Are intact dogs less likely to get cancer

Dr. Fan welcome oncology questions from veterinarians and veterinary technicians.

Q. I've heard about studies linking neutered dogs with an increased cancer risk. Should I take this into consideration before performing routine spays and castrations?

A. The epidemiologic findings in a recent study provide indirect and foundational evidence for the participation of gonadal status in susceptibility to or protection from various categorical causes of death in companion dogs.1 Based on the retrospective analysis of a very large cohort of female and male dogs, which were either gonadally sterilized (neutered) or intact, the findings of the study indicate that gonadal sterilization not only significantly impacts when companion dogs might die, but also provides novel information pertaining to why individuals die.

The study's specifics

Specifically, gonadal sterilization significantly increased life expectancy in both male and female dogs by 13.8% and 26.8%, respectively, in comparison to sexually intact individuals. Importantly, the study findings identified a substantial effect of gonadal sterilization on the cause of death, with sterilization of dogs being significantly protective for fatality associated with various categorical pathologic processes including infectious, traumatic, vascular, and degenerative disease processes.

In contrast, sterilized dogs were significantly more likely to experience fatality associated with select neoplastic and immune-mediated processes. The identified association between increased fatalities of sterilized dogs from either neoplastic or immune-mediated diseases has the potential to direct future hypothesis-driven experiments that specifically address the participatory roles of chronic gonadal hormone exposure on tumorigenesis and immune surveillance.

In the context of cancer, sterilized dogs had a significantly increased risk of death, independent of age, associated with transitional cell carcinoma, osteosarcoma, lymphoma, and mast cell tumors; however, the increased death risk from cancer was not preserved across all tumor histologies, as sterilization status did not significantly influence the incidence of mortality in dogs with other common cancers such as prostate carcinoma, squamous cell carcinoma, and melanoma.

More research needed

Based on these initial epidemiologic study observations, prospective investigations addressing the putative and mechanistic roles of chronic gonadal hormone exposure and specific cancer-related death risks are well-justified. However, at this point before additional hypothesis-driven experiments can be conducted, it would be premature and imprudent to recommend the avoidance of elective gonadal sterilization because of concerns of increased death risk from cancer in companion dogs. Future rigorous and definitive cause-and-effect scientific studies are required before changes in sterilization practices should be considered.

Timothy M. Fan, DVM, PhD, DACVIM (internal medicine, oncology)
Department of Veterinary Clinical Medicine
College of Veterinary Medicine
University of Illinois
Urbana, Ill.

REFERENCE

1. Hoffman JM, Creevy KE, Promislow DE. Reproductive capability is associated with lifespan and cause of death in companion dogs. PLoS One 2013;8(4):e61082.