Equine Neoplasia Study Guide

Overview and Clinical Importance

Neoplasia in horses accounts for approximately 3% of all equine presentations for treatment or necropsy. Unlike small animals, horses have relatively low tumor incidence rates overall. However, cutaneous neoplasms are the most common tumor type, representing 45-80% of all equine cancers. The three most frequently diagnosed skin tumors in horses are sarcoids (accounting for approximately 40-51% of all equine tumors), squamous cell carcinoma (18-20%), and melanoma (particularly in grey horses). Understanding the clinical presentation, diagnosis, and treatment options for these tumors is essential for the NAVLE examination.

Section 1: Equine Sarcoid

Equine sarcoids are the most common skin tumor in horses, donkeys, and mules, accounting for approximately 40-51% of all equine cutaneous neoplasms. These are locally aggressive, non-metastatic fibroblastic tumors associated with bovine papillomavirus types 1, 2, and possibly 13 (BPV-1, BPV-2, BPV-13). BPV-1 predominates in Europe while BPV-2 is more common in the western United States.

Etiology and Pathogenesis

Sarcoid development requires viral exposure combined with host genetic susceptibility. BPV DNA and transcripts are detectable in virtually 100% of lesions. Unlike in cattle where BPV causes self-limiting warts, in horses the virus causes persistent fibroblastic proliferation without viral particle production. This makes transmission routes unclear and explains why virus-based vaccines are not feasible. Genetic factors play a significant role, with certain MHC (Major Histocompatibility Complex) alleles associated with susceptibility.

Clinical Presentation and Classification

Sarcoids can occur anywhere on the body but have predilection for the head (especially periocular regions), ears, ventral abdomen, limbs, and areas of previous wounds or repeated trauma. Approximately 32% occur on the head and neck. Six clinical types are recognized, and multiple types may occur simultaneously on the same horse.

Sarcoid Classification Table

Diagnosis

Clinical appearance often allows presumptive diagnosis, but definitive diagnosis requires histopathology. CRITICAL: Biopsy carries risk of exacerbating sarcoids due to their reactivity to trauma. Incisional biopsy should be avoided if possible; excisional biopsy with wide margins is preferred when diagnosis and treatment can be combined. PCR detection of BPV DNA is not diagnostic as virus is present in normal skin of affected horses.

Treatment Options

Section 2: Squamous Cell Carcinoma (SCC)

Squamous cell carcinoma (SCC) is the second most common skin tumor in horses, accounting for approximately 18-20% of all equine cutaneous neoplasms. Unlike sarcoids, SCC is locally invasive and can metastasize, primarily to regional lymph nodes. It is also the most common tumor of the equine gastrointestinal tract (particularly stomach) and the most common tumor of male external genitalia.

Risk Factors and Etiology

• UV light exposure: Primary risk factor for periocular and mucocutaneous SCC
• Lack of skin pigmentation: Pink/white skin areas at highest risk
• Breed predisposition: Appaloosas, Paints, draft breeds (Belgians, Clydesdales), Haflingers
• Chronic irritation: Smegma accumulation (genital SCC)
• Viral association: Equus caballus papillomavirus-2 (EcPV-2) linked to genital SCC (found in 43% of penile SCC); NOT found in periocular SCC
• Age: Typically older horses (greater than 9 years); genital SCC average age 16 years

Clinical Presentation by Location

Diagnosis

Histopathology is required for definitive diagnosis. Biopsy should include assessment for depth of invasion and differentiation grade. Regional lymph node evaluation (aspiration or palpation) is recommended to assess for metastasis. For genital SCC, rectal examination can detect sublumbar lymph node enlargement.

Treatment

Section 3: Equine Melanoma

Equine melanoma arises from melanocytes and is the third most common skin tumor in horses. It has a unique relationship with coat color, as approximately 80% of grey horses over 15 years of age develop melanomas. Unlike human melanoma, UV radiation is NOT considered a significant factor in equine melanoma development.

Genetics and Pathogenesis

All grey horses inherit a mutation in the STX17 gene (syntaxin 17) that causes progressive greying and predisposes to both vitiligo and melanoma. Additional risk is conferred by ASIP and MC1R coat color genes. Relative melanoma risk based on pre-greying coat color: Black > Bay > Chestnut. Melanomas typically begin developing at around 4 years of age in grey horses.

Classification of Melanocytic Tumors

Clinical Presentation

Common locations: ventral tail (most common), perineum/perianal region, external genitalia, lips, eyelids, parotid salivary gland (throatlatch region). Tumors appear as firm, black, dome-shaped nodules. They may be solitary or multiple, ranging from small (less than 1 cm) to very large confluent masses.

Clinical signs depend on location: Large perianal masses may impair defecation; parotid masses may limit head movement; internal metastases cause weight loss, respiratory difficulty, or neurological signs.

Treatment Options

Section 4: Equine Lymphoma

Lymphoma (lymphosarcoma) is the most common hematopoietic neoplasm in horses, accounting for 1.3-2.8% of all equine neoplasia. It is the most common malignant tumor of the equine gastrointestinal tract and thorax. There is no benign form of lymphoma in horses, hence pathologists prefer the term 'lymphoma' over 'lymphosarcoma.'

Anatomic Forms

Diagnosis

Diagnosis is challenging due to nonspecific signs. Common clinicopathologic findings: anemia (chronic disease), hyperfibrinogenemia, elevated LDH, hypercalcemia (paraneoplastic), hypoproteinemia. Definitive diagnosis requires identification of neoplastic lymphocytes via cytology (FNA) or histopathology. Rectal palpation may detect intestinal masses; ultrasound reveals thickened bowel wall, enlarged lymph nodes, or thoracic masses. Immunophenotyping (T-cell vs B-cell) helps with prognosis.

Treatment and Prognosis

Prognosis is generally poor to grave. Median survival is approximately 4-8 months after diagnosis. Treatment options include chemotherapy (corticosteroids, cyclophosphamide, doxorubicin), surgical excision of localized masses, and radiation for accessible tumors. Cutaneous lymphoma has the best prognosis and may respond to corticosteroids alone. Approximately 93% of horses show some response to chemotherapy, but complete remission is achieved in only 33%.

Section 5: Granulosa Cell Tumor (GCT)

Granulosa cell tumor (GCT), also called granulosa-theca cell tumor (GTCT), is the most common ovarian tumor in mares. These tumors arise from sex cord-stromal tissue and are almost always unilateral, benign, but hormonally active. The contralateral ovary is typically small and inactive due to inhibin-mediated suppression of FSH.

Clinical Presentation

Clinical signs vary based on the hormonal activity of the tumor:

• Stallion-like behavior (testosterone elevation): Mounting, aggression, herding mares, squealing, striking
• Nymphomania (estrogen elevation): Continuous/frequent estrus behavior
• Anestrus (inhibin elevation): No cycling, failure to conceive
• Performance issues: Vague lameness, back pain from ligament tension

Diagnosis

Rectal palpation and ultrasound: One ovary enlarged (can reach 40 cm), often with 'honeycomb' multicystic appearance; contralateral ovary small and inactive.

Hormone testing:

Treatment and Prognosis

Ovariectomy (surgical removal of affected ovary) is curative. Can be performed via flank laparotomy, ventral midline, or laparoscopic approach. The contralateral ovary resumes normal function within 6-12 months in most cases. Prognosis for return to breeding is excellent; mares can carry pregnancies to term with the remaining ovary.

Summary: Comparison of Major Equine Tumors