Study results looking at long-term outcome of gonadectomies performed at an early or traditional age in cats and dogs were published in JAVMA in the Dec. 1, 2000 and Jan. 15, 2001 issues, respectively.
The studies involved 269 dogs and 263 cats from animal shelters and were conducted by Dr. Lisa Howe at Texas A&M’s Veterinary College. The conclusion for dogs was that “with the exception of infectious diseases, prepubertal gonadectomy may be safely performed in dogs without concern for increased incidence of physical or behavioral problems during at least a 4-year period after gonadectomy.”
Shelters that held puppies long-term encountered problems with parvovirus. However, the authors of this paper did not conclude that pediatric neutering caused parvovirus. The puppies in the study were at an age where they were highly susceptible to parvovirus and housed in a shelter environment where parvovirus can be common; they developed parvovirus for these reasons, and not because they were sterilized as pediatric patients.
The conclusion for cats was that “prepubertal gonadectomy may be performed safely in cats without concern for increased incidence of physical or behavioral problems for at least a 3-year period after gonadectomy.”
Another study addressing the long-term effects of pediatric spay/neuter was published in JAVMA in the February 1, 2004 issue. This study, which was coordinated by Dr. Vic Spain at Cornell University’s College of Veterinary Medicine, looked at the records of shelter animals (1,842 dogs and 1,660 cats) who were sterilized as pediatric patients.
This study provided follow-up for as long as 11 years, and the conclusion for dogs was that “because early-age gonadectomy appears to offer more benefits than risks for male dogs, animal shelters can safely gonadectomize male dogs at a young age and veterinary practitioners should consider recommending routine gonadectomy for client-owned male dogs before the traditional age of 6 to 8 months. For female dogs, however, increased urinary incontinence suggests that delaying gonadectomy until at least 3 months of age may be beneficial.”
It is important to note that the female dogs with reported urinary incontinence remained in their homes and were not relinquished to the shelter. The long-term Texas A&M study did not find similar results about urinary incontinence, and another study in 1992 by Arnold showed a higher incidence of urinary incontinence in female dogs spayed after the first estrus cycle.
The conclusion for cats from the Cornell study was that “gonadectomy before 5.5 months of age was not associated with increased rates of death or relinquishment or occurrence of any serious medical or behavioral condition and may provide certain important long-term benefits, especially for male cats. Animal shelters can safely gonadectomize cats at a young age and veterinarians should consider recommending routine gonadectomy for client-owned cats before the traditional age of 6 to 8 months.”
It should be noted that similar focused studies had not been conducted to establish the long-term safety of gonadectomies performed on dogs and cats at 6 months of age. In a review of the veterinary literature by Olson and Root Kustritz, as of 2001 when the article was published, adverse side effects are apparently no greater in animals neutered at early ages (7 weeks) than in those neutered at the conventional age (7 months).
Other Health Factors
Much of the information here on the long-term safety of pediatric neutering is a summation of the findings in a 2007 article on determining the optimal age for gonadectomy by Dr. Margaret Root Kustritz, a veterinary theriogenologist (reproductive expert) at the University of Minnesota College of Veterinary Medicine.
In some cases this information touches on the impact of neutering in general, not just pediatric neutering. (Readers who desire more detailed information are referred to the author’s original article, which is listed in the reference list, along with other suggested reading materials.)
Obesity is such a common problem of dogs and cats that many veterinary organizations and pet-food companies provide multiple resources and special diets to help clients reduce the weight of their pets.
Obesity is influenced by a number of factors, and while veterinarians report that neutered animals do have a tendency to weigh more than intact animals, it can occur regardless of the animal’s age when the surgery was performed. (A published study in 1991 indicated that dogs did not develop obesity when they were neutered pre or post puberty. Another study from 1996 showed that cats can gain weight after pre or post pubertal gonadectomy. The 2004 Cornell study found a decrease in obesity for both male and female dogs who had undergone pediatric neutering compared to those neutered after 5 months of age.)
So obesity is a multi-factorial problem and not an automatic consequence of neutering – even an intact animal can become obese if a proper diet and exercise regimen is not followed. Just as in humans, dietary indiscretions and lack of activity are the real culprits in this case.
Many veterinarians erroneously believe that pediatric neutering will stunt the growth of animals. This concern has been refuted by multiple studies. The removal of hormonal influences on the growth plates of the long bones results in delayed closure, resulting in bones that are actually a little longer. However, no clinical significance to this difference in size has been found thus far.
Cranial Cruciate Ligament Rupture
The reported incidence of rupture of the CCL in dogs is about 1.8%, and this is reportedly more common in neutered female and male dogs than in intact dogs. The exact cause and effect relationship has not yet been defined, but in addition to a suspected hormonal influence, heredity, body weight, and body condition score may all play a role in CCL rupture. More research is needed.
The reported incidence of hip dysplasia is 1.7%, with an increased incidence in large and giant breed dogs. Hip dysplasia is a hereditary condition that is affected by environmental factors as well, such as diet. Long term studies have looked at the incidence of hip dysplasia in dogs and the association of hip dysplasia with pediatric spay/neuter.
In the long-term Cornell study, puppies who underwent pediatric neutering before 5.5 months of age had an increased incidence of hip dysplasia. However, an additional finding of this study was that dogs who were neutered at 6 months of age were 3 times more likely to be euthanized for the condition as compared to the pediatric neutered group. The authors suggest that pediatric neutering may be associated with a less severe form of hip dysplasia.
The effects of pediatric neutering on behavior remain largely unknown. Neutering and the subsequent decrease in related hormones have been correlated with a decrease in gender-specific, sexually dimorphic behaviors. Neutering at any age reduces the urge of male animals to spray urine to mark territory or to roam and fight with other male animals seeking females in estrus, making them more desirable pets.
Also, the trainability of working dogs is not altered by gonadectomy and does not vary with the age of the dog at time of surgery (Root Kustritz).
The Cornell study of dogs neutered before 5.5 months of age indicated an increase in noise phobias and sexual behaviors and a decrease in escaping, separation anxiety, and urination in the house when frightened. However, the 2001 Howe study showed no difference in the incidence of overall or specific behavioral problems between pediatric and traditional-age neutering.
Several studies have shown a breed-specific increase in aggression and reactivity after spaying certain breeds while they are in estrus. The exact cause of this tendency remains unknown. Hart reported a slowing of the progression of cognitive dysfunction in intact male dogs compared to neutered males, but the sample size in this study was very small and not specifically linked to pediatric neutering. There is some early evidence that animals who are neutered at 7 weeks or 7 months of age are more active and excitable, and that male and female cats may be more affectionate than those left intact, but this is a fairly subjective observation.
It is clear that much more research is needed to explore the impact of neutering on behavior.
Lower Urinary Tract Disease
Although some veterinarians continue to believe that pediatric neutering contributes to a higher rate of urinary tract obstructions in male cats, this is not the case. Studies have been conducted on male cats to determine the incidence of urinary tract obstructions in all populations, and no correlation has been found between the age at neutering and the incidence of feline lower urinary tract disease. It was found that the diameter of the penile urethra did not vary between cats neutered at 7 weeks or 7 months of age, and that of intact male cats.
Secondary Sexual Characteristics
The vulva of spayed females is smaller than that of intact bitches, but there is no evidence that there is any clinical significance to this size difference. Perivulvular dermatitis occurs in intact as well as spayed females, and is related more to obesity than sexual status. Mammary glands and nipples are also smaller.
The penis and prepuce of male animals will retain a juvenile appearance, but again, there is no evidence of any clinical significance in animals who are not sexually active. There is a reduction in the male cat's ability to extrude the penis from the prepuce, but there is no knowledge of any clinical problems associated with this. It can occur regardless if the surgery is performed at 7 weeks or 7 months of age.
Estrogen-responsive urinary incontinence, now known as urethral sphincter mechanism incompetence (or more simply, the inability to control urination), is common in spayed female dogs regardless of their age when spayed. However, the Cornell study showed a significant increased risk of urinary incontinence for female dogs spayed before 12 weeks of age (12.9% vs. 5.0%) although other risk factors for developing this problem include age, obesity and breed. Older, intact female dogs may experience incontinence naturally as a result of the decrease in circulating estrogen, which has an effect on the external urethral sphincter. Incontinence may be seen soon after the surgery has been performed, years later, or not at all.
It is clear tat there is a need for more research, as the Texas A&M study did not show an increased risk and another study from 1992 showed a higher incidence of urinary incontinence in female dogs spayed after the first estrus cycle (20.1%).
Some shelters have found an increased incidence of infectious disease (in particular, upper respiratory infections in cats and parvovirus in dogs) in animals who undergo pediatric neutering, but the stress of shelter life, anesthesia and surgery affects adult animals as well, not just kittens and puppies. Many shelter animals would probably have developed disease anyway because of the increased presence of infectious agents in shelters.
Infectious disease should not be a problem in the private-practice clinical setting.
Pyometras, or infection of the uterus, affects 15.2% to 24% of bitches between the ages of 4 and 10 years in countries (Some countries allow spaying for therapeutic reasons only) where spaying is not commonly practiced. Because spaying that removes both the ovaries and uterus and thus prevents the development of pyometras is so common in the U.S., data for its incidence here is difficult to obtain.
Ovariectomy, or removal of the entire ovary, will also prevent the development of pyometra even though the uterus is left behind.
Hypothyroidism occurs more commonly in neutered dogs than in intact dogs, but a direct cause and effect has not been established. The overall incidence of hypothyroidism in dogs is 0.2% to 0.3% and certain breeds, such as Doberman Pinschers, Golden Retrievers and Dachshunds, have a predilection for this disease.
Neutered male and female cats have been shown to have an increased risk of developing diabetes mellitus compared to intact male and female cats. Other risk factors for cats in developing diabetes mellitus include breed (Burmese have a higher incidence), sex (males have a higher incidence), obesity and old age.
Recent theories suggest a high-carbohydrate, dry-food diet may also be a contributing factor to the development of diabetes in cats.
A possible increased risk of developing diabetes was found in neutered male dogs, but this may also be associated with obesity. Much more research is needed.
There has been concern that pediatric neutering may increase the risk for certain types of neoplasia. In order to be balanced, any discussion of neoplasia must also discuss the decreased risk for other types of neoplasia. For example, mammary gland tumors are the most common type of tumor of female dogs, with a reported incidence of 3.4%. Mammary gland tumors are the third most common tumor of cats, with a reported incidence of 2.5%. In female dogs, 50% of mammary gland tumors are malignant; the percentage rises to 90% in female cats.
Sexually intact dogs and cats have a much greater risk of developing mammary gland tumors than neutered animals. Studies have shown that the risk of developing mammary gland tumors in dogs spayed prior to the first estrus is 0.5%, while the risk after the first estrus increases to 8.0%, and the risk after the second estrus increases to 26%.
Many veterinarians believe that castration lowers the risk of prostatic neoplasia in dogs, but studies show that castrated dogs are actually 2.4 to 4.3 times more likely to develop prostatic neoplasia (usually prostatic adenocarcinoma) than intact dogs. However, the reported overall incidence of prostatic tumors in dogs is only 0.2% to 0.6%.
An exact cause and effect is not known, but castration does protect against other prostatic diseases seen much more commonly in intact male dogs, such as benign prostatic hyperplasia (BPH), cystic hyperplasia, squamous metaplasia, paraprostatic cysts, prostatitis, and prostatic abscesses.
Another tumor that has been associated with neutering is hemangiosarcoma. Spayed females have 2.2 times the risk of developing splenic hemangiosarcoma and 5 times the risk of developing cardiac hemangiosarcoma compared to intact females, while castrated male dogs have 2.4 times the risk of intact males. However, the overall incidence of cardiac tumors in one study was only 0.19%, making them very uncommon compared to other tumor types.
The incidence of osteosarcoma (OSA) is believed to be .2%, but neutering can increase the risk of its development by 1.3 to 2.0 times. In a limited study of Rottweilers by Cooley, there was a significant increase of OSA in neutered dogs who had undergone neutering at younger than 1 year (which is not considered to be pediatric neutering), but the overall incidence of OSA in Rottweilers is much higher than that in the general population. Also, in this study, although the life span of sexually intact and neutered male dogs did not differ, the life span was actually increased in spayed female dogs compared to intact female dogs. It is not possible to make generalizations about all dogs based on this study.
Transitional Cell Carcinoma (TCC) is the most common tumor of the urinary tract of dogs. Neutered animals have a 2-4 times increased risk of developing TCC compared to sexually intact animals. However, a cause and effect relationship has not been defined, and TCC in dogs is reported to be, at most,1.0% of all malignant tumors.
Testicular tumors account for 90% of all cancers originating from the male reproductive system. Although many factors contribute to their development, cryptorchidism – the failure of a testicle to descend into the scrotum – is a major contributing factor. Metastasis is considered low for these types of tumors, and castration is preventative and usually curative.
Ovarian and uterine tumors are not common in dogs and cats, and although malignant tumors of the female reproductive tract have been reported, spaying is also usually curative.
Many studies show an association between pediatric neutering and neutering at any age with various health conditions; however, in many cases, a direct cause and effect has not been determined.
Furthermore, many of the serious problems such as hemangiosarcomas are fairly rare and impacted by many other factors such as heredity and breed, age, diet, weight, environment, etc. They are also outweighed by the positive health benefits of pediatric spaying on much more common conditions such as mammary gland neoplasia, pyometra and benign prostatic neoplasia.
Based on current knowledge, veterinarians should feel comfortable assessing their patients to determine the best age to neuter dogs and cats.