My dog was not spayed until she was six years old. We did not breed her. We have been informed that she is very susceptible to breast cancer and advised to have her checked once a year. If she had been spayed at a younger age, would her chances of developing breast cancer be lower?

Yes, if performed before the first heat cycle.

Unfortunately, there are many dog owners who do not know that spaying a dog before her first heat cycle will decrease her chances of mammary cancer by over 98 percent. This is one reason that veterinarians recommend that female dogs be spayed before they are six months old. Your pet is no more susceptible to mammary cancer than any other female dog that is spayed after her second heat cycle. Do not regret what wasn't previously done but rather feel good that you have prevented your dog from going through some other potentially dangerous conditions by having her spayed at this time. Pyometra, or infection of the uterus, can be a fatal illness that is common in older, unspayed females. As far as watching for breast cancer, the best thing you can do is to check your dog's mammary glands for lumps monthly. Female dogs have left and right mammary chains that run from the front leg area all the way back to the inguinal area. You can start at each nipple and feel her skin and subcutaneous tissue around each one. Many owners are able to find a lump on their own if they are in the habit of petting their dog's belly area. If you find a lump, you should set up an appointment with your veterinarian to have it evaluated and removed if necessary. Mammary growths have about a 50 percent chance of being malignant and spreading to other parts of the body. The earlier you address a growth, the less chance it has to spread if it happens to be of the malignant type. Time frames for this vary but usually they don't occur until the primary cancer has been present for a certain period of time. Your veterinarian will also check your dog thoroughly at each yearly exam. However, do not hesitate to have her seen sooner than that if you find a growth. You have taken a big step in preventing problems by having her spayed and being aware that you should watch for abnormal growths.