Disease of the scrotal skin

It has been often said that scrotal skin reacts just like the skin elsewhere, so it is not ‘special’. This is true, or at least is considered so. There is little point listing all the diseases of skin. Instead, the diseases that only affect scrotal skin, or have a predilectin for scrotal skin will be mentioned.

In stallions, scrotal dermatitis is caused by contact with irritant substances from the environment, ectoparasites such as biting insects (Simuliidae and Culicoides), lice (Haematopinus eurysternus), mites (Chorioptes bovis); fungi; protozoa (Besnoitia besnoiti, Trypanosoma spp); bacteria (Dermatophilus congolensis); and viruses (pox viruses) to name a few. These may occur as diffuse or focal lesions.

Range stallions and occasionally rams, exposed to extreme cold may develop scrotal frostbite (Fig 5.7). A higher incidence of lesions in old stallions is attributed to their more pendulous scrotums. Lesions are well demarcated regions of necrosis of skin on the ventral aspect of the scrotum and can involve up to 75% of the scrotum. Scrotal swelling, tunic adhesions, and decreased semen quality accompany the more severe lesions. The scrotal skin of stallions is especially susceptible to infection by Dermatophilus congolensis. Severe, prolonged infection may result in thickening of the skin to 1 cm, with advanced testicular degeneration and infertility. Infection of scrotal skin by Besnoitia besnoiti also may lead to severe testicular degeneration. This infection is not limited to skin, and the small nodules containing white gritty material are seen grossly in the subcutis of the scrotum as well as in the tunics, the parenchyma of the testis, and in the epididymis. Vascular damage is important in the development of lesions, and large numbers of Besnoitia cysts may be present in the intima of vessels in the pampiniform plexus. Neoplasia is rare in stallions, but papilloma, melanoma and hemangioma occur. Varicose dilations of scrotal veins occur in older stallions. They appear as flattened and irregular thickenings of the scrotal skin. The overlying epithelium usually is normal unless it ulcerates and bleeds. The affected venous channels are irregular in shape, widely dilated, and have thickened walls; thrombosis occurs in many.

The cavity of the vaginal tunics communicates with the peritoneal cavity and is susceptible to the accumulation of ascitic fluid. Fluid within the cavity around the scrotal contents is called hydrocele. It forms for exactly the same reasons as ascites. Hydrocele may lead to severe testicular degeneration on the involved side and to a lesser extent on the opposite side. Scrotal hydrocele, with diminished sperm quality, is a herd problem in stallions in some regions. Clinically, the fluctuant nature of the enlarged scrotum, with a scrotal circumference up to 60 cm, is indicative of edema, and if affected stallions are examined at autopsy, there is obvious ascites. If hydrocele persists, the tunics become increasingly fibrotic. Ostertagiasis and hemotropic mycoplasma (Mycoplasma wenyonii) infection are incriminated as causes of this syndrome. Hydrocele in other species occurs with ascites of heart failure or any other cause of fluid buildup in the peritoneal cavity. It can also occur in conditions that cause edema of the scrotum (as listed above).

Severe periorchitis is a complication of epididymitis irrespective of cause, but is particularly found in stallions with Trypanosoma brucei infection, in rams infected with Brucella ovis or Actinobacillus seminis (Fig 5.9), and dogs with Escherichia coli epididymitis (Fig 5.10). Setaria labiatopapillosa causes granulomatous periorchitis in stallions, and lesions occur particularly after adults are killed by anthelmintic treatment. Ectopic parasites, such as migrating equine strongyle larvae (Strongylus edentatus), can be found on the tunics. A high occurrence of eosinophilic, then granulomatous, periorchitis in water buffaloes is attributed to Setaria sp. Periorchitis in rams is very common in infectious epididymitis. Pyocele or hemipyocele is reported rarely as a spontaneous condition and is reported in the ram with Staphylococcus capitis infection. Cysts of taenid larvae, mostly Cysticercus tenuicollis, may be found in the scrotal cavity of rams, without inflammation. In dogs, rare peritoneal cestodiasis caused by larvae of Mesocestoides sp. may involve the vaginal tunics, with saccular dilations within the tunics, thickening of the spermatic cord, and pyogranulomatous inflammation involving the testicular capsule and extending into the testes.

Neoplastic disease involving the vaginal tunic is rare, even as secondary extension from the testis or scrotum. Mesotheliomas with primary involvement of the vaginal tunic occur in the dog and stallion (Fig 5.11). Extension of the tumor into the peritoneal cavity has been observed in the dog. Ultrastructural study or immunohistochemistry is necessary to differentiate mesotheliomas from metastatic adenocarcinoma. The origin of intrascrotal mesothelioma from the paramesonephric duct remnant is suggested. There is a report of a lipoma occupying the right scrotal cavity in a ram, causing compression of, but apparently not directly involving, the testis.

Scrotal hyperplasia

Masses (including neoplasms)

One may expect any neoplasm of skin to affect this region. Likewise, non neoplastic lesions would surely occur. Here are some examples of lesions that can occur

Scrotal dermatitis and non neoplastic disease

Vaginal tunics (tunica vaginalis) and testicular capsule (tunica albuginea)

The tunics are continuous with the peritoneum, so they are affected by the same influences as the abdominal cavity. Disease of the tunics can originate from the abdomen, or from the scrotal sac itself.

Fluid accumulation

Hematocele

An accumulation of blood within the cavity of the vaginal tunics is haematocele. Local trauma is the most likely cause, but any disease or condition resulting in hemorrhage can be the cause.

Hydrocele

An accumulation of fluid within the cavity of the vaginal tunic is hydrocele. This fluid is typically a transudate and identical to ascitic fluid. It will form for all the same reasons that ascites forms. Hydrocele with fluid restricted to the scrotum is seen secondary to other conditions such as metastatic Sertoli cell tumors that obstructed vessels, and caused varicosities and hydrocele (McNeil and Weaver 1980).

Parasitism

Scrotal (inguinal) hernia

Inflammation (periorchitis)

Periorchitis is inflammation around the testis and it involves the tunics. It can arise from extension of peritonitis, from penetrating injury to the scrotum, but most commonly, it arises from epididymitis. The dependent nature of the scrotal sac means that exudates tend to remain in the sac. Organisation and fibrosis of exudates, and granulation tissue formation leads to adhesions.

Neoplasia

There are an infinite number of potential neoplasms that could possibly be found on the vaginal tunics, or extending into the cavity. Neoplasia is very rare, however.

Carcinomatosis

Leiomyoma

Mesothelioma

There is a single case report of a sarcomatoid mesothelioma of the pampiniform plexus of a young stallion with spread to the peritoneum.

MacMillan KM, Martinson SA, Smith M, Lofstedt J. Malignant Mesothelioma of the Vaginal Tunic and Peritoneum in a Young Stallion. J Equine Vet Sci. 2019; 80: 76-79.

Sarcoma