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Equine Health

Watch him drop – Penile cancer in horses

It’s a good idea to be observant when your older male horse urinates in his golden years. Penile cancer in horses typically affects geldings and stallions greater than 14 years of age. They can be located on the prepuce (sheath), although they are more commonly found on the penis. Some horses show no concurrent clinical signs with a penile mass. If the mass is extensive, then you may observe sheath swelling or difficulty urinating if the mass is compressing the urethra. Some cancers can also be ulcerated and may have an odor if they have become secondarily infected with bacteria.

The most common cancer that targets the penis is squamous cell carcinoma (SCC). Light colored horses tend to be predisposed to this type of cancer. They have less pigment in their skin, and sunlight affects the genetic material in the cells causing cancer. There are likely multiple factors, as horses can also get this type of cancer in their upper airway that is not exposed to sunlight.

When you observe a mass on the penis, you should call your veterinarian for an evaluation. The first thing that should be done is a biopsy. Other possibilities for penile masses include parasitic nodules, papillomas (warts) and other tumors (sarcoids, melanoma). It’s important to know what you’re treating, as no one wants to pay for surgery if they can treat a parasitic nodule with a $10 dewormer. If possible, 2-3 biopsies should be taken, especially if there are multiple masses. Although not common, additional diseases can be found on masses that also have SCC. A biopsy of the mass may also tell you how likely the tumor is to metastasize or spread to other organs in the body.

Once you know what you’ve got, then you need to discuss how to go about getting rid of it. There are basically six different treatment options depending on the size and location of the mass, the first two are the most common methods.

1. Surgical excision - This involves excising or cutting the mass away from the normal tissue. This is usually attempted for small tumors or tumors that do not extend into the deeper tissue of the penis.
2) Cryotherapy “cold therapy” - This procedure involves liquid nitrogen, a chemical that freezes the cancer tissue. It freezes the cells, then they die when they thaw after the chemical is removed. This treatment is usually used in conjunction with #1 or for very small masses to decrease the chances of recurrence.
3) Topical chemotherapy - The success depends on the size of the tumor and is variable. Basically it is a topical chemo treatment that is applied daily for a recommended period of time.
4) Surgical removal of mass and healthy tissue -If the tumor extends into the sheath tissue, then this requires cutting off part of the sheath to limit the chances of cancer recurrence.
5) Penile amputation, where a section of the penis containing the mass is removed.
6) Massive penis and prepuce resection and penile retroversion – This procedure is not done often, but is an option for extensive penile tumors that are high up on the penis and/or the horse is having difficulty urinating. Basically the major portion of the penis as well as the whole prepuce (sheath) are removed. The urethra that carries urine from the bladder is significantly shortened and needs a new outlet. This “reconstructive surgery” sutures the new opening below the anus; in effect the male horses start urinating like a female.

Although they have the capability to spread to other organs, squamous cell carcinoma tends to be locally invasive meaning that they just spread to the local lymph nodes. But metastasis to other organs is possible nonetheless. Recurrence rates depend on the extensiveness of the cancer, but in some cases have been as high as 50% and necessitate more than one round of treatment. In effect, it is important to be observant of the older male and to initiate treatment sooner than later with SCC of the penis and sheath. Routine and regular sheath cleaning by your veterinarian constitutes an excellent screening technique.

If you have any questions regarding penile masses, contact your veterinarian or any of the veterinarians at New England Equine.

Karyn Labbe, DVM
Omar Maher, DV, DACVS
Jacqueline Bartol, DVM, DACVIM

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