Diagnosis and Medical Management of Canine Prostatic Disease
Melissa S. Wallace, DVM, Dipl. ACVIM
The canine is the only species other than man that has a male accessory sex gland, the prostate, which commonly develops spontaneous disease. The canine prostate gland is a bilobed structure with a palpable median raphe, which completely surrounds the urethra just distal to the internal sphincter. The gland is mostly in the retroperitoneal space, and usually resides within the pelvic canal at the pelvic inlet. The male cat also has a prostate gland, but it is small and located more distally, and does not entirely surround the urethra, unlike the canine prostate. The feline prostate gland is rarely associated with clinical disease.
There are four major disorders of the canine prostate gland. Benign prostatic hyperplasia (BPH) is the most common, and this disorder has hormonal, pathophysiological, and clinical similarities to BPH in man, although there are some interesting differences. Dogs can also develop bacterial prostatitis, paraprostatic cysts, and prostatic neoplasia. Subcategories of disease are prostatic cysts (usually a complication of BPH), and prostatic abscesses (a complication of bacterial prostatitis and/or infected cysts).
Clinical Presentation of the Dog with Prostatic Disease
The typical history is a male dog, usually older than 6 years, with either lower urinary tract signs (pollakiuria, dysuria, hematuria) and/or lower bowel signs (tenesmus, hematochezia, constipation). In addition, a hemorrhagic and/or purulent urethral discharge is a common sign of prostatic disease. Some dogs with prostatic disease may exhibit a wide-based gait in the hind limbs, called the 'prostatic shuffle', which is an attempt to ease discomfort while walking. In breeding dogs, decreased libido (due to discomfort), hemospermia and reduced fertility may be the historical complaints.
The prostate gland is palpated per rectum. Insert a gloved, lubricated index finger into the rectum, and use the other hand to palpate the caudal abdomen simultaneously. If necessary, you can gently push the prostate gland into the pelvic canal to examine it more completely per rectum. The normal gland is smooth and moderately firm but not hard in texture, is somewhat movable, and has a palpable median raphe. The gland is located within the pelvic canal or at the brim of the pelvis. The normal gland is larger in older dogs than juvenile dogs, and the Scottish terrier is known for having a larger than average prostate gland. If the gland is abnormally enlarged, it will often fall into the abdominal cavity. Other abnormalities include irregularity, uneven texture (fluctuant areas or very firm areas), lack of a palpable median raphe, adherence to the floor of the pelvis, and pain on palpation. The lymph nodes located in the sublumbar area should be palpated, as should rest of the pelvic canal and perineum. An enlarged gland can sometimes be palpated per abdomen. Urinary retention secondary to prostatic disease may be appreciated as a distended, painful urinary bladder. Constipation from an enlarged prostate gland may also be appreciated on abdominal palpation and rectal examination. The prepuce and urethral orifice should be examined for an abnormal (bloody) urethral discharge. Collection of semen with an artificial vagina, if possible, is important in evaluation of the breeding male dog with suspected prostatic disease.
Evaluation of dogs with suspected prostatic disease is similar to dogs with other lower urinary tract diseases, and should include a minimum data base of: urinalysis, urine culture (by cystocentesis or sterile catheterization), serum biochemistry profile, complete blood count, and survey abdominal radiographs. Other diagnostics that can be added on a case-specific basis include: abdominal ultrasound, semen evaluation, prostatic fluid culture (ejaculate or prostatic wash), prostatic fluid cytology (ejaculate or prostatic wash), prostatic fine needle aspirate, prostatic needle biopsy or surgical biopsy, and chest radiograph.
Abdominal radiographs are helpful in evaluating the size and location of the prostate gland, looking for evidence of metastatic neoplasia, sublumbar lymphadenopathy, mineralization of the prostate, or evidence of localized peritonitis. Prostatomegaly is defined as a dorsoventral prostate dimension > 70% of the distance from the sacral promontory to the pubis on the lateral radiograph. The size and position of the urinary bladder and colon are also evaluated. Since dogs with urolithiasis often show signs similar to those with prostatic disease, the radiograph should be carefully evaluated for calculi.
Abdominal ultrasound has become indispensable for evaluation of the abnormal canine prostate gland. Cysts, abscesses, size, echogenic texture, and changes suggestive of BPH or neoplasia can all be evaluated. The ultrasound does not diagnose the cause of the prostatic disease, but it is an important tool along with other diagnostic tests. Ultrasound can be used to guide aspirates of abnormal areas within the prostate gland for cytology and culture, and can be used to guide a needle biopsy (tru-cut) of the gland, if warranted.
Contrast radiography is often helpful in cases of prostatic or other lower urinary tract disease, and can also be used in practices that do not have access to ultrasound. Typically, a double contrast cystogram, positive contrast cystogram, and retrograde urethrogram are all performed under general anesthesia following colon cleansing with enemas. The normal prostatic urethra is usually somewhat wider in diameter as compared to the internal sphincter located just proximal to it and the membranous urethra distal to it. Evidence of prostatic disease may be seen as excessive (> 1 x urethral diameter) reflux of contrast agent from the urethra into the prostatic tissue. The identification of paraprostatic cysts can usually be made with contrast radiography, by accurately delineating where the urinary bladder is in relationship to the cyst.
Benign Prostatic Hyperplasia
This disorder develops in older intact male dogs, and is a natural result of aging and hormonal influence on the prostate gland. Testosterone secreted by the testes is converted within the prostatic epithelial cells by an enzyme (5-alpha reductase) to dihydrotestosterone (DHT). DHT regulates prostatic gland growth by interacting with DHT receptors within the prostate. As dogs age, the number of receptors for DHT seem to increase, as does the percentage of testosterone that is converted to DHT. Also with age, testosterone levels fall, while estrogen levels remain the same. This altered androgen to estrogen ratio is felt to play a role in the number of DHT receptors and their sensitivity to DHT. The result of these incompletely understood hormonal alterations is that the canine prostate develops both glandular hyperplasia and hypertrophy, which is often cystic. In man, the same endocrine factors are at play, but the hyperplasia involves the periurethral stromal tissues more than the glandular tissues, which may account for differing clinical signs and responses to pharmacologic agents in man versus the dog. In spite of these differences, the dog has served as an excellent research model for the human disease, and research to benefit man has significantly increased our knowledge of this disease and its potential treatment in the dog.
Dogs with BPH may have no clinical signs, but some dogs will exhibit a bloody urethral discharge, pain on ejaculation, blood in the ejaculate, tenesmus, or hematuria. The signs may be intermittent. Dogs with BPH are not usually sick. Physical examination is normal except for an enlarged prostate gland on rectal palpation, which may be somewhat irregular and mildly uncomfortable.
The urinalysis may be normal or show red blood cells and/or increase squamous epithelial cells. The urine culture should be negative (cystocentesis) or grow < 10,000 colonies/ml (catheterization) unless the dog has secondary prostatitis. Radiographs show prostatomegaly and no other abnormalities. Ultrasound reveals fairly homogenous texture that is normal to slightly hyper-echoic. Small fluid-filled cysts may be present.
The most effective treatment is castration. It is interesting to note that this is a common (inevitable) disease of older men, who are never castrated for this problem. If no clinical signs are present, re-evaluation every 6 months is probably warranted, and client education regarding signs of prostatitis is important. If clinical signs are present and the owner of the dog does not wish him to be castrated, then medical management, with monitoring for progression or complications, is warranted. There are no FDA approved drugs for the treatment of BPH in dogs. Megestrol acetate will reduce the size of the prostate, but it causes squamous metaplasia of the prostatic epithelium and is therefore not a good long-term treatment.
Therapies marketed and prescribed for men can be used for BPH in the dog, as long as the client understands the off-label nature of the treatment, is willing to bear the expense, and will return for routine monitoring of the condition. Finasteride (Proscar) is a 5-alpha reductase inhibitor. It reduces prostatic DHT concentration, causing atrophy of glandular and stromal tissues in the prostate without significantly altering testosterone concentrations. A significant reduction in prostatic size occurs over 1 - 2 months, but the results are not as dramatic as with castration, and cysts may not resolve. I have treated a few dogs with this medication long-term with good success, meaning failure of the disease to progress and resolution of clinical signs of prostatic disease. It is important to instruct the owners that women of reproductive age should not be exposed to this drug, because it can interfere with normal sexual development of the human fetus. Concerns about using this drug in male dogs used for breeding have been expressed by some specialists, but because dogs do not continue breeding during pregnancy this concern is probably unwarranted. Stopping the drug temporarily when the prospective bitch comes into proestrus and during the breeding period is a good compromise, and will also restore some of the prostatic fluid in the ejaculate, which is diminished in dogs taking finasteride. The ideal dose for the dog is not yet known, but since research on dogs using much higher doses have not resulted in serious side effects, I use one tablet (5 mg) daily, so as to avoid splitting the tablets and exposing the client to the drug.
Flutamide is an anti-androgen that binds to DHT receptors in the prostate. Similar to finasteride, this drug reduces glandular hyperplasia without causing infertility, since testosterone levels are not affected. The reported dose is 2.5 - 5 mg/kg/day. The disadvantage of both finasteride and flutamide is expense.
Infection of the prostate gland can occur with a male dog of any age, but it is more common in older dogs with BPH, and is rare or nonexistent in castrated dogs due to atrophy of the prostate gland. The glandular changes and disruption of normal urine flow and/or prostatic fluid flow associated with BPH predisposes the gland to infection. The urethra communicates with the prostate gland via the prostatic ducts, and ascending infection is therefore the most common etiology. The bacteria involved are those which also typically cause ascending infections of the urinary system of the female dog, such as E. coli, Klebsiella sp., Staphylococcus sp., Streptococcus sp., Proteus sp., Pseudomonas sp., etc. Mycoplasma sp. and/or Ureaplasma sp. are also possible opportunistic pathogens. Brucella canis can infect the prostate. Mycotic infections of the prostate gland occur rarely.
Clinical signs of bacterial prostatitis are more severe than BPH, and can include urethral discharge, hematuria, pollakiuria, dysuria, tenesmus, fever, caudal abdominal or pelvic discomfort, a painful prostate on rectal palpation and partial anorexia. Sometimes the dog will dribble discolored or bloody urine.
The urinalysis will reveal pyuria, hematuria, bacteriuria, and perhaps an increase in squamous epithelial cells. The urine culture will be positive. Except in mild cases, collecting an ejaculate is usually not possible. Performing a prostatic massage or wash may be helpful. To do this, empty the urinary bladder via a sterile urinary catheter. Place the tip urethral catheter within the prostatic urethra, and then gently massage the prostate gland for a few minutes. Aspirate the catheter and use any fluid recovered (presumably prostatic fluid) for quantitative culture and cytology. A small amount of saline (5 cc) can be instilled into the catheter prior to the massage to try to increase the recovery of fluid. This technique is sometimes useful and is sometimes frustrating. If the dog is very painful an analgesic should be administered before the procedure. The serum biochemistry panel is usually normal unless the dog has become dehydrated due to illness. The CBC may reveal a leukocytosis with a left shift, but this depends on the duration and severity of the infection.
Radiographs show prostatomegaly, and in some cases a hazy appearance below the bladder neck indicating localized peritonitis. The ultrasound will show changes in echogenicity that are more severe than with BPH, and the texture may be more complex. Hypoechoic abscesses may be seen, and cannot always be differentiated from cysts. Needle aspiration of the fluid for cytology and culture may be performed, but should only be done if the diagnosis is in question, as the procedure may not be totally benign.
Bacterial prostatitis is treated with antibiotics based on culture of the bacteria, as well as how well the drug penetrates prostatic fluid. Drugs that are highly lipid soluble, have low protein binding, and are either weak bases or amphoteric are the ones which penetrate the blood/prostate barrier the best. Excellent initial choices are enrofloxacin or trimethoprim-sulfamethoxizole. Penicillins, cephalosporins and aminoglycosides do not penetrate prostatic tissue well. In severe prostatitis with peritonitis, the blood prostate barrier is disrupted, and a beta-lactam antibiotic used in combination with a fluoroquinolone may be effective. Treatment of bacterial prostatitis should be long-term (4 - 6 weeks), and follow-up evaluation is important. If BPH is also present, which is typical, it should be treated by castration or appropriate medical management, because the condition is likely to recur.
A subset of dogs with bacterial prostatitis will develop bacterial abscesses of the prostate gland. These infections can be life-threatening, because the abscess can result in septicemia, endotoxemia, and localized peritonitis. Similar to pyometra in the bitch, a bacterial abscess of the prostate gland is a medical and surgical emergency, requiring rapid medical stabilization (i.e. treat for septicemia and shock) of the patient and surgical drainage of the abscess. Usually, the abscesses are broken down and surgical drains are placed into the prostate gland that exit the caudal abdomen for a few days. Sterile laparotomy pads are used to cover the drains, and these must be changed frequently. This condition has a high mortality rate, because septicemia, DIC, and hypoalbuminemia from peritonitis are common complications. If a dog has this condition, castration at the time of surgical drainage of the abscess, or after recovery, is advisable.
Unlike true prostatic cysts, which are often seen in association with BPH, paraprostatic cysts are located in the vicinity of the prostate but not within it. They are sometimes attached to the prostate, but do not have to be. The origin of these cysts is obscure, but they may be remnants of the müllerian duct system (i.e. uterus masculinus). The cysts are often so large that they resemble a second urinary bladder on radiographs. They are thin walled and contain sterile fluid that may have necrotic debris. They do not interfere with function unless they become so large that they obstruct flow of urine, but they should be surgically addressed by marsupialization to prevent complications such as infection.
Only 5 - 7 % of dogs with prostatic disease have prostatic neoplasia. Of the dogs with neoplasia, most will have prostatic adenocarcinoma. Other cancers that can involve the prostate gland are transitional cell carcinoma, leiomyosarcoma, and hemangiosarcoma.
Prostatic adenocarcinoma is most common in older male dogs, and it occurs in both intact and castrated dogs. Limited studies indicate that castration may be a risk factor for development of this neoplasia, which is contrary to the popular belief that castration may reduce the risk of prostatic carcinoma. More research into this association is needed.
Dogs with prostatic cancer have clinical signs typical of other prostatic diseases, such as tenesmus, hematochezia, bloody urethral discharge, hematuria and strangiuria, Additional signs in more advanced cases are lumbar or caudal abdominal pain, rear limb weakness, weight loss, anorexia, and cachexia. On rectal palpation, the prostate gland will be enlarged, it may be irregular, and it may be adhered to the floor of the pelvis. Radiographs show prostatomegaly, and in some cases areas of mineralization within the gland. Metastasis to the lumbar vertebrae and/or pelvis is common, and metastasis to the lungs can also occur.
The prognosis for dogs with prostatic adenocarcinoma is poor, with typical survival of 1 - 2 months after diagnosis. The disease is usually diffuse within the gland, and often has metastasized at the time of diagnosis. Surgical resection is not a good option for the dog, because it results in permanent urinary incontinence, and is unlikely to cure the cancer. Radiation therapy may improve the survival time, but local side effects such as colitis, urethritis and cystitis make this option less than ideal. Chemotherapy may be the best option, but current protocols are not very successful. If urethral obstruction occurs, a cystostomy tube is the most effective way to improve quality of life for the dog that is not yet debilitated by the disease.