There are other, less common conditions that can cause paraphimosis in the stallion. Certain drugs, such as acepromazine tranquilizer
and other phenyl-thiazine agents, will cause potentially devastating paraphimosis and should never be used on stallions. Liver
disease can occasionally cause penile swelling, and weakened, debilitated stallions may also suffer this complication.
Tumors of the equine penis are not that common but are seen often enough to be important. Additionally, as advances in equine
science allow horses to live longer and advances in reproductive areas make these older horses capable of breeding at later
stages of their lives, veterinarians will be seeing an increase in the presenting cases of equine cancer.
Seminoma of the equine testicle is usually a benign neoplasia in the horse and the most common testicular cancer seen. Benign
teratomas are tumors of pluripotential germ cells seen infrequently and usually in retained testicles. Sertoli cell and Leydig
cell tumors are also seen but are really quite rare. Melanomas of the penis and/or prepuce are also seen, as are sarcoids,
but the most common penile tumor in horses is squamous cell carcinoma (SCC; See photo). This is a locally aggressive metastatic
cancer that requires early diagnosis and usually aggressive treatment for a successful outcome. Squamous cell carcinoma is
thought to derive from either the epidermis or the epithelium of the outer root sheaths of hair follicles, which is paradoxical
because this tumor tends to occur most frequently in non-haired, non-pigmented areas. Clinically, SCC appears as red to whitish
raised plaques that rapidly take on an irritated appearance and grow to resemble granulation tissue in later stages. Often
breeding managers are alerted to the presence of these lesions during routine cleaning of stallions prior to breeding/collection
or by the presence of blood on the penile shaft following breeding/collection. Abrasion of these lesions during reproductive
use causes bleeding and in advanced stages, the stallion might be unwilling to mount and/or breed.
Biopsy of these lesions is necessary to make the diagnosis. If the lesion is solitary and in an accessible area, then surgical
excision is recommended. Though SCC of the tip or head of the penis is the easiest for the equine surgeon to approach, Love
urges caution with excision in this area.
"Be careful about losing elasticity in this area as the penile head tends to rip or tear constantly on ejaculation following
some surgical excisions," he says. "Surgical repair or removal of lesions near the tip of the penis is often less rewarding
on breeding stallions."
Various topical anti-cancer creams and preparations, such as 5-fluorouracil (5-FU) have been advocated for SCC treatment by
various clinicians, and the general opinion is that this treatment approach works best if used early and often when lesions
first appear. Use of these topicals typically will cause swelling of the penis and some local irritation, so breeding is not
possible during this period.
"Veterinarians should be concerned with 5-FU and other anti-cancer drugs, especially with long-term administration because
these agents affect rapidly dividing cells (cancer cells and sperm cells) and they may affect fertility," Love says.
Surgical laser ablation is becoming more commonly used on SCC of the penis, especially if the lesion is widespread and extensive.
Diode or CO2 lasers are most frequently used and the procedure is done under general anesthesia.
Coital exanthema is another problem seen in both mares and stallions. This condition is a benign venereal disease that probably
occurs worldwide and is caused by equine herpes virus type 3 (EHV-3). Stallions with EHV-3 generally exhibit multiple circular
red nodules up to 2 mm in diameter seen on the penis or prepuce. Intromission is painful with these lesions and the stallion
may be reluctant to collect or breed naturally. Though coital exanthema can be confused with other penile lesions and can
affect a stallion's performance, it has no effect on fertility. Sexual rest for a period of about three weeks, isolation from
other horses and antibiotic cream applied topically to prevent secondary infection is usually curative.
Bacterial infections of the penis and testicles are infrequently seen but have been reported. Pseudomonas, Klebsiella, Streptococcus equi, Salmonella and E. coli are the bacterium most commonly involved in these cases. Contagious Equine Metritis (CEM) is usually seen in mares but can
be found in stallions and equine viral arteritis (EVA) can also cause infection and testicular swelling.