Viscum album L., a Therapeutic Option for Neoplastic Diseases in Companion Animals? A Systematic Review

Cancer is a common disease in humans and in companion animals and treatment is challenging. The aim of this systematic review was to identify and assess the potential use of Viscum album L. extracts (VAE) for treatment of neoplastic diseases in companion animals. Peer-reviewed animal, in vivo and in vitro studies were included, considering the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement and A Measurement Tool to Assess Systematic Reviews (AMSTAR). Overall, 6,148 references were identified. Following a predefined protocol, 114 full-text references were assessed. Ultimately, 61 references were included for further assessment, 25 references included in vitro experiments, 26 included in vivo and clinical experiments, and 10 references included both in vitro and in vivo experiments. These 61 references comprised data of 193 in vitro and 67 in vivo and clinical experiments. Most of the 67 in vivo and clinical experiments were conducted with mice (59), followed by rats (4), dogs (3), and horses (1). So far, oral melanomas, mammary tumors, and sticker sarcomas in dogs, as well as sarcoids in horses, have been investigated in controlled clinical trials. A scoring system was established to evaluate the outcomes of each study based on defined effect levels. The efficacy of VAE treatment was most pronounced for melanomas, sarcomas, mammary carcinoma, and equine sarcoids. The limited number and quality of published studies on VAE treatment in companion animals impede drawing definitive conclusions regarding the efficacy of VAE in the treatment of cancer. Thus, further research is needed to elucidate the impact of VAE on the treatment of cancer in companion animals and possible underlying mechanisms.