Reproductive diseases in male dogs are fairly common and include functional abnormalities, congenital anomalies, disturbancies of growth, and other diseases and disorders.
Phimosis is the inability to extrude the penis. Stenosis of the orifice of the penis is the most common problem, which 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 is protrusion of the penis with an inability to retract the penis into the prepuce. Many cases are idiopathic 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 is persistent erection of the penis. Persistently erect penises become traumatised, dry, or undergo necrosis. Little is reported about the pathogenesis.
Posthitis is inflammation of the prepuce; balanitis is inflammation of the head of the penis; 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).
In a study of normal stud dogs aerobic bacteria were found in about 86% of preputial samples with Pasteurella multicida, beta hemolytic streptococci and Escherichia coli being the most common species. 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. Canine herpesvirus infects the penis and prepuce. Many dogs are asymptomatic. Rarely, multiple ulcers occur in the typical pattern of herpesviruses. Mycoplasma and ureaplasma have been recovered from the prepuce of 85% of 136 dogs. More infertile male dogs had positive cultures. The prepuce is infrequently colonised with Malassezia pachydermitis. Nodules in the penis and prepuce of dogs cane be infected with canine visceral leishmaniasis. Dogs with visceral leishmaniasis had more inflammation of the penis and prepuce than noninfected dogs, especially in those that had organisms identified by immunohistochemistry.
The main clinical sign of physiochemical injury is a bleeding or pus-filled discharge from the prepuce that are caused by trauma, prostatic disease, neoplasia, foreign body, prolapsed urethra, urolithiasis, urethritis, bleeding disorders, ureteral duplication, and persistent frenulum (the fold under the penis connecting it with the prepuce). 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 (penis bone) is a well recognized in dogs.
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. Ulceration is an indication of epithelial cell death and is a common finding. In severe cases the ulcerated regions may 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.
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 bleeding.
One of the most frequent proliferative lesions of the penis (or prepuce) are polyps, projections of the tissues on the penis. Little is know of the causes 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 overgrown epithelium. Where there is erosion or ulceration of the surface, there will be underlying granulation tissue. 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.
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 is identified as a red, bleeding, circular 'donut' shaped mass at the end of the penis.
A congentially small penis (also called an infantile penis) is occasionally seen. Micropenis (hypoplasia of the penis) is a small penis. Megalopenis is an unusually large penis.
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.
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.
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. There is a general belief that mast cell tumors of the scrotum and inguinal area are more aggressive and 'malignant' than those elsewhere on the body.
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. Below is the list of male dog reproductive diseases:
SUMMARY OF THE DISEASES