When (and Whether) to Spay or Neuter your dog

New information may impact your decision about fixing your pet.


“I will not be renewing my subscription,” one Your Dog reader wrote, “because of the misinformation you gave on the appropriate age to spay/neuter. Not only that, I will also encourage all my dog club members to cancel any subscriptions that they may have. Shame on you!!!”

This person is referring to our comment in the August issue that most female dogs “should be spayed by the age of six months.”

“Don’t you read the most recent studies that recommend spay/neuter be deferred until the dog is 18 months to two years,” the reader admonished.

We stand by our advice — most female dogs should be spayed early. But we understand where the disgruntled subscriber is coming from. There are studies that suggest spaying and neutering can increase the risk for certain illnesses—but that the risk can be mitigated somewhat by delaying the surgery until beyond a year of age. How to make sense of it all?

For some perspective on the matter, we sat down with Your Dog editor-in-chief John Berg, DVM. He makes clear that the advice he gives here on certain aspects of the issue consists of hisopinions. His thinking does not necessarily reflect a consensus of the veterinary community in general, and there is much that is not yet known. But the considerations he lays out will help you make informed decisions as you think about spaying or neutering your own dog.

Your Dog: How can it be that most female dogs should be spayed early in life if there’s evidence that spaying can increase the risk for certain diseases?

Dr. Berg: It’s a complicated topic. As recently as 10 years ago, veterinarians pretty much automatically recommended spaying of every dog that wasn’t going to be bred. There were a number of reasons: to prevent unwanted puppies and to prevent a severe infection of the uterus later in the dog’s life called pyometra, which often requires an emergency hysterectomy.

Most of all, however, the advice to spay early was to prevent mammary cancer, a kind of cancer that is often ultimately fatal. We’ve known for a long time that if you spay prior to a dog’s first heat, which commonly occurs by the time she’s six months old, it virtually eliminates any mammary cancer risk. If you spay between the first and second heat, there’s still a sparing effect, but the risk does rise, and it rises more between the second and third heat. After the third heat, spaying may not confer any protection against mammary malignancies, although there are conflicting data about that.

Your Dog: What are the diseases that are more likely in spayed dogs? It seems straightforward.

Dr. Berg: For one thing, spayed dogs are more prone to obesity and also develop a small but statistically significant risk for urinary incontinence. Five percent of spayed dogs end up with it, anywhere from months to years after their ovaries are removed. But urinary incontinence is treatable with medicine, and obesity is treatable with lifestyle changes — even preventable — so those two do not make the timing of spaying a dog so much of a concern.

There’s also some evidence, albeit rather scant, that spayed dogs may be more likely to get cruciate ligament ruptures in their knee, which are painful. But that, too, is fixable, with surgery.

However, there has been some information emerging over the last decade that spaying — as well as neutering male dogs — may predispose them to certain other diseases that can be more difficult to cure. Some of the data are fairly weak. But there are important illnesses on the list. These include various other cancers, like mast cell tumors, lymphoma, transitional cell carcinoma, which is cancer of the bladder — Scotties are prone to that one — and osteosarcoma, which is bone cancer. Many of these cancers, including lymphoma, transitional cell carcinoma, and osteosarcoma, are almost always eventually fatal, even though treatment can provide additional high-quality time. Only large-breed dogs are prone to osteosarcoma. If it’s a small breed of dog, we don’t worry about that one.

Your Dog: How worried should people be?

Dr. Berg: Well, studies have been coming out to suggest that there’s maybe a two- to three-fold increased risk for these various cancers in spayed dogs, and that the risk may be highest if the dogs are spayed while young, for reasons we don’t completely understand. Not all of the evidence is solid. But if you get enough evidence saying the same thing, you start to worry that there may be some truth in it.

Your Dog: So why do you feel that dogs should be spayed before the age of six months to prevent just one type of cancer when evidence is coming to light that the risk for a number of other cancers may increase as a result of early spaying?

Dr. Berg: Mammary cancer is quite common among unspayed dogs, and often fatal. And we know for a fact that early spaying practically eliminates the risk. For the other cancers, we don’t yet know for certain that early spaying increases the chances for their occurrences. Remember, too, that a two- to three-fold increase in risk for some of the cancers could still amount to a very small risk. For instance, if the risk for a specific type of cancer goes up two-fold, that might translate to a difference of one in every 10,000 dogs to one in every 5,000, making it statistically detectable but not very relevant in the real world. Until we have a better handle on whether there is an increased risk of certain cancers, and what the level of risk actually is, my bias is to prevent the known, established risk for mammary cancer.

Your Dog: What about male dogs? Should they continue to be neutered as puppies, or are there emerging data to suggest it’s better to wait?

Dr. Berg:Okay, so let’s look at what happens with males. What do we prevent by neutering young, and by young I don’t even necessarily mean as puppies but simply early in life, before middle age?

One thing is benign prostate diseases, that is, not prostate cancer but other, non-fatal prostate issues. These include prostate infections, prostatic abscesses, and benign prostate hypertrophy, which is an enlargement of the prostrate that creates difficulty urinating. Men get it, too.

Neutering a male dog also prevents perianal adenomas — benign tumors around the anal area. They are almost always a condition of intact male dogs.

Castration prevents perianal hernias, too. That’s a herniation in the pelvic area that can make it difficult for a male dog to defecate.

Your Dog: That sounds like a lot of illnesses that neutering a male dog can prevent.

Dr. Berg: Yes, but here’s the thing. All of these diseases are treatable, and generally not life-threatening to begin with. If you castrate a dog with a prostate problem, it will cure benign prostate hypertrophy and be part of a treatment plan, along with medication, for prostate infections and abscesses. Castration is also a component of effective treatment for a dog with a perianal adenoma, as it is for perianal hernias. That is, if a male dog gets a condition as a result of not having been neutered, you can solve the problem without fear that the disease will kill him. That’s also true for the obesity that is more apt to affect castrated male dogs, just like with females.

On the other hand, evidence has come to light that castrating a male dog young can potentially predispose him to diseases that might not be curable. The data are imperfect, just as they are for females, but they’re there.

Your Dog: What do you mean?

Dr. Berg: There’s some evidence for males that neutering them while young may predispose them later on to cognitive dysfunction, the canine version of Alzheimer’s disease. There are also some data to suggest that the same cancers that might affect spayed females to a greater degree than intact females might also occur more often in neutered males than in those who have not been castrated. And neutered males, too, may be more prone to cruciate ligament ruptures — not life-threatening, but serious and expensive to treat.

Your Dog: So what does this mean for the owner of a male dog?

Dr. Berg: While I believe most female dogs should be spayed by the age of four of five months — prior to their first heat — to essentially erase the risk for mammary cancer, which can kill them, to my mind it’s more questionable whether we should be neutering male dogs as puppies speaking strictly from a disease standpoint. Males not castrated are not more prone to life-threatening illnesses but rather illnesses that can be dealt with, whereas castrated males might be more likely to get diseases that could prove fatal.

I want to make clear that the emphasis is very much on might. It’s possible that further research will show that males neutered while young — and females spayed during their first half year, for that matter — are not in fact more prone to various cancers. But still, it’s something worth considering.

Certainly for large-breed male dogs who, according to the early research, might be prone to getting osteosarcoma — a highly fatal kind of cancer — waiting to neuter them until they’re at least a year old could be a reasonable option. Researchers have detected an increased risk for osteosarcoma only in those large-breed males neutered prior to the age of one year. After that, no increased risk could be detected — which is different from saying there is no increased risk. And neutering after a full year will still have the added benefit of protecting against all those diseases of the prostate.

Your Dog: What do you mean when you say we may need to reconsider neutering male puppies young “strictly from a disease standpoint?”

Dr. Berg: There are also behavioral components to neutering, and they are important.

Your Dog: How so?

Dr. Berg: For one thing, un-neutered dogs are almost three times as likely to bite as those who have been neutered, says the Humane Society of the United States. And the American Veterinary Medical Association’s Task Force on Canine Aggression and Humane-Canine Interactions says that intact male dogs are “involved in 70 to 76% of reported dog bite incidents.” Because of such statistics, the Centers for Disease Control and Prevention recommend neutering male dogs to lessen the risk that they will bite.

Your Dog: Okay, but are there really that many people who are bitten by dogs?

Dr. Berg: There are close to 5 million dog bites occurring each year in the United States, most of them sustained by children younger than 15. You could definitely make a reasonable argument that this becomes as important in the mix of deciding whether to neuter a dog as the dog’s own health outlook.

Your Dog: What are the other behavioral components involved with neutering?

Dr. Berg: Research shows that castration reduces urine marking, mounting, and roaming by more than 50 percent in six out of 10 male dogs. In many of those dogs, the unwanted behavior drops by more than 90 percent.

Your Dog: Anything else?

Dr. Berg: Neutering helps reduce aggression in more than one in three dogs. That includes aggression toward human family members, aggression toward unfamiliar people, aggression toward other dogs in the household, and also toward unfamiliar dogs. For all of the behavior changes I’ve mentioned, neither the age of the dog nor the duration of the problem behavior at the time of castration appears to affect the outcome. Thus, for instance, if you want to neuter a large breed dog but fear that he will fall victim to osteosarcoma, you can wait till he’s at least one year of age. The waiting won’t dampen the effect on behavior.


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