Concepts to consider when guiding owners on the appropriate age for sterilizing their dog:

1. Association does not confirm cause and effect. Much of the spay-neuter literature documents association but not cause and effect.
2. Research confirms breed differences in risk factors for many diseases and conditions. Therefore, it may not appropriate to apply research findings on the timing of sterilization in one breed of dog to all breeds of dogs. Additionally, the population of dogs evaluated in one study may also not be equivalent to that evaluated in another study.
3. The incidence of the conditions attempting to be prevented by sterilization timing should be considered. For example, osteosarcoma is reported to have an overall incidence of 0.2%.¹⁶⁴ In comparison, the incidence of mammary neoplasia in female dogs who are allowed to have one estrus cycle prior to ovariohysterectomy is reported to be 8 percent and 26 percent after a second estrus cycle.¹¹⁷ However, even this example is complicated by other studies that show the population of dogs that are prone to developing osteosarcoma (primarily large-breed dogs) may not be the same as those with a predisposition to developing mammary neoplasia (primarily small-breed dogs).^165,167
4. The morbidity and mortality rates of the conditions attempting to be prevented by sterilization timing should be considered. Allowing estrus to occur through delaying ovariohysterectomy increases the risk of mammary neoplasia, a disease with high prevalence, higher morbidity and mortality than the diseases with lower incidence and less causation (cruciate disease, hip dysplasia, urethral sphincter mechanism incompetence, lymphoma, osteosarcoma, hemangiosarcoma)
5. Knowledge of individual breed variation of timing of first estrus will help in determining optimum age for ovariohysterectomy. The age at which first estrus occurs varies among breeds from 4 months in some small breeds to 24 months in some large breeds, but in general occurs later in large-breed dogs than small-breed dogs.¹⁶⁷ Therefore, categorically delaying ovariohysterectomy in female dogs to 6 months or later, may be after the first estrus in some breeds.