Cancer constitutes a significant clinical challenge in veterinary medicine, with a high prevalence among companion animals. While cutaneous and mammary neoplasia account for a large proportion of malignant lesions in companion animals, other malignant tumors are also frequently found, for instance, in the neurological, gastrointestinal, and urological systems [1]. Despite recent advances concerning the early diagnosis of cancer and its treatment [2], it remains among the most common causes of death in companion animals. Furthermore, companion animals with naturally occurring cancers are widely utilized as a natural model for cancer research [3]. In fact, dogs and cats develop multiple types of cancer with numerous clinical, molecular, histopathological, and genetic similarities to human cancers [4]. Such similarities constitute opportunities to develop new diagnostic and prognostic markers and new therapies for human and animal diseases [3,4,5]. Despite the availability of numerous treatment approaches for cancer, advancements in the understanding of cancer mechanisms and the development of innovative treatments are still needed to improve outcomes for human and animal oncologic patients. Work on improving methods for earlier diagnosis and stratifying cancer patients according to their response to available therapies is also ongoing. Accordingly, this Special Issue, entitled “Comparative Oncology of Companion Animals”, aimed to publish original research and reviews concerning the diagnosis, treatment, and prognosis of cancer in companion animals, highlighting new advances in this field.
This Special Issue included four research articles and one review article. Two and one of these articles deal with canine and feline diseases, respectively, one examines both, and the fifth article deals with an experimental rat model of mammary cancer, which remains a major health issue for companion animals and humans. The single review article included in this Special Issue [6] addresses key aspects of the tumor microenvironment in feline mammary carcinomas. The authors focus on tumor necrosis, fibrosis, angiogenesis, adipose tissue-associated inflammation, extracellular vesicles, the epithelial–mesenchymal transition, and their prognostic implications. Overall, the review findings indicate that collagen remodeling, cancer-associated fibroblasts, regulatory T lymphocytes, and elevated serum leptin levels are associated with poor prognosis, while the presence of stromal cytotoxic T lymphocytes is associated with a more favorable outcome. The authors did not find consistent data regarding necrosis and pro-angiogenic factors, and report that research on the roles of extracellular vesicles on this type of cancer is still at a very early stage.
Schwinn et al. [7] contributed a research article on canine cutaneous mast cell tumors occurring in 849 dogs aged up to 3 years old. In this relatively young dog population, the authors reported on most affected breeds and anatomic locations, the tumor’s histological grade, and Ki67 and c-KIT immunohistochemical staining patterns. Danish–Swedish farm dogs and English Setters were over-represented and German Shepherd dogs were under-represented in this population. The authors concluded that cutaneous mast cell tumors in this young dog population share important similarities and differences compared with the general canine population, although survival data was not yet available.
Lisjak et al. [8] also reported on dogs with mast cell tumors (MCTs) but studied the fecal microbiota of a specific dog population whose tumors were treated with electrochemotherapy and IL-12 gene electrotransfer. The authors reported that dogs with MCTs showed differential relative abundance of specific bacteria compared with normal controls, although there were no differences in the dysbiosis index, alpha diversity, or beta diversity, nor between the expression levels of immunological markers such as PD-1 and granzyme B. The authors also performed analyses before and after the treatment with electrochemotherapy and IL-12 gene electrotransfer and found no differences associated with this treatment.
A research article by Juodžiukynienė [9] described the prevalence of feline and canine tumors in Lithuania. Mammary and cutaneous tumors were most commonly diagnosed. In agreement with global trends, cutaneous squamous cell carcinomas were more common in cats.
Silva et al. [10] contributed an experimental research article describing the effects of resistance training on the development of chemically induced mammary cancer in Wistar rats, a relevant model of cancer. The authors observed that exercise training thrice a week on a ladder resulted in earlier tumor onset, a slightly higher tumor incidence, and a lower mortality rate. Additional studies are required to clarify the potential role of exercise training on the risk and biopathological characteristics of mammary cancer in companion animals.
Taken together, the articles published in this Special Issue reflect the very diverse approaches to companion animal oncology, and the integration of concepts from other scientific areas. Future research in companion animal oncology should consider gray areas and incompletely solved issues, such as the biopathological role of microvesicles in the tumor microenvironment and body fluids and their potential as diagnostic and prognostic markers. The role of the microbiome as a tumor-enabling characteristic [11] is also poorly understood in companion animals and deserves further study.
Funding
This work was supported by national funds through FCT/MCTES (PIDDAC): CITAB [UID/04033/2020] (https://doi.org/10.54499/UIDB/04033/2020); CERNAS [UIDB/00681/2020] (https://doi.org/10.54499/UIDB/00681/2020); LEPABE [UIDB/00511/2020] (https://doi.org/10.54499/UIDB/00511/2020); [UIDP/00511/2020] (https://doi.org/10.54499/UIDP/00511/2020); ALiCE [LA/P/0045/2020] (https://doi.org/10.54499/LA/P/0045/2020); LAQV-REQUIMTE [UID/50006/2025] (http://doi.org/10.54499/UID/50006/2025). [UIDP/50006/2020] (http://doi.org/10.54499/UIDP/50006/2020) and [UIDB/50006/2020] (http://doi.org/10.54499/UIDB/50006/2020). This work was supported by CAPES finance code 001, by FAPEMA under grant APP-01393/25 (EDITAL FAPEMANº15/2024—PLANOMARANHÃO2050: SOLUÇÕES INOVADORAS) and the National Scientific and Technological Development Council (Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq) grant 404177/2025-5 (CHAMADA CNPQ/MCTI Nº 44/2024). RMGdC is a research productivity fellow of CNPq [grant number: 301729/2025-5]. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Conflicts of Interest
The authors declare no conflicts of interest.
List of Contributions
- Schwinn, J.; Törner, K.; Beitzinger, C.; Klopfleisch, R.; Aupperle-Lellbach, H. The Characteristics of 924 Cutaneous Mast Cell Tumours in Dogs ≤3 Years of Age—With a Short Literature Review of Feline, Equine, and Human Proliferative Mast Cell Disorders. Vet. Sci. 2026, 13, 500. https://doi.org/10.3390/vetsci13050500.
- Lisjak, A.; Correa Lopes, B.; Pilla, R.; Nemec, A.; Lampreht Tratar, U.; Suchodolski, J.S.; Tozon, N. Assessment of Fecal Microbiota in Healthy Dogs and Dogs with Cutaneous Mast Cell Tumors Treated with Electrochemotherapy Combined with Gene Electrotransfer of IL-12. Vet. Sci. 2026, 13, 241. https://doi.org/10.3390/vetsci13030241.
- Juodžiukynienė, N. Retrospective Analysis of Prevalence of Tumors in Dogs and Cats in Lithuania. Vet. Sci. 2025, 12, 1038. https://doi.org/10.3390/vetsci12111038.
- Silva, J.; Azevedo, T.; Aires, I.; Medeiros, C.; Neuparth, M.J.; Seixas, F.; Ferreira, R.; Faustino-Rocha, A.I.; Oliveira, P.A.; Duarte, J.A. Effects of Ladder-Climbing Exercise on Mammary Cancer: Data from a Chemically Induced Rat Model. Vet. Sci. 2025, 12, 303. https://doi.org/10.3390/vetsci12040303.
- Rodrigues-Jesus, J.; Canadas-Sousa, A.; Vilhena, H.; Dias-Pereira, P. Inside the Tumor: Decoding the Feline Mammary Tumor Microenvironment and Its Prognostic Value—A Review. Vet. Sci. 2025, 12, 959. https://doi.org/10.3390/vetsci12100959.
References
- Meuten, D.J. Tumors in Domestic Animals, 5th ed.; Wiley Blackwell: Oxford, UK, 2020. [Google Scholar]
- Christensen, J.; Johnson, K.; Ettinger, S.; Garrett, L.; Gordon, I.; Ireifej, S.; Love, A.; Wisecup, M. AAHA oncology guidelines for dogs and cats. J. Am. Anim. Hosp. Assoc. 2026, 62, 1–37. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Fenger, J.M.; London, C.A. Dogs as a model for cancer: An update. Annu. Rev. Anim. Biosci. 2026, 14, 371–374. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Rasteiro, A.M.; Sá e Lemos, E.; Oliveira, P.A.; Gil da Costa, R.M. Molecular markers in urinary bladder cancer: Applications for diagnosis, prognosis and therapy. Vet. Sci. 2022, 9, 107. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Chow, L.; Ammons, D.; Dow, S.W. Role of companion dogs in cancer immunotherapy development. J. Immunol. 2025, 214, 2515–2522. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Rodrigues-Jesus, J.; Canadas-Sousa, A.; Vilhena, H.; Dias-Pereira, P. Inside the tumor: Decoding the feline mammary tumor microenvironment and its prognostic value—A review. Vet. Sci. 2025, 12, 959. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Schwinn, J.; Törner, K.; Beitzinger, C.; Klopfleisch, R.; Aupperle-Lellbach, H. The characteristics of 924 cutaneous mast cell tumors in dogs ≤3 years of age—With a short literature review of feline, equine, and human proliferative mast cell disorders. Vet. Sci. 2026, 13, 500. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Jisjak, A.; Lopes, B.C.; Pilla, R.; Nemec, A.; Lampreht Tratar, U.; Suchodolski, J.S.; Tozon, N. Assessment of fecal microbiota in healthy dogs and dogs with cutaneous mast cell tumors treated with electrochemotherapy combined with gene electrotransfer of IL-12. Vet. Sci. 2026, 13, 241. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Juodžiukynienė, N. Retrospective analysis of prevalence of tumors in dogs and cats in Lithuania. Vet. Sci. 2025, 12, 1038. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Silva, J.; Azevedo, T.; Aires, I.; Medeiros, C.; Neuparth, M.J.; Seixas, F.; Ferreira, R.; Faustino-Rocha, A.I.; Oliveira, P.A.; Duarte, J.A. Effects of ladder-climbing exercise on mammary cancer: Data from a chemically induced rat model. Vet. Sci. 2025, 12, 303. [Google Scholar] [CrossRef] [Scilit] [PubMed]
- Hanahan, D. Hallmarks of cancer: New dimensions. Cancer Discov. 2022, 12, 31–46. [Google Scholar] [CrossRef] [Scilit] [PubMed]
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content. |
© 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.