Effect of spaying and timing of spaying on survival of dogs with mammary carcinoma

The risk of developing mammary gland tumors in dogs is significantly decrea sed by ovariohysterectomy at an early age. However, previous studies have n ot found a benefit to ovariohysterectomy concurrent with tumor removal in d ogs with established mammary gland tumors, suggesting that the progression of these tumors is independent of continued estrogen stimulation. The purpo se of this study was to evaluate the effect of spaying and of the timing of spaying on survival in dogs with mammary gland carcinoma. Signalment, spay status and spay age, tumor characteristics, treatment, survival, and cause of death of 137 dogs with mammary gland carcinoma were analyzed. The dogs were classified into 3 groups according to spay status and spay time: intac t dogs, dogs spayed less than 2 years before tumor surgery (SPAY 1), and do gs spayed more than 2 years before their tumor surgery (SPAY 2). Dogs in th e SPAY 1 group lived significantly longer than dogs in SPAY 2 and intact do gs (median survival of 755 days, versus 301 and 286 days, respectively, P = .02 and .03). After adjusting for differences between the spay groups with regard to age, histologic differentiation, and vascular invasion, SPAY 1 do gs survived 45% longer compared to dogs that were either intact or in the S PAY 2 group (RR =.55; 95% CI.32-.93; P =.03). This study reveals ovariohyst erectomy to be an effective adjunct to tumor removal in dogs with mammary g land carcinoma and that the timing of ovariohysterectomy is important in in fluencing survival.