Prostate disease in the dog and cat

Prostatic diseases include benign or cystic hyperplasia, squamous metaplasia, paraprostatic cysts, prostatitis and prostatic neoplasias. In view of their greater sensitivity to genital and urinary tract infections and the presence of prostatic hyperplasia the predisposition for prostatic diseases is greater in dogs than in cats.

ANATOMY AND PHYSIOLOGY

The prostate is the only accessory sexual gland in the dog: it is located in the retroperitoneal space, it is a bilobar organ, with a dorsal median septum and it encircles the proximal urethra at the level of the bladder. The location may vary depending on the age, on the degree of bladder filling and on the pathologic status of the gland. Dorsally, it is connected to the rectum by a fibrous septum and it is surrounded by a capsule from which smooth muscle septa branch out and extend through the parenchyma to reach the urethral muscle layer; numerous ducts open up at the level of the seminal colliculus, an elongated elevation located on the posterior part of the prostatic urethra. The deferent ducts enter at the level of the cranio-dorsal surface of the gland, extend caudo-ventrally and open up on each side of the colliculus. Blood supply is provided by numerous branches of the urogenital artery (a branch of the internal pudendal artery) which, once crossed the dorso-lateral surface of the gland, then penetrate into the parenchyma dividing into many small arterioles. Venous drainage is through the urethral vein and the small venules which follow the course of arterioles; lymphatic drainage is mediated by the medial iliac lymph nodes. The pelvic and hypogastric nerves provide the connection with the parasympathetic and sympathetic nervous system, which determine the increase in the production of prostatic fluid and its ejection within the urethra._^1,2,3

In order to achieve and maintain its final dimensions, the prostate gland requires the presence of testosterone, and specifically of dihydrotestosterone; for this reason, if a dog is castrated before sexual maturity, the normal growth of the gland is inhibited. In dogs castrated in adulthood, the prostate undergoes an involution equal to around 20% of its physiological volume._⁴  The function of the prostate is to produce prostatic fluid, which is a transport medium for sperm cells during ejaculation. In the absence of ejaculation a basal level of prostatic secretion is still maintained, which constantly enters into the urethra through the prostatic ducts. In the absence of ejaculation or micturition this fluid moves in cranial direction and collects into the bladder.

In the cat, bulbourethral glands are also present in addition to the prostate. The most important anatomical difference between the cat and the dog is in the relationship existing between prostate and urethra; in the cat the urethra is in fact encircled by the gland only dorsally and laterally. The gland is bilobar, as in the dog, however its location is more caudal with respect to the neck of the bladder. Furthermore, some disseminated prostatic tissue is present within the thickness of the urethral wall.

EXAMINATION OF THE PROSTATIC GLAND

Transrectal palpation allows a rapid examination of the prostate, however this technique only permits the examination of the dorsal or dorsocaudal portions of the gland. Bi-manual palpation (transrectal and abdominal) may be of great help, allowing an easier palpation of also the cranial portion of the prostate. This because the hand which palpates the abdomen may exert a gentle cranial pressure, allowing to push the prostate towards the pelvis and hence making it more accessible with rectal palpation.

This examination allows to assess the size, contours, surface and mobility of the gland, as well as the symmetry or asymmetry of the two prostatic lobes and the eventual presence of pain. In normal conditions the prostate is bilobar, symmetric, with rounded contours, smooth, mobile and not painful.

The examination of the prostate requires numerous collateral examinations; among these, one of the most used is transabdominal ultrasonography, as this technique is safe, non invasive and capable of supplying important information on prostate structure._⁵dog has an echo pattern which is medium-fine, uniform and moderately hyperechoic; the capsule is more echoic and presents smooth margins, consequently the urethra, which has an inferior echogenicity, is easily identifiable (Fig. 1).

The prostate of an orchiectomised dog is instead totally different: compared to the surrounding adipous tissue it is hypoechoic and, for this reason, the identification of the urethra may result more difficult. In both cases, when assessing the gland it is important to assess its echostructure in search of focal and/or multifocal lesions or of diffuse alterations.

The prostate may be assessed with plain X-rays, using latero-lateral projections of the abdomen with the hind limbs extended caudally and ventro-dorsal abdominal projections. In males castrated when young the prostate is small and intrapelvic, while in intact male cats the gland is not usually visible. Radiographically, in physiological conditions the prostate presents itself with the opacity typical of soft tissues, with a round form, smooth margins and with a location just caudal to the neck of the bladder. The size of the gland varies considerably among single subjects, increasing gradually with age to then decrease in size following atrophy of the gland in advanced ages. Contrast studies allow the assessment of the contours and integrity of the urethra. A uniform increase in the size of the prostate is compatible with the presence of benign prostatic hyperplasia, squamous metaplasia, cysts, abscesses or neoplasia. An asymmetric prostate is more likely correlated to cysts, abscesses or neoplasias. The presence of parenchymal calcifications may be indicative of chronic prostatitis or neoplasia. The loss of definition of the gland margins may be indicative of the presence of prostatitis or neoplasia. The identification of a sublumbar tumefaction, as well as the presence of lythic lesions affecting the vertebrae or the bones of the pelvis, is indicative of a neoplasia.

COLLECTION AND ANALYSIS OF PROSTATIC FLUID

Prostatic fluid must be collected in all those patients in whom the presence of a pathologic condition affecting the gland is suspected; cytological and a microbiological tests are necessary, and the fluid can be collected with various techniques:_^6,7,8

• Urethral discharge: it must be differentiated from preputial discharge. Urethral discharge is commonly associated with the presence of prostatic diseases, however its origin may be caused by the presence of urinary incontinence or of urethral diseases (uroliths  or neoplasias). Urinary incontinence may be excluded by comparing the characteristics of the discharge with those of a sample of urine obtained by cystocentesis. Urethral diseases may be excluded on the basis of the clinical history and clinically: these diseases are always characterised by the presence of dysuria, which is only rarely associated to prostatomegaly. This said, radiography and endoscopy may be useful in excluding urethral diseases. A microbilogical examination of urethral discharge is instead not usually performed, as it is contaminated by the microbial flora which is usually present within the distal urethra and the preputium. In the presence of purulent discharge and if an ejaculate sample cannot be collected, a culture examination may supply important information. To collect urethral secretions the penis must be exposed, cleaned gently with a chlorhexidine or iodopovidone based solution, rinsed and dried with sterile gauzes. The discharge should be able to flow freely into a sterile container; should a microbiological test be requested, a quantitative culture should be done,_⁸ to be eventually  compared to the microbiological examination done with an urethral swab. During the interpretation of the results it is important to consider that a bacterium may more likely be the cause of infection if it is isolated also from a sample of urine collected by cystocentesis, if it is present in large quantities (>100.000/ml) or if fluid cytology identifies the presence of an active inflammation.
  
  
• Ejaculation:  in intact dogs, samples of prostatic fluid may be obtained with ejaculation, as 95% of the ejaculate consists of prostatic fluid (first and third fraction) The first fraction (pre-sperm) is clear in colour and of reduced volume (from 0.1 to 2 ml); the second fraction is white and opaque, as it contains a high number of sperm cells, the volume varies between 0.1 and 4 ml and it is collected in those cases in which a semen examination is necessary. For a diagnostic assessment of the prostate it may be useful to collect 2-3 ml of the third fraction (volume ranging from 1 to 16 ml), which is usually clear in colour and is ejaculated in 3-35 minutes. This is the fraction to be analised with a quantitative culture and a cytologic examination. In order to allow the dog to ejaculate and collect the sample, the dog must be manipulated gently and very calmly during the entire procedure. The animal is contained gently and, if sexually expert, the sample can be harvested with only manual stimulation. Should the animal be shy or be totally inexperienced, a useful aid consists in presenting to the dog a bitch in oestrus or to apply canine methyl-p-hydroxybenzoate pheromone on the genitalia of a bitch in anaestrus. The operator must stay on the side of the dog and apply a gentle pressure on the penis through the sheath just caudally to the glans penis: this stimulates the erection; the penis is exposed with the other hand and it must be examined for any eventual presence of preputial discharge, which must be delicately removed. During ejaculation some pulsations are felt; after the initial thrusts the dog will try to pass over the arm of the operator with one or both hind legs, however the pressure must be maintained. Once the animal has finished with the thrusts the prostatic fraction will drip out from the urethral meatus for a variable time lasting several minutes. By being careful it will be therefore possible to collect the prostatic fluid with an aseptic technique, using a sterile container and paying attention so that the penis of the dog does not come into contact with the walls of the container. The pH of the prostatic fluid of a healthy dog is mildly acidic (6-6.7) and occasionally contains granulocytes and some squamous cells. Contaminating bacteria may be free or superimposed on squamous cells; for this reason it is important that the cytologic examination of the fluid should be done at the same time of the microbiological test. Significant findings may include the presence of a large number of granulocytes, erythrocytes, hemosiderin-containing macrophages or the presence of bacteria, especially if contained within the granulocytes. The contaminating bacteria are usually Gram positive, consequently the identification of a large number of Gram negative microorganisms associated with the presence of granulocytes is indicative o a prostatic infection; collection of the third fraction of the ejaculate is the elective technique whenever a chronic prostatitis is suspected, and this is to be done together with a microbiological examination of the urine.
  
  
• Prostatic massage: this technique is used whenever it is not possible to induce ejaculation of the dog because of its inexperience, temperament or the presence of pain. Paying attention to the rules of asepsis, a urinary catheter is introduced into the bladder and all the urine present is removed; as an alternative the dog can be made to urinate preventively. The bladder is then totally emptied by means of the catheter; the residual volume of urine present in the bladder is measured and a sample is collected for a complete urinalysis.A bladder lavage is then performed using a sterile saline solution (5 – 10 ml). The catheter is then withdrawn from the bladder until reaching the prostate, which is at the same time massaged transrectally and/or transabdominally for about 1 minute. At this point the external urethral meatus is occluded, 5 ml of sterile saline solution are injected and, while continuing to gently suction with the syringe, the catheter is pushed back into the bladder. The urine and the samples collected before and after the massage must be used for cytology and for the quantitative microbiologic examination. The interpretation of results may not be easy, especially when the urinary culture is also positive: in such case it may be useful to treat the animal with an antibiotic which concentrates within the urine, but which cannot penetrate into the prostate; then, once the lower urinary tract infection has been resolved, the prostatic massage is to be repeated. As an alternative, depending on the antibiogram, the patient may be treated with an antibiotic which concentrates within the urine but which can also penetrate into the prostatic gland: in such cases the antibiotic therapy must be continued for at least 6-8 weeks (see later). It is worth recalling that in the presence of prostatic abscesses the prostatic massage should be carried out with great caution, as abscesses may rupture or there may be the release of septic emboli.

HARVERSTING OF PROSTATIC TISSUE

The collection of prostatic tissue becomes necessary when less invasive techniques have not been able to establish a diagnosis. It should be recalled that in those patients in whom the presence of prostatic abscesses is suspected, a needle aspiration biopsy should better be avoided, in view of the possible dissemination of bacteria along the needle tract and the consequent appearance of a localised peritonitis.

In the dog, prostatic aspiration is easily performed by means of a perirectal or transabdominal approach. In the perirectal approach the needle is guided by means of transrectal palpation. In the abdominal approach ultrasound guidance in indicated. In both cases, a mild sedation of the patient is sufficient.

In order to confirm the type of pathology present, an histologic prostatic biopsy is necessary. In the case of acute prostatitis or of prostatic abscesses, percutaneous biopsies are to be avoided. In the presence of cystic lesions a needle aspiration is to be preferred over a biopsy. Biopsies can be done with both a percutaneous or a surgical approach. The percutaneous approach is similar to that of a cytologic biopsy, and the approach can be perirectal or transabdominal. The needle may be guided by means of only palpation, although in this case ultrasound guidance would be preferable. After the biopsy the patient must be hospitalised and monitored for a few hours. A mild, self-limiting haematuria is commonly found. The surgical approach is via a para-preputial laparotomy. The sample may be harvested with a Tru-Cut needle or, in alternative, with an incisional/excisional biopsy. Potential complications are haemorrhage, the dissemination of infection or of neoplastic cells and traumas to the urethra; with regards to this latter issue, a preventive catheterisation of the bladder may be useful.

CLINICAL SIGNS INDICATIVE OF PROSTATIC DISEASE

The more common clinical signs indicative of a prostatic disease are tenesmus, the presence of urethral discharge independent from micturition, haematuria and recurrent urinary tract infections. Non-specific signs such as fever, malaise and caudal abdominal pain may also be present. In the presence of neoplasias, acute prostatitis or prostatic abscesses a rigid gait on the hind limbs may be observed. Less common are urethral obstruction, incontinence and infertility.

References

1. Ettinger SJ, Feldman EC: “Textbook of Veterinary Internal Medicine, 6^th edn”. Elsevier-Saunders, St. Louis 2005.
2. Couto CG, Nelson WR: “Medicina interna del cane e del gatto” Elservier-Masson, 2010.
3. BSAVA Manual of Canine and Feline Nephrology and Urology Second edition, Edited by Johnathan Elliot and Gregory F. Grauer, 2007.
4. EvansHE: “Miller’s Anatomy of the dog, 3rd ed.” WB Saunders, Toronto, 1993
5. OsborneCA, Finco DR: Canine and feline nephrology and urology. Williams & Wilkins, Philadelphia, 1995

Suggested readings

1. Arver S, Sjostrand NO.: “function of adrenergic and cholinergic nerves in canine effectors of seminal emission” Acta Physiol Scand. 1982 May;115(1):67-77.
2. Bruschini H, Schmidt RA, Tanagho EA.: “Neurologic control of prostatic secretion in the dog.” Invest Urol. 1978 Jan;15(4):288-90.
3. Bruschini H, Schmidt RA, Tanagho EA.: “The male genitourinary sphincter mechanism in the dog.”Invest Urol. 1978 Jan;15(4):284-7.
4. Huggins C, Clark PJ:  “Quantitative studies of prostatic secretion: II. The effect of castration and of estrogen injection on the normal and on the hyperplastic prostate glands of dogs.” J Exp Med. 1940 Nov 30;72(6):747-62.
5. Ruel Y, Barthez PY, Mailles A, Begon D.: “Ultrasonographic evaluation of the prostate in healthy intact dogs.” Vet Radiol Ultrasound. 1998 May-Jun;39(3):212-6.
6. Barsanti JA, Finco DR.: “Evaluation of techniques for diagnosis of canine prostatic diseases.”J Am Vet Med Assoc. 1984 Jul 15;185(2):198-200.
7. Barsanti JA, Prasse KW, Crowell WA, Shotts EB, Finco DR.: “Evaluation of various techniques for diagnosis of chronic bacterial prostatitis in the dog.J Am Vet Med Assoc. 1983 Jul 15;183(2):219-24.
8. Ling GV, Branam JE, Ruby AL, Johnson DL.: “Canine prostatic fluid: techniques of collection, quantitative bacterial culture, and interpretation of results.”J Am Vet Med Assoc. 1983 Jul 15;183(2):201-6.