In 2010 I published an extensive narrative review of the research concerning the benefits and risks of neutering. Since then, there has been a proliferation of studies and reviews attempting to refine and strengthen the evidence and allow better individualized recommendations about neutering individual pets. I have collected my original review and links to articles updating the evidence on this subject here, and I will endeavor to keep this collection updated as more and better evidence is available on this complex and important subject.
As a general practice veterinarian, I think this subject provides a perfect example of the process of evidence-based medicine. Using the totality of the available evidence, with critical appraisal and an awareness of the limitations of each study and review, veterinarians can make informed recommendations to clients about the care of individual pets.
A one-size-fits-all policy about neutering can never be optimal as the particular risks and benefits of this procedure depend on many factors: the breed, age, size, health status, and purpose of the pet; the values, behaviors, and socioeconomic status of the owner; the independent risks for specific health problems that might also be associated with neuter status; the time and place in which the pet lives; and the availability and utilization of preventative and therapeutic veterinary care.
Science makes the care we provide better, but it may not always make it simpler or easier. Individualization of care requires as comprehensive an understanding as possible about the real factors that influence health and disease. We cannot simply make up systems of diagnosis and therapy, such as those of homeopathy and so-called Traditional Chinese Veterinary Medicine, and then claim that we are providing individualized care if we have not validated the specific diagnostic and therapeutic methods we are using through controlled scientific research.
Conversely, we must make decisions based on the evidence we have, not the evidence we want. This means we have to be honest about what we know and don’t know, and we have to accept that there will always be significant uncertainty about the outcome in any individual case. Science can reduce this uncertainty but cannot eliminate it. And we must make the best effort possible to integrate all the complex and conflicting information available into as rational and evidence-based a recommendation as possible, rather than relying on simply, reflexive rules (such as always neuter before 6 months of age or never neuter until after 1 year of age) or the traditional but weak foundations of clinical experience and the advice of our teachers and mentors.
**In 2014, I updated my 2010 review, with the addition of scientific reports published in the intervening years. This led to a number of changes in my conclusions with regard to specific issues. The new version is available here, and while I will keep the link active to the previous version, this update supersedes the older version.
1) McKenzie, B. Evaluating the benefits and risks of neutering dogs and cats. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources 2010 5, No. 045
2) Beauvais W, Cardwell JM, Brodbelt DC.The effect of neutering on the risk of urinary incontinence in bitches – a systematic review. J Small Anim Pract. 2012 Apr;53(4):198-204.
3) Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs–a systematic review.J Small Anim Pract. 2012 Jun;53(6):314-22.
4) Torres de la Riva G, Hart BL, Farver TB, Oberbauer AM, Messam LLM, et al. (2013) Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers. PLoS ONE 8(2): e55937. doi:10.1371/journal.pone.0055937
5) Hoffman JM, Creevy KE, Promislow DEL (2013) Reproductive Capability Is Associated with Lifespan and Cause of Death in Companion Dogs. PLoS ONE 8(4): e61082. doi:10.1371/journal.pone.0061082
6) Lefebvre, SL. Yang, M. Wang, M. Elliott, DA. Buff, PR. Lund, EM. Effect of age at gonadectomy on the probability of dogs becoming overweight. Journal of the American Veterinary Medical Association. 2013;243(2):236-43.
7) Zink, MC. Farhoodly, P. Elser, SE. Ruffini, LD. Gibbons, TA. Riegr, RH. Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas. J. Amer Vet Med Assoc. 2014;244(3):309-319.
8) Hart BL, Hart LA, Thigpen AP, Willits NH. Long-Term Health Effects of Neutering Dogs: Comparison of Labrador Retrievers with Golden Retrievers. PLoS ONE 2014;9(7): e102241. doi:10.1371/journal.pone.0102241
9) O’Neill DG, Church DB, McGreevy PD, Thomson PC, Brodbelt DC. Longevity and mortality of cats attending primary care veterinary practices in England. J Feline Med Surg. 2015 Feb;17(2):125-33. doi: 10.1177/1098612X14536176. Epub 2014 Jun 12.
10) M.M.E. Larsen, B. Børresen, A.T. Kristensen. Neuter status and risk of cancer in a Danish dog population.
11) Salas Y, Márquez A, Diaz D, Romero L (2015) Epidemiological Study of Mammary Tumors in Female Dogs Diagnosed during the Period 2002-2012: A Growing Animal Health Problem. PLoS ONE 10(5): e0127381. doi:10.1371/journal.pone.0127381
12) Graf R. et al. Swiss Feline Cancer Registry 1965-2008: the Influence of Sex, Breed and Age on Tumour Types and Tumour Locations. J Compar Pathol. 2016.
13) Hart, B. L., Hart, L. A., Thigpen, A. P. and Willits, N. H. (2016). Neutering of German Shepherd Dogs: associated joint disorders, cancers and urinary incontinence. Veterinary Medicine and Scienc. doi: 10.1002/vms3.34