter
In addition, a variety of treatment modalities are provided, including surgical debulking, cryotherapy, intra-lesional chemotherapy
injections and topical chemotherapy treatment.
"The incidence of SCC seems to be high in the state of Florida, and this may be due to the ideal environmental conditions,"
Porter says. "However, there is no definitive proof of this theory. My personal experience with treating SCC suggests that
early and aggressive debulking followed by either topical or intra-lesional chemotherapy is critical for complete resolution
of clinical signs."
Pythiosis is not a tumor but rather a granuloma that develops secondary to invasion of the soft tissues by the aquatic organisms
Although rare, metastatic SCC does occur and usually results in the development of squamous cell carcinomas in the lungs.
They have seen several such cases at the University of Florida that originally presented for a SCC on the penis.
Equally important to cases of SCC seen at UF, yet not metastatic, are sarcoids. These are considered the most common type
of skin tumor in horses and can be locally aggressive. The majority of sarcoid cases that present to UF have been removed
and/or topically treated at least once prior to their visit. However, due to their refractory and sometimes aggressive pattern
of growth, sarcoids on some equine patients are referred for more aggressive treatment. "The university provides surgical
debulking that often requires general anesthesia, cryotherapy and chemotherapy. Similar to SCC, sarcoids require early and
aggressive treatment to reduce the likelihood of re-development," Porter says.
Skin tumor types
A variety of cutaneous neoplasms are seen in horses, with sarcoids, melanoma, squamous cell carcinoma and papilloma being
most common. Seen less frequently are cutaneous lymphosarcoma, mastocytoma (mast-cell tumors), vascular tumors (hemangioma
and hemangiosarcoma), keratoma and basal-cell tumors. Though the latter are less common, they still should be considered part
of the differential diagnosis of skin masses and should be treated if identified.
Intestinal lymphoma: The spleen and GI tract appear to be most commonly affected by lymphoma, whereas the kidneys and stomach
are common sites of adenocarcinoma.