Prostatic Diseases in Small Animals
Disease of the prostate gland is relatively common in intact dogs but less common in other domestic animal species. Benign prostatic hyperplasia is by far the most common disease of the prostate in intact male dogs. Bacterial prostatitis (acute or chronic), prostatic abscesses, prostatic and paraprostatic cysts, and prostatic adenocarcinoma are seen much less frequently and can be seen in castrated males.
When prostatic calculi occur (rarely), there is usually some other prostatic disease as well. Radiopaque prostatic calculi are rare incidental findings on abdominal radiographs.
Depending on the disorder, clinical signs may include:
However, many intact males with benign prostatic hyperplasia (with or without chronic prostatitis) are asymptomatic or present with signs of hemospermia and/or infertility only. Additional nonspecific signs, such as fever, malaise, anorexia, severe stiffness, and caudal abdominal pain, can be seen with acute bacterial infections, abscesses, and neoplasia. Prostatic adenocarcinoma with bony involvement of the pelvis and lumbar vertebrae may cause hindlimb gait abnormalities. Less commonly, prostatic diseases may cause urinary incontinence. Prostatic adenocarcinoma may cause complete urethral obstruction.
Physical examination of the prostate gland should include abdominal and rectal palpation. An enlarged prostate typically is located further cranial than usual and can be found in the caudal abdomen rather than within the pelvic canal. Simultaneous abdominal and rectal palpation allows not only for the cranial aspects of the prostate to be palpated but also for better palpation per rectum because the prostate can be pushed into or near the pelvic canal, which is especially important in large-breed dogs and in males with very enlarged prostates. Size, shape, symmetry, consistency, mobility, and the presence or absence of pain can be assessed by palpation. The normal dorsal sulcus (depression) aids in assessment of shape and symmetry. More than 90% of the protein secreted in prostatic fluid is canine prostatic-specific esterase, which is an arginine esterase found mainly on the apical regional of prostatic secretory epithelial cells. Canine prostatic-specific esterase has increased serum concentrations with all forms of prostate disease but is not significantly different between benign prostatic hyperplasia, prostatitis, or neoplasia.
Abdominal radiographs may help define the size, shape, and position of the prostate gland. The sublumbar lymph nodes, lumbar vertebrae, and bony pelvis should be evaluated radiographically for evidence of periosteal new bone and bony metastases. A positive-contrast retrograde urethrogram can be done when an abnormal prostate or paraprostatic cyst is difficult to differentiate from the bladder. However, transabdominal ultrasonography is the best imaging modality for evaluation of the prostate, because it allows for evaluation of the prostatic parenchyma and adjacent soft-tissue structures (see video). Intraprostatic cysts appear as anechoic areas within the prostatic parenchyma (see images). Increased echogenicity (see images) is associated with benign prostatic hyperplasia, chronic bacterial prostatitis, and prostatic neoplasia, whereas areas of mineralization may be secondary to chronic bacterial prostatitis or prostatic neoplasia. Mass lesions within the prostatic urethra and discontinuity of the prostatic urethral wall are both highly suggestive of prostatic neoplasia.
The best way to obtain a representative sample of prostatic fluid for cytology and culture is by manual ejaculation of the dog (see video) and separate collection of the third (prostatic) fraction of the ejaculate into a sterile container (see photo). If the dog will not ejaculate, material for cytologic and microbiologic examination can be obtained by prostatic massage. Using aseptic technique, the bladder is catheterized, and all urine removed. The bladder is flushed with saline, and this sample is saved. The catheter is then withdrawn so that the end is caudal to the prostate. The prostate is subsequently massaged per rectum for about 1 minute to release prostatic fluid into the urethra, where it can be collected with the catheter. While occluding the urethral opening, saline is slowly injected. The catheter is then advanced into the bladder as aspiration is performed and another sample is collected. Results of cytologic and microbiologic examination from both prostatic wash specimens should be compared.
Prostatic massage may produce septicemia in dogs with acute bacterial prostatitis or a prostatic abscess. Neoplastic cells are often not recovered in specimens obtained by ejaculation or prostatic massage.
Fine-needle aspiration of the prostate gland can be performed transrectally or percutaneously, with or without ultrasonographic guidance. Although generally safe and simple, this is not without some risk of penetration of surrounding structures and iatrogenic peritonitis. Biopsy is the most definitive, but also the most invasive, diagnostic procedure to differentiate prostatic diseases. To obtain diagnostic samples, a prostatic biopsy should be performed via celiotomy or by a skilled ultrasonographer.
Alternatively, if a prostatic urethral transitional cell carcinoma is suspected, a catheter biopsy of the prostate can be performed with minimal chemical restraint. To perform a catheter biopsy, the largest-bore catheter (with side holes) that can be advanced into the urethra is chosen. The bladder is catheterized and emptied of urine. The catheter is attached to a 12 mL syringe filled with 3–10 mL of normal saline and guided to the level of the lesion as indicated by radiographs or ultrasound, or by a finger inserted in the rectum. With a finger in the rectum, the lesion is pushed ventrally against the urinary catheter. Negative pressure is applied by retracting the syringe plunger, and the catheter is rapidly moved back and forth over a short distance to secure a piece of tissue. The negative pressure is gradually released, and the catheter is withdrawn. If the lesion involves the prostate gland, it should be massaged rectally before and during the aspiration phase. Tissue pieces are fixed in 10% buffered formalin and evaluated histologically. Slides are made of the remaining liquid portion of the sample and examined cytologically.