Dogs and cats have a greatly improved chance of long life, good health, and contentment if they are sterilized. The most reliable cure for numerous health and behavior problems, sterilization also acts as a powerful preventative.
As with unneutered male dogs, an urge to breed increases the chances that a male cat will slip out of the house in search of a mate and suffer fight wounds and other injuries. By far, most serious cat fights occur between unneutered males. The resulting wounds frequently develop into abscesses that must be surgically drained and treated with antibiotics. Worse, even a single bite can transmit deadly diseases - most often, Feline Immunodeficiency Virus (FIV) or Feline Leukemia (FeLV) - from one cat to another. FIV, for which no vaccine or cure currently exists, causes fatal failure of the immune system.
Spaying removes the ovaries and uterus from female animals and so eliminates the possibility of ovarian and uterine infection or cancer. Bacterial infection of the uterus (pyometra) commonly afflicts older unspayed dogs and cats. As pyometra advances, bacterial poisons enter the bloodstream, causing general illness and often kidney failure. If the uterus ruptures, the dog or cat will almost certainly die. Pyometra requires emergency spaying, which may fail to save an animal already severely weakened. The best preventative is to spay dogs and cats while they are young and healthy.
Spaying can also prevent mammary gland tumors, the most common tumor in unspayed female dogs and the third most common in female cats.(1) A high percentage of mammary tumors are malignant: in dogs, nearly 50 percent; in cats, nearly 90 percent. Once a mammary tumor spreads to the lungs or bones, the cancer will be fatal. An unspayed dog is approximately 4 times more likely to develop mammary tumors than a dog spayed after only two heats, 12 times more likely than a dog spayed before her first year (by 6 -8 months of age).(1) An unspayed cat is seven times more likely than a spayed cat to develop mammary tumors.
Spayed dogs and cats avoid the dangers of giving birth. A birth canal that is overly narrow - due to injury (such as a broken pelvis) or, as in bulldogs, to a breed trait of narrow hips - make giving birth perilous. So does inadequate body size, which can leave a Chihuahua, toy poodle, Yorkshire terrier, or other small dog too weak to deliver puppies naturally.
Such disabilities often necessitate Caesarian section to save the dog or cat's life. When a small dog begins to nurse her puppies, she is also vulnerable to eclampsia, in which blood calcium plummets. Initial symptoms include panting, high fever, and trembling. Unless given an emergency intravenous injection of calcium, the dog will suffer seizures and die.
Neutering (surgical sterilization) removes the testicles and so prevents testicular tumors in male dogs. A dog who develops a testicular tumor must be treated before the tumor spreads by the only effective means - neutering. Especially prevalent in older dogs, testicular tumors are the second most common tumor in male dogs. (Some dogs have one or two "undescended" testicles, which remain inside the body; these dogs have a particularly high risk of testicular tumors.) Although only a small percentage of testicular tumors are malignant, even non-cancerous ones can threaten a dog's life. One type of nonmalignant testicular tumor sometimes secretes the hormone estrogen at a toxic level that destroys the bone marrow's ability to produce blood cells - a fatal outcome.
By eliminating the sexual drive that can cause a dog to bolt from the house or yard, neutering helps protect dogs from injuries and diseases associated with roaming in search of a mate. Neutering decreases roaming, one study found, in 90 percent of male dogs.(3) On the loose, a dog may be hit by a car, harmed by an act of cruelty, or infected with a disease transmitted by another animal. He can also be seriously wounded in a dog fight - always less likely if a dog has been neutered since neutering reduces aggressiveness toward other male dogs.(3)