Spay/neuter: What does the science say?

January 8, 2008

It’s become an absolute tenet in the canon of good pet ownership that spaying and neutering our cats and dogs is healthier than leaving them intact. We’ve all heard the dire warnings about mammary tumors and uterine infections in bitches, unwanted marking and aggression in male dogs, and of course, the various societal or public health benefits of preventing the unwanted reproduction of dogs and cats.

But with all that talk, facts have been a bit scarce. Last month, the Journal of the American Veterinary Medical Association took a big step towards changing that, bringing together in one article a great deal of the research on the pros and cons of spaying and neutering pet dogs and cats.

The AVMA this week published the article publicly, so that pet owners could access it without paying the usual $10 fee. They have asked that if you choose to share the article, that you do so by giving out the link to it (which is at the end of this post) rather than copying the PDF file, so please honor that request — if not because it’s the right thing to do, then because it might lead to even more articles being made public in the future.

Author Margaret V. Root Kustritz, DVM, PhD, DACT presents a comprehensive overview of research on the varying effects of sterilization surgery on male and female cats and dogs, as well as animals of different ages at the time of surgery. She begins by pointing out something that will be news to a lot of people:

Currently, most veterinarians in the United States recommend that elective gonadectomy [spay/neuter surgery] be performed in dogs and cats at 6 to 9 months of age. However, there does not appear to be any scientific evidence to document that this is the optimal age.

In other words, while veterinarians commonly recommend spaying and neutering be done on pet dogs and cats at around 6-9 months of age, that’s just a habit, and not based on medical fact.

Pet Overpopulation and Social Benefit

After an overview, Kustritz tackles the societal perspective and the contentious issue of pet overpopulation. Obviously, animals who have been sterilized cannot reproduce, so their offspring will never find their way into the shelter system. In addition, she cites multiple studies showing intact dogs and cats are more likely to be surrendered at shelters than spayed or neutered dogs and cats — although one study showed the opposite; she speculates that the animals in that study may have been altered as part of the treatment of their behavioral problems.

She then uses numbers from 18 years ago to offer support for a hypothesis of “pet overpopulation.” Her figures are given as a range, but the top of that range would be 18.6 million dogs and cats euthanized in shelters each year, when the figures I’ve seen for last year were more along the lines of 4-5 million… still 4-5 million too many and a tragedy, but her argument on the societal benefit of spay/neuter would have been stronger if it wasn’t based on such outdated statistics. She indicated that owner ignorance about canine and feline reproduction was behind many relinquishments to shelters, citing in this case a 2004 study.

She then cited some more early 90s studies:

Owners that adopt animals from humane organizations routinely sign a spay-neuter contract. However, compliance with such contracts is typically [less than] 60%. Up to 90% of veterinarians support mandatory gonadectomy of dogs and cats prior to adoption.

The “gold standard” for communities now is to not adopt unaltered pets, and while I’m sure it does still happen, it’s certainly far less common than it was 15 years ago, when the studies she cites were conducted.

She concludes this section by saying that veterinarians are largely untrained in early-age sterilization of kittens and puppies, and that remedying that, as well as requiring the sterilization of all animals adopted from shelters before adoption and “increased education of dog and cat owners about small animal reproductive physiology can only be of benefit in addressing these societal issues.”

I want to remind everyone that this section deals with societal issues, not the individual health issues of pets — the medical aspects of early spay/neuter are addressed later in the article, along with a full discussion of the issues around spay/neuter surgery — after the jump.

The Effect of Spay/Neuter on Behavior

Spaying and neutering have been shown to reduce aggression between animals that is directly caused by the presence of a female in heat (estrus). That’s very straightforward and doesn’t really need any explanation.

She states that the trainability of working dogs is not altered by sterilization, and that age of sterilization also has no effect on the trainability of working dogs. The citation for this assertion was simply “personal communication” with R. Daniels at Canine Companions for Independence, which is disappointing, as it is completely contrary to the experience of many working dog trainers. Service and assistance dogs are working dogs, certainly, but dogs do many kinds of work in many different settings, and I don’t believe you can extrapolate from one type of work to another.

Which actually brings to mind another “societal” issue — that of the continued existence of working dogs. Even if spaying and neutering them didn’t impact their trainability, it would make it pretty hard to use your best working dogs to produce another generation of working dogs — and where will our police dogs, drug dogs, search and rescue dogs, guide dogs, stock dogs, and other working animals come from if they’re all altered?

I think that each type of work and the needs of each breeding population needs to be looked at individually, and far more comprehensively, before a blunt statement in a veterinary article that sterilization has no effect on the trainability of working dogs is made — particularly when it’s backed up by nothing more than a personal communication. I’d like to see this statement qualified quite a bit, or even removed.

She then looks at the sexual behavior of male cats, which she says makes them “undesirable, and often unsafe, household pets.” Neutering tomcats decreases their unwanted behavior. Intact female cats and intact dogs of both sexes may still have some undesirable traits, but not to the extent of intact male cats. I’m sure a thousand cat breeders will yell at me, but I can’t really argue with that. I’ve had intact male dogs for almost a quarter century, but managing an intact male cat is a task I’d never want to take on.

She says that other types of behavior problems are not “typically affected” by sterilization, although she cites one large study that found that noise phobias increased while separation anxiety and submissive urination decreased in dogs altered prior to the age of 5 months.

She then says that “several studies” have seen an increase in aggression both against humans and other dogs after spaying, and adds that “The reason for this possible tendency has not been defined but may be attributable to a decrease in estrogen and oxytocin concentrations, both of which may exert antianxiety effects in some species. This tendency also may be a breed-specific phenomenon.”

She then looks at how the ability to think — cognitive function — is affected by spaying and neutering. One small study (only 6 dogs) found that intact dogs had a slower progression toward cognitive problems than neutered males. Male hormone deprivation has been associated with similar problems in other species, including humans. But in another study, investigators looked directly at the brains of laboratory beagles between 9 and 10. 5 years old, and found more neurons with DNA damage in the intact dogs than neutered dogs.

Physical Health and Spay/Neuter

Dr. Kustritz then moves into the literally massive amount of literature on the positive and negative health impacts of spaying and neutering, including cancer and orthopedic disease.


First is some well known territory: Mammary gland tumors in female dogs, which are the most common type of malignant tumors in the dog — half of all canine mammary tumors are malignant, and up to 77 percent of those metastasize, often to the lungs. This risk increases as the dogs and cats get older, and some breeds are at greater risk than others, including “the Boxer, Brittany, Cocker Spaniel, Dachshund, English Setter, English Springer Spaniel, German Shepherd Dog, Maltese, Miniature Poodle, Pointer, Toy Poodle, and Yorkshire Terrier.”

Only the Japanese domestic breeds and Siamese cats are stated as being at increased risk for feline mammary tumors.

Unspayed dogs and cats have an overall 7 times greater risk of developing mammary tumors than spayed dogs and cats. When you break that down, however, it’s much clearer:

Dogs have an overall incidence of mammary tumors of 3.4 percent. Dogs spayed before their first heat have only a 0.5 percent of the chance of getting a mammary tumor that intact female dogs have; this is a 1.7 percent chance overall, although it may be more or less depending on breed and other factors. Dogs spayed after they’ve had two heats have 26 percent of the risk of intact female dogs, or around 1.3 percent overall (again, dependent on breed and other factors). [Update: See this comment for additional discussion of this statistic.] I didn’t see any mention if pregnancy changed those statistics, as it does, for instance, in human females.

On the other hand, ovarian and uterine tumors are uncommon in dogs and cats, and rarely metastasize or are malignant. Spaying cures them.

On the male side of things, neutered male dogs are at increased risk of prostatic cancer. The incidence of prostate tumors in dogs is between 0.2 and 0.6 percent, with almost all of them being malignant adenocarcinomas. Neutered males have 2.4 to 4.3 times the risk of intact dogs.

Testicular tumors are, of course, not found in dogs who have had their testicles removed, but testicular tumors in dogs tend to appear late in life and are rarely malignant; neutering at that time will cure them in most cases.

Spayed and neutered animals have a 200 to 400 percent greater risk of developing a form of bladder cancer (transitional cell carcinoma, or TCC) than intact animals, but the reason is unknown.

Spaying and neutering can increase the risk of osteosarcoma (bone cancer) by 1.3 to 2 times, although the cause and effect relationship is unknown.

Hemangiosarcoma is a tumor of the heart or spleen; the heart tumor has a fairly high incidence in dogs. Spayed and neutered dogs are at greater risk of developing these tumors. Spayed females have 2.2 times the risk of a spleen tumor and 5 times the risk of a heart tumor, compared with the risk for unspayed females.

Neutered males have 2.4 times the risk of a heart tumor than intact males. The reason for this increased risk is unknown.

Orthopedic Issues

Unlike in humans, there is no increased risk of osteoporosis in spayed animals.

Early age sterilization delays the closure of the covering on the long bones, and makes them a little longer than they would have been. There is no increased rate of fracture associated with this.

One study did find an increased incidence of fractures in overweight neutered male cats.

There was an increased incidence of hip dysplasia in dogs who had been spayed or neutered prior to 5 months of age in one large study of 1,842 dogs, however, the diagnosis of hip dysplasia may have been in doubt with some of the dogs in the study.

In humans, rupture of the cranial cruciate ligament (or CCL, a ligament in the knee) is more common in women than men and more likely to occur during certain phases of the menstrual cycle, which leads researchers to believe there is a hormonal cause for this condition. In dogs, CCL rupture is more common in spayed females and neutered males.


Despite the common refrain that “altering doesn’t make your pets gain weight,” Dr. Kustritz states that “obesity is the most common nutritional disorder of cats and dogs,” and goes on to say:

The most commonly reported risk factor for obesity is gonadectomy, with spayed or castrated dogs and cats much more commonly designated by veterinarians as being overweight or obese, compared with the weight designations for sexually intact animals.

One study found that there was no difference in food intake, body weight, or depth of back fat in two groups of dogs, some altered at 7-8 weeks old, and the others at 7 months.

However, another large study — a retrospective study, which means they “looked back” at dogs who had already been altered rather than having a test population specifically for the study — found that dogs altered prior to 5 months of age had a lower rate of obesity than those altered at greater than 5 months.

Two studies found that the metabolic rate of spayed and neutered cats is lower than intact cats, but no such clear-cut cause and effect relationship is known in dogs.

Female dogs do experience increased appetite after being spayed when compared to dogs of the same age who are unspayed — including those who had a “sham surgery.” This contests the common explanation that spaying doesn’t make female dogs fat, it’s just that they’re getting older. Dr. Kustritz writes:

Estrogen may act as a satiety factor, which would explain these changes. This does not address the correlation between obesity and castration in male dogs.

Urinary Tract and Genital Abnormalities and Disorders

For unknown reasons, female dogs have a greater risk of bladder infections.

Female dogs spayed before puberty may have some abnormalities of their vulvas, and Dr. Kustritz writes:

It is the author’s experience that bitches spayed as adults will have vulvar atrophy, which achieves the same result. A juvenile vulva in an otherwise healthy dog is of no clinical relevance.

Female dogs who are overweight and have a recessed vulva, especially if they also suffer from urinary leaking (incontinence), are more likely to develop a chronic rash in the area.

Male dogs neutered at 7 weeks of age had a smaller penis than dogs neutered when older, and male cats neutered prior to puberty had less ability to extrude the penis. It’s not known if this is a health issue or not.

I’m a little confused by her statements on feline lower urinary tract disease (FLUTD), She first states neutering has no effect on the rate of FLUTD in male cats:

Despite numerous vehemently declared anecdotes of an increase in the incidence of urethral obstruction in male cats castrated when young, numerous studies have failed to detect a correlation between gonadectomy of cats at any age and a decrease in diameter of the urethra or an increase in incidence of FLUTD, with or without urethral obstruction.

She then goes on to apparently contradict that:

In 1 large study,136 investigators identified gonadectomy as a risk factor for development of FLUTD in both female and male cats and also identified an increased risk of development of FLUTD in overweight or obese cats. In that study, sexually intact female cats had a relatively reduced risk for development of FLUTD.

She offers no explanation for this apparent contradiction.

The condition usually called “spay incontinence” in female dogs, which she calls “urethral sphincter mechanism incompetence, formerly known as estrogen-responsive urinary incontinence,” is common — as many as 20 percent of spayed dogs suffer from it. Here we have another one of those contradictions I can’t quite resolve.

She first says that “Studies have failed to detect a correlation between age at time of OHE (spay) and likelihood of developing incontinence.”

Yet her very next sentence contradicts that:

In a study of 983 female dogs, bitches were significantly less likely to develop incontinence when spayed at [greater than] 3 months of age…. An exact cause-and-effect relationship has not been defined, with research currently focusing on altered gonadotropin secretion after gonadectomy.

Endocrine System and Hormonal Issues

Dr. Kustritz states she’s not aware of any increase in the incidence of adrenal gland disease (Cushing’s, Addison’s) in dogs or cats related to altering, even though “the incidence of adrenal gland disease in ferrets is higher in the United States than in European countries where ferrets are not routinely spayed or castrated.”

Pyometra is an infection of the uterus. She somewhat surprised me by saying that there are no statistics on the rate of pyometra in dogs and cats in the United States, due to high rates of spaying. I certainly hear about quite a bit of pyo among breeders who I know, although I’m not saying that comes close to any kind of scientific documentation. But I know quite a few bitches who have had to be spayed due to a uterine infection.

She continues to say that in “other countries, 15.2% and 23% to 24% of bitches developed pyometra by 4 and 10 years of age, respectively. Pyometra is more common in [bitches who have never had a litter] than in bitches with a history of carrying a pregnancy successfully to term.”

She adds, “There is a significant likelihood that cats will have clinical evidence of uterine disease when queens reach 5 years of age.”

She says that spay is curative for pyometra, and morality rates are as low as zero in dogs and 8 percent in cats.

Prostate disease other than cancer — benign prostatic hypertrophy-hyperplasia (BPH) — is common in intact male dogs. One study found that more than 60 percent of intact male dogs had BPH, while by contrast, all neutered dogs in the study had very atrophied prostate glands.

By the age of 9, nearly every intact male dog will have some degree of BPH, which can make them more susceptible to prostatitis, or inflammation or infection of the prostate. Treatment for both usually includes neutering.

One study found that neutered male dogs have a greater risk of diabetes, but this might have been due to obesity.

Spayed and neutered cats have an 8.7 times greater risk of developing diabetes than intact cats — she doesn’t say whether obesity is a factor here, but I would expect it must be.

Spayed and neutered dogs have an increased risk of hypothyroidism, for unknown reasons.

Life Span

Even though obesity is more common in spayed and neutered dogs and cats and is correlated with shorter life span, spayed and neutered dogs and cats nonetheless tend to live longer than intact animals. She speculates, “In dogs and cats, this may be a reflection of enhanced care of animals by owners who have made the investment of surgery or a decrease in risk-associated behaviors (such as roaming)” in altered animals, rather than an actual medical result of being spayed or neutered.

Her Conclusions

Dr. Kustritz makes a distinction between the effect of spaying and neutering on individual, owned pets, and animals in shelters. As many veterinarians understandably do, she considers shelter animals to be “a population,” and recommends making decisions based on the good of the population and societal benefit rather than the individual animal. For that reason, she believes all dogs and cats in shelters who are offered for adoption should be spayed or neutered.

On the other hand, she believes that pets should be evaluated on an individualized basis by their owner and the pet’s veterinarian:

Pets should be considered individually, with the understanding that for these pets, population control is a less important concern than is health of each animal. Dogs and cats should be maintained as household pets. Responsible owners should ensure that their pets are provided appropriate and regularly scheduled veterinary care.

Because the behavior of unaltered male cats is so extreme, she recommends that all tomcats be neutered prior to puberty unless intended for breeding, and all breeding males be neutered as soon as their breeding use is over.

She then outlines a veterinarian-client interaction that I would give money to see in real life, and that I sincerely hope is the kind of thing actually taking place somewhere, even though I’ve never seen anything remotely like it. Imagine, she says, an owner with a new puppy, an 8-week-old Labrador female, coming into the vet’s office.

The owner has no intention of breeding.

Spaying the puppy before her first heat would reduce her chance of mammary gland tumors, which are both common and deadly.

On the downside, because of her breed, spaying would increase her risk of cruciate ligament rupture, hemangiosarcoma, and obesity. But the rate of hemangiosarcoma is low, and obesity can be controlled if the owner is alerted to watch out for it and told how to prevent it, which leaves only the risk of the cruciate rupture.

The risk of CCL injury is lower than that for mammary gland tumors, and can possibly be staved off by keeping the dog in good physical condition. For this puppy, spaying before her first heat is clearly in her best interest — but after 3 months of age, to reduce the risk of spay incontinence.

Her final words:

The information provided here on the risks and detriments of gonadectomy is not intended to promote or to minimize the importance of gonadectomy as a means of controlling animal populations or possible impacts on animal health or behavior of a specific animal. The veterinary profession recognizes the need for individual assessment of risk and benefit when evaluating vaccination protocols for animals. Elucidation of the genome in various species may lead to individualized diagnostic and treatment plans for each animal in the future. It behooves us as veterinarians dedicated to the provision of the best possible care for animals to educate clients and evaluate each animal carefully when making recommendations regarding gonadectomy.

The idea that “one size fits all medicine” can ever be good medicine is a false one, when disucssing spay/neuter or anything else. I’m very glad the AVMA made this article, with all its references, available to the public. Even though I believe most dog and cat owners will continue to spay and neuter their animals, the age at which they do so, and their awareness of some health complications they need to be aware of, may well be influenced by the information it contains.

The article, in two different PDF formats, is here.

Filed under: pets, connected,veterinary medicine — Christie Keith @ 10:51 pm


  1. Our local shelter requires neutering and if the animal is old enough it is done before the animal leaves. Otherwise in the case of puppies or kittens it is part of the contract and is always included in the price of adoption so in effect it is prepaid.

    One dog recently came in with NINE puppies. While all the pups found homes the poor old girl is still there but at least will not produce any more. Hopefully she will find a home soon. My old friend Brandy was from a litter like that. I used to frequently see people walking dogs in town that looked exactly like her and expect that they were her siblings.

    In one respect I look at Scout and think – wouldnt it be great to have some of his kids? And then I realize that the world will not run out of fine dogs who desperately need homes, any one of which could and would be forever grateful to become my friend and companion just as Scout and all the other dogs who have come into my life before him.

    As Ive said before, you cant have a discussion with your dog about abstinence or pretty much anything else on that order. It also seems that you cant have that discussion with a good number of humans – including celebrity bimbos. Even if there was a slight advantage to leaving them “intact” the suffering that would inevitably result from the population boom that would result could not possibly compensate for individual longer life. Ive spent too much time at the shelter to see it any other way. Fortunately it appears that the scales seem to tip the other way.

    In the end, I dont think they care. Everything a dog is is right on the surface and long thoughts of missing offspring and lost horizons do not seem to trouble them. Those are human traits. Scout’s idea of longing for companionship is in his eyes when I have to leave him at home when I see him peering out the window hoping I will turn around and take him with me after all. And when I return I see it renewed in his joy and excitement in the renewed possibilities of the moment.

    How many eyes will never see that moment?

    Comment by Bernard J. (Bernie) Starzewski — January 9, 2008 @ 7:23 am

  2. Like many people, I am influenced by personal experiences. I currently have an 8 year old Beagle whom I never spayed (or bred) but has been in for surgery twice in the last six months to remove fast growing mammary tumors. I can’t help but look back at my decision to leave her intact. (She is now spayed.)

    Comment by slt — January 9, 2008 @ 8:57 am

  3. Christie: Great work here. I agree that our science is behind the curve on the societal issues spaying and neutering represent. For too long it’s been held as dogma among us vets that cats and dogs be neutered for population control, health and behavior issue. It seems we’ve always considered it a no brainer to undertake the procedure at six months of age.

    Though I’d argue it’s more than just habitual, as you state, it’s possibly no more than “arbitrarily advisable,” based more on empirical evidence than on solid research.

    But the research is not likely to come at us fast and furiously, as this worthy but flawed article intimates. There’s not much of an impetus, financial or social, to respond to the needs of the individual owner who wants to determine the ideal situation for their pet.

    Most significantly, that’s because (regardless of the actual stats) the leading cause of dog and cat death in this country is still shelter euthanasia.

    *Sigh.* We can only hope the next decade sees greater enlightenment on this issue so we can get to the nitty gritty of practicing medicine over population control.

    Comment by Dr. Patty Khuly — January 9, 2008 @ 9:02 am

  4. Hmm the vet who wrote the reviewed article missed this too from 2002

    Almost sounds like maybe someone updated a college paper on a topic and republished it, what with all the outdated shelter info!

    Comment by Nancy H. — January 9, 2008 @ 9:12 am

  5. “She states that the trainability of working dogs is not altered by sterilization, and that age of sterilization also has no effect on the trainability of working dogs. The citation for this assertion was simply “personal communication” with R. Daniels at Canine Companions for Independence, which is disappointing, as it is completely contrary to the experience of many working dog trainers.”

    It’s also completely contrary to what Canine Companions for Independence is saying today.

    I spent a day traveling around the California state Capital with Paul Mundell, National Director of Canine Programs at Canine Companions for Independence. I listened as Paul told one staffer after another about the controlled prospective research study that CCI did in 1990 that showed that dogs s/n before 6 mo. of age had higher failure rates as working assistance dogs than those s/n at CCI’s traditional age, which is over 12 mo. and averaging 17 mo. of age. CCI will not s/n dogs before 6 mo. of age as a result of this research. You can find mention of this study in CCI’s letter opposing AB 1634 (the mandatory spay/neuter bill). Quoting from CCI’s letter, which is on CCI’s website:

    *Calling AB 1634 the ‘California Healthy Pets Act’ is a misnomer*
    Surgical sterilization of preadult dogs has been shown to increase the risk for several significant behavioral and health problems. CCI did a study on the effects of prepubertal gonadectomy (i.e., sterilization) in 1990, and found significant increases in failure rates due to both medical and behavioral reasons in those dogs that had been sterilized early. This research has been repeated elsewhere with the same results. Increased incidence of health problems such as urinary incontinence, osteosarcoma, hemangiosarcoma, obesity and orthopedic problems as well as behavioral problems such as environmental fear and interdog aggression are strong arguments against prepubertal sterilization for any dog, but especially those destined for a working role.”

    Pretty unambiguous, it’s it?

    I don’t know how Dr. Kustritz could have received a personal communications from CCI indicating that s/n doesn’t affect trainability in working dogs when CCI’s research done in 1990… as far as I know the most rigorous such study ever done… found just the opposite.

    Comment by LauraS — January 9, 2008 @ 10:18 am

  6. While much of Dr. Kustritz’s article is welcome and long overdue, it is yet another example of the superficial understanding that many (not all) veterinarians have about dog training and working dogs in particular.

    Many experienced working dog trainers find that dogs who are s/n as puppies, especially puppies under 6 mo. of age, have lower odds of success as working dogs. One will find this among herding dog trainers, scent detection dog trainers, and it is most pronounced among police patrol dog trainers.

    Comment by LauraS — January 9, 2008 @ 10:32 am

  7. Here’s an interaction I would like to see between vet and client:

    Client brings in a newly acquired bitch puppy. She wants to breed the puppy when it is old enough (or maybe she just wants to wait until after the first heat to spay her.)

    Vet explains the importance of testing for diseases prevalent in the breed and allowing the bitch to grow up before breeding her. Vet then explains how the bitch’s reproductive system works, how to tell when she will be coming into heat, what to do to prevent her from becoming pregnant, and gives the client a sheet to take home, with illustrations, reiterating the information.

    I have kept intact dogs for more than ten years and I have never had a new (to me) vet ask me if I knew how to keep my bitches from getting pregnant. And I’ve been to a lot of vets. I’ve never had a vet ask me if I understood the heat cycle. I’ve been been on mailing lists and message boards with people who either had an unintended pregnancy because they were ignorant about the heat cycle and bitch physiology, or had a planned pregnancy in a breed with lots of health problems, but it’s okay because their vet said ‘the dog is healthy.’ People who are not ‘dog people’ often look to their vet as the repository of dog knowledge. It behooves vets to educate their clients about breeding healthy dogs, even if the vet is not a breeder themselves. If the client doesn’t listen, at least an effort was made.

    Comment by Jessica — January 9, 2008 @ 11:00 am

  8. “Dogs spayed before their first heat have only a 0.5 percent chance of getting a mammary tumor. Dogs spayed after they’ve had two heats have a 26 percent risk”

    This is actually not correct. These numbers come from the landmark 1969 epidemiological study by Schneider, examining the association between age at spaying and risk of mammary cancer in dogs. The risks that were reported in that paper were all very clearly stated to be relative risks, odds ratios, not absolute risks as the above would suggest. The risk that a female dog spayed after two or more heats will develop mammary cancer was found to be 26% of the risk that intact female dogs will develop mammary cancer.

    For example, in a breed where intact female dogs have a 10% lifetime risk of developing mammary cancer, the lifetime risk for those spayed after two or more heats would be 26% of 10%, or 2.6%. Quite a bit less than 26%.

    From my reading of it, the 1969 Schneider paper appears to be a well done study and is as important today as it was in 1969 [for those who believe that medical and scientific research comes with an expiration date, it does not]. But this study is almost always misquoted. Apparently most veterinarians and most academics who quote it don’t bother to actually read the paper but rather just repeat what others have written about it. Somewhere along the line this paper was misquoted, and this misquote has propagated widely. I used to misquote it too, until I trekked over to UC Davis and photocopied the thing, and read it. Oops.

    Comment by LauraS — January 9, 2008 @ 11:27 am

  9. I’m happy to see it has become a realization that spayed females do experience a decrease in hormones and probably go through a type of menopause. I’ve often wondered if they do. From a human point of view, it’s been a terrible experience for me. I can only imagine how hard it must be on dogs (and cats) that have been spayed so early in life. Of course, I do believe in spay and neuter for pets but we need to consider the downfalls also and what our options are.
    About pyometra, my ex mother-in-law had female dogs. She never spayed them and all 3 at seperate times developed this terrible infection. One died, one almost died and the other was caught early and was spayed and treated. I have a hard time believing the percentages that were mentioned. Personally, from working at a vet office, I think it should be higher.

    Comment by Cindy — January 9, 2008 @ 1:08 pm

  10. “spayed and neutered dogs and cats nonetheless tend to live longer than intact animals.”

    I’ve found five retrospective studies of longevity in dogs as a function of s/n status, and they all found the same things: intact and neutered male dogs have the same average lifespan, while spayed females outlive intact females by an average of 1.2-2.6 years, depending on the study. So the oft repeated claim that s/n increases lifespan in dogs is only supportable in female dogs, not male dogs.

    OK, now this issue gets more complicated. The studies that looked at this question were all retrospective studies, not prospective studies. For an issue like longevity and s/n, there is a built in selection bias that will exaggerate the longevity of s/n dogs or s/n cats.

    Glickman briefly mention this selection bias issue in their health survey report of Akitas: “In bitches, it appears that the later a dog is neutered the longer it lived. However, this finding may be biased by the fact that unless a bitch lives to an older age it cannot be neutered at that age. In contrast, any bitch could be neutered earlier in life.”

    I was curious what this might amount to, so I did a Monte Carlo simulation of a model dog population s/n at different ages. I used a real longevity distribution curve and distributed the s/n over the age ranges. The results show that the selection bias in retrospective longevity studies of s/n can be quite large. In my analysis, it exaggerated the median longevity advantage for s/n by 2.5 years. YMMV, depending on at what ages s/n is distributed over the longevity distribution curve. The more dogs who are s/n as adults and especially as older adults, the larger this selection bias becomes.

    The point is, I think if somebody were to do a controlled prospective research study of longevity as a function of s/n in dogs it would probably find that intact male dogs outlive their neutered male counterparts, and that the longevity advantage observed in retrospective studies for spaying female dogs would shrink and perhaps disappear entirely.

    Controlled prospective research studies are a lot more expensive to do than retrospective studies, and in a field like veterinary medical research that isn’t funded to the degree that human medicine is, controlled prospective research studies are few and far between.

    Comment by LauraS — January 9, 2008 @ 1:41 pm

  11. In a few instances I did raise questions, but for most of the information I just explained what the article was saying. I think I should clarify that, so people don’t think this is the proven gospel. I was mostly just trying to restate what the author said in less technical terms, but since I did offer some commentary on things I knew more about, I should probably disclaim the blog post a bit more.

    Thanks, everyone! Great comments!

    Comment by Christie Keith — January 9, 2008 @ 2:55 pm

  12. It is amazing to me that the same old stuff keeps surfacing in different ways. I think some of the old citations (other than being irresponsible) and viewpoints are really common in most veterinary practices–at least in my area they are.

    The problem in many situations is that anecdotal information is often not substantiated BUT also popular culture (whether you are talking dog training community or not) often stumbles onto trends that either don’t have the numbers to study or to gain the interest of researchers.

    The difficulty is that you can make statements on particular trends or cite studies that support them while there is probably another one that conflicts elsewhere.

    It is something that has made me cranky at some scientific presentations…

    I don’t remember when the really early altering procedures began to be promoted–1990s? But most veterinarians insist on delaying up here until after 6 months of age, and many breeders here want their animals to develop the physical traits that they believe are solely based on hormones in the system.

    My point? (Other than I probably need to finish my coffee before posting!) Is that you did a great job in giving an overview and pulling out important statements for reader review.

    So, TY2U for tackling this!

    Comment by Diana Guerrero — January 10, 2008 @ 8:31 am

  13. As a dog rescuer, I could go on for days about the s/n controversy. Health issues aside… There is no doubt that altering an animal that is no longer wanted in one way or another should be the ONLY option. Everyone will have their own opinions about all of the issues that have been talked about… Here’s mine…

    I completely agree with the statement made by Jessica. It should be a Vet’s DUTY to explain an owners role if they choose to NOT alter their pet. A Vet should explain in detail what can happen if a female gets pregnant or how a male can wander and inpregnate females… Also, how to prevent unwanted pregnancies should they choose to not alter. There are many ‘backyard’ breeders who have contacted our rescue asking for help… it’s always the same scenerio… “I thought it would be easy to find homes for all of them”… Then we are called on to fix the problem they created. If we can’t help… it’s off to the shelter to either get saved and find a great loving home, go to a hell hole, or be euthanized.

    Breeders too (in all forms) should make people aware of these issues. I am going to assume that honest reputable breeders not only screen applicants but educate potential adopters about altering vs not altering. But I know, not all of them do. For some, it’s about MONEY and not the BREED.

    All lives are worth the effort in saving. It is not the animals fault, I place the blame on negligent owners mostly.. However Vets and Breeders must realize how vital their role is in the education of people who are not as aware of the potential outcomes of not altering animals.

    I am not trying to say that ALL animals should be fixed and I hope it isn’t coming across that way. A good breeder will breed the right dogs to get a litter of healthy pups who will further that partiular breeds’ standards and make it a good solid/strong stock.
    Then there are others…We have taken adult dogs from breeders because they are of no use to them anymore and I am so sick of people ‘creating’ these new hybrid dogs. Gee… lets put these two together and see what happens…. They end up at the shelter or in rescue groups… that is what happens. We recently helped a litter of Pit/Chihuahua pups b/c of that reason… will it ever stop?

    I’m tending to jump around here, but I get so worked up on this issue because I deal with the aftermath on a DAILY basis…and let me remind everyone.. Rescue Groups do not get any monetary benefit from doing so.

    Education is a powerful tool and it’s high time that EVERYONE in the position to do so… Does it!

    Comment by MM — January 16, 2008 @ 11:18 am

  14. Can any advocate of this BARBARIAN — and nevertheless, so popular in this country — procedure explain what’s wrong with vasectomy or tubal ligation in dogs for the reasons of overpopulation control?

    Of course, if one is the owner who only needs a fluffy toy, castration has all desirable bonuses: the energy level drops, and if you do it at a very young age, so does the IQ. As a result, you don’t need to provide your dog with walks and work, challenging their mind and body. The dog will be happy with the bag of food and a couch, no need to train, as there are no agression issues, no need to take a stroll in the rain, no need to become the dog’s pack leader — why bother?

    At least, don’t stay brain-washed and have courage to admit that you want to alter your dog’s personality, so it fits your lifestyle and turns your devoted companion into a stuffed animal, that it is convenience you care about — not the health issues and overpopulation!

    How many dogs have you seen who would run away and quickly impregnate a run-away bitch who happened to be around and in heat? Even if you know a few, think about how little responsibility their owners had. And again — what’s wrong with sterilization that only eliminates the reproductive ability but dones’t dosuch horrendous damage to the hormonal system and the whole mind and body as a result?!

    And if this is such a healthy procedure that only has benefits — why not try on ourselves? Instead of poisonining our body with hormonal contraception or going through the inconvenience of other other contraception, after we have the kids and don’t want to have more — let’s get rid of those organs! Get an early menopause, become a castrate — why not, if it is so good for the body?

    Comment by Irina — February 14, 2008 @ 8:50 pm

  15. I just got a puppy from the rescue place, and i have had many puppies,, they made him become nurterd before i could take him home, at 12 weeks,, I can already see a hugh difference in his bone struture, and his mass, I have had him for two months, noises make him scared, and he can not be left alone, so i am all for neutering, but it was way to early, I feel sorry for the little guy,, he is getting so short changed,, his bones are growing longer, and he is not getting any mass,, so people should listen to dog owners, they can see the difference,, I really wanted to rescue the dog, but i feel for his health, the neutering should of been after he was a year old,, after all when we rescue these dogs, they become or family, and we want them to grow the way they should, i can only imagine the feelings that go on in there,,, it has to be messed up,, I think its the wrong answer,,

    Comment by jo king — March 1, 2008 @ 6:49 pm

  16. Irina I so agree – tubal ligation and vasectomy has NEVER been on offer from the vets I have seen, and I would have used it for my pets. My personal experience in rescue dogs includes intact and neutered male dogs and spayed female cats. The dogs were different in metabolism and muscle tone; the intact male being more athletic and muscular, but in all cases the first principle was the same; good training, attentive care and a keen eye spotted any medical issues and they were dealt with promptly. Also a well fenced yard and care when out exercising meant no roaming and no unwanted pups.
    I do not believe in blanket s/n of our pets and think that subtle alternatives (vasectomy/ligation) and attentive care are the best of all worlds for the animals. We call them “our pets” – in fact, they are creatures in their own right, not just belongings, and we owe them respect.

    As regards cross breeding, I think all dogs chould be cross bred, preferably until they are all wild-dog-shaped, normal-legged, shortish haired, athletic looking dogs. No Bulldog caesareans, no docked tails, no clipped ears, no hip dysplasia, no droopy infected eyes, no ears prone to infection, no breed weaknesses. Just canine athletes, with respectful and truly caring human companions.

    Anything less is a crime.

    Comment by tracey — June 22, 2009 @ 3:11 pm

  17. We call them “our pets” – in fact, they are creatures in their own right, not just belongings, and we owe them respect.

    This is a bit of creeping PETA-ism that annoys me whenever I encounter it, and I’ll object every time no matter how much I agree with the substance of the comment. “Pet” is the English word that refers to animals that are NOT “just belongings”, animals that are valued for emotional reasons beyond their practical and economic value. That’s why “pet” is also used as a term of endearment for human loved ones, and “cow”, “donkey”, and “sheep” aren’t.

    As regards cross breeding, I think all dogs chould be cross bred, preferably until they are all wild-dog-shaped, normal-legged, shortish haired, athletic looking dogs.

    Because, of course, no one is justified in wanting for any reason, any dog that doesn’t look and act like a dingo. It’s an outrage that people should have smart, steady, responsible guide dogs; reliable, dedicated shepherds or cattle dogs; and especially no small, medium-energy, affectionate housepets, right?

    No Bulldog caesareans,

    My personal opinion is that any bitch that needs a caesarean for her first whelping, should be spayed, and the litter should not be registerable–placed as pets on spay/neuter contracts only.

    no docked tails, no clipped ears,

    I’m opposed to these things, too, but we don’t have to end the existence of purebred dogs in order end these things. Just ask the British.

    no hip dysplasia, no droopy infected eyes, no ears prone to infection, no breed weaknesses.

    Because, of course, all these things will magically disappear if we just forbid the existence of purebred dogs, and mandate that only crossbreeds can exist. Right. Sure.

    Just canine athletes, with respectful and truly caring human companions.

    Because of course, I am not a respectful and truly caring companion to the Twisted Abomination of Nature that’s sitting here beside me. (Okay, you didn’t say that, and I bet have no idea what dog I’m talking about, but I’m pretty confident that you’d agree with the ego with a keyboard that said it, if you did know. Follows logically from what you describe as the only acceptable dog.)

    Anything less is a crime.

    Having a dog that you personally don’t approve of or value is a crime. Right. Got it.

    Comment by Lis — June 22, 2009 @ 4:29 pm

  18. I think that this is totally correct. I believe in what this says. Thanks for spreading the word.

    Comment by sam — September 29, 2009 @ 7:13 pm

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