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The Spay/Neuter Controversy – Dr. LeeAnn DuMars, ABVP

Let’s discuss the recent controversy surrounding early spay-neuter status in dogs (primarily) and cats. Veterinarians have been taught to recommend early spay-neuter sterilization for positive health benefits and population control. Recent studies have documented an increased risk to some very common cancers in dogs that are sterilized. The three main studies that are referred to include Rottweilers and bone cancers; Golden Retrievers and various cancers, and Vizslas. Like all studies, some of the evidence is strong; other evidence is less so. The studies were only conducted in purebred dogs, making interpretation to other breeds and mixed breeds challenging. Cats are more straight forward in recommendations and there have been no recent studies to document ill effects of sterilization. Unfortunately, veterinarians do not have all the answers yet. However, it is important to discuss the issues surrounding sterilization procedures with owners so that they may make an informed decision.

In female dogs, spaying reduces the risk of mammary (breast) cancer. The chart below shows the relative risk of breast cancer compared to intact dogs:

Time of Spay                        Relative risk of mammary cancer compared to intact dogs
Before first heat                   0.05%
Before second heat              8%
Before third heat                  26%
After third heat                     100%

In summary, after the third heat cycle, the incidence of mammary cancer is the same in spayed or intact female dogs.
Other reasons to spay female dogs include population control, prevention of pyometra (serious uterine infection that requires exploratory surgery for immediate spay and intensive hospitalization); no heat cycles, less roaming and attraction of males, and absence of ovarian or uterine cancers.
Spaying female dogs may increase the risk of obesity, urinary incontinence, cranial cruciate rupture and possibly hip dysplasia.

Castrated male dogs have a decreased risk for perineal hernia, benign prostatic diseases, perineal adenomas, interdog aggression, and less urine marking. There may be a decreased risk for human bite injuries.

Spayed female cats have a decreased risk of pyometra, mammary tumors and roaming as well attraction of male tomcats. Of course there is the obvious lack of problems with difficult births and overpopulation. Spayed female cats may have an increased risk of obesity, diabetes and feline lower urinary tract diseases.

Castrated male cats have decreased roaming, less intercat fighting, and less house soiling problems. They have an increased risk of obesity, lower urinary tract disease, and diabetes.

The following chart summarizes the evidence associated with sterilization of dogs as far as predisposition to some common serious cancers according to the recent studies:
Type of Cancer                                 Relative Risk Castrated Males                 Relative Risk Spayed Females
Osteosarcoma (bone cancer)        3.8                                                                  3.1
Bladder Cancer                                2-4                                                                 2-4
Prostate Cancer                               2.4-4.3
Splenic hemangiosarcoma                                                                                    2.2
Mast Cell tumors                                                                                                    4.1

According to the chart, neutered pets have 2-4 times greater risk of developing these cancers compared to intact dogs. Why is this so? Could obesity associated with sterilization predispose dogs to cancer? Could owners that do a better job of caring for their pets sterilize dogs and then visit a veterinarian to make the diagnosis of cancer? Purebred breed clubs have been diligent in tracking health issues regarding their breed and contributing to studies and perhaps owners of these breeds have been owners who followed through on good veterinary care? The answers are not clear yet and we need more studies to confirm or refute these findings. And what is the incidence in our mixed breed dogs? The Vizsla study in 2014 showed increased fear and aggression in both male and female dogs that were altered at less than 12 months of age. For mast cell tumors, lymphosarcoma, and hemangiosarcoma, in general, the younger the dog was sterilized, the younger the dog was when he/she developed cancer.

In shelter/rescue pets, we will likely continue early age sterilization as we were not able to slow overpopulation with old methods of entrusting adopters to follow through with sterilization of pets. This was a large factor in passing laws to provide mandatory spay-neuter of pets prior to adoption. From a monetary and humane perspective, communities were not able to keep up with the rising cost of sheltering and euthanizing excess pets. However, with private pet owners of both purebred and mixed breed dogs, we do have the duty to discuss the health issues controversy with them and allow them to make an informed decision.

Some guidance thus far… Sterilize all male and female cats prior to sexual maturity (by 7 months of age). Small and medium-sized dogs – spaying females is recommended prior to first heat cycle (by 7-8 months of age). For male dogs, the recommendations are less clear on when to neuter. Small and medium-sized dogs are less prone to bone tumors than larger breed dogs, so behavioral considerations rise to the top of the list. Neutering decreases male-on-male aggression and urine marking, less roaming, and less dog bites to humans (per the Center for Disease Control (CDC).
Large-breed male dogs are prone to hemanagiosarcoma and osteosarcoma bone tumors. Consider delaying neutering until after 18 months of age, if neutering at all. What?? Again, it depends on each pet owner’s ability to keep male dogs away from intact females, confined to a yard to eliminate roaming and possible injuries because of roaming. And then there is the human issue of increased dog bites – especially to children – with intact male dogs.

Large-breed female dogs are more prone to mammary cancer, so spaying prior to first estrus (usually by 12 months) is a good option. However, early spaying may increase the risk of bone tumors, lymphoma, mast cell tumors, splenic hemangiosarcoma, urinary incontinence and bladder cancer. Large-breed, obese dogs are not easy to spay and therefore, costs and risk both increase. And clients do not relish a 3-week bloody heat cycle in a large-breed female dog. Be aware of the controversy and discuss with your veterinarian for individual pet recommendations.

It is difficult to sort through all the issues relating to sterilizing privately owned pets. It likely depends on genetics, purebred vs mixed breed dogs, sizes of dog and other health issues that are not identified. Further, behavior issues come into play that are not included in these studies…. I.e. play group/doggie daycare participation, ability to keep dogs confined in yard to eliminate roaming and injury, increased dog bites to humans (75% of human dog bites are from male intact dogs per the CDC and 37% of dog bites in 2013 were to children 5-9 yrs. of age).
Quality of life issues are also not included in these studies – i.e. maybe my large-breed Golden Retriever intact male won’t succumb to bone cancer, but what if he gets out of the yard and gets hit-by-a-car and sustains pain and injury instead? What are the mortality rates secondary to behavior problems by keeping male dogs intact? We have no studies to investigate these issues and it is likely dependent on the breed of dog, regional differences in geography and cultural differences, as well as other factors.

No one answer satisfies every individual owner, dog or living situation. Be aware of the controversy so that you can have a conversation with your veterinarian and make an informed decision for your pet and living situation. More studies over the next 5-15 years will hopefully give additional guidance.