Advantages and Disadvantages to Spay and Neuter Surgery In the Dog and the Cat

Filed Under: Dogs, Cats, General Care

When is the best age to have your pet spayed or neuteredShould you even get your dog spayed or neutered?  Does having your dog or cat spayed or neutered enhance their life and prevent the onset of disease?   The answer may be more complicated than you think.  Do you think there are only advantages to having your pet altered?  Guess again.  There may be a larger downside than you expected. 

Most veterinarians in the United States currently recommend that elective gonadectomy (spay or neuter surgery) be performed ideally at 6 to 9 months of age for dogs and cats.  Most of these same veterinarians would also recommend that the majority of female pets be spayed before their first estrus.  Most doctors prefer not to perform elective ovarian hysterectomies or spays during an estrus cycle, for the reproductive organs to be removed are swollen and bleed easier, resulting in an increased risk for complications to occur.  Neutering or castration of the male is not affected by reproductive cycles.

In the past several years many humane organizations have lowered the ideal age for gonadectomy, or removal of the reproductive organs, to as early as 6 weeks of age.  The primary concern of these humane organizations is not the health of the individual pet, but rather the prevention of an unwanted pregnancy or litter.  Most of these humane organizations have found that the compliance rate with mandatory spay and neuter contracts is less than 60% when gonadectomy surgery is conducted at a later, more acceptable age after the pet has already been placed in the home.  In an effort to increase owner compliance with gonadectomies these same organizations have continued to lower the age at which spay and neuter surgeries are conducted to prevent their adoptees from causing an unplanned pregnancy.  According to one study, 56% of dog litters and up to 68% of cat litters are unplanned.  Neutering dogs and cats regardless of age before adoption prevents accidents from occurring.   In the United States alone there was an estimated 5.4 to 9.1 million dogs and an additional 5.7 to 9.5 million cats euthanized in 1990.  The primary focus of these humane organizations is to reduce the number of unwanted animals and to prevent this senseless euthanasia.

Is gonadectomy at an early age what is physically best for the life of your individual pet?  That answer may be a resounding, “No!”  Purdue researchers have documented that female dogs that keep their ovaries are also found to live longer.  It has long been recorded that human females have a distinct survival advantage as compared to their male counterparts.  By the age of 100, female humans outnumber men by 4:1, and yet there are more males born than females in any given year.  In the Purdue study it was found that when female dogs had their ovaries removed in the first four years of life, their natural advantage in life expectancy verses their male counterparts was completely nullified.    

The biggest threat to the life of a dog or cat is that of euthanasia.  Multiple studies have shown that intact dogs and cats are more likely to be relinquished to animal control or to humane organizations than those that have been previously spayed or neutered.  The number one reason pets are relinquished to animal control facilities is for behavioral issues.  

The majority of the United States public is ill-informed about the reproductive cycles of their pet.  Almost 60% of female dog owners do not realize that their female dog will cycle twice a year and an amazing 83% of female cat owners are unaware that their queens are seasonally polyestrous.  Seasonally polyestrous animals like cats will continue to cycle in and out of heat in the spring and fall seasons until such time that they are either bred by a male cat or spayed.   Up to 60% of the American public still believes that their pet is better off physically if they actually have a litter before being spayed.  

One of the more commonly fielded phone calls on emergency rotation for a veterinarian come from clients with reproductively intact felines concerned that their beloved cat is in severe pain when they are under the influence of hormones during an estrus cycle.  These amorous felines will roll around in the floor, arch their back, and howl and moan to their horrified owners.  Although the preceding behaviors are normal and indicate a cat in estrus, or a heat cycle, it is a behavior not well-appreciated by much of the pet-owning public.  Not to be outdone, the male cat will engage in howling courting rituals and will also actively fight competitors.  Due to their quite vocal and combative breeding seasons more cats are neutered and spayed than are dogs.     

Noted advantages for the early spay or neuter of a pet include a decrease in the most common neoplasia in female dogs:  that of mammary gland tumors.  The reported incidence of mammary tumors in intact dogs is 7 times higher than that of spayed dogs and cats with a reported incidence of 3.4% overall.  Performing an ovarian hysterectomy will help reduce the incidence of mammary cancer in animals as old as 9 years of age.  

Testicular tumors are commonly found in older unaltered male dogs and are typically seen as an enlargement of the testicle or a portion of the testicle involved.   The most commonly affected testicle is on the right side, which is also the more commonly retained testicle as well.  Retained or cryptorchid testicles are more commonly affected than are testicles found in the normal scrotal position.  The malignancy rate (ability of the tumor to spread to other body areas) is considered to be low for all types of testicular tumors.  Castration is curative or preventative for this type of tumor.  

Although rare, ovarian and uterine tumors can occur.  Performance of an ovarian-hysterectomy will be curative in these situations.  Commonly seen in dogs from 4 to 10 years of age is pyometra.  With pyometra the uterus has an increased sensitivity to progesterone and the uterus will start filling with pus.  The incidence for pyometra development is from 15.2% to 24% of all intact female dogs, and those dogs with a history of carrying a pregnancy successfully to term are also at an increased risk.  Here again, spaying a dog would prevent this condition from occurring and the performance of an ovarian-hysterectomy in an intact dog would be curative-especially when caught early.  Unfortunately when a pyometra is advanced when detected or the pet is already toxic it can have up to a 17% mortality rate depending on the condition of the pet and the surgeon involved.  

Male dogs that are intact have an increased occurrence of benign prostatic hypertrophy (BPH). It is estimated that up to 63.4% of intact male dogs will suffer from BPH and that incidence is positively correlated with the age of the dog:  80% of male dogs over 6 years of age are affected and that number progresses to nearly 100% for dogs over 9 years of age.  BPH will predispose these male dogs to prostatitis, or inflammation of the prostate gland.  Castration will decrease the incidence of BPH.

The most common complaint following spaying or neutering of a pet is that of obesity.  In one study 34% of neutered or castrated male pets and 38% of female pets that are spayed are considered to be overweight or obese.  Cats are more likely to become obese following gonadectomy than are dogs.  It is felt that the basal metabolic rate decreases in cats once they have been spayed or neutered.  This increased tendency toward obesity can be controlled with an appropriate diet and an exercise regimen.  

There are several medical downsides to having a pet spayed or neutered and including an increase in the following tumors:  prostatic tumors, transitional cell carcinomas of the bladder, osteosarcomas and hemangiosarcomas.  There is also an increase in orthopedic problems such as, rupture of the cranial cruciate ligament, and hip dysplasiaUrinary incontinence is common following ovarian hysterectomy in female dogs as well as an increased risk for the development of urinary tract infections or perivulvar dermatitis (dermatitis surrounding the vulva area).  Male dogs castrated at 7 weeks of age were found to have a smaller penis than those of comparable dogs’ neutered following puberty.  Male cats neutered before the onset of puberty were often found to be unable to extrude their penis.  Androgen deprivation, or lack of testosterone, has also been associated with cognitive dysfunction.

Contrary to popular myths, dogs that are neutered are actually at an increased risk for the development of prostatic neoplasia.  The most common prostatic neoplasia is malignant adenocarcinoma, and castration has been shown to increase the risk for its development.  Dogs suffering from a prostatic tumor will often have difficulty during defecation, become emaciated or exhibit pain, or have problems with locomotion (moving the rear legs).   It is common for this tumor to metastasize to nearby bone.   It is believed that the deprivation of androgens does not act to initiate the neoplasia, but rather allows for the progression of the disease.  Castrated dogs have an increased risk of 2.4 to 4.3 times greater than those of sexually intact male dogs for the development of prostatic adenocarcinoma.

The most common type of bladder cancer is that of transitional cell carcinoma (TCC).  The reported incidence of TCC is approximately 1% of all malignant tumors.  Neutered pets have a 2 to 4 times higher incidence for the development of TCC than do their unaltered counterparts.  

Osteosarcoma is another highly malignant tumor of the bone.  The general incidence in dogs for osteosarcoma development is 0.2%.  The risk of osteosarcoma increases with the age and weight of the pet.  Gonadectomy increases the risk of osteosarcoma development 1.3 to 2 times that of unaltered dogs.   Male dogs are also at greater risk for development of this tumor than are female dogs at a ratio of 1.5 to 1.0.   

Hemangiosarcoma is a commonly found tumor in the spleen or heart of dogs.  The reported incidence is 0.2%, although some breeds of dogs are more commonly affected.  Spayed females have a 2.2 times greater risk of splenic hemangiosarcoma and 5 times the risk of cardiac hemangiosarcoma than do unaltered dogs.  Castrated male dogs and unaltered female dogs have 2.4 times the risk of cardiac hemangiosarcoma than their unaltered male counterparts.  

Hip dysplasia is more commonly seen in dogs especially if spayed or castrated before they are 5 months of age.  Rupture of the cranial cruciate ligament in the stifle (knee) is more commonly seen in neutered or spayed animals.  It is postulated that body weight and body condition have a direct cause-and-effect relationship.  Cranial cruciate rupture occurs more commonly in female intact pets during certain phases of their estrus cycle which also suggests a hormonal effect on joint stability.  

There is an increased risk of hypothyroidism (a lack of thyroid hormones) and cognitive dysfunction following gonadectomy.  Androgen deprivation has been shown to increase the amyloid deposition in the brains of humans and rodents, which is the characteristic lesion seen in Alzheimer’s Disease.  There is also a decrease in the number of synapses that occur in the brains of rodents and nonhuman primates once these animals have been neutered or spayed.  

The choice of whether to spay or neuter a pet should be based on careful consideration of all the facts.  One should certainly contemplate all the pros and cons of their decision in careful consideration of their particular lifestyle and situation. The intended function of a pet and anticipated behavior one is likely to encounter should help determine if and how to affect the reproductive status of one’s pet.  For most situations, spaying and neutering is still a wonderful option, but there is not a one size fits all solution.  


Heflin, Marissa.  “Study Links Ovaries and Longevity.”  Veterinary Practice News.  Vol. 22/No. 2.  Pp. 1, 37.

Kustritz, Margaret, “Determining the Optimal Age for Gonedectomy of Dogs and Cats.”  JAVMA. Vol 231, No. 11.  December 1, 2007.  Pp. 1665-1675.

Maxie, Grant Ed.  Pathology of Domestic Animals. Vol. 3.  5th Edition.  Saunders/Elsevier. 2007. P. 594, 609-610.

“New Dog Study Reveals Link between Retaining Ovaries and Longevity.”  Pet Product News.  February 2010.  P. 23. 

Rosenberger, Julie and Norma Pablo, et al.  “Prevalence of and intrinsic Risk Factors for Appendicular Osteosarcoma in Dogs:  179 cases (1996-2005).  JAVMA. Vol. 231, No. 7. October 1, 2007.  Pp. 1076-1080.

Topics: neuter, spay

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