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.