The most common bladder tumor in dogs is a malignant tumor called transitional cell carcinoma. This cancer usually arises from the inside surface of the urinary bladder or urethra and less commonly from the muscular wall of the urinary tract. Transitional cell carcinoma accounts for 0.5 percent of all cancers in dogs.

The cause of transitional cell carcinoma is unknown, but carcinogens, or cancer causing chemicals, that are excreted in the urine may cause the cells that line the bladder and urethra to become cancerous. Exposure to insecticide dips applied to kill fleas and ticks may increase the risk of developing this type of cancer. Similarly, exposure to sprays used to control mosquitoes in marshy or wetland areas also may increase risk.

Cyclophosphamide, a drug used to treat cancer and certain immune diseases, is metabolized to a carcinogenic chemical called acrolein, which is excreted in the urine. Exposure to cyclophosphamide may increase a pet's risk for development of urinary bladder cancer.

Breeds predisposed or reported to be at a higher risk for bladder cancer include Shetland sheepdogs, Scottish terriers, West Highland white terriers, beagles and wirehaired fox terriers.

Female dogs are affected more commonly than males. Neutered dogs are at a higher risk than are intact dogs. Obesity also may predispose to development of this type of cancer.

Urinary bladder cancer is life-threatening. Left untreated, it can result in obstruction of the urinary tract and inability to urinate. This form of cancer can also metastasize, or spread to other parts of the body. At the time of diagnosis, transitional cell carcinoma is estimated to have spread in more than 50 percent of dogs in which it is diagnosed. Survival of dogs with this type of cancer is dependent on the location of the tumor in the bladder, extent of disease and whether it has metastasized, and what treatments are prescribed. Survival time can range from weeks to more than a year.

What to Watch For

Blood in the urine

Straining to urinate

Increased frequency of urination with passage of small amounts of urine

Straining while defecating

Signs of exercise intolerance

Difficulty breathing or coughing

In many cases, these signs can be present for many months before diagnosis. If your pet is showing any of these signs and does not seem to be getting better with treatment, additional tests should be done to rule out cancer as the cause.