Today's Veterinary Practice

MAY-JUN 2018

Today's Veterinary Practice provides comprehensive information to keep every small animal practitioner up to date on companion animal medicine and surgery as well as practice building and management.

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PEER REVIEWED 40 THE NEUTERING CONTROVERSY Nonetheless, repeatable trends provide guidance for future studies investigating the pathophysiology and influence of sex hormones on cancer. Prospective studies including a greater number of patients and attention to decreasing potential confounding factors should be performed. Urogenital and Mammary Neoplasia Decreasing the risk of mammary cancer, along with eliminating the risk of ovarian, uterine, or testicular cancer, are among the most common reasons cited to encourage spaying and neutering of companion animals. Mammary cancer in dogs carries a 50% chance of malignancy, with surgery being the treatment of choice. 3 Removal of the ovaries before the first heat cycle reduced the risk of mammary cancer to 0.5% in one study, compared to a 26% risk when a dog is spayed after 2 or more estrous cycles. 4 This protective effect is decreased after the first estrous cycle and lost once a mammary tumor has developed. 3,5 A recent systematic review of literature evaluating the impact of ovariectomy on the risk of canine mammary neoplasia determined that evidence supporting the protective link is weak, largely because of the potential for bias in the majority of the studies investigated. 6 Although castration greatly reduces the risk of canine prostatic diseases such as benign prostatic hyperplasia (BPH), castration is not protective against prostatic neoplasia. Several studies have shown an increased incidence of prostatic cancer in castrated dogs compared to intact male dogs, with a risk increase 2 to 4 times that of intact males. 7 One study found an increased risk for all types of prostatic neoplasia in castrated dogs, including transitional cell carcinoma (TCC), adenocarcinoma, and carcinoma. 8 Limitations to these studies include that no case controls were included and the impact of age at castration was not evaluated. TCC of the urinary bladder is a relatively uncommon but devastating neoplasia in dogs. TCC is generally more common in female dogs than male dogs; however, gonadectomy has been associated with an increased incidence in both sexes. 9 One study found castrated males to have a 3.6 times higher risk than intact males for development of urinary bladder TCC. 8 Hemangiosarcoma Splenic hemangiosarcoma (HSA) is a common disease in dogs. Early retrospective studies demonstrated a predisposition for development of splenic HSA in spayed female dogs compared to intact female dogs. 10 A recent retrospective study evaluating golden retrievers presenting to a single referral institution found a higher incidence of HSA in females spayed after 12 months of age (rate of diagnosis, 7.4%) compared to intact females (1.6%) and females spayed before 12 months of age (1.8%). 11 This suggests that either early spaying is protective against HSA in golden retrievers, or, more likely, that duration of sex hormone exposure is not a causative factor in development of the disease. Neuter status did not affect the rate of diagnosis of HSA in male golden retrievers in this study. The study evaluated more than 700 golden retrievers; however, the number of dogs with HSA was small: 10 females and 10 males. In this study, patients diagnosed with HSA at ages older than 9 years were excluded, which may have introduced bias as splenic HSA has previously been documented to be more common in dogs between 8 to 13 years of age. 10 A recent retrospective study, in which the data were obtained by questionnaires available to owners of vizslas through the Vizsla Club of America website, found spayed female vizslas had a higher incidence of HSA than intact female vizslas; however, diagnoses were not confirmed by review of medical records. The study found spayed female vizslas to have 9 times the risk of intact females for developing splenic HSA. 12 Neuter status did not affect odds of HSA diagnosis in male vizslas. The number of affected dogs was moderate in this study, with a total of 43 female and 30 male vizslas diagnosed with HSA. Despite the pitfalls of retrospective studies, collectively these data suggest a possible link between gonadectomy and the incidence of HSA in female dogs. This was corroborated in a review of the Swiss Canine Cancer Registry; over 1900 cases of Castration is associated with improvement in some undesirable behaviors, such as roaming, mounting, and urine marking in male dogs.

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