Discovering the Cause of Urethral Plugs in Dogs and Cats

Published May 16, 2014

Cat owners are very familiar and concerned about urethral plugs in their male cats. This obstruction of urine flow from the bladder to the penis opening can be life threating if not treated. Although male dogs can become obstructed by stones, only recently has it been reported that they can also develop a plug in their urethra.

The findings were just published in the March/April edition of the Journal of Veterinary Internal Medicine.

What is a Urethral Plug?

Urethral plugs are solid, soft masses composed of crystals, mucous protein and cellular debris. The mucous and cellular debris are thought to be a result of infection or chronic inflammation of the bladder lining. The male urethra in dogs and cats is much smaller in diameter than the females. These crystal accumulations become lodged in the small urethra and block the outflow of urine. That is why females seldom block. The backed-up urine increases pressure in the kidneys, causing potentially permanent dysfunction. Electrolyte (sodium, potassium) imbalances develop, endangering heart function and ammonia levels in the blood rise to toxic levels.

Treatment requires relieving the obstruction and re-establishing urine flow through the urethra and penis with a urethral catheter. This typically requires anesthesia. Intravenous fluid therapy is initiated to “flush” the kidneys and prevent further damage. This also promotes the production of large amounts of dilute urine to help dissolve plugs that were forced back into the bladder with urethral catheterization or plugs that may be forming in the bladder.

Provided the condition is treated soon enough, 24-48 hours of hospitalization returns body function to normal with minimal, if any, permanent kidney damage. Affected cats are generally released from the hospital with specific treatment for bladder infections or, more commonly, interstitial cystitis (chronic bladder wall inflammation of unknown cause) and dietary management.

Urethral Plugs in Dogs

Because this new study is the first to report urethral plugs in dogs, less is known about the cause and treatment for this condition. The study was a result of 42 samples of urethral plugs submitted to the Veterinary Medical Center at the University of Minnesota. The facility is known for its leadership in veterinary urinary stone and crystal research and is used worldwide by veterinarians seeking urinary stone analysis. Nine of the samples were from dogs treated at the Minnesota Veterinary Medical Center, the rest were submitted by other institutions or private practitioners.

Eighty-one percent of the plugs contained crystal formations called struvite. Struvite crystals are composed of magnesium, ammonia and phosphorus. In dogs, these crystals are most commonly associated with bladder infections. The nine dogs treated at the veterinary center had no evidence of bacteria in their urine or bladder infections. Researchers did not have similar data from the 33 submitted plugs.

Bladder biopsies were not performed on any of the nine treated dogs so it is not known if the cause could be related to chronic bladder infection as it is in cats.

The most interesting finding was that 71 percent of all plugs were collected from Pugs. Researchers were unable to confirm that certain breed anomalies of Pugs were associated with the struvite plugs.

The dogs were treated in the same way as cats and placed on special urinary diets to manage the condition. Follow-up of the cases revealed that one dog reobstructed within nine weeks and was euthanized. Another obstructed four years later due to oxalate, not struvite stones. Researchers did not comment on the follow-up of the other six dogs (one was lost to follow-up).

This study is important in that it gives veterinarians another consideration in male dogs with urination difficulties. Unlike stones, plugs may not show up on X-ray due to the lower concentration of crystals. A negative X-ray would not rule out urethral obstruction. It also highlights a red flag for Pugs with urination difficulties.