Past president of the California Veterinary Medical Association, Dr. John Hamil, wrote:

DVM Newsmagazine reports that the CVMA withdrew support for a mandatory spay/neuter law because of opposition from CVMA member veterinarians and the veterinary community, and:

A recent American Veterinary Medical Association report invalidates the claim by mandatory spay/neuter supporters that pets should be spayed or neutered for population control reasons, and invalidates the claim that spay and neuter is always healthy for pets. The report finds adverse effects from spay and neuter include increased risks of aggression, prostate cancer, bone cancer, bladder cancer, hemangiosarcoma, obesity, diabetes, ligament rupture, and complications from surgery.

After reviewing the risks and benefits associated with spay/neuter of cats and dogs, the AVMA report concludes:

The recommendation from America's leading association of veterinarians is that the best interests of each individual patient should determine whether or when a pet should be spayed or neutered, because there are adverse health and behavioral effects that need to be weighed against benefits. This is a medical decision, to be decided by a pet owner in consultation with their veterinarian. One size does not fit all, and should not be mandated by government.

The The full version of the paper summarized below, complete with all references to the veterinary medical research cited, is available. This paper reports some of the adverse behavioral impacts of early spay/neuter.

California AB 1634 is named "The California Healthy Pets Act". It is deceptively – and incorrectly named. Mandatory spay/neuter will not improve the health of pets. This is not a "health" initiative. The evidence indicates that spay/neuter, especially early spay/neuter as would be mandated by AB 1634, in fact hurts the health of many dogs.

An objective reading of the veterinary medical literature reveals a complex situation with respect to the long-term health impacts of spay/neuter in dogs. The evidence shows that spay/neuter correlates with both positive and adverse health effects in dogs. It also suggests how much we really do not yet understand about this subject.

On balance, it appears that no compelling case can be made for neutering most male dogs, especially immature male dogs, in order to prevent future health problems. The number of health problems associated with neutering may exceed the associated health benefits in most cases.

On the positive side, neutering male dogs

• eliminates the small risk (probably <1%) of dying from testicular cancer
• reduces the risk of non-cancerous prostate disorders
• reduces the risk of perianal fistulas
• may possibly reduce the risk of diabetes (data inconclusive)

On the negative side, neutering male dogs

• if done before maturity, increases the risk of osteosarcoma (bone cancer) by a factor of 3.8; this is a common cancer in medium/large and larger breeds with a poor prognosis
• increases the risk of cardiac hemangiosarcoma by a factor of 1.6; this is a common cancer and major cause of death in some breeds
• triples the risk of hypothyroidism
• increases the risk of geriatric cognitive impairment
• triples the risk of obesity, and with it many of the associated health problems
• quadruples the small risk (<0.6%) of prostate cancer
• doubles the small risk (<1%) of urinary tract cancers
• increases the risk of orthopedic disorders
• increases the risk of adverse reactions to vaccinations

For female dogs, the situation is more complex. The number of health benefits associated with spaying may exceed the associated health problems in some (not all) cases. On balance, whether spaying improves the odds of overall good health or degrades them probably depends on the age of the female dog and the relative risk of various diseases in the different breeds.

On the positive side, spaying female dogs

• if done before 2.5 years of age, greatly reduces the risk of mammary tumors, the most common tumors in female dogs
• nearly eliminates the risk of pyometra, which otherwise would infect about 23% of intact female dogs; pyometra kills about 1% of intact female dogs
• reduces the risk of perianal fistulas
• removes the very small risk (<0.5%) from uterine, cervical, and ovarian tumors

On the negative side, spaying female dogs

• if done before maturity, increases the risk of osteosarcoma by a factor of 3.1; this is a common cancer in larger breeds with a poor prognosis
• increases the risk of splenic hemangiosarcoma by a factor of 2.2 and cardiac hemangiosarcoma by a factor of more than 5; this is a common cancer and major cause of death in some breeds
• triples the risk of hypothyroidism
• increases the risk of obesity by a factor of 1.6 - 2, and with it the many associated health problems
• causes urinary spay incontinence in 4-20% of female dogs
• increases the risk of persistent or recurring urinary tract infections by a factor of 3-4
• increases the risk of recessed vulva, vaginal dermatitis, and vaginitis, especially for female dogs spayed before puberty
• doubles the small risk (<1%) of urinary tract tumors
• increases the risk of orthopedic disorders
• increases the risk of adverse reactions to vaccinations

One thing is clear -- much of the spay/neuter information that is available to the public is unbalanced and contains claims that are exaggerated or unsupported by evidence. Rather than helping to educate pet owners, much of this has contributed to common misunderstandings about the long-term health impacts of spay/neuter in dogs.

The traditional spay/neuter age of six months as well as the modern practice of pediatric spay/neuter appear to predispose dogs to health risks that could otherwise be avoided by waiting until the dog is physically mature, or (perhaps in the case of many male dogs) foregoing it altogether unless medically necessary.

The balance of long-term health risks and benefits of spay/neuter will vary from one dog to the next. Across-the-board assertions that spay/neuter will improve the health of all pet dogs do not appear to be supportable from findings in the veterinary medical literature. This is especially true of spay/neuter before physical maturity.