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Neoplasia, or cancer, is abnormal growth of tissue. Benign tumors tend to stay in one location and cause disease by altering the single tissue involved and compressing tissue around it. Malignant tumors tend to spread in the area from which they arise and to spread to distant tissues, causing widespread disease. Virtually all tumors are more common in aged than in young animals, with average reported age at time of diagnosis of about 10 years. For the tumor types described below, exact cause-and-effect relationship between gonadectomy and development of tumors is unknown.
Mammary neoplasia, or breast cancer, is a very common disorder of female dogs, with a reported incidence of 3.4%; this is most common tumor type in female dogs. Of female dogs with mammary tumors, 50.9% have malignant tumors. Risk factors for mammary neoplasia in female dogs include age, breed (Table 1), and sexually intact status. Multiple studies have documented that spaying bitches when young greatly decreases their risk of developing mammary neoplasia when aged. Compared with bitches left intact, those spayed before puberty have a 0.5% risk, those spayed after one estrous cycle have an 8.0% risk, and dogs spayed after two estrous cycles have a 26.0% risk of developing mammary neoplasia later in life. Overall, unspayed bitches have a seven times greater risk of developing mammary neoplasia than do those that are spayed. While the benefit of spaying decreases with each estrous cycle, some benefit has been demonstrated in bitches even up to 9 years of age. The exact cause-and-effect relationship between intact status and development of mammary neoplasia in female dogs has not been identified. The genetic and hormonal causes of breast cancer identified in women have not been consistently identified in female dogs despite extensive research.
Prostatic cancer in dogs is uncommon, with a reported incidence of 0.2 to 0.6%. Prostatic adenocarcinoma is a highly malignant tumor that cannot be cured medically or surgically. A 2.4 to 4.3 times increase in incidence in prostatic neoplasia with castration has been demonstrated, with that information verified in multiple studies.
Testicular neoplasia is a very common tumor in dogs, with a reported incidence of 0.9%. Unlike in humans, testicular tumors occur late in life in dogs, are readily diagnosed, and are rarely malignant. Ovarian and uterine tumors are very uncommon in bitches.
Several tumors of non-reproductive tissues have been reported to be increased in incidence after gonadectomy. Transitional cell carcinoma, a malignant tumor of the urinary tract, was reported in two studies to occur 2 to 4 times more frequently in spayed or castrated dogs than in intact female or male dogs. Exact incidence is not reported; estimated incidence is less than 1.0%. A breed predisposition exists (Table 1). Surgical removal of transitional cell carcinoma may or may not be possible, depending on site of the primary tumor.
Osteosarcoma is a low incidence (0.2%), highly malignant tumor of bone. It is reported to be more common in large breed dogs with some specific breeds predisposed (Table 1). Two studies have documented a 1.3 to 2.0 times increased incidence of osteosarcoma with gonadectomy. However, one study evaluated solely Rottweilers, a breed with a reported genetic predisposition. Treatment often includes limb amputation and radiation or chemotherapy.
Hemangiosarcoma is a malignant tumor of vascular tissue, including the heart, major blood vessels, and spleen. Large breeds in general are at increased risk with some breeds specifically predisposed (Table 1). Two studies have documented increased incidence, from 2.2 to 5 times, in gonadectomized males and females compared to intact animals. Overall incidence of hemangiosarcoma is low, at 0.2%. Surgical removal is the treatment of choice, if possible.
Long bones grow from growth plates on either end. The growth plates close after exposure to estrogen and testosterone, explaining why growth in height is largely completed after puberty. In bitches and dogs, removal of the gonads before puberty slows closure of the growth plates, leading to a statistically significant but not overtly obvious increase in height. There is no evidence that after gonadectomy some growth plates will close on time and some late, however most studies have only examined long bones of the forelimb. No studies have demonstrated increased incidence in fractures or other abnormalities of the growth plates associated with age at time of spay or castration.
Hip dysplasia is abnormal formation of the hip joint with associated development of arthritis. Genetic, hormonal, and environmental factors, including diet, are involved (Table 1). In the one study describing increased incidence of hip dysplasia in female or male dogs spayed or castrated before 5 months of age, it is not clear that the diagnosis of hip dysplasia was made by a veterinarian in all cases.
The paired cruciate ligaments form a cross within the knee (stifle) joint. The cranial cruciate ligament (CCL) undergoes tearing or complete rupture when the stifle is stressed from the side, especially if the animal twists while bearing weight on that limb. CCL injury is very common, with reported incidence of 1.8%. Large breed dogs are generally at risk, with some breeds predisposed (Table 1). Overweight female and male dogs also may be at increased risk. It has been demonstrated that CCL injury is more common in spayed or castrated animals than in intact animals. The basis may be hormonal, as it has been demonstrated that CCL injury in humans is more common in women than in men with incidence varying with stage of the menstrual cycle. A very recent study documented change in anatomy of the stifle joint of female and male dogs with CCL injury with gonadectomy prior to 6 months of age; further research is pending. CCL injury is treated with surgery and rehabilitation; treatment is costly and recovery protracted.
Obesity is very common in dogs, with reported incidence of 2.8% in the general dog population; incidences of 34% of castrated male dogs and 38% of spayed female dogs were reported in one study. Multiple risk factors exist, including breed (Table 1), age, and body condition and age of the owner. A very commonly reported risk factor for development of obesity is gonadectomy. In cats, it has been demonstrated that gonadectomy causes a decrease in metabolic rate. There are no reports documenting metabolic rate in female or male dogs relative to gonadectomy. Obesity is itself a risk factor for some forms of cancer, CCL injury, diabetes mellitus, and decreased life span. Obesity is controllable with appropriate diet and exercise.
A very common form of urinary incontinence, formerly termed estrogen-responsive urinary incontinence and now more commonly called urethral sphincter mechanism incompetence, occurs in spayed female dogs. Urine leaks from the spayed female dogs when they are relaxed and so most often is seen by the owners as wet spots where the dog sleeps. Reported incidence ranges from 4.9 to 20.0%, with female dogs weighing more than 44 pounds and some specific breeds predisposed (Table 1). While multiple studies have documented correlation between gonadectomy and occurrence of this disorder, only one has demonstrated a correlation between incidence and age at gonadectomy. In that study, it was demonstrated that spaying before 3 months of age was significantly more likely to be associated with eventual occurrence of urinary incontinence in a given female dog than was spaying later. Urethral sphincter mechanism incompetence is easily controlled medically in most female dogs.
Pyometra is uterine infection overlying age-related change in the uterine lining. Incidence increases with age; 23 to 24% of dogs developed pyometra by 10 years of age in one Swedish study. Specific breeds are at increased risk (Table 1). This very common disorder of aged intact bitches is treated surgically.
Benign prostatic hypertrophy/prostatitis
Benign prostatic hypertrophy (BPH) is age-related change in prostate size. By 6 years of age, 75 to 80% of intact male dogs will have evidence of BPH; by 9 years of age, 95 to 100% of intact male dogs will have evidence of BPH. The increased size of the prostate is associated with increased blood supply. The most common clinical signs are dripping of bloody fluid from the prepuce and blood in the semen. Development of BPH predisposes the dog to prostate infection (prostatitis). Medical therapy for BPH can be used to control clinical signs but surgical therapy (castration) is curative.
Only one study has demonstrated a possible increased incidence of diabetes mellitus in dogs associated with gonadectomy. That study did not consider the effect of obesity, a known risk factor for diabetes mellitus.
Two studies have demonstrated increased incidence of hypothyroidism in female and male dogs after gonadectomy. Genetic factors also are involved (Table 1). Cause-and-effect has not been described, nor has a specific numerical factor for increased incidence been reported.
Several studies have demonstrated that spayed and castrated female and male dogs live longer than do intact bitches or dogs. Cause-and-effect has not been described. It is possible that gonadectomized dogs are less likely to show risky behaviors or that owners who have invested in animals by presenting them for spay or castration continue to present them for consistent veterinary care.
So how do you reconcile all this information in helping make decisions for individual animals? Considerations must include evaluation of incidence of various disorders, breed predisposition, and health significance of the various disorders (Table 2 and Table 3).
For female dogs, the high incidence and high percentage of malignancy of mammary neoplasia, and the significant effect of spaying on decreasing its incidence make ovariohysterectomy prior to the first heat the best recommendation for non-breeding animals. The demonstrated increased incidence of urinary incontinence in bitches spayed before 3 months of age and possible effect of CCL injury in bitches spayed before 6 months of age suggest that spaying bitches after 6 months of age but before their first heat is most beneficial. For bitches of breeds predisposed by ovariohysterectomy to highly malignant tumors and for breeding animals, spaying at a later age may be more beneficial.
For male dogs, castration decreases incidence of disorders with little health significance and may increase incidence of disorders of much greater health significance. For non-breeding animals, evaluation of breed and subsequent predispositions to disorders by gonadectomy should guide when and if castration is recommended.
As dog breeders, you are a source of information for people seeking a dog for companionship, to show or work as a hobby, or to grow up with their children. As veterinarians, we are one of the guardians of safety and good health for all animals in our society. It behooves all of us to thoughtfully consider why we recommend spay or castration for dogs, to ensure we are not putting our own convenience above their good health. For every individual bitch or dog, careful consideration of their breed, age, lifestyle, and suitability as a breeding animal must be a part of the decision as to when or if they should undergo gonadectomy.
Used with permission from the AKC Canine Health Foundation, a nonprofit organization dedicated to advancing the health of all dogs and their owners by funding sound scientific research and supporting the dissemination of health information to prevent, treat, and cure canine disease.