There is a much lower risk (about 1 percent) in male dogs and a risk in cats about half that of dogs. In dogs
Causes
The exact causes for the development of canine mammary tumors are not fully understood. However, hormones of the estrous cycle seem to be involved. Female dogs who are not spayed or who are spayed later than the first heat cycle are more likely to develop mammary tumors. Dogs have an overall reported incidence of mammary tumors of 3.4 percent. Dogs spayed before their first heat have 0.5 percent of this risk, and dogs spayed after just one heat cycle have 8 percent of this risk. The tumors are often multiple. The average age of dogs with mammary tumors is ten to eleven years old. Obesity at one year of age and eating red meat have also been associated with an increased risk for these tumors, as has the feeding of high fat homemade diets. There are several hypotheses on the molecular mechanisms involved in the development of canine mammary tumors but a specific genetic mutation has not been identified. Biology
Historically, about 50 percent of mammary tumors in dogs were found to be malignant, although taking into account tumor behavior, one study has estimated true malignancy in mammary tumors to be 21 to 22 percent. Adenomas and fibroadenomas make up the benign types. Malignant mammary tumors are divided into sarcomas, carcinosarcomas, inflammatory carcinomas (usually anaplastic carcinomas), and carcinomas (including adenocarcinomas), which are the most common.
Most seem to be benign and occur most frequently in neutered males. Surgery is recommended. Guinea pigs: Mammary tumors in guinea pigs occur in males and females. Most are benign, but 30 percent are adenocarcinomas. They usually do not metastasize, but aggressive surgery is necessary to prevent recurrence. Hamsters and gerbils: Mammary tumors tend to be benign in hamsters and malignant in gerbils. Hedgehogs: Mammary gland adenocarcinoma is the most common tumor of the hedgehog. == References ==