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Whether unspayed dogs get cancer is something many dog owners may be wondering about. Perhaps, you are wondering whether your dog is better off being spayed or left intact. The spay and neuter debate is often a subject of controversy, with mixed information that creates confusion. Veterinarian Dr. Ivana provides some clarity by pointing out various types of cancer seen in unspayed dogs. 

Do Unspayed Dogs Get Cancer?

When talking about spaying, the first thing that comes to mind is prevention of unwanted pregnancies. However, spaying is much more than population control. 

Spaying reduces or completely eliminates the risk of certain types of cancer, thus prolonging the dog’s life. Intact or non-spayed female dogs can develop several types of tumors.  Following is a list of certain types of cancer common in unspayed female dogs. 

Mammary Tumors in Unspayed Dogs

Mammary tumors in dogs account for 42 percent of all tumors and most commonly develop in female dogs over the age of 6 years old. They are often multiple, occurring near one or more of the dog’s 10 to 12 nipples. Usually, they are mixed type tumors with a potential to become malignant.

Most mammary tumors, whether benign or malignant, are painless, mobile masses. Some dogs, however, develop an invasive, painful, inflammatory form of breast cancer, often in the glands that are closest to the groin. This form of cancer is very difficult to differentiate from an acute bacterial infection. 

Malignant breast cancer tends to spread locally or nearby lymph nodes, and sometimes also more widely, especially to the lungs.

Spaying is the single most important factor influencing the risk of mammary tumors developing. An intact female has a 1 in 4 risk of developing a mammary tumor. The risk is eliminated if the female dog is spayed before her first heat cycle. Even spaying after one season reduces the risk of mammary tumors by over 90 percent.

Certain dog breeds are predisposed to mammary tumors. Those breeds include: Boston Terriers,  Dachshunds, poodles, spaniels and retrievers.

Mammary tumors can usually be felt under the skin. If you feel a lump in your dog’s breast tissue, do not delay. See you veterinarian as soon as you can.

Breast lumps, together with some surrounding normal tissue, are always surgically remove and the removed tissue is then examined by a pathologist. 

Because breast cancer in dogs spreads, chest x-rays are taken before breast surgery to determine the extent of the cancer. Surgery offers a high cure rate for benign tumors, a moderate rate for malignant tumors less than 2 centimeters in diameter, but a poor rate for large or inflammatory forms of breast cancer.

 When surgery is not practical due to poor health of the dog or the extent of cancer spread, chemotherapy or immunotherapy may enhance an individual’s quality of life as part of palliative care.

The best prevention for mammary tumors is spaying. If a female dog has not been spayed at the time of surgery to remove a mammary tumor, this procedure should also be considered to prevent recurrence. However, spaying at this stage does not increase the likelihood of a cure if the tumor has been found to be malignant. 

Vaginal Tumors in Unspayed Dogs

Vaginal tumors are not very common in female dogs (they only account for 2.4 to 3.0 percent of all canine tumors, but are most likely to occur in intact individuals over 10 years old. Most are benign, being lipomas or leiomyomas (tumors arising from smooth muscle tissues) but some may be malignant. Malignant types include leiomyosarcomas, mast cell tumors and squamous cell carcinomas.

Vaginal bleeding or discharge, excessive licking of the vulval area, or a visible mass inside the vulva are all signs of a vaginal tumor.

Diagnosis of vaginal tumors is by physical examination and sometimes also by biopsy. Surgical removal is the usual treatment and is successful in most cases. Spaying is also highly recommended because, if left intact, the tumors are likely to reoccur.

Ovarian Tumors in Unspayed Dogs

Ovarian tumors are relatively rare in female dogs (they only account for 0.5to 1.2 percent of tumors in dogs) and usually trigger no clinical manifestation. Although generally rare, certain dog breeds such as German Shepherds, English Bulldogs, Boxers and Yorkshire Terriers are predisposed to developing ovarian tumors.

Based on the cell origin, there are several different types of ovarian tumors in dogs:

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· Epithelial tumors

· Sex cord stromal tumors

· Germ cell tumors

· Mesenchymal tumors

o Ovarian hemangiosarcomas

o Ovarian fibromas.

As stated, unless they grow to be noticeably large, ovarian tumors rarely cause clinical manifestation. In the more advanced stages they may cause hormonal imbalance which will lead to a lack of heat. However, the lack of heat is often attributed to other causes and the ovarian tumor remains undiagnosed.

Most ovarian tumors are diagnosed during routine ovariohysterectomy operations in older dogs. More common are ovarian cysts, which may also be detected at the time of surgery.

The treatment of choice is ovariohysterectomy. Chemotherapy is also an option but there is no standardized protocol, each case is individually assessed and the treatment individually tailored.

Uterine Tumors in Unspayed Dogs 

Uterine tumors account for 0.3 to 0.4% of all canine tumors. The uterus consists of several layers of tissue and abnormal growths can arise from each of those layers. 

However, the most common type of uterine tumor is the benign, non-cancerous leiomyoma which arises from smooth muscle cells. The exact cause is unknown but it is believed that several environmental and hereditary factors play important roles in the tumor’s origin.

A dog with uterine tumor is likely to have distended abdomen, vaginal discharge and difficulty urinating. As the condition progresses, the risk of developing pyometra (pus filled uterus) increases.

An experienced vet will be able to feel a mass upon abdominal palpation. The blood tests usually show increased levels of calcium and estrogen. An abdominal x-ray and ultrasound confirm the diagnosis.

The treatment involves ovariohysterectomy. If the tumor has spread to other tissues the vet will recommend chemotherapy.

 Transmissible Venereal Tumors (TVT) in Unspayed Dogs 

TVT, also known as Stricker’s sarcoma, is a relatively rare type of tumor usually transmitted during a dog's mating. However, biting, licking and even sniffing TVTs can lead to transmission.

The most important clinical manifestation is the development of cauliflower-like growths in the dog's vagina or vulva. Other signs are epistaxis, epiphora, halitosis, tooth loss, facial deformation, skin masses and enlarged lymph nodes. Transmissible venerable tumors occur in males dogs as well.

Diagnosis is by physical examination and biopsy. Weekly treatment for up to six weeks with the chemotherapeutic drug vincristine sulfate usually produces a cure. Radiation therapy is another possible and effective form of treatment. These treatments are often combined with complete surgical excision. However, chemotherapy remains the treatment of choice for TVT. 

About the Author

Dr. Ivana Crnec is a graduate of the University Sv. Kliment Ohridski’s Faculty of Veterinary Medicine in Bitola, Republic of Macedonia.


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