There are many things to consider when trying to decide if you want to spay or neuter your pet. The issues break down into three categories: humane, medical, and behavioral.
Humane Reasons to Spay or Neuter your pet. Every year a simply horrible number of dogs and cats are put to sleep. Estimates range from 8 to 12 million.
This is enough to give any sane person nightmares. The number one reason to spay or neuter you pet is to prevent this senseless slaughter.
So how, you ask, is spaying my dog, who lives at home with me, going to prevent unwanted puppies? You'd never let her have puppies unless they were wanted. But let's say your dog gets pregnant anyway. It happens in the best families. But you are responsible and you find good homes for all the puppies. Or so you think. What if one (or more) of those homes isn't so good after all, and your grandpuppies end up having litter after litter of unwanted puppies? What if one (or more) of those homes don't neuter the males grandpuppies, and they wander the neighborhood sireing litter after litter on unwanted puppies? In my opinion, unless you have definite plans for breeding your dog or cat, it is the socially-responsible thing to do to have them spayed or neutered.
Medical Reasons to Spay or Neuter your Pet. It is well-established that there are definite medical benefits to spaying and neutering.
Let's start with neutering. The big issue here is preventing prostate disease and eliminating any chance of testicular cancer. Neutering does not protect against the development of prostatic cancer. However, it is well documented in the scientific literature that neutering does help prevent other prostatic diseases commonly seen in intact male dogs including benign prostatic hyperplasia, cystic hyperplasia, squamous metaplasia, paraprostatic cysts, prostatitis, and prostatic abscessation. It is estimated that 80% of un-neutered dogs over the age of eight will develop some form of prostate disease. While these prostate diseases are benign and non-life-threatening, they can be quite problematic for your pet. Prostate disease is very rare in cats.
The next benefit is decreased risk of perianal adenocarcinomas. These are tumors whose growth is stimulated by testosterone. These occur near the anus, hence the name peri-anal. While exact risk estimations are not readily available, perianal adenocarcinomas are much more common in un-neutered dogs than in neutered dogs. Perianal adenocarcionomas are very rare in all cats, however.
Now on to Spaying. Here the health benefits are even more clear-cut. Lets start with mammary (breast) cancer.The scientific literature states that the risk of developing mammary cancer in dogs spayed prior to the first heat is only 5 in 1000, but the risk rises dramatically to 80 in 1000 in dogs spayed after first heat and before the second heat. In dogs spayed after the second heat or not at all, the risk jumps to 260 in 1000. That means that one in four un-spayed dogs will develop mammary cancer in their life.
In cats, mammary tumors occur with about one-half the frequency of mannary tumors in dogs, but unlike dogs, 85% of mammary cancer in cats is malignant. That means 1 in 10 unspayed
cats will develop malignant mammary tumors. Like dogs, spaying cats before they come in heat the first time dramatically lowers the incidence of breast cancer.
Pyometra is the next health concern. Pyometra means uterine infection. While this condition is rare in humans, it is common in dogs, and it is often life-threatening. Because dogs don't have a monthly menstrual cycle like people do, the infection grows and grows undetected until it becomes an emergency condition. While there is a treatment for pyometra, it is at best expensive, and dogs can and do die from pyometra even with treatment. A Scandanavian study estimated that at least 25% of un-spayed dogs in Scandanavia will develop pyometra by 10 years of age, while an American study put the number closer to 66% by nine years of age for American dogs.
While the data are not as complete for cats as for dogs, pyometra is another common medical condtion in un-spayed cats, and similar protective benefits are accrued by spaying cats.
Behavioral Reasons to spay or neuter your pet. Females in heat, both dogs and cats, exhibit a number of undesirable characteristics that make spaying a good idea. Females of both species will actively search out males and may attempt to escape from the house or yard. This puts them at high risk for getting hit by a car, and for fighting with other animals.
Often there is a sudden influx of males around the home and yard, as the unspayed females in heat will attract males from near and far.
Female dogs in heat bleed for two to three weeks, posing a hygeine problem for indoor pets. In addition, unspayed female cats may spray urine when they are heat.
While females only exhibit undesirable characteristics when they are in heat, males often exhibit undesirable characteristics all the time. Testosterone makes un-neutered males much more lilely to roam the neighborhood, get hit by cars, get in fights with other dogs, damage property, hump human's legs, and be aggressive towards humans.
Neutered male cats also have a much lower incidence of urine spraying.
As you can see, there are lots of good reasons to spay or neuter your pet. Done properly, with safe anesthesia and excellent pain medication, your pet will come through surgery with minimal risk or discomfort.