Veterinarians can give you many reasons to spay or neuter your dog or cat: The procedure reduces aggression, lessens territorial behavior, cuts back on the desire to roam and the chance of being hit by a vehicle and – if your animal goes outside or gets loose – prevents it from producing unwanted puppies or kittens. Spaying can also reduce the risk of some cancers. There, though, the relationship between spaying/neutering and cancer becomes much more complex.

Pros and Cons of Spaying

Theriogenology is the branch of veterinary medicine concerned with obstetrics and with diseases and physiology of reproductive systems. The American College of Theriogenologists and The Society for Theriogenology believe companion animals not intended for breeding should be spayed or neutered. But the decision to spay or neuter a pet must be made on a case-by-case basis between the pet’s owner and the vet, considering the pet’s age, breed, sex, health status, intended use, household environment and temperament.

Spaying decreases risk of mammary, testicular and ovarian cancers. On the other hand, it may bring increased risk of osteosarcoma, hemangiosarcoma, prostatic adenocarcinoma, transitional cell carcinoma and other non-cancerous diseases. Vets frequently use this research study to calculate the risks of spaying, especially in certain dog breeds.

Yet the evidence remains inconsistent and contradictory from study to study: Various cancers have seemed more prevalent in de-sexed animals in some studies, but not in others. The small numbers of animals in study groups may indicate a trend where one does not exist in the larger population. So it’s hard for veterinarians to give concrete, unbiased recommendations.

Spaying/Neutering and Cancer in Dogs

The clearest link is between sex-hormone exposure and the development of mammary tumors. A dog spayed prior to the first heat has a risk of only 0.5%. This risk increases to 8% if spayed after the first estrus, then 26% after the second.

Other cancers of the reproductive tract can be reduced significantly by spaying and neutering. Removing the gonad eliminates the risk of uterine, ovarian, and testicular tumors.

On the other hand, intact dogs have noticeably less risk of developing lymphoma, one of the most common cancers in dogs. A large, multi-institutional retrospective study using the Veterinary Medical Database compared about 15,000 canine lymphoma patients with 1.2 million dogs from the general population. Intact female dogs had about half the likelihood of spayed females, intact males, or neutered males.

And less common cancers may become a likelier problem, too. For instance, prostatic carcinoma increases in risk from two to eight times in neutered dogs.

Spaying/Neutering and Cancer in Cats

Fewer studies have been done on cats. Cancer in general afflicts an estimated 30 to 40 percent of all cats, and one-third of these malignancies involve the mammary glands. Mammary cancer occurs more than 95 percent of the time in females and is the most frequently diagnosed cancer in cats over 10 years of age. Unfortunately, 85 percent of feline mammary cancers are lethally malignant. (The rate is more like 50 percent in dogs.)

Underlying causes of feline mammary gland cancer remain unknown. Genetic influence has not been found to play a role. Links between external agents— environmental carcinogens, exposure to sunlight, viruses, vaccines, etc. — have been established for other feline cancers, but not mammary cancer.

Yet hormone status is significant, specifically the roles played by estrogen and progesterone. Cats should be spayed before their first heat cycle: One study indicated that cats spayed prior to 6 months of age have only a 9% percent risk of developing mammary tumors, whereas the risk increases to 14% in cats spayed between seven and 12 months.

CARE is the animal equivalent of a medical center for humans, offering all forms of treatment for pets through emergency care 24 hours a day and 365 days a year. It also provides board-certified specialty care, once you get a referral from your primary veterinarian. You can take a visual tour of the practice at

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