Like most surgical interventions, spay and neuter, or collectively “castration”, has both benefits and risks associated with it. The procedure is most often performed prior to puberty, around 6 months of age but is sometimes done as young as 6 weeks old. The practice of surgically castrating pets became widely available in the 1930s. After intensive campaigning by shelters and rescue groups, spay/neuter programs began to spring up everywhere. Since then the euthanasia rate of unwanted pets has dropped steadily. Even in recent years we have seen a massive decrease in euthanasia from 15 million in the 1970s to 1.5 million in 2017 as reported by the American Society for the Prevention of Cruelty to Animals (ASPCA). The castration programs of shelters around the USA are credited with this decline as many shelters castrate the animal prior to adoption. While castration is an effective tool in decreasing the number of stray pets, recent research indicates that when performed prior to puberty castration may not be ideal for responsible pet owners. It is ultimately up to each owner to look at the data and decide what is best for their particular pet.
A commonly cited benefit of castration is an increase in the animal’s lifespan. A study of 80,000 dogs out of the University of Georgia used data out of Banfield Pet Hospital, a nationwide veterinarian clinic. The study showed castrated female dogs lived 23% longer than intact females, male dogs lived 18% longer, female cats lived 39% longer and male cats lived a whopping 62% longer. Castration is also associated with a decreased likelihood of infectious diseases such as pyometra. Pyometra is an infection of the uterus that affects 25-66% of intact dogs over the age of 9.
Early castration is associated with a decreased chance of some cancers, most notably mammary cancer in female dogs and benign prostatic hyperplasia and testicular cancer in male dogs. The risk of malignant mammary cancer in intact females ranges from 23% to 63% with a 50% chance of malignancy. If castrated after her second cycle there is a 26% of developing tumors but if she is castrated before her first cycle the risk drops to 0.5% chance. Testicular cancer on the other hand kills less than 1% of intact dogs each year and benign prostatic hyperplasia is not life threatening but may predispose the animal to other health risks.
While the chance of developing reproductive cancers decrease with castration, other cancer types increase in prevalence. Osteosarcoma is the most common cancer to affect dogs at a rate 15 times more than what is seen in humans. It accounts for 85% of bone tumors and is the leading cause of death in medium sized dogs. About 10,000 new cases are diagnosed every year and castration before the age of 1 increases the chance of osteosarcoma 3 to 4 times. This means that castrated males have a 28% chance of developing osteosarcoma, castrated females have 25% chance and intact dogs have an 8% chance. However, increased height and weight are risk factors for osteosarcoma and dogs castrated at an early age have a tendency to be both heavier and taller. Other cancers have been touted as having an increased prevalence after castration, however the results of these studies often contradict each other.
Another disorder that increases with castration is urinary incontinence. This can happen in both female and male dogs but tends to be more severe and last longer in females. Three separate studies found that urinary incontinence increases in prevalence when a female was castrated before 3 months and developing this condition was more likely in medium to large dogs. Other studies have found that urinary incontinence in females is a continuous scale as the age of castration decreases, the rate of acquired incontinence increases. Females also show an increased incidence of cystitis when castrated before they turn 6 months.
There is a clear connection between the function of the immune system and the sex hormones produed before castration. Immune tissues such as the thymus, lymph nodes and spleen have receptors for sex hormones. One study on dogs found that there was a significant increase in immune disorders in both castrated males and females, specifically hypoadrenocorticism, autoimmune hemolytic anemia, atopic dermatitis, hypothyroidism, inflammatory bowel disease, and immune-mediated thrombocytopenia. It was also found that Lupus erythematosus was more prevalent in castrated females compared to intact females. This may be due to the fact that females are more susceptible to autoimmune disorders because the female immune system is more sensitive to hormonal changes.
Another aspect of the effects of castration on a dog’s immune system is an increase in negative reactions to vaccination. One study looked at 1,226,159 dogs and 4,678 adverse vaccine responses over 3,439,576 doses of vaccines. The data came from 360 Banfield pet hospitals and found that the dogs at most risk of having adverse effects to a vaccine are young adult small-breed castrated males that received multiple vaccines per office visit. The risk of a vaccine reaction increased with each additional vaccine by 27% in dogs under 22 lbs and by 12% in dogs over 22 lbs. However, the adverse effects the study looked for only included allergic reactions, hives, anaphylaxis, cardiac arrest, cardiovascular shock and sudden death. Other risks not assessed in the study were vaccine-associated immune-mediated hemolytic anemia, vaccine-induced fibrosarcoma, and acute canine polyradiculoneuropathy (ACP). ACP is also called Coonhound paralysis, and is most often seen with the attenuated rabies vaccine. In humans this disease is called Guillain Barre Syndrome and has serious effects on quality of life.
The most common and predictable change seen in dogs castrated before puberty is bone and joint issues. Puberty initiates a release of sex hormones that help close a bone’s growth plates. The last growth plate to close is located in the front legs and, on average, closes a little after a dog’s first birthday, around 375 days. When dogs and cats are castrated they lose the ability to signal when growth plates should close and the bones grow beyond their intended length. Muscles provide 80% of stability across a joint. When the relationship between bone length and muscle development changes it can lead to a variety of orthopedic issues such as cranial cruciate ligament rupture, patellar luxation and hip dysplasia.
Multiple studies have looked into the incidence, severity, and mortality of hip dysplasia in dogs and compared it to the age of castration. Out of almost 2,000 dogs one study found that castration before 6 months of age was associated with a significantly increased incidence of hip dysplasia. The chance of developing the condition and the age of diagnosis was earlier in dogs castrated prior to 6 months which means the condition progressed more rapidly. However, dogs castrated after 6 months were 3 times more likely to be euthanized for the condition. In another study of 759 golden retrievers the risk of hip dysplasia in females was not found to not be influenced by castration or the timing of the surgery. However, males castrated before 1 year of age had twice the risk of hip dysplasia compared with intact dogs. Yet another study looked at 1,500 Labrador retrievers and found there was no difference in the risk of hip dysplasia in males. In females they found that dogs castrated before 6 months had the highest risk of developing hip dysplasia and the risk decreased linearly the older the dog was when they underwent the surgery. However, females that were castrated between the ages of 2 and 8 years old had a lower risk of the condition compared to intact dogs.
Similar findings are seen when studying elbow dysplasia and cruciate ligaments tears. Two studies, one of 10,000 dogs and the other of over 3,000 dogs, looked at the incidence of cranial cruciate ligament injuries in dogs castrated versus intact. Both found that the castration was associated with a two fold chance of injury. Two other studies reported the risk of cruciate ligament damage was increased in dogs castrated at a younger age. One reported castration before age 6 months meant a risk of 7.6% and the second study found a risk of 7.7% in dogs castrated before their first birthday. Fewer studies have been done on elbow dysplasia but from the data we have it follows a similar pattern. In all these orthopedic studies males were more likely to develop orthopedic issues.
The development of any condition is influenced by many environmental and genetic factors as well. All of the studies mentioned were retrospective and some aspects of how the dog grew up can not always be accounted for. Knowing there is a correlation does not tell us what exactly is responsible for the correlation. A major factor that influences the incidence of developing these conditions is obesity. Obesity is common after castration because the dog requires 25% fewer calories than they did prior to castration. All suggested doses on pet food labels are calculated for intact dogs. As such, owners should adjust their pet’s food accordingly. Castration is a large reason why shelters are euthanizing fewer abandoned pets. However, if given the choice it is up to every dog owner to weigh the risks and benefits of early castration.
Other Resources:
Sundburg CR, Belanger JM, Bannasch DL, et al. Gonadectomy effects on the risk of immune disorders in the dog: a retrospective study. BMC Vet Res. 2016;12(1):278. doi:10.1186/s12917-016-0911-5.
Bushby PA. The optimal age for spay/neuter: a critical analysis of spay neuter literature. Presented at the Southwest Veterinary Symposium; San Antonio, TX; 2018.
Howe, Lisa. “Current Perspectives on the Optimal Age to Spay/Castrate Dogs and Cats.” Veterinary Medicine: Research and Reports, 2015, p. 171., doi:10.2147/vmrr.s53264.
https://www.dvm360.com/view/reexamining-early-spay-neuter-paradigm-dogs
https://veterinary-practice.com/article/the-neutering-of-dogs-and-bitches-in-the-uk-and-europe
Howe LM, Slater MR, Boothe HW, et al. Long-term outcome of gonadectomy performed at an early age or traditional age in cats. J Am Vet Med Assoc. 2000;217(11):1661-1665. http://