So when should you spay or neuter your pet? As soon as possible, right? Maybe not. Some new studies have come out recently that question the ideal time to spay or neuter a pet. The idea of spaying and neutering (we’ll refer to it as neutering for both sexes) came about due to issues with an out-of-control pet population. Shelters were overrun with unwanted dogs and cats, and huge numbers of unwanted pets were being euthanized. Preventing uncontrolled reproduction was essential.
We also knew that neutered pets were less likely to roam (males can smell a female in heat a mile away, literally!), less aggressive, and less likely to pee on your things to mark their territory. The more we spayed and neutered the more we noticed a decline in mammary cancer and pyometra (pus-filled uterus); and obviously removing ovaries and testicles eliminates the risk of ovarian and testicular cancers. We found neutered pets also tended to live longer than their intact counterparts.
Most veterinary clinics have traditionally recommended neutering pets prior to sexual maturity (so around 5-6 months of age) to avoid behaviors associated with sexually mature pets. It also dramatically reduces the risk of mammary cancer, and surgical complications are less likely in young puppies than large dogs.
Shelters started neutering pets prior to adoption, as young as 8 weeks, because they found that owners were unlikely to bring the pet back later for the procedure, even if given vouchers to have it done for free. These young puppies and kittens recovered well and did great, and it was better to neuter that pet early than have an owner bring that pet back with behavior issues, or even a litter of unexpected pups they couldn’t keep.
For a long time we have been aware of some negative effects of neutering, such as decreased metabolism leading to weight gain, and urinary incontinence in females. These are generally manageable conditions. There are also the concerns with anesthetic and surgical complications, which can range from minor inconveniences to the rare possibility of death.
The recent studies that have come out look at how neuter status relates to frequency of joint disease and incidence of certain cancers over the course of a pet’s lifetime. Many of the studies presented thus far focus on individual breeds of dogs, and the results vary between breeds, therefore we cannot apply these results to mixed breed dogs or breeds that have not been studied. They are also retrospective studies, meaning researchers gathered data from pets previously seen at teaching hospitals (primarily UC-Davis). These studies varied in whether they addressed how old the pet was when neutered (1 year), which also complicates comparisons. For example, sex hormones influence the closure of growth plates, so joint diseases may not be a problem if the pet was neutered after growth plates closed, while it could be a factor if neutered very young.
There are inherent issues with retrospective studies, including the fact that you can’t have a true control group. You can determine correlation of diseases to neuter status, but you cannot prove causation (Just because neutered pets have more hip dysplasia doesn’t necessarily mean that it is because they are neutered. There may be other factors involved). The fact that the data was from a teaching hospital inherently biases for pets whose owners presumably do not have financial barriers to treating their pets. The care their pets receive over the course of a lifetime may be different from pets whose owners cannot afford advanced medical care. So the population of pets seen at a teaching hospital may not adequately represent the population of the breed as a whole.
Despite these inevitable flaws, several studies did find significant increases in cranial cruciate ligament (CCL) tears, hip dysplasia, and certain types of cancers in neutered vs. intact dogs. One study even looked at a correlation between neuter status and risk of immune mediated diseases. Estrogen and testosterone have both been found to be immune suppressive, which may be part of why intact pets are at a higher risk of death due to infectious diseases.
I am providing links to some breed-specific studies below, with a brief summary. If you have questions about when to neuter your own pet, please consult with your veterinarian to go over all the pros and cons. Keep in mind that these studies have created more questions than answers, so there may not be a clear “right” answer for your pet.
Rottweiler study: This study specifically looked at the risk of osteosarcoma (OSA) and how that was affected by exposure to gonadal hormones. It showed a dramatic increase in incidence of OSA for dogs neutered prior to 1 year of age compared to dogs neutered after 1 year or left intact. Because OSA is such a devastating diagnosis, and more frequent in Rottweilers than other breeds, it is currently recommended to wait until after 1 year of age to neuter this breed.
Golden Retriever Study: This study found a significant increase in the rate of hip dysplasia and cruciate ligament tears in early neutered males (1 yr) vs. early neutered females, and none were seen in intact females (no difference in males).
German Shepherd Study: This study found a significant increase in the incidence of CCL disease in early neutered (<12 mo) vs late neutered or intact dogs. No significant change in incidence of hip dysplasia or any of the cancers studied (mammary cancer, lymphosarcoma, mast cell tumors, hemangiosarcoma, osteosarcoma).
