By Christie Keith
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.
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 authorâ€™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.
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.
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.