New Views on Neutering

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By Ruth Marrion, DVM

Everyone seems to have an opinion when it comes to neutering dogs. Some opinions are based on fact while others could not be farther from the truth. How many times have you heard pet owners say, "I don't want to spay my bitch because she'll become fat and lazy," "I want her to have one litter before she's spayed because that will improve her personality," or "I don't need to spay (or castrate) my dog because there aren't other dogs around"? Breeders should have rational, factual arguments ready at their fingertips to respond to these types of misguided statements.

Yet despite the fact that people have been neutering* animals since ancient times (writings that discuss canine castration date back as far as 284 BC!), information on the physiological and behavioral effects of neutering has been sparse until recent years. With little scientific data available on the subject, misinformation regarding neutering has been freely disseminated.

This article will discuss some recent scientific studies regarding the physical and behavioral effects of neutering dogs, and explore the concept of early neutering—that is, prior to five to seven months of age. Early neutering was developed to help alleviate the pet overpopulation problem, and may prove to be of further benefit to dog breeders.

*Neutering is a term applicable to an animal of either sex that has had both ovaries or testes removed. This includes dogs who have undergone an ovariohysterectomy (removal of the ovaries and uterus), ovariectomy (removal of the ovaries alone) or orchieclomy (removal of the testes). Unneutered animals are referred to as being sexually intact

 Behavioral Effects of Neutering
         The estrous cycle of the intact bitch is divided into several stages. The first stage of the cycle is proestrus, in which the bitch is attractive to males but is not receptive to mating. This is followed by estrus (derived from the Greek meaning "mad desire"), in which the bitch is receptive to dogs. The onset of the next stage, diestrus, is defined as the first day after an estrus period when a bitch will no longer accept a dog.

The female hormones estrogen and progesterone, which are produced by the ovary, control the estrous cycle. Estrogen promotes the bitch's desire to mate (behavioral estrus), stimulates growth of the uterus and mammary gland, and sensitizes the uterine muscle to oxytocin, a hormone, during estrus and whelping. Progesterone enhances the effect of estrogens in causing behavioral estrus, and acts in concert with estrogens to promote uterine and mammary growth.

Removal of both ovaries also removes the source of estrogen and progesterone, thus resulting in the abolition of estrous cycles. Since these hormones are responsible for sexual activity, bilateral ovariectomy almost always results in cessation of copulatory behavior for the bitch. The reason that copulatory behavior is not always abolished is not completely clear, but mounting behavior in females has been found not to depend on the presence of estrogen. For example, my Sheltie bitch was spayed as a puppy and at four years of age, she still engages in mounting activity during play with her neutered brother.

Lots of people believe that inactivity and weight gain follow ovariohyslerectomy in the bitch. These changes, however, are difficult to distinguish from changes due to normal maturation. Recent studies have tried to uncover the truth about these supposed effects of spaying.

One report on gonadectomy (removal of the ovaries or testes) in immature dogs' determined that neutered dogs were actually more active than sexually intact dogs. Gonadectomy was found to neither affect food intake nor weight gain. But another group of researchers, comparing food intake in sexually intact and neutered Beagle bitches fed ad libitum, concluded that neutered bitches ate much more than their sexually intact counterparts.

In yet another study5, activity level and weight fluctuations in spayed bitches were compared to those of intact females. The spayed bitches did not gain weight on a diet of a fixed amount of commercial dog food. No data on the effect of orchiectomy on food intake and weight gain in male dogs, other than the study on immature dogs, has been published to date.

The bottom line is that the available scientific evidence about whether neutering influences weight gain and activity in dogs shows that the question is, as yet, unresolved. In other words, it's too early for breeders to jump to either conclusion and therefore, one can't assume the worst.

Eliminating undesirable behavior is often cited as a reason for neutering male animals. Behavior patterns that are altered by neutering are generally male-specific actions such as urine marking, mounting, and intermale aggression. These three types of behaviors, in fact, have been shown to be markedly reduced or eliminated in 50 to 60 percent of dogs as a result of neutering. Behavior patterns common to both males and females, such as watchdog barking, playfulness and attention seeking, are not affected by neutering. No basic personality or behavior changes occur as a result of neutering, with the exception that undesirable male behaviors may be reduced or eliminated.

It is interesting to note that the age at which an animal is neutered does not affect the likelihood that neutering will have an impact on a particular behavior. The chance that neutering will prevent an objectionable male behavior in a pre-pubertal animal is approximately equal to the likelihood that the same behavior will be abolished by neutering an adult animal2.

Experience, on the other hand, does appear to play some role in determining how quickly an undesirable behavior ceases to exist. Copulatory behavior in experienced male dogs may not end completely for months to years following castration. Residual testosterone is not a contributing factor for persistent male-specific behavior patterns, since the hormone is so rapidly metabolized by the body that it is virtually undelectable eight hours following castration.

Finally, many pet owners nurture the belief that bitches become more even-tempered if they produce one litter before being spayed. Neither clinical observations nor animal behavior research bear this out. True, a bitch's behavior will change temporarily as a result of the hormonal changes during pregnancy and lactation, but these effects are transient—not permanent, as some believe.

But if the owners of a bitch manage to convince themselves that her behavior will change as a result of whelping, their way of dealing with the bitch may change, and thereby influence the bitch's ensuing behavior. This is known as the placebo effect2.

There are other circumstances where an owner's expectations and resulting behavior can influence a dog. The placebo effect also plays a role, for instance, when people anticipate certain changes to take place following neutering— weight gain, for example, or decreased activity. Without realizing the implications of their actions, many owners will actually start to feed the neutered animal more food and place fewer physical demands on it. The result: an overweight, sedentary (neutered) animal. Just what owners expected!

The specialized training of behavioral scientists helps them differentiate between various influences on an animal's behavior. Data from their controlled studies are therefore generally more valuable than observations made by casual observers on a limited number of animals.

 Physical and Medical Effects

Intact bitches are susceptible to several diseases of the reproductive tract and mammary glands. Removing the ovaries and uterus greatly decreases or eliminates the risk of a bitch developing the following disorders:

1) Mammary cancer. Mammary gland cancer is the most common tumor of the sexually intact bitch. The beneficial effects of bilateral ovariohysterectomy in preventing the development of mammary cancer have been well documented4.

Sexually intact bitches have three to seven times the risk of developing mammary gland cancer compared to neutered bitches. The timing of neutering is critical if mammary cancer is to be avoided. Bitches spayed prior to their first estrus have been reported to have 0.5 percent risk of developing mammary cancer. If spaying is delayed for one to two heat periods, however, the chance of a female developing a tumor jumps to 8 and 26 percent, respectively. Little sparing effect is seen in bitches neutered later in life.

Regardless of age, ovariohysterectomy is often performed concurrently with mammary tumor removal. The reason for this is that a number of these tumors have the potential to metastasize to other areas of the body. The cells which have metastasized, like the parent mammary cancer cells, are often stimulated by female sex hormones. Ovariohyslereclomy removes the source of female hormones and thereby decreases progression of the disease.

2) Pyometra. Meaning "pus in the uterus," pyometra is a common disease of intact bitches, usually affecting individuals over six years of age. This condition develops as a result of the influence of progesterone on the uterus during diestrus. Progesterone promotes growth of the endometrium (the lining of the uterus), and secretion of uterine glands. These secretions provide a me-dium for bacterial growth. In addition, progesterone inhibits the infection-fighting activities of uterine white blood cells.

Signs of pyometra include varying degrees of illness along with an increase in drinking and urination in an older intact bitch. Blood analysis and abdominal radiographs are used to confirm clinical suspicion. Ovariohysterectomy is the only acceptable treatment for pyometra. Medical treatment using antibiotics, prostaglandins, hormones and supportive therapy is generally unsuccessful. Removal of the uterus and ovaries at any age will prevent development of pyometra.

3) Vaginal prolapse. This condition occurs in response to the effects of estrogen on the lining of the vagina. Estrogen causes this tissue to become edematous (swollen) and protrude from the vulva, becoming subject to trauma or self-mutilation by the bitch. Ovariohysterectomy prevents this condition from developing.

Potential adverse effects of ovariohysterectomy are uncommon and of less significance than the beneficial effects. A condition of estrogen-responsive urinary incontinence develops in a small proportion of spayed bitches (one researcher found a 4 percent incidence10). The incontinence condition generally responds to treatment with oral medication.

Many individuals feel that their bitch should go through one estrous cycle prior to being spayed. There is one situation in which this should be considered—the case of puppy vaginitis. Some puppy bitches develop a vaginitis because the lining of their vagina is thin.  This is often alleviated after one estrous cycle. The effect of the estrogen produced during the estrous cycle is to thicken the vaginal lining, making it less susceptible to infection. If a bitch who has been spayed before her first estrous cycle continues to exhibit signs of vaginitis, antibiotic and hormone therapy may be initiated to treat the problem.

The Early Sterilization Program

Dr. Michael Aronsohn, director of the Early Sterilization Program (ESP) at the Massachusetts SPCA's Angell Memorial Animal Hospital, sees .the. ESP as a boon to pure-bred dog breeders.

"The program is useful to breeders because they can get pet quality puppies neutered as early as six weeks of age,.prior to placing them in homes," says Dr. Aronsohn, a Diplomate of the American College of Veterinary Surgeons and head of the Department of Surgery at Angell Memorial's affiliated Boston shelter. "Breeders can bring in litters to be neutered and the puppies will be ready to go to their new homes•a few days after surgery.

Dr. Aronsohn and anesthesiologist Dr. Alicia Faggella have ! neutered several hundred puppies and kittens with no apparent ill effects. At another animal shelter, in Medford, Oregon, a study of 8,000 puppies and kittens neutered at eight to 12 weeks of age also reported no undesirable effects.1

When asked about the safety of early neutering, which has been in existence for nearly 20 years, Dr. Aronsohn replied, "Anesthetic protocols have been worked out, and are as safe in the younger puppies as they are in the ones five to eight months old. We previously thought that, hypoglycemia was going to be a problem, but it has turned out not to be."

The Angell Memorial team has neutered litters of puppies "and kittens as young as six to eight weeks old for several area breeders. Dr. Aronsohn recommends that interested breeders contact their local humane association for information on shelters and hospitals in their area that perform early neutering.—RM

l. Kellington E, Hannawalt EH. Study of the effects of early spaying and neutering. Medford, Oregon SPCA Rep. May 1985.

In the absence of puppy vaginitis, there is no reason that a bitch should not be spayed prior to her first estrus. The sparing effect on mammary cancer development alone is reason enough not to delay ovariohysterectomy.


Beneficial Effects of Castration

Neutered dogs are at less risk of developing some diseases than their sexually intact counterparts. The most important of these are prostatic disease and testicular cancer.

Benign prostatic hypertrophy is an androgen (male sex hormone) related disorder of the prostate gland in which the gland enlarges and becomes more susceptible to infection.  Prostatic infection may spread to involve the bloodstream (sepsis) or abdominal cavity, and may result in death. Benign prostatic hypertrophy is both prevented and treated by castration.

          Testicular cancer is a potential danger to intact male dogs. Cryptorchid dogs are actually at greater risk of developing testicular cancer than normal intact dogs. In one study of 410

dogs cryptorchids were found to have 13.6 times the risk of normal dogs for developing testicular cancer11. Sertoli cell tumors are a type of cancer more often seen in cryptorchids than in intact males. This condition can cause a life-threatening bone marrow suppression.


Early Neuters

The pet overpopulation problem in the United States has reached enormous proportions. In 1987, for example, between 6.3 and 10.4 million dogs were euthanatized in shelters in the United Stales. Most animal care facilities have instituted mandatory neuter policies as an attempt to reduce the number of unowned and unwanted animals. They require that owners who adopt kittens and puppies have them neutered at five to eight months of age. As one would expect, follow-up and enforcement of these policies are difficult at best. Compliance rates average only 50 to 60 percent, with the result that many animals remain sexually intact and reproduce prolifically.

Current practice among veterinarians in the United States is to neuter puppies between five and eight months of age. However, there is little information in the veterinary literature regarding the optimal age for performing spays and castrations. Since some dogs reach sexual maturity prior to six months of age, a number of dogs are neutered after reaching sexual maturity and therefore, they may already have bred fertile females. For sterilization programs to be effective, all non-breeding animals should be neutered prior to the onset of puberty, and compliance rates must be improved.

Some animal shelters, in responding to these problems, have instituted early neuter programs. Under these programs, puppies and kittens are neutered before they leave the shelter, bringing compliance rates to 100 percent. Widespread adoption of early neuter programs by shelters will likely have a significant positive impact on the pet overpopulation problem.

Since the advent of early neuter programs, a number of questions have been raised regarding the appropriate age at which gonadectomy should be performed, as well as the safety of anesthetizing young puppies. Data on gonadectomy in immature dogs have recently been published, including anesthetic and surgical techniques and effects on behavioral and physical development. One study7, comparing the effects of neutering puppies at seven weeks versus seven months of age, found that neutering at either age produced similar effects on physical, skeletal and behavioral development. Gonadectomy did not affect food intake or weight gain, once again arguing against the idea that neutering animals predisposes them to obesity. Neutering did not result in inactivity or lethargy in this study; in fact, all neutered dogs were assessed by their caretakers to be more active than their sexually intact counterparts. They also found that prepubertal gonadectomy does not stunt growth; indeed, it contributes to growth enhancement!

Bone growth ceases when the physiologic growth plates located at the ends of immature bones "close." Growth plate closure of the radius and ulna (forelimb bones) occurred three months later (in seven-month-old neuters) to four months later (in seven-week-old neuters) in neutered puppies versus their sexually intact litter males. The result is that the forelimb bones of neutered puppies were a fraction of an inch longer than those of the unneutered pups.

Also of significance is the fact that the seven-week-old puppies in this study tolerated anesthesia well. Surgeons actually found that spaying the younger puppies was easier than spaying bitches at the traditional age due to the smaller amount of fat, within the abdomen and the relative lack of vasculature. Surgical time and risk of hemorrhage were greatly reduced.

The benefits of early neutering to dog breeders are obvious. By having pet quality puppies neutered before they leave for their new homes, breeders would no longer have to rely on their puppy buyers to carry out their wishes. Early neutering virtually guarantees that undesirable genes will not affect future generations, and it also pays heed to every responsible breeder's concern about the problem of pet overpopulation.

Some shelters and veterinarians in private practice are currently performing the early neuter procedure. But it's also important for breeders to understand why individual veterinarians may choose not to perform early neuters. An eight-week-old puppy is not just a smaller version of an eight-month-old puppy. There are important differences between the two in factors such as respiratory and cardiovascular physiology, drug metabolism and thermoregulation. Few practitioners have accumulated a significant amount of experience in anesthetizing very young puppies on a regular basis, since there are not many situations which call for it.

On the other hand, most practitioners have neutered many older puppies and young adults, feel comfortable with the anesthetic and surgical protocols they have developed, and may not want to change. These individuals may feel it is neither time- nor cost-efficient to develop new protocols for early neuters, unless they have a clientele composed substantially of breeders or other individuals who would be interested in such a program.

In my experience as a veterinarian, I am often able to alleviate people's fears about neutering and clear up their misconceptions by presenting the objective data discussed here. Responsible breeders can—and should—make the same effort to ensure that pet puppies are neutered. Fulfillment of this duty brings all of us closer to breeding fewer dogs and conquering the pet overpopulation problem.

Dog Owners' Observations

Between 1974 and 1980, • thousands of 8 to 12-week-old puppies were neutered at the Medford, Oregon, SPCA shelter. Did early neutering cause long-term undesirable after effects, as some people believed? Shelter personnel decided to find out for themselves by looking at the dogs in their area.

  Questionnaires were sent out to owners of 200 randomly selected dogs aged 3 to 14 years that had been neutered when they were 6 to 12 weeks old. Identical questionnaires were also mailed to owners of 200 dogs that had not been neutered or had been neutered after 6 months of age.

Their responses (see the charts at right) were published in an article by Leo L. Lieberman, D.V.M., in the Journal of the American Veterinary Medical Association on September 1, 1987. Owners of both male and female dogs in this survey reported less aggressive behavior, weight gain and medical problems for those dogs neutered at an early age, as compared to those dogs that were sexually intact or neutered after, six months.—Elizabeth Bodner, D.V.M.

The results of the survey that asked owners to comment on after effects of neutering. Numbers on horizontal axis indicate: (1) owner not pleased; (2) undesirable sexual behavior of pet; (3) aggressive behavior of pet; (4) overweight pet; (5) medical problems with pet; (6) intelligence rating of pet. Taken from JAVMA, Vol. 191, No. 5, September 1, 1987.

Dr. Bodner is the executive editor of the GAZETTE.



1. Grandy JL, Dunlop CI. Anesthesia of pups and kittens. JAVMA 1991;198:1244-1249.

2. Hart BL. Effects of neutering and spaying on the behavior of dogs and cats: Questions and answers about practical concerns. JAVMA 1991;198:1204-1205.

3. Houpt KA. Coren B, Hintz et al. Effect of sex and reproductive status on sucrose preference, food intake, and body weight of dogs. JAVMA 1979; 174:1083-K185.

4. Johnston SD. Questions and answers on the effects of surgically neutering dogs and cats. JAVMA 1991; 198:1206-1213.

5. LeRoux PH. Thyroid status, oestradiol level, work performance and body mass of ovariectomised bitches and bitches bearing ovarian autotransplants in the stomach wall.JS Afr Vet Assoc 1977:48:115- 117.

6. Olson PN, Nett TM. Reproductive endocrinology and physiology of the bitch, in Morrow DA (ed.) Current Therapy in Theriogenology. 1986, W.B. Saunders, Philadelphia. 453-457.

7. Salmeri KR, Bloomberg MS, Scruggs SL, Shille V. Gonadectomy in immature dogs: Effects on skeletal, physical, and behavioral development. JAVMA 1991;198:1193-1203.

8. Salmeri KR, Olson PN, Bloomberg MS, Elective gonadectomy in dogs: A review. JAVMA 1991:198:1183-1191.

9. Johnston SD. Chapter 181, Male Reproductive System. In Slatter DH (ed.), Textbook of Small Animal Surgery, Volume 11. 1985, W. B. Saunders. Philadelphia. 2535-2545.

10. Thrusfield MV. Association between urinary incontinence and spaying in bitches. Vet Rec 1985,116:695.

Dr. Marrion is affiliated with Angell Memorial Animal Hospital in Boston