In patients in which risk for mammary cancer is the underlying reason prepubertal gonadectomy is recommended, breed predisposition to the disease should be considered. According to a Swedish insurance study of >260,000 female dogs, mammary tumors were most often reported in Leonbergers, Doberman pinschers, Bernese mountain dogs, Welsh terriers, English springer spaniels, American cocker spaniels, and boxers.16 For each of these breeds, the estimated likelihood of developing mammary tumors over a lifetime was ≥35%; conversely, estimated likelihood was ≤5% for basenjis, collies, Finnish Lapphunds, Lancashire heelers, Norwegian Buhunds, Norwich terriers, Pomeranians, pugs, and Siberian huskies.16
In another study of >7,000 female dogs in the United States, no mammary cancers were reported in intact or gonadectomized Bernese mountain dogs, boxers, miniature schnauzers, pugs, Saint Bernards, Shetland sheepdogs, or West Highland white terriers, but a high incidence was reported in American cocker spaniels and English springer spaniels.4 Based on these studies, owners of cocker spaniels and English springer spaniels may be counseled to consider ovariectomy before the first or second estrous cycle; however, mammary cancer may not play a decision-making role for owners of West Highland white terriers or Shetland sheepdogs. The discrepancy regarding Bernese mountain dogs could reflect differences in regional genetics or follow-up: the study of Swedish dogs16 followed patients to 10 years of age, whereas in the US-based study,4 mean age at follow-up was <6 years.