The Role of Neutering in Cancer Development

By Annette N. Smith, DVM, MS
Vet Clin Small Anim 44 (2014) 965-975

Many PBGVs in the PBGVCA community are reproductively intact because they are being actively shown or are in breeding programs. What about the dogs who are retired from the ring, are no longer being bred or are family pets? The advantages of neutering the latter group of dogs are well documented. A spayed bitch doesn’t worry about pyometra, isn’t surprised by an unplanned pregnancy and no longer has to shop in the feminine hygiene aisle of the supermarket. A castrated male is less likely to roam, fight or mount inappropriately. In addition, neutering leads to decreased risk of mammary, ovarian and uterine tumors of the female, and testicular tumors of the male. What’s not to like?

Dr. Annette Smith is Professor of Clinical Sciences at the Auburn University College of Veterinary Medicine. Her clinical specialty is oncology. Dr. Smith has gathered information from published studies on the effect of surgical sterilization on the occurrence of cancer.

Her article cites several studies that have found an increased risk of certain tumor types in surgically altered dogs. For example, osteosarcoma is more common in large and giant breeds, and neutered dogs of those breeds have a two-fold high risk for this diagnosis. Several studies have found the risk of hemangiosarcoma is many-fold greater in neutered females than intact, whereas the risk for males was less. The risks for both lymphoma and transitional cell carcinoma are higher in neutered dogs of both sexes.

Dr. Smith cites a study published in 2013 that examined the causes of death among more than 40,000 dogs presented to North American veterinary teaching hospitals from 1984 to 2004 (Hoffman, Creevy, and Promislow, 2013, PLOS ONE 8:e61082). The authors found a slight increase in life expectancy in sterilized dogs as compared to intact; however, they found large differences in the cause of death. Sterilized dogs of both sexes were much less likely to die of infectious disease (i.e., parvovirus, heartworm, canine distemper, intestinal parasites), trauma, vascular disease and degenerative disease. In contrast, the sterilized dogs were more likely to die of cancer (i.e. transitional cell carcinoma, osteosarcoma, lymphoma, mast cell tumors) and immunemediated disease. The relationship between sterilization and risk of cancer was seen in all size classes of dogs.

The bottom line is that neutering has positive effects on behaviors, noncancerous diseases and lifespan that outweigh the increased risk of cancer. It is unclear why sterilization has an effect on the risk for cancers outside of the reproductive system. Owners should be aware of the risk of cancer in their neutered hounds and discuss this with their veterinarians. Dr. Smith concludes by saying “In un-owned, shelter or rescue populations, the population benefits of neutering likely outweigh any potential for increasing cancer risk. For owned animals, veterinarians will need to discuss the pros and cons for each individual and determine the best strategy for that pet based on breed, lifestyle, longevity expectations, concurrent diseases, cancer risks, other considerations for intact and sterilized dogs, and owner preferences.”

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