When should you neuter your dog? Scientists recommend you use this chart

Getting dogs neutered or spayed is a routine part of pet ownership. Experts say it can keep pets healthier — it can also lead to increased health risks in some dogs, including joint disorders and some cancers, according to new research.

Neutering dogs at the right age can help to prevent putting the animals in danger. But exactly when you should neuter your dog — or if you should even neuter it at all — depends on its breed, scientists report.

Over a 10-year span, scientists studied 35 dog breeds to determine their risk of developing five types of cancer and three joint disorders, in relation to being neutered.

In the majority of cases, the age at which a dog was neutered, or if it was neutered, didn't affect its chance of becoming ill. Overall, they discovered:

• In most cases, it doesn't matter at what age a dog gets neutered.
• In small dogs, on average, neutering was not linked to an increase in joint disorders.
• In only two small dog breeds — the Boston Terrier and the Shih Tzu — there was an increase in cancer risk linked to neutering.
• Large dog breeds who were neutered were more likely to have an increased risk for both cancer and joint disorders, with the exception of Great Danes and Irish Wolfhounds.

The results appear in a study published on July 10 in the journal Frontiers in Veterinary Science.

Neutering and disease risk — Previous research has looked at the risk of disease in neutered dogs. One 2018 study, conducted by some of the same research team, found that cancer risk in Golden Retrievers increases by 5 percent or higher after neutering.

The new research adds complexity to those findings by studying a wider range of dog breeds, which vary in their risk of disease.

"There is a huge disparity among different breeds," lead study author Benjamin Hart, professor emeritus at the UC Davis School of Veterinary Medicine, explained. That means that, for pet owners, one size does not fit all.

The size of the dog, however, could be a major factor.

Smaller breeds tended not to have joint issues after being neutered, and showed lower rates of cancer overall, compared to larger dogs. But two small breeds — Boston Terriers and Shih Tzus — each had a significant increase in cancers after being neutered.

The majority of larger dog breeds did have joint disorders, the researchers found. They found that, in general, vulnerability was related to body size. There were few exceptions, however: Great Danes and Irish Wolfhounds each showed no joint disease risk increase post-neutering, despite their large size. The authors write:

Vulnerability to joint disorders associated with neutering is generally related to body size. Small-dog breeds – Boston Terrier, Cavalier King Charles Spaniel, Chihuahua, Corgi, Dachshund, Maltese, Pomeranian, Poodle-Toy, Pug, Shih Tzu, Yorkshire Terrier – do not appear to have an increased risk in joint disorders with neutering compared to the breeds of larger size.

The sex of a dog can also affect health risks. In Boston Terriers neutered at six months old, females did not experience increased risks of joint disorders or cancers, but males did.

What should dog owners do? — To help dog owners determine the best way to keep their pets healthy, the researchers developed a chart for each of the dog breeds they studied.

In general, there is support for neutering pets among the veterinary community. Neutering can prevent tumors, ease behavioral problems, and, of course, prevent unwanted puppies, the ASPCA reports.

But deciding when to put your pup under the knife is an important part of the decision. In the United States, most dog owners neuter their pets before they are six months old.

The new long-term study suggests that might not always be the best option.

"We think it's the decision of the pet owner, in consultation with their veterinarian, not society's expectations that should dictate when to neuter," Hart said. "This is a paradigm shift for the most commonly performed operation in veterinary practice."

Abstract: Neutering (including spaying) of male and female dogs in the first year after birth has become routine in the U.S. and much of Europe, but recent research reveals that for some dog breeds, neutering may be associated with increased risks of debilitating joint disorders and some cancers, complicating pet owners' decisions on neutering. The joint disorders include hip dysplasia, cranial cruciate ligament tear or rupture, and elbow dysplasia. The cancers include lymphoma, mast cell tumor, hemangiosarcoma, and osteosarcoma. In previous studies on the Golden Retriever, Labrador Retriever and German Shepherd Dog, neutering before a year of age was associated with increased risks of one or more joint disorders, 2–4 times that of intact dogs. The increase was particularly seen with dogs neutered by 6 months of age. In female Golden Retrievers, there was an increase in one or more of the cancers followed to about 2–4 times that of intact females with neutering at any age. The goal of the present study was to expand and use the same data collection and analyses to cover an additional 29 breeds, plus three varieties of Poodles. There were major breed differences in vulnerability to neutering, both with regard to joint disorders and cancers. In most cases, the caregiver can choose the age of neutering without increasing the risks of these joint disorders or cancers. Small-dog breeds seemed to have no increased risks of joint disorders associated with neutering, and in only two small breeds (Boston Terrier and Shih Tzu) was there a significant increase in cancers. To assist pet owners and veterinarians in deciding on the age of neutering a specific dog, guidelines that avoid increasing the risks of a dog acquiring these joint disorders or cancers are laid out for neutering ages on a breed-by-breed and sex basis.