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.