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Prostatomegaly (Enlarged Prostate) in Dogs

By: Dr. Douglas Brum

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Prostatomegaly is a common clinical sign in older intact male dogs. As intact dogs age, testosterone and estrogen levels change, and with time, lead to a prostate gland that gradually enlarges. The prostate cells become larger and more numerous and often will form multiple small cysts throughout the prostatic tissue. This change (BPH) is a normal physiologic response to hormonal changes in the body and usually does not cause any clinical signs.

With intact male dogs, prostatomegaly is, many times, an incidental finding. Occasionally, if the growth of the prostate is excessive, the symptoms of prostatomegaly will be noted. If a dog is neutered prior to reaching sexual maturity, the usual prostatic growth is inhibited.

Dogs that have been previously castrated that present with prostatomegaly are at greater risk of having prostatic pathology (disease) than intact dogs with enlarged prostates. Some degree of prostatomegaly might be considered normal in the intact male. A more aggressive diagnostic approach is recommended in the neutered male dog with prostatomegaly since this is not a usual finding.

The physical examination of the prostate is important in providing useful information regarding the depth of the work-up required in evaluating prostatomegaly. The prostate generally can be palpated (felt) rectally or externally just in front of the dog's pelvis. If the enlarged prostate is painful or asymmetrical, further diagnostics are warranted. Asymmetrical prostates are more commonly associated with prostatic neoplasia or infections. Any history of weight loss or poor general body condition may indicate a more chronic (long term) condition.

Ill animals with large and painful prostates may have prostatitis or a prostatic abscess and would require more rapid attention. Acute or sudden infections of the prostate can be serious and may even spread systemically, causing septicemia (a blood infection). Rapid diagnosis and treatment will improve the prognosis and minimize the potential of sepsis.

Dogs that are straining to urinate or defecate should also be treated rapidly, as these animals are usually quite uncomfortable, and delays could lead to other problems. Severe constipation or a urinary obstruction might result, requiring emergency intervention. Fortunately, prostatomegaly is usually not an emergency situation, being a more chronic condition. This enables veterinarians to evaluate the patient carefully and to choose an appropriate diagnostic plan.


The location and anatomy of the prostate is important in understanding the clinical signs associated with prostatomegaly. The prostate is located just behind the urinary bladder and under the colon. The prostate encircles the urethra, which is the tube that carries urine from the bladder through the penis and out of the body, as it exits the bladder.

The prostate is made up of two symmetrical parts, or lobes, located on either side of the urethra. When the prostate is small it sits within the pelvic canal, but as it increases in size, it moves forward into the abdomen.

The clinical signs of prostatomegaly vary with the type and severity of the prostatic disease. A urethral discharge is commonly seen with prostatic disease. Since the prostate communicates with the urethra, any increase in prostatic secretions may lead to a noted discharge.

Infection from prostatitis or a prostatic abscess may lead to a cloudy or purulent or pussy discharge. Bloody discharges may be seen with infections, tumors, or even BPH as an increased blood supply to the prostate may lead to hemorrhage. Clear or yellow discharges may be seen with cystic disease.

Your dog may also strain when he defecates when the prostate enlarges to the point of putting pressure on the colon and decreasing its functional diameter. The occasional "ribbon-like" appearance to the stools is a result of this compression. The decreased diameter can also result in constipation.

Paraprostatic cysts and prostatic abscesses are most commonly associated with this sign. The prostate generally enlarges outward, but if there is inward enlargement, pressure on the urethra may cause difficulty in urinating. This is an unusual presentation that generally only occurs with very significant enlargement as seen with paraprostatic cysts or abscesses. If the cysts or abscess is very large, distention of the abdomen may even be noted. Finally, animals with infections or tumors may feel ill, have a loss of appetite or have a more chronic weight loss.

The most common causes of prostatomegaly include:

  • Benign prostatic hyperplasia. BPH is the most common form of prostatomegaly, with virtually all intact male dog acquiring the condition as they age. It is caused by an increase in number and size of the prostate cells as the intact dog ages and is exposed to normal hormonal influences. It is a benign condition that usually does not cause any clinical signs. The majority of the time this condition is found incidentally on routine physical examination.

  • Prostatitis. Bacterial infection of the prostate gland causes pain in the prostate on palpation, and many times the dog acts ill. A large prostate and a bloody discharge from the penis or blood in the urine are common signs. Prostatitis may be acute (sudden) or chronic (long term).

  • Prostatic abscess. An abscess is a walled off pocket of infection containing white blood cells, bacteria, and cellular debris. Abscesses occasionally form within the prostate gland in cases of chronic prostatic infections. They may get to be quite large and cause compression of both the colon and urethra. Your dog may strain when he defecates or urinates and will probably feel ill.

  • Paraprostatic cyst. These fluid-filled sacs are connected to the prostate by a thin stalk. The cysts may be developmental in origin arising from remnants of fetal tissue that normally degenerates (uterus masculinus). The cyst can also be directly of prostatic origin. Single or multiple cysts may be seen, and they can get very large. Animals usually only feel ill if the cysts become large enough to compress other internal organs.

  • Prostatic neoplasia (cancer). Cancer of the prostate may closely mimic other types of prostatomegaly, but usually dogs with prostatic cancer have an asymmetric enlargement or firm nodules in one of the lobes of the prostate. Animals with prostate cancer also tend to be ill systemically, and have a history of weight loss. Tumors of the prostate are almost always malignant. The most common tumors involving the prostate are adenocarcinoma and transitional cell carcinoma. In contrast to most other types of prostatic disease, prostatic cancer occurs with the same frequency in both intact and neutered dogs. In a neutered male dog with significant prostatomegaly, prostatic neoplasia would be high on the list of potential causes.

  • Squamous metaplasia. Squamous metaplasia is a change in the prostate gland due to elevated blood estrogen levels. The prostate gland generally becomes symmetrically enlarged bilaterally. The main cause of this is an estrogen-producing tumor (Sertoli cell tumor). Long-term oral estrogen supplementation can also cause these changes.

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