Risks of Neutering
1.Neutering Surgery Risks-
Like all surgeries, neutering involves some risks. Total complication rates for routine castration or spaying have been reported from 2.6%-20% of cases.[64-67]. The majority of these are minor and require no treatment. [64,67] Complication rates vary considerably from practice to practice and are generally reported to be higher in studies of surgeries performed by students in training.[64,67] Reported death rates are less than 0.1%.
Prostate cancer in dogs has previously been reported to have an low incidence of less than 1%, but several recent studies have suggested it may be more common, though not always clinically recognized, and these papers have reported rates of 3.6-13%.[32,35] Most such cancers are malignant, with metastases reported in 40-80% of cases at the time of diagnosis.[13,36]. There is some uncertainty about the role of castration in prostate cancer development. While some reports have found fewer prostate cancers in castrated dogs than in intact dogs [36-38], most recent studies have found either no effect of castration on the rate of prostate cancers  or an increased risk for castrated dogs.[35,40] Most canine prostate cancers examined seem to lack receptors for male 8
hormones, so it appears that unlike in humans these hormones are not responsible for the initiation or progression of prostatic cancers, but it is unclear whether castration is overall beneficial, neutral, or a risk factor for their development.[30,40] Prostate cancer is an aggressive cancer with a poor long-term prognosis.
Osteosarcoma is a bone tumor usually seen in large breed dogs.[68,69] Overall incidence has been reported as 0.2%, but for at risk breeds rates of 4.4%-6.2% are often reported.[70,72] A rate of 12.5% was reported in one study, though the authors suggested this might have been an overestimate.  Neutered dogs have been reported to be at higher risk for osteosarcoma than intact dogs.[68,71]. In one study, no difference was found in overall risk for intact versus neutered animals of either sex, but neutering before 1 year of age was found to increase the risk, and it was found that the longer an individual had been intact the lower their osteosarcoma risk. However, the neutered animals in this study (especially the spayed females) lived longer than the intact animals, which may have contributed to an increased incidence of cancer in the neutered group.
It is possible that neutering, especially before sexual maturity, raises the risk of osteosarcoma, at least in predisposed breeds. Osteosarcoma is an aggressive cancer with a poor long-term prognosis, and it is generally treated with surgery and chemotherapy.
Hemagiosarcoma is a cancer of the cells that normally form blood vessels. The overall incidence has not been reported, but it makes up 5% of all non-skin cancers in dogs. It is less common in the cat, found in 0.5% of cats autopsied and 2% of cancers in this species. It most commonly occurs in the spleen, and certain breeds (such as German Shepherds, Labrador Retrievers, and Golden Retriever) are at greater risk than others.[73,74,76] Hemangiosarcoma can also develop in the heart, with a reported incidence of 0.19%.
Spayed females have been reported to have 2 times the risk of splenic hemangiosarcoma and 5 times the risk of cardiac hemangiosarcoma of intact females.[74,75] Castrated males have either been found to have no increased risk of splenic hemangiosarcoma and only a slightly higher risk (1.55 times) than that of intact males for cardiac hemangiosarcoma. Hemangiosarcoma is an aggressive cancer with a poor long-term prognosis, and it is usually treated with removal of the spleen (if this is the primary site) and chemotherapy.
Transitional cell carcinoma is a cancer of the lower urinary tract, usually found in the bladder and uncommonly in the urethra of dogs. It represents 1%-2% of canine cancers and is rare in the cat.[77,78] It is more common in females than males, prevalence varies by breed, and neutered animals have been reported to be at 2-4 times greater risk than intact animals.[78,79] Transitional cell carcinoma is an aggressive cancer with a fair long-term prognosis, and it is usually treated with chemotherapy and sometimes surgery or radiation therapy.
3. Orthopedic Disease- 9
Rupture of the cranial cruciate ligament in the knee is a common problem of large breed dogs, with a reported incidence of 1.8%-4.5%, though the incidence in predisposed breeds has been reported to be as high as 8.9%.[80-82,86] In addition to breed and obesity, neutering increases the risk of cranial cruciate ligament rupture, [80,81,82,86] One study suggested neutering may increase the angle between the bones in the knee in a way that promotes cruciate rupture, but this effect was only seen in dogs neutered earlier than 6 months of age. Cruciate ligament rupture is treated with a variety of surgical approaches, and it has an excellent long-term prognosis.[83,84]
Hip dysplasia is a developmental abnormality of the hip joint that can result in arthritis and clinical discomfort. It is rare in small breeds, with rates of affected dogs less than 1%, but it can be seen in as many as 40%-75% of large breed dogs.[86-89] Hip dysplasia is estimated to lead to clinically significant arthritis is fewer than 5% of affected dogs, but there are many factors involved including breed, weight, and the degree of anatomic abnormality of the hip joint, which makes predicting the outcome for any individual difficult. The incidence of hip dysplasia is most strongly associated with breed and family history.[86,90,91].
Some studies have identified neutering as increasing the risk of hip dysplasia.[86,92]. As discussed below, the age at neutering may also be a factor influencing the development of hip dysplasia. It is unclear if the increased risk is directly due to the effects of neutering or due to an increased incidence of obesity in neutered dogs. Hip dysplasia can be treated if detected early with surgical therapies that reduce the chances of clinically significant arthritis later in life.[94,95] In older dogs who have already developed arthritis and clinical symptoms, these can be managed surgically or medically, with medications, weight reduction, and other therapies.[96-98] Because of the genetic basis of the disorder, the ideal approach to eliminating it is to neuter those dogs that carry the predisposing genes to eliminate the disease from the population.[99,100]
Fractures of the capital physis of the femur (the growth plate where the femur attaches to the pelvis at the hip joint) can occur in growing animals both due to trauma and spontaneously. A number of studies have found a large majority of spontaneous capital physeal fractures in cats occur in obese neutered males.[179-182] It is clear that neutering delays closure of the growth plates in male cats, and so it may be an independent risk factor for such fractures, though neutering also increases the incidence of obesity, and the relative contribution of obesity and neutering to the risk of these fractures has not been elucidated.
4. Behavioral Risks-
Though neutering has been associated with a decreased incidence of some kinds of aggression, there is limited evidence that it may sometimes be associated with an increase in aggressive behavior. There is one study that identified more owner-directed aggression reported in Springer Spaniels that were neutered than in intact Springers. How reliable such an owner survey might be or how applicable to other breeds is unclear. Similarly, one study found evidence of an increase in aggression towards owners among spayed female dogs who were spayed before 11 months of age and who had already 10
showed some aggressive behaviors before neutering. However, there were some differences between the control group and the spayed dogs in addition to having surgery, and these make the results less reliable.
One study found female German Shepherds who were neutered were more reactive to the presence of unfamiliar humans and dogs than were intact dogs.. Another study found neutered dogs to be more active than intact dogs and castrated males to be more excitable than intact males but found no other measurable behavioral differences between the groups. The clinical significance or applicability of these findings to behavior problems is unclear.
One study has examined the relationship between neutering and the development of age-related behavioral changes thought to be similar to Alzheimer's disease or other forms of senile dementia in humans. Such changes are relative common, being reported in 28% of dogs 10-12 years old and 68% of dogs 15-16 years old. When multiple comparisons were made between intact males, castrated males, and spayed females (no intact females were included in the study), the only association found was for castrated males who had already shown signs of behavioral impairment when first assessed to progress to more severe impairment at a higher rate than intact males or spayed females. The significance of this finding is not clear.
Urinary incontinence is common in middle-aged to older female dogs associated with spaying, with a reported incidence of 5-30%. Rates are lower in small dogs and higher in large breed dogs.[93,108-111] It can usually be successfully treated with medication.[108,112]
Several reports have found spayed females to be at increased risk for urinary tract infections compared to intact females[113,114], but other studies have not found such a relationship. No associated with urinary tract infections has been found for neutering of male dogs. Most urinary tract infections can be successfully treated with antibiotics.
Feline Lower Urinary Tract Disease (FLUTD) is a collection of symptoms ranging from mild bloody urine and straining while urinating to potentially life-threatening urinary tract obstruction. Causes include bladder inflammation (cystitis), urinary tract infection, urinary tract stones, tumors, and others. FLUTD has been reported to occur in 1.3%-4.6% of cats in private practice and 7%-8% of cats in veterinary teaching hospitals.[117,118] While some studies have found no association between FLUTD conditions and neutering [70,119], and it does not appear that neutering affects the size of the urethra in male cats (a possible risk factor for obstruction), several epidemiologic studies have found that neutering status does raise the risk of some causes of FLUTD.[121,122]. Castrated males were at an increased risk compared to intact males for all causes of FLUTD except infection and urinary incontinence. Spayed females had an increased risk for urinary tract stones, urinary tract infections, and urinary tract tumor, 11
but not other causes of FLUTD. Intact females had a decreased risk for most causes. While most cases of FLUTD are treatable and not life-threatening, urinary tract obstruction in males is a very serious condition. This occurred in about 12% of cats with FLUTD symptoms, and the risk is higher in castrated males cats.[122,123]
Hypothyroidism is a condition in which the thyroid gland atrophies or is damaged by the immune system and fails to produce adequate amounts of thyroid hormone.[124,125] It occurs in an estimated 0.2%-0.3% of dogs.[126,127] Some studies have found that neutered dogs are at higher risk than intact dogs for developing hypothyroidism.[126,127] However, other studies have not found any such association. Supplementation of thyroid hormone resolves the disease in most cases.[127,128]
Diabetes mellitus is a complex disease that comes in a variety of forms and has a variety of causes. Briefly, an affected animal will have blood sugar levels that are too high and will usually need insulin injections to control their blood sugar and prevent the many serious secondary problems associated with uncontrolled diabetes. Incidence in cats has been reported from 0.08%-2%, with Burmese cats having a higher rate of occurrence than other breeds or mixed-breed cats.[131-134] Incidence in dogs is estimated at 0.19-0.64%, with significant breed variations.[135,136] Diabetes is more common in male cats than females, and neutering is associated with an increased risk of diabetes in both male and female cats in some studies. However when age and weight are controlled for no effect of neutering is seen in others. For dogs, diabetes is usually believed to be more common in females than males [130,136] though this is not found in all populations.
Castrated males were at higher risk for diabetes than intact males in one study, though weight was not controlled for. Some authors have suggested that intact females may be at greater risk of diabetes due to the antagonistic effects of ovarian hormones on insulin, and spaying is an important part of regulating diabetes in female dogs.  Weight is clearly a risk factor for diabetes in cats, thought there is some debate about whether this is true in dogs, and since neutered animals are prone to be heavier than intact animals matched by breed and age, this may be a confounding factor creating the appearance of a direct effect of neutering on diabetes risk.[130-133,136] Diabetes is a serious chronic disease that can often be managed for long periods but cannot be cured.
Pancreatitis is an inflammatory condition of the pancreas, an organ involved in digestion and also insulin production. It can occur as a sudden severe disease or as a long-term chronic, waxing and waning disease. The true incidence of pancreatitis is unknown, and although autopsy surveys have found evidence of inflammation in anywhere from less than 1% to more than 50% of dog pancreases, no study has yet examined how common clinical pancreatitis is.[139-141] In dogs, there is some evidence that neutered animals may be at higher risk than intact animals for sudden-onset form.[142,143]
Obesity is a common and growing clinical problem in dogs and cats. Though clear and consistent definitions do not exist, various reports have suggested that among dogs 18%-12
44% are overweight and 2.9%-7.6% are obese.[154-156] Among cats, an estimated 19%-40% are overweight and 7.8% are obese.[157-159] Being overweight is a significant risk factor for many serious diseases.[134,160-162] Almost all studies agree that neutered cats and dogs are more likely to be overweight or obese than intact cats and dogs.[154-156,158-160,163-169] However, the exact relationship between neutering and excess body weight has not been clearly established.
Some studies have indicated that neutered animals have a lower metabolic rate and so burn fewer calories regardless of activity, which would make them prone to being overweight.[170-173] But other studies, which controlled for the proportion of an animal's weight made up of fat, which is not very metabolically active, have found comparable metabolic rates in intact and neutered animals.[167-169,174] There is evidence that the reason neutered animals gain excess weight is that they eat more and expend less energy than intact animals despite having the same resting metabolic rate.[53,165,169,171,172] There are also many other risk factors for obesity, including sex, breed, and variables associated with owners and their habits, that affect the chances of an animal becoming overweight regardless whether it is neutered or intact.[154,159,160,175]
It is clear that obesity is preventable. Proper restriction of the amount of food, and hence the number of calories available to dogs and cats is all that is necessary to prevent obesity regardless of neuter status. [154,160]
Optimal Age for Neutering
For decades, the traditional age for neutering dogs and cats has been 6-9 months. There is no clear scientific basis for choosing this age, and it has been suggested that this practice arose as a response to anesthetic mortality in very young animals in the first half of the 20th century. Anesthetic procedures have evolved dramatically since that time, and it has since been demonstrated that not only is the procedure safe in puppies and kittens 7-12 weeks of age, but these younger patients actually recover faster and have fewer complications than those neutered at the traditional age.[65,102,176]
A large scale trial found no significant differences in the week immediately after surgery between patients neutered at the traditional age and earlier, apart from more minor surgical complications in the traditional age group. Another study followed cats neutered at 7 weeks and at 7 months for 1 year and found no differences in any outcome. Two large studies followed puppies and kittens neutered before and after 24 weeks of age for approximately 3 years.[177,178] For cats, of the numerous measures of health, behavior, and relationship with owner, the only difference detected was a greater incidence of urinary tract problems in the cats neutered at the traditional age. In the dog study, puppies neutered earlier than 24 weeks did have a higher rate of infections, primarily parvovirus. This may have been due to differences in the management policies of the two shelters in which the subjects were neutered since the rate of parvovirus infections was higher at the shelter where most of the early neutering animals were spayed or castrated. Dogs in the traditional age group had more 13
gastrointestinal problems than dogs in the early neuter group. Interestingly, there was no difference in the incidence of urinary incontinence in female dogs in this study, which contrasts with another paper that found urinary incontinence occurred twice as often in females spayed after their first heat as those spayed before having a heat cycle.
By far the largest, best designed studies in dogs and cats involved following over 1800 dogs and 1600 cats after neutering (either before or after 5.5 months of age) for an average of 4-4.5 years, but as long as 11 years in some cases.[93,149] For dogs, 7 out of 14 behavioral measures appeared affected by age at neutering, with early-neutering worsening 3 problem behaviors and improving 4. Animals in the early-neuter group exhibited higher rates of noise phobia and sexual behaviors. The early-neutered group also exhibited less separation anxiety, fearful urination in the house, and escaping. Early-castrated males (but not females) showed more aggression towards humans in the household and more barking. When only problems considered by owners to be serious were analyzed, the reduced risk of escaping for the early-neuter group was the only behavior still significantly associated with age at neutering.
For medical conditions, 4 were significantly associated with age at neutering. Dogs neutered early had higher rates of hip dysplasia, though the dysplasia seen in the traditional-age group was clinically worse and this group was far more likely to be euthanized for the problem than the early-neuter group. Rates of cystitis and urinary incontinence were higher for females neutered before 5.5 months of age. The early-neuter group had lower rates of respiratory infections but higher rates of parvoviral infection. And finally, the early-neuter group had a lower rate of obesity than those dogs neutered at the traditional age. The remaining 43 outcome measures studied showed no difference between the two groups.
For cats, early neutering increased shyness around strangers for both sexes, and it increased hiding behavior for males but not females. Early-neutered cats were showed less hyperactivity, and early-neutered males showed less aggression towards veterinarians, less urine spraying, and fewer sexual behaviors. There may also have been a decreased rate of scratching furniture in early-neutered cats, but these cats were more likely to be declawed so the effect may be an artifact. When only problems considered serious were analyzed, none of these behaviors was significantly associated with age at neuter.
Early-neutered cats experienced lower rates of asthma and gingivitis, and males experienced fewer abscesses in the first 5-6 years after neutering. Cats neutered early may have experienced lower rates of cancer, but when only malignancies confirmed by a veterinarian were considered this effect was not significant. For the other 38 outcome measures studied, no difference between the groups was observed.
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