For the vast majority of American dog owners, the question is a no-brainer: unless you breed dogs or participate in dog shows—or don’t mind being persona non grata at the dog park—you have your dog “fixed.” About 80 percent of U.S. dogs have been altered—relieved of their sex hormone-producing organs, a.k.a. gonads (testicles for males, ovaries for females), through a surgical procedure known as a gonadectomy (castration for the boys, spaying for the girls).
Of course, the primary objective of surgical sterilization is to prevent unwanted litters, and in that regard, the procedure is extremely effective. It became standard practice in the U.S. in the 1970s and has had a huge impact on the numbers of puppies roaming the streets or filling shelters. Animal welfare organizations continue to endorse it as the best and only way to get—and keep—shelter numbers down, even as many advocates also push for more research into alternative methods of contraception.
Shelters also promote—and practice— early sterilization, says Joyce Briggs, president of the Alliance for Contraception in Cats and Dogs (ACC&D), an advocacy group that promotes non-surgical methods. While veterinarians have traditionally recommended altering a dog between the ages of six and nine months (HSUS now recommends four months), that’s a bit late for a puppy who’s ready to be adopted at eight weeks.
Plus, most shelters and rescue groups would rather send an animal home already neutered or spayed than ask the adopters to have the procedure done at a later date (in many areas, shelters can’t legally place intact dogs). “Pediatric spay/neuter is a critical component of shelter dog placement,” says Briggs. After all, she says, the goal is to get dogs out of the shelter, and to ensure they can’t produce litters that could add to future shelter populations.
The procedure also reduces a dog’s chances of being relinquished to a shelter later, regardless of whether the dog started out as a shelter pet or not. Statistics are clear on this one: intact dogs of both sexes are surrendered to shelters roughly twice as often as those who have been sterilized.
While here in the U.S., this is generally accepted as the right thing to do, it’s not seen that way everywhere. In other parts of the world, it ranges from generally accepted to culturally taboo. In many European countries, neutering and spaying are less common than in the U.S., but not entirely unheard-of (with the exception of Norway, where it’s illegal unless done explicitly for the animal’s health). In other places, including much of Latin America, neutering is considered both physically and socially emasculating (for both dog and owner, apparently) and therefore anathema.
Forest vs. Trees
But while there’s little argument that spay/neuter programs benefit the general dog population, there’s growing debate over the merits for owned dogs, says Stephen Zawistowski, PhD, a certified applied animal behaviorist and science advisor to the ASPCA. Because surgical sterilization is so effective at preventing unwanted breeding, the animal welfare community, as well as veterinarians and other advocates, “might have oversold some of it,” he says, by stressing its health and longevity benefits for individual dogs as well.
In reality, he says, the health effects are a mixed bag: some research shows clear benefits, other shows disadvantages, at least for certain subsets of dogs. As a result, many vets and other experts are stepping away from the standard advice— to alter every animal as quickly as is safe and practical—in favor of a more case-by-case (or dog-by-dog) approach.
Crunching The Numbers
Looking at studies and statistics is not for the faint of heart (or the math-challenged). But to calculate the odds most accurately, you’ve got to separate what the experts call relative and absolute risk.
Say a study shows that altered dogs of a certain breed get a certain disease twice as often as intact dogs. That would make the relative risk of altering those dogs 200 percent, which sounds huge. But that number would apply only if the disease occurred in virtually every dog of that breed. If the disease is uncommon—maybe it occurs in only one out of every 1,000 intact dogs (0.001 percent)—then doubling that number wouldn’t make the absolute risk much greater (0.002 percent) for any one dog. However, if it’s a common condition—it occurs in 10 percent of all intact dogs—then doubling the relative risk would mean that altering puts the dog at a greater absolute risk (20 percent) for that disease.
Why So Complicated?
Although it might seem that the whole cause-and-effect thing would be easy —dogs who are surgically sterilized are X times more (or less) likely to get disease Y or condition Z than dogs who aren’t—it’s actually tougher than it looks, says Brennen McKenzie, DVM, a veterinarian at Adobe Animal Hospital in Los Altos, Calif., and author of the “SkeptVet” blog. “There’s no such thing as ‘simple’ when it comes to medicine, and anything you do to an animal—and all the big and subtle changes that go along with it— carries both risks and benefits.”
One big problem, McKenzie says, is the shortage of quality research on the subject: The studies that do exist are regressive, meaning they look back at the data and try to draw conclusions; also, they’re typically small and don’t always control for critical variables, such as the age at which an animal was altered. Some studies are based on owner surveys, which are notoriously unreliable (some owners describe a dog as “aggressive” because it growls when playing, for example).
Take the concept of longevity. Most research indicates that neutering and spaying increases canine lifespan. One recent study, which tracked 2.2 million dogs across the country, found that they live an average of 11 years, with neutered males living 18 percent longer (and spayed females 23 percent longer) than intact animals. But other studies have found the opposite: altered dogs (or dogs who were neutered or spayed at a younger age) actually died earlier than did intact dogs (or those neutered or spayed later in life).
This discrepancy can be explained in part by the fact that study data is just that—data—that can be interpreted in many ways, some more valid than others. So, although the raw numbers say that intact dogs die sooner than their altered brethren, that doesn’t mean that the altered dogs lived longer because they’d had their gonads removed (what experts call a “causal relationship”). These numbers include deaths from all causes; dogs who were hit by cars or died of distemper or other preventable diseases are counted along with dogs who succumbed to cancer or another disease with a significant genetic (read: not preventable) component.
This big-picture view also misses some key socio-economic factors: statistics show that dogs who are surgically sterilized typically belong to humans who are relatively wealthier, better educated and more likely to provide quality veterinary care. Thus, these dogs are already at an advantage; they’ve had their shots, they aren’t wandering the streets (dodging traffic, picking through trash, getting into fights), they’re eating a healthy diet. All of which might mean that the procedure was just part of the dogs’ better, and more-conducive-to-longevity, standard of living.
The same questions can be applied to other research as well. For example, a handful of studies have found higher rates of certain cancers in altered dogs, even when those same dog populations live longer overall when they’ve been neutered or spayed. How can that be? One theory is that, in dogs as in humans, many of the deadliest diseases occur later in life; the longer one lives, the more likely one is to develop a serious disease. In this scenario, altered dogs would automatically have higher rates of age-related diseases simply because they’re living longer. Thus, the dogs didn’t necessarily get cancer because they were altered—they lived long enough to get cancer.
Likewise, studies that look at behavior are also open to interpretation. For example, a 2011 study of dogs who had bitten children found that nearly all (93 percent) were altered. Observers might infer that sterilization causes aggression. But they might also conclude that the dogs in the study were likely to bite regardless of their status, and that this aggression might have been at least one reason they were altered in the first place.
Hormones: Friend or Foe?
Of course, the whole issue of surgical sterilization (or not) is one of hormones: Aside from their role in reproduction, are they good for the dog’s health, or are they bad? And is it a question of dosage —how much is too much?—or an allor- nothing situation?
The age at which a dog is altered can have big implications on the chance of developing several diseases, simply because the longer a dog is in possession of his or her gonads, the more sex hormones the dog’s body will receive. But the ideal level of exposure to those hormones is anything but clear. In humans, the understandably limited data on castrated vs. intact men show that eunuchs live longer (up to 20 years longer, according to a recent study of 16th-to-19th century Korean men who’d been castrated before puberty). But for women, the opposite seems to be true; women who keep their ovaries past the age of 50 live longer than those who undergo a non-ovary-sparing hysterectomy before that age.
Research has shown that female Rottweilers live longer—and stay disease-free—when their ovaries are left in place or removed at a later-than-average age. In Rotties, the ovaries appear to increase lifespan, at least in part because they may protect against certain types of cancer. (Another study found that male and female Rottweilers who were altered before their first birthday were three to four times more likely to develop osteosarcoma, or bone cancer.)
But even these studies are oddly inconclusive: should we conclude that Rottweilers are unique among dog breeds, or that this research simply illustrates the need for more research? Lumping all dogs who died without their ovaries into one category— “spayed”—ignores the fact that dogs who were altered late in life (perhaps because of infection or another non-breeding- related reason) had years of exposure to sex hormones, which may very well have affected their health and longevity. Put another way, research that doesn’t consider the age at which an animal was sterilized may be (incorrectly) crediting the procedure with benefits that are actually related to keeping an animal intact, thus getting it backwards.
Meanwhile, research on Golden Retrievers analyzed dogs of both sexes, categorizing individuals as intact, sterilized early (before age one) or late, and tracking the development of several diseases common to Goldens, including five types of cancer. The study found that early-neutered males were three times as likely to develop lymphosarcoma (or lymphoma, a malignant cancer of the lymphoid system), while late-spayed females were four times as likely to develop hemangiosarcoma (malignant tumors that form in blood vessel cells, most often in the spleen). Overall, the likelihood of an intact dog developing one of the diseases being studied was typically one-fourth to one-half that of an altered dog. But the advantages of late sterilization over early (or vice versa) were inconclusive.
We know that removing gonadal hormones can affect everything from neurological and immunological functioning to blood clotting and metabolism. Research also shows a connection to orthopedic disease: in the Golden Retriever study, early-sterilized dogs had a greater chance of hip dysplasia and cranial cruciate ligament (CCL) rupture. Because the sex hormones have an impact on bone growth and development, altering dogs before puberty can create what’s known as differential growth. In this situation, some bones are longer (and lighter) than they would have been if the dog had matured with his or her gonads in place, potentially causing joint misalignments and other orthopedic problems.
When it comes to behavior, sterilization can definitely have an impact, says Zawistowski, although not always in the expected direction; some studies show that spayed females show more aggression than those who are intact, for instance. But what is clear is that the procedure will affect only those behaviors that are mediated by sex hormones. This should dispel one of the oldest myths about sterilization: that it will harm a hunting or working dog’s abilities. “Working behaviors have been genetically installed in certain breeds—they’re not part of the sex hormone equation,” Zawistowski explains. “A hunting dog will hunt, a guard dog will guard, a herding dog will herd regardless of whether he or she has been altered.”
The Bottom Line
If you’re looking for a nice clean conclusion to this discussion, you’re going to be disappointed, McKenzie says. Like any other surgery or treatment, sterilization is a double-edged sword, reducing the chances of certain conditions while raising the risks of others. Consider the research on women and hysterectomies: over the 24-year span of a major study, women who had their ovaries removed were less likely to develop breast cancer (and had almost no risk of other reproductive cancer), but were more likely to develop heart disease—and to die— than women who retained their ovaries. The research on dogs and surgical sterilization is similar. For example, an often-cited study from 2013 concluded that sterilization was “strongly associated with an increase in lifespan.” But while it “dramatically” decreased the risk of death from infectious disease and trauma, it increased a dog’s chances of dying from other causes, most notably cancer.
The goal, it seems, is one of balancing the needs of the general dog population (read: population control) with those of individual pets (read: optimal health and longevity). And the bottom line, if there is one, seems to be this: when you’re talking about preventing litters, surgical sterilization is the best option. And having it done early, insofar as it prevents breeding while preserving each animal’s health, is preferable to postponing the procedure.
Looking at the rest of the dog population, the research shows a fairly clear benefit for spaying any female dog who won’t be bred, in most cases before her first heat, McKenzie says (when it will do the most to reduce the risk of mammary tumors). As for males, he says, there’s no clear health benefit for neutering, at least for neutering before the dog is fully grown.
“I don’t think we’ll ever have a simple rule that can be applied in all cases— always alter, never alter, alter before this or that age—because biology is simply more complex and nuanced than that,” McKenzie says. Instead, he advises dog owners to do their homework: research conditions and diseases that are common in dogs like theirs, then look at the impact that sterilization might have. As he notes, “You have to understand the real risk to your dog.”
Other Sterilization Options
Zeuterin. The Food and Drug Administration recently approved the use of Zeuterin, the first and (so far) only nonsurgical form of neutering dogs in the U.S. Designed for 8-to-10-month-old males, Zeuterin is a solution of zinc gluconate that’s injected directly into each testicle (vets and the manufacturer swear it’s essentially painless), where it kills sperm and triggers the formation of scar tissue that blocks the tubes that would otherwise carry the sperm. According to research trials, the results are permanent, and the process works in 99.6 percent of dogs. “Zeutered” dogs get to keep their testicles, although they typically shrink a bit after the procedure; the dog will get a “Z” tattoo to show he’s been sterilized. Dogs treated with Zeuterin also keep an average of 50 percent of their testosterone, as the testicles will continue to produce at least some sex hormones. This might be a good thing— or not—as testosterone appears to help on some fronts, hurt on others.
Gonadotropin-Releasing Hormone (GnRH ) agents. In Australia, New Zealand and parts of Europe, male dogs can be treated with Suprelorin (also known as Deslorelin), an implant that sterilizes male dogs for up to a year by neutralizing the production of the GnRH reproductive hormones. Gonacon is an immunocontraceptive injection currently approved in the U.S. for use in wild horses and deer (no word on when or if the government will okay it for dogs) that can sterilize an animal for up to four years by blocking the production of GnRH.
New spay surgery. For female dogs, some U.S. vets are replacing the standard spay surgery—the ovariohysterectomy, or OVA, which removes both the ovaries and the uterus—with an ovaries-only procedure called ovarioectomy (OVE), which has been used for many years in Europe. Leaving the uterus in place means a much less complicated surgery, with a speedier (and likely less painful) recovery, while still achieving the goals of spaying (infertility, prevention of “heat” cycles and various reproductive diseases).