This is a more challenging question than you may think.
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Spaying, neutering, “fixing”, desexing, altering and sterilizing are all terms used to describe surgical removal of a pet’s reproductive organs. Prior to the 1970s, neutering dogs and cats was relatively uncommon. At that time, a large effort was made to reduce the number of unplanned litters and reduce the number of unwanted animals being euthanized. Since then, neutering at 6 months of age has been the most commonly recommended age. For most breeds, this is just prior to sexual maturity/puberty.
Neutering reduces undesirable behaviors such as roaming, marking and unwanted sexual behaviors. It also eliminates the risk of pyometra, an infected uterus, which affects 25% of unspayed females by the age of 10 years. Spaying before the first heat cycle also greatly reduces the risk of mammary cancer (a rate of 0.5%). Spaying later in life still reduces the risk of mammary cancer, but the benefit decreases. Mammary cancer has a rate of 8% when spayed between first and second heat cycles, and over 20% after the second heat. There is evidence that neutered animals, especially females, live longer, although we don’t fully understand why.
Beyond 6 months of age, altering is more challenging surgically with greater risk of complications. These risks include increased bleeding and prolonged healing, not to mention unwanted pregnancy. In order to prevent unwanted pregnancies, shelters and rescue organizations will sometimes spay/neuter as early as 8 weeks of age or even younger. Development of urinary incontinence is of greater concern at such a young age. Neutering at 6 months of age strikes a good balance.
Between 2010 and 2020, 3 papers came out of the University California at Davis (UCDavis) that seemed to demonstrate desexing Golden Retrievers increased the risk for certain types of cancer; however, one of these papers looked only at younger dogs. Mammary carcinoma (cancer) has a low incidence prior to 8 years of age, and they didn’t look at old enough dogs. A different study out of UCDavis (Kent et al 2018) found that age, not sexual status determined the probability of Golden Retrievers dying from cancer.
In July 2020, another paper was published from UCDavis: Assisting Decision-Making on Age of Neutering for 35 Breeds of Dogs: Associated Joint Disorders, Cancers, and Urinary Incontinence. The authors looked at a variety of joint disorders (Hip Dysplasia and Cranial Cruciate Ligament disease), cancers (Lymphoma, Mast Cell Tumor (MCT) and Hemangiosarcoma), as well as urinary incontinence by reviewing the timing of neutering. While helpful, this paper also has its limitations. Once again, the age of dogs in the study was low (mean between 4.5 and 5.5 years with the oldest being 7 years of age); but cancer and many joint disorders do not occur or become apparent until later in life. The overall numbers of animals included was small and the population does not represent the average pet– these are all animals that were seen at UCDavis; the majority of pets in the US are not treated at a university.
One interesting tidbit that comes from a study involving a database of 168,000 dogs (Shoop et al 2015) found that being insured raised the risk of Mast Cell Tumors by 5 times. Since insurance plays no direct role in whether a lump is, or is not, a MCT, this points to a significant “selection bias”. The take home message is that great care should be taken when basing recommendations on the results of studies that only include pets seen at referral hospitals and universities.
Our Current Recommendations:
To avoid unwanted pregnancies and minimize unwanted behaviors, balanced with risks for cancers, joint disease and urinary issues, we recommend spaying/neutering at 6 months of age. If your dog comes from a pedigree with significant joint disease and you are willing to accept the responsibility of preventing unwanted pregnancies as well as potential behavioral issues, we would then advise spaying/neutering closer to 12 months of age.