BCG Immunotherapy in Equine Sarcoid Treatment: A Narrative Review of Mechanisms, Clinical Efficacy, and Challenges in Veterinary Oncology

Equine sarcoids, skin tumors linked to bovine papillomavirus (BPV), are the most common neoplasms in horses, with recurrence rates up to 50% after conventional therapies like surgery or cryotherapy. Their molecular complexity, driven by viral integration and inactivation of tumor suppressor genes (p53/pRb), demands innovative approaches in veterinary oncology. This review evaluates Bacillus Calmette-Guérin (BCG) immunotherapy as a cost-effective alternative, focusing on its immune mechanisms, clinical efficacy, and challenges in protocol standardization, while exploring its translational potential. A systematic literature search (1980–2023) across PubMed, Web of Science, and CAB Abstracts identified 45 studies (clinical trials, case series, and translational research), prioritizing peer-reviewed articles with quantitative BCG data and excluding non-systematic reviews or clinically irrelevant studies. Intralesional BCG therapy induced tumor regression in 40–70% of cases by activating innate (macrophages, TNF-α) and adaptive (CD8+ T cells) immune responses. However, adverse effects like severe inflammation (15% of cases) and protocol heterogeneity (dose, strain) limit its application. Compared to conventional therapies, BCG offers cost and non-invasiveness advantages, though long-term recurrence data remain scarce. BCG shows promise as an accessible immunotherapy but requires randomized trials, standardized protocols, and predictive biomarker validation. Integrating strategies like checkpoint inhibitors and One Health approaches could amplify its impact, benefiting both animal health and translational oncology research.