Why Spay/Neuter



Race Foster, DVM

Holly Nash, DVM, MS

Drs. Foster & Smith, Inc.

Mammary tumors are the most common tumors in female dogs who have not been spayed. Mammary tumors can be small, simple nodules or large, aggressive, metastatic growths. With early detection and prompt treatment, even some of the more serious tumors can be successfully treated. Cats also suffer from mammary tumors and they have their own unique set of problems that are discussed in a separate article.

Which dogs are at risk for developing mammary tumors?

Mammary tumors are more common in unspayed, middle-aged female dogs (those between 5 and 10 years of age), although they can, on rare occasions, be found in dogs as young as 2 years. These tumors are rare in dogs that were spayed under 2 years of age. Occasionally, mammary tumors will develop in male dogs and these are usually very aggressive and have a poor prognosis.

Spaying greatly reduces the chances of a female dog developing this condition. In those females spayed prior to their first heat cycle, breast cancer is very, very rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat. It is believed that the elimination or reduction of certain hormonal factors causes the lowering of incidence of the disease in dogs that have been spayed. These factors would probably be estrogen, progesterone, a similar hormone or possibly a combination of two or more of these.

The risk of breast cancer is almost eliminated in dogs that are spayed before their first heat.

What are the types of mammary tumors in dogs?

There are multiple types of mammary tumors in dogs. Approximately one-half of all mammary tumors in dogs are benign, and half are malignant. All mammary tumors should be identified through a biopsy and histopathology (microscopic examination of the tissue) to help in the treatment of that particular type of tumor. The most common benign form of canine mammary tumors is actually a mixture of several different types of cells. For a single tumor to possess more than one kind of cancerous cell is actually rare in many species. This combination cancer in the dog is called a 'benign mixed mammary tumor' and contains glandular and connective tissue. Other benign tumors include complex adenomas, fibroadenomas, duct papillomas, and simple adenomas. The malignant mammary tumors include: tubular adenocarcinomas, papillary adenocarcinomas, papillary cystic adenocarcinomas, solid carcinomas, anaplastic carcinomas, osteosarcomas, fibrosarcomas, and malignant mixed tumors.

What are the symptoms of mammary tumors?

Mammary tumors present as a solid mass or as multiple swellings. When tumors do arise in the mammary tissue, they are usually easy to detect by gently palpating the mammary glands. When tumors first appear they will feel like small pieces of pea gravel just under the skin. They are very hard and are difficult to move around under the skin. They can grow rapidly in a short period of time, doubling their size every month or so.

The dog normally has five mammary glands, each with its own nipple, on both the right and left side of its lower abdomen. Although breast cancer can and does occur in all of the glands, it usually occurs most frequently in the 4th and 5th. In half of the cases, more than one growth is observed. Benign growths are often smooth, small and slow growing. Signs of malignant tumors include rapid growth, irregular shape, firm attachment to the skin or underlying tissue, bleeding, and ulceration. Occasionally tumors that have been small for a long period of time may suddenly grow quickly and aggressively, but this is the exception not the rule.


It is very difficult to determine the type of tumor based on physical inspection. A biopsy or tumor removal and analysis are almost always needed to determine if the tumor is benign or malignant, and to identify what type it is. Tumors, which are more aggressive may metastasize and spread to the surrounding lymph nodes or to the lungs. A chest x-ray and physical inspection of the lymph nodes will often help in confirming this. Mammary cancer spreads to the rest of the body through the release of individual cancer cells from the various tumors into the lymphatics. The lymphatic system includes special vessels and lymph nodes. There are regional lymph nodes on both the right and left sides of the body under the front and rear legs. They are called the 'axillary' and 'inguinal' lymph nodes, respectively. Mammary glands 1, 2, and 3 drain and spread their tumor cells forward to axillary lymph nodes, while cells from 3, 4, and 5 spread to the inguinal ones. New tumors form at these sites and then release more cells that go to other organs such as the lungs, liver, or kidneys.

What is the treatment?

Surgical Removal: Upon finding any mass within the breast of a dog, surgical removal is recommended unless the patient is very old. If a surgery is done early in the course of this disease, the cancer can be totally eliminated in over 50% of the cases having a malignant form of cancer. The area excised depends on the judgment and preference of the practitioner. Some will only remove the mass itself. Others, taking into consideration how the cancer spreads, will remove the mass and the rest of the mammary tissue and lymph nodes that drain with the gland. For example, if a growth were detected in the number 2 gland on the left side, we would therefore remove glands, 1, 2, and 3 and the axillary lymph node on that side. If it were found in the number 4 gland on the right side, then glands 3, 4, 5, and the inguinal lymph node on that side would be completely removed. With some tumor types, especially sarcomas, complete removal is very difficult and many of these cases will have tumor regrowth at the site of the previously removed tumor. Owners may confuse a surgical removal of a mammary gland in the dog with a radical mastectomy in humans, with all of the associated problems. In humans, this type of surgery would affect the underlying muscle tissue which complicates the recovery. In the dog, however, all of the breast tissue and the related lymphatics are outside of the muscle layer, so we only need to cut through the skin and the mammary tissue. This makes the surgery much easier and recovery much faster. A radical mastectomy in a dog means all the breasts, the skin covering them, and the four lymph nodes are all removed at the same time. Although this is truly major surgery, suture removal usually occurs in 10 to 14 days with normal activity resuming at that point.

Many veterinarians will spay a dog having a mastectomy (unless she is very old). The value of this in decreasing the recurrence of tumors is still controversial. Chemotherapy and Radiation Therapy: Chemotherapy has not been a very successful nor widely used treatment for mammary tumors in dogs. However, with the constantly changing and improving drugs available, a veterinary oncologist should be consulted to find out if there is an effective drug available for your dog's particular type of mammary cancer. The effectiveness of radiation therapy has not been thoroughly researched. Some anti-hormonal drug regimens are being tested in dogs. At this point in time, surgical removal of the tumors is the treatment of choice.

How can I prevent mammary cancer in my dog?

There are few cancers that are as easily prevented as mammary cancer in dogs. There is a direct and well-documented link between the early spaying of female dogs and the reduction in the incidence in mammary cancer. Dogs spayed before coming into their first heat have an extremely small chance of ever developing mammary cancer. Dogs spayed after their first heat but before 2.5 years are at more risk, but less risk than that of dogs who were never spayed, or spayed later in life. We all know the huge benefits of spaying females at an early age, but every day, veterinarians still deal with this easily preventable disease. Early spaying is still one of the best things pet owners can do to improve the health and ensure a long life for their dogs.


Mammary cancer is a very common cancer and can often be successfully treated, if caught early. If all non-breeding dogs and cats were spayed before their first heat this disease could be almost completely eliminated. If you find a growth or lump in the mammary tissue of your dog, you should inform your veterinarian immediately and not take a "wait and see" attitude.

References and Further Reading Bonagura, J. Current Veterinary Therapy 12. W.B. Saunders Co. Philadelphia, PA; 1995. Ettinger, S. Textbook of Veterinary Internal Medicine. W.B. Saunders Co. Philadelphia, PA; 1989. Rutteman, GR; Withrow, SJ; MacEwen, EG. Tumors of the mammary gland. In Withrow, SJ; MacEwen, EG (eds). Small Animal Clinical Oncology. W.B. Saunders Co. Philadelphia, PA; 2001455-477.

© 2007 Drs. Foster and Smith, Inc.

Reprinted as a courtesy and with permission from

PetEducation.com www.PetEducation.com

On-line store at www.DrsFosterSmith.com

Free pet supply catalog: 1-800-323-4208

Neutering-Why it's a Good Idea

Race Foster, DVM

Holly Nash, DVM, MS

Drs. Foster & Smith, Inc.

Other than population control, there are lots of very, very good reasons to castrate (remove the testicles from) male dogs. They basically fall into one of two categories – they are either behavioral or medical. Regardless of which category we are talking about, most of the unwanted characteristics or conditions are caused by the male hormone testosterone, which is produced within the testicle. That is the major reason vasectomies have never been that popular in veterinary medicine. A vasectomy eliminates successful breeding, but it does not reduce any of the undesirable problems of the intact male, since it does affect testosterone production or its distribution throughout the rest of the dog's body.

Behavioral advantages of neutering

Decreased Aggression: One of the most important behavioral advantages of castration is that as adults, these dogs will tend to be less aggressive both toward other male dogs and also people. The androgen (male) hormones, of which testosterone is the most important, are responsible for the development of many behavioral patterns. When young puppies are sexually mounting their 7 and 8-week old litter mates this is because of androgen surges in their bodies. The same is true with aggressive behavior. Some medications that have androgenic hormonal activity often cause increased aggression (an example would be the birth control medication, Cheque Drops, which contains one of these androgen-type chemicals). The degree castration has on suppressing aggression varies between animals and the age at which it is done. Its effect is greatest if it is done before one year of age. Decreased Roaming: A second behavioral advantage of neutering is that these dogs will not 'roam' when they sense a female in heat. Male dogs can sense females in heat through pheromones. These are airborne chemical attractants that are liberated from the female when she is cycling. They travel through the air for great distances. We grew up on a farm where the next closest house was over a mile away, but when one of our female dogs was in heat, the males would come for miles from upwind, downwind, and crosswind. Pheromones are, to say the least, very effective stimuli. In the seventies, it was briefly popular to do vasectomies on dogs thinking that we would not be taking the 'joy of sex' away from our canine counterparts. The problem with this reasoning was that many of us keep our dogs restricted in our homes, a kennel, or on a chain. Now think of the psychological stress the vasectomized male is under when he is locked up, but yet smells that female in heat four blocks away. There is no joy of sex, as he is trapped on your property unable to go and mate with her. He is, in effect, teased continuously for three to fourteen days while the female is in estrus and he is unable to mate with her. If dogs are neutered at an early age, they will not sense or respond to pheromones, and would certainly be less stressed and tend to stay home. Increased Concentration: A third behavioral advantage occurs when you are training or working your dog, or using him for field work. If neutered, he will be a much better student with a much longer attention span when there are females nearby that are in heat. This is because he will not be constantly distracted by pheromonal stimuli.

Medical advantages

The medical advantages are numerous and even more significant. Again, all are caused by the effects of testosterone on the body or are physical problems that arise within the testicles themselves. Here again, a vasectomy would not serve any real or meaningful purpose.

There are numerous behavioral and medical benefits to neutering your dog.

No Testicular Tumors: There are several different tumor types, both benign and malignant, that arise within the testicles. As with most cancers, these usually are not noted until the animal reaches 5 or more years of age. Therefore, these would not be a problem in those individuals castrated at the recommended age. Improved Genetics: We all agree that a male carrying a harmful genetic trait like hip dysplasia or epilepsy should be neutered. We must do all that is possible to prevent the spread or continuation of these conditions and others like them. Fewer Hernias: A hernia is a protrusion of an organ or parts of an organ or other structure through the wall of a cavity that normally contains it. Perianal hernias occur when the colon, urinary bladder, prostate, or fat protrude from the abdominal cavity, through the muscular wall by the anus and then lie just under the skin. This type of hernia is far more common in older, unneutered male dogs. The levels of testosterone and other hormones appear to relax or weaken the group of muscles near the anus. When the animal then strains to defecate or urinate, the weakened muscles break down and the abdominal organs and fat bulge out under the skin. In shorthaired breeds, this large bulge is noted by the owner almost immediately, but in the longhaired dogs, the problem may go on for months before anyone realizes there is an abnormality. Left untreated, these organs may become damaged, unable to function or even die from loss of blood supply. Additionally, because of the displacement of organs into this area, the animal may not be able to defecate or urinate correctly or completely and may become constipated or have urinary incontinence (dribble urine). The surgery to repair this condition is not simple and today can easily cost $700 to $1500 or more, depending on the severity.

Fewer Perianal Tumors: There are tumors whose growth is stimulated by testosterone. These occur near the anus and are called perianal adenomas (benign) or perianal adenocarcinomas (malignant). As with the hernias, these usually do not occur until the dog is at least 7-years old. They require surgical treatment and should be caught early in their development to prevent recurrence. These tumors and the above hernia are very, very rare in those individuals castrated at 7 to 8-months of age. Fewer Prostate Problems: The most common medical problems eliminated in dogs neutered at an early age are those involving the prostate. Over 80% of all unneutered male dogs develop prostate disease. Prostate conditions such as benign enlargement, cysts, and infection are all related to the presence of testosterone.

Early neutering

In the United States, most dogs are neutered between 5 and 8 months of age. Many humane shelters and veterinarians are starting to neuter male animals at a younger age, even 6-14 weeks of age. This early neutering does not affect the growth rate, and there are no appreciable differences in skeletal, physical, or behavioral development between those animals neutered early than those neutered at a more traditional age. It must be remembered that younger animals may need different anesthetics and are more prone to hypothermia (lower than normal body temperature) during surgery. As long as procedures are modified to account for these differences, early neutering is very safe. In fact, animals neutered at a younger age often have faster recoveries than those neutered when they are older.


None of the behavioral or medical problems caused by testosterone are rare. Veterinarians deal with them on a daily basis. To say it in a way that may not sound very nice but is certainly true – veterinarians would make a lot less money if everyone neutered their male dogs before they were a year of age.

© 2007 Drs. Foster and Smith, Inc.

Reprinted as a courtesy and with permission from

PetEducation.com www.PetEducation.com

On-line store at www.DrsFosterSmith.com

Free pet supply catalog: 1-800-323-4208

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