When to spay or neuter your dog

by Dr. Kate Lawton

Over the last couple of years there have been a few studies that look at the many long term effects of neutering our pets. It has been common practice in the United States to neuter cats and dogs by six months of age to reduce both overpopulation and the risk of undesirable behaviors.

Since establishing spay/neuter programs, we have seen a huge improvement in overpopulation. During the 1980s over 17 million pets were euthanized annually; currently, 4-6 million pets enter shelters annually with half still resulting in euthanasia.

Historically we have recommended spaying female dogs before their first heat cycle (estrus) to greatly reduce the risk of developing malignant mammary cancer later in life. The risk increases exponentially after each estrus cycle. While early spayed females have a 0.05% incidence of developing mammary cancer during their lifetime, it goes up to 8% after the first estrus, and up to 26% by the second estrus. The incidence of mammary cancer appears to be dramatically higher in European countries in which dog populations are predominantly sexually intact. In these countries, mammary tumors account for 50–70% of all tumors; meanwhile, mammary cancer is relatively uncommon in the U.S., where the majority of female dogs are spayed prior to their first heat cycle. Spaying also eliminates the risk of developing a pyometra, a life-threatening infection of the uterus.

The main reason we neuter males at a younger age is to prevent unwanted behaviors such as aggression, urine marking, and roaming. Unneutered dogs are twice as likely to be hit by a car. While not as closely related to the age of neutering, castration reduces the risk of prostatic enlargement and eliminates the risk of testicular cancer.

The study that received the most media attention in 2014 was conducted at UC Davis. They looked at a group of 759 Golden Retrievers over a 10-year period and found that there was a greater incidence of hip dysplasia, cranial cruciate ligament rupture, and two types of cancer - lymphosarcoma and hemangiosarcoma - when neutered at less than one year of age. The limitations of this study were the small sample size of a single breed. Golden retrievers are already known to be predisposed to all of those above-cited diseases.

UC Davis did a follow up study in 2015 involving 1,170 German shepherds. The findings show that 7% of intact males were diagnosed with one or more joint disorders, as contrasted with 21% of males that were neutered prior to 1 year of age. Five percent of intact females were diagnosed with one or more joint disorders, while in females neutered before the age of 1 the percentage diagnosed rose to 16%.

There was also an internet-based study of 2,505 Vizslas showing that neutered dogs, regardless of age at neuter, were predisposed to MCT, lymphoma and all types of cancer combined.

A University of Georgia study found a strong association between neutering and a longer lifespan. The cause of death was looked at for over 40,000 dogs over a 20-year period. They found a high correlation between a longer lifespan and a decreased risk of death from infectious disease and trauma in neutered animals. However, this study also saw an increase in death from cancer with dogs neutered at a younger age, similar to the Golden Retriever study. Since cancer is a disease that develops over time, could the connection between cancer and sterilization be the result of neutered dogs living longer?

In July 2014, Banfield Pet Hospital looked at data from 800 of their hospitals nationwide to include 2.2 million dogs. In their study, spayed female dogs live 23% longer and castrated male dogs 18% longer than their unneutered counterparts.

Banfield also looked at the higher risk of unneutered dogs becoming overweight and did not find an association with the age at which the animal was neutered. Neutered animals indeed have a lower metabolic rate than their intact counterparts but this is taken into account when establishing diet recommendation and daily caloric requirements. It may take a bit more restraint and diligence on the owner’s part but there is no physiological reason why a neutered animal cannot remain at an appropriate body weight.

So the question remains, when is the most appropriate time to neuter a pet? We think the answer varies from pet to pet and owner to owner. In our opinion, the protective benefits of spaying a female before her first heat cycle should receive heavy consideration. There is also a higher risk of complications during and following the spay surgery in older females due to increased uterine blood supply and size.

For males, we feel comfortable allowing them to mature until 12 – 18 months of age. However, if any inappropriate behaviors arise then a neuter should be considered immediately, before the behavior becomes learned. Also, please be advised that there is increased risk of post-operative hemorrhage in mature males. This can lead to the temporary development of a scrotal hematoma, a large blood clot filling the now empty scrotal sac. Scrotal hematomas typically self-resolve with rest and cold compresses!

Please talk to your veterinarian about the best age to neuter your pet.