In the healthcare and scientific communities, systematic reviews are utilized to guide medical decisions as they provide an exhaustive summary of high-quality peer-reviewed studies relevant to a research question. The foundation for our segments which outline the positive and negative effects of spay/neuter is a 2007 summary of a systematic review of more than 50 peer reviewed studies ⁸³ in the veterinary medical literature with respect to the long term physical risks and benefits associated with spay/neuter in dogs. Since 2007, there have been significant studies performed which have expanded the knowledge base. We will include the results of these new studies as well. Most significant of the new studies is the 2012 meta-analysis of published studies ⁷² which concludes:

“A commonly-stated advantage of …[spaying] bitches is a significant reduction in the risk of mammary tumours, however the evidence for this has not previously been assessed by systematic review… Due to the limited evidence available and the risk of bias in the published results, the evidence that …[spaying] reduces the risk of mammary neoplasia, and the evidence that age at …[spaying] has an effect, are judged to be weak and are not a sound basis for firm recommendations.”

We would suggest with respect to the concern regarding mammary tumors in dogs not spayed, it would be prudent to provide annual or semi-annual mammary exams of these dogs to screen for mammary tumors, just as we do for humans.

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 malignant tumors in female dogs 
• nearly eliminates the risk of pyometra, which otherwise would affect about 23% of intact female dogs; pyometra kills about 1% of intact female dogs ^44,45
• reduces the risk of perianal fistulas ⁴⁶
• removes the very small risk (0.5%) from uterine, cervical, and ovarian tumors ^21,22

On the negative side, spaying female dogs:

• if done before 1 year of age, significantly increases the risk of osteosarcoma (bone cancer); this is a common cancer in larger breeds with a poor prognosis ^{13,14,60,61,109}
• increases the risk of splenic hemangiosarcoma by a factor of 2.2 and cardiac hemangiosarcoma by a factor of >5; this is a common cancer and major cause of death in some breeds^{24,25,60,61,109}
• increases the risk of lymphoma and lymphosarcoma   ^{60,61,66,109,190}
• quadruples the risk of cutaneous mast cell tumors  ^60,61,87,109
• triples the risk of hypothyroidism ^26,27,28,29
• increases the risk of obesity by a factor of 1.6-2, a common health problem in dogs with many associated health problems ^30,31,32
• causes urinary “spay incontinence” in 4-20% of female dogs ^{35,36,37,38,39,40,108}
• increases risk of persistent or recurring urinary tract infections by a factor of 3-4^41,42,43
• increases the risk of recessed vulva, vaginal dermatitis, and vaginitis, especially for female dogs spayed before puberty ⁴³
• doubles the small risk (<1%) of urinary tract tumors ²³
• increases the risk of orthopedic disorders ^{30,42,48,49,50,51,52,53,60,61,64,67,68,89,108}
• increases the risk of adverse reactions to vaccinations ³⁴
• triples the risk of atopic/allergic dermatitis ⁹²
• triples the risk of patellar luxation ⁵⁴
• increases the relative risk of brain tumours 11-fold ¹⁷⁵

As a footnote to this discussion, we want to point out that conventional spay surgeries are invasive procedures, and involve removal of the ovaries and uterus. Multiple studies show that the total complications (intraoperative and postoperative) for spay surgeries are consistently about 20%.^1,2,3,4  Further, the positive outcome listed above (nearly eliminates the risk of pyometra) is qualified because it is not uncommon for surgeons to inadvertently leave some portion of the uterus behind. The “stump” (of uterine tissue) as it is termed by veterinarians, is very susceptible to pyometra. For a more in-depth analysis of the complications of spay surgery please see our page with the same title.

Alternative sterilization procedures would be ovary sparing spay (hysterectomy) and tubal ligation. Both of these procedures are generally considered to be hormone sparing as the ovaries are left intact. However, new evidence seems to indicate removal of the uterus in ovary sparing spay (hysterectomy) does alter the hormone balance, whereas tubal ligation is truly hormone-sparing. Therefore, tubal ligation would be our choice over ovary sparing spay, but both procedures could be expected to have fewer complications as they are much less invasive procedures.