Disease of the Canine Penis and Internal Sheath of the Prepuce
One of the most common diseases is nonspecific posthitis, and closely following this is trauma. These will be mentioned in due course. The listing of diseases are done with the view to the clinical prevalence.
The anatomy of the penis of the dog is different to many other species and is outlined in the section on normal anatomy.
Functional abnormalities
Phimosis
Phimosis is the inability to extrude the penis. Stenosis of the orifice of the penis is the most common problem. This stenosis can be congenital or acquired. It is seldom reported, primarily because it is usually a secondary problem. It is usually impossible to extrude the penis of a prepubertal animal as the penile epithelium is fused to the internal preputial sheath until puberty.
Paraphimosis
Paraphimosis is protrusion of the penis, with an inability to retract the penis into the prepuce. Many cases are idiopathic (Papazoglou 2001) and therefore have no known cause. There has been a suggestion that it is related to an abnormality with the preputial muscles. The reasons for paraphimosis in the dog include a small preputial oriface, shortened prepuce, weakened preputial muscles and trauma.
Priapism
Priapism is persistent erection of the penis. Persistently erect penises become traumatised, dry, or undergo necrosis. Little is reported about the pathogenesis.
Inflammation of the penis and prepuce
Inflammation of the prepuce is posthitis. Inflammation of the head of the penis is balanitis. Inflammation of the penis is phalitis, but most of the time the inflammation is of the head of the penis so balanoposthitis is appropriate for inflammation of the head of the penis and the internal layer of the prepuce.
Nonspecific Posthitis
While dogs do get venereally transmitted diseases, the tendency of owners to assume that a purulent discharge from the prepuce is a sign of a sexually transmitted disease (STD) makes for some interesting clinical situations! It is a very common, but the prepuce is seldom sampled unless there is some obvious lesion, the amount of discharge is excessive, or has not responded to therapy. With increasing severity of the response to antigens within the preputial cavity, the follicles become more numerous, and lymphocytes and plasma cells will accumulate beneath the epithelial surface. This 'lichenoid' infiltrate is reminiscent of an immune mediated disease of skin, but while there may be exocytosis of lymphocytes, there is no destruction of the epithelial cells. This lesion is truly nonspecific as there is not specificity to agent or antigenic stimulus. It is a very steriotyped reaction.
Immune Mediated Posthitis
It is extremely rare for dogs to develop a true immune mediated disease of the prepuce. When it does occur, the penis and prepuce are affected as part of a more widespread disease wherein the anus and mouth are also involved. Comfort in diagnosing an immune mediated disease comes with an appropriate clinical history, and evidence of attack to the epithelial layer. Dramatic thickening of the basement membrane, single cell death of epithelial cells and direct targeting of epithelial cells makes the diagnosis more likely. Always be cautious as severe chronic posthitis can have a similar appearance. In the Yager-Best Histovet database, there are 2 cases. One was probably systemic lupus erythematosis and the other was probably pemphigus vulgaris.
Infectious Posthitis
In a study of normal stud dogs aerobic bacterial were found in about 86% of preputial samples with Pasteurella multicida, beta hemolytic streptococci and Escherichia coli were the most common bacterial isolates (Bjurstrom and Linde-Forsberg 1992). With bacterial such as this, identifying a pathogenic populaton becomes problematic especially when there is a lesion.
Viral Infection
Canine herpesvirus infects the penis and prepuce. Many dogs are asymptomatic. Rarely, multifocal ulceration occurs in the typical pattern of herpesviruses. A calicivirus was recovered from the penis and prepuce of a male dog with vesicular lesions (Crandell 1988). No gross or microscopic findings were published.
Infectious Posthitis
Mycoplasmal Infection
Mycoplasma and ureaplasma can be cultured from male dogs. It is only rarely associated with any disease, but occasionally a 'nonspecific' posthitis occurs (see above). Mycoplasma and ureaplasma have been recovered from the prepuce of 85% of 136 dogs. More infertile male dogs had positive cultures.
Mycotic Infection
The prepuce is infrequently colonised with Malassezia pachydermitis.
Protozoal Infection
Nodules in the penis and prepuce of dogs cane be infected with visceral leishmaniasis.
Dogs with visceral leishmaniasis (Leishmania donovani) had more inflammation of the penis and prepuce than noninfected dogs, especially in those that had organisms identied by immunohistochemistry. The reaction was histiocytic and intraepithelial histiocytes containing the organisms were found. No macroscopic lesions were reported.
Phalitis
Phalitis is inflammation of the penis. The penis proper can be affected and this section is dedicated to inflammatory disease of the root, body or head of the penis, apart from the penile epithelium (covered with posthitis)
Physiochemical injury (including trauma) to the penis and prepuce
The main clinical sign of physiochemical injury is a hemorrhagic or purulent discharge from the prepuce. The following are causes of hemorrhage and or purulent discharge from the prepuce:- Trauma, prostatic disease, neoplasia, foreign body, prolapsed urethra, urolithiasis, urethritis, bleeding disorders, ureteral duplication, and persistent frenulum.
Urine, Bacteria and Ulceration
While there is little information about the effects of urinay ammonia on preputial epithelium, it has been shown that urease producing bacteria in the prepuce catalyse the reaction that splits urea into ammonia. Ammonia is a toxic product that causes epithelial cell death, and subsequently ulceration.
Erosion and Ulceration
It is obvious from the table of preputial diseases above that ulceration is a common finding. Ulceration is an indication of epithelial cell death. The ulcerated regions vary from focal to be circumferential around the oriface of the prepuce.. In severe cases they extend into the preputial cavity. Some are covered with fibrin and debris, but most are not. Histologically they are regions without epithelium on the surface, and with linear necrosis of the superficial connective tissue. The only nflammation present is a thin layer of neutrophils at the junction of the viable and dead area. Granulation tissue forms in the healing phase. The ulcers have no indication of viral infection, or of a primary inflammation. Thermal or chemical injury has this pattern and so it is placed in the 'physiochemical' category. Of the cases where the sex of the dog is known, all were neutered, so a 'self induced traumatic injury' is improbable. It reasonable to assume the necrosis and ulceration arise from a local event and the effects of urine, urea, urease and ammonia seem likely.
Foreign Bodies
The potential for the number and types of foreign material in the prepuce is large. Constricting rubber bands, cords, string and hair (hair ring) around the penis of dogs. Substances such as stray, cedar shaving, grass, urinary calculi and other small objects are also mentioned. Dogs with short legs in particular, will acquire sand in the preputial cavity. This causes irritation and hemorrhage.
Trauma
Traumatic injury to the penis is relatively common, but not often reported in the refereed literature. Motor vehicle accidents and fight wounds cause significant injury to the penis and prepuce in some instances. Injuries sustained during masterbation, mating injury such as occurs when dogs tie through a fence, or degloving injury when owners try to separate dogs that are tied are clinical conditions not usually within the domain of surgical pathologists. They are noteworthy when considering lesions of a physicochemical nature that have no apparent cause.
Fracture of the os penis Fracture of os penis is a well recognised clinical entity in dogs. Samples of a previously fractured os penis may be submitted to exclude a pathological fracture or other anomaly.
Disturbances of Growth
Polyps and Tags
One of the most frequent proliferative lesions of the penis (or prepuce) is a polyp. These solitary lesions are projections of the tissues on the penis, and they are mostly composed of stroma that is often edematous. Little is know of the pathogenesis of these, although some may arise as the result of local trauma such as a cut or similar physicochemical injury. They typically have an overlying layer of intact and either normal or hyperplastic epithelium. The hyperplastic epithelium is well differentiated. The stroma is indistinguishable from surrounding subepithelial connective tissue. Where there is erosion or ulceration of the surface, there will be underlying granulation tissue.
Epithelial Hyperplasia
The epithelium of the penis reacts just like stratified squamous epithelium elsewhere. Insult or injury will, if there is sufficient time, result in proliferation of the cells and the epithelium will become thicker. The more diffuse hyperplasia may occur with chronic balanoposthitis, but similar changes also occur in the apparent absence of inflammatory disease. Diffuse hyperplasia may be seen as a roughening of the surface, or as a plaque. These latter structures appear as white slightly raised lesions. Histologically, the nonkeratinizing epithelium of the penis becomes thicker and will, on occasion become keratinized. Periodically, the hyperplastic epithelium will have an area that has progressed to squamous papilloma, or to carcinoma.
Miscellaneous Masses
Vascular Hamartoma
There is one cases of a vascular hamartoma in the YagerBest Histovet database
Arteriovenous fistula
An arteriovenous fistula occurs when an artery communicates directly with a vein. The resultant increased pressure in the vein causes it to dilate, become tortious and for edema to develop. An AV fistula in the penis can cause priapism. In the prepuce it will cause localised edema and subsequent swelling.
Urethral Prolapse
A rare but occasionally seen lesion is prolapse of the urethra. This is identified as a circular 'donut' shaped mass at the end of the penis. It is red/haemorrhagic because of venous infarction and excoriation.
Neoplasia
- Squamous cell carcinoma
Squamous cell carcinomas of the penis are reported periodically (Wakui et al 1992). There are 4 in the Yager-Best Histovet database. These proliferative lesions of the penis have a plaque like appearance, or are raised papillomatous lesions. Histologically, some are seen arising from areas of hyperplasia or papilloma. The appearance of dysplastic and neoplastic cells within the epithelium (in situ carcinoma) and then squamous cell carcinoma suggests progression from one to the other. The carcinomas are well differentiated, and have invasion of the cells into the underlying stroma. The majority of the cells resemble those of the stratum spinosum having a large amount of cytoplasm and a prominent nucleus with a central nucleolus. Keratinisation is unusual. There is no follow up on the animals with penile squamous cell carcinoma, so prognosis is unknown.
- Canine Transmissible Venereal Tumor
- Chondrosarcoma
- Haemangiosarcoma
- Lipoma
- Lymphoma - Lymphoma of the penis and prepuce is a very rare manifestation of lymphoma. Lymphoma is typically a multisystemic disease, but there are several reports of the penis being involved primarily or as the first manifestation.
- Ossifying fibroma of the penis
- Osteosarcoma of penis
- Mesenchymoma
- Plasmacytoma
- Soft tissue sarcoma
Congenital Anomalies
Hypoplasia and hypertrophy
A congentially small penis (also called an infantile penis) is occasionally seen. Micropenis or hypoplasia of the penis is a small penis. Megalopenis is an unusually large penis.
Hypospadias and Fistulas
Hypospadias is when the urethra opens on the ventral surface of the penis (or on the perineum). It is the result of a failure of complete closure of the urethra during development. There are also many different fistulas where the urethra opens into other structures - these include urethrorectal and urethroperineal fistulas. Some of these do not involve the penis.
Balanopreputial Fold or Band (Persistent frenulum)
The internal sheath of the prepuce and the epithelium of the penis are fused in neonates, and separate after birth. In some animals this only occurs at puberty. Failure of a portion to separate results in a balanopreputial band. A band from the frenulum of the penis to the prepuce is a persistent frenulum. The frenulum is located on the ventral surface of the free portion of the penis, but not the head of the penis. According to the NAV, the part of the penis of the dog within the prepuce is the head of the penis (including the bulb) so a true frenulum would be difficult! There are many individual reports of a 'persistent frenulum' in the literature, and the recognition and treatment is simple and does not require the expertise of a surgical pathologist.
Disease of the External Sheath of the Prepuce
The external sheath of the penis is haired skin, and as such shares its diseases with the rest of the skin. This is the domain of the dermatologist and the dermatopathologist, and is beyond this review. There are some diseases that are unique to this area and these will be highlighted.
Must Cell Tumor
There is a general belief that mast cell tumours of the scrotum and inguinal area are more aggressive and 'malignant' than those elsewhere on the body.
Hepatoid Gland Tumors (Preputial Gland)
The prepuce of dogs may contain hepatoid glands. These glands are well known around the anus where they are called perianal glands. Neoplasms of perianal hepatoid glands are common and well recognized.
SUMMARY OF THE DISEASES
Penile diseases
- Polyp or tag
- Papilloma
- Ulceration
- Squamous cell carcinoma
- Fibrosarcoma
- Hemangiosarcoma
- Plasmacytoma
- Lymphoma
- Epithelial hyperplasia
- Lymphoid follicular hyperplasia
- Urethral prolapse
- Vascular hamartoma
- Hemosiderosis
Preputial disease - internal shealth
Preputial disease - external sheath
- Neoplasia
- Hepatoid adenoma
- Hepatoid carcinoma
- Mast cell tumour - cutaneous
- Mast cell tumour - subcutaneous
- Lipoma
- Melanocytoma
- Hemangiosarcoma
- Sebaceous adenoma
- Plasmacytoma
- Histiocytoma
- Cutaneous acanthoma
- Pyoderma
- Furunculosis
References:
1.Surgical Pathology of the Canine Male Reproductive Tract. Dr Rob Foster
OVC Pathobiology University of Guelph
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