Stallion reproductive tract anatomy; some major aspects

Keywords: testicle, cremaster, equine, ductus deferens, pudendal, penis, corpus, spongiosum, cavernosum, parietal, visceral, vaginal, accessory, vesicular, prostate.

Note: the term testicle and testes are used interchangeably in peer reviewed literature but testes is about four times more commonly used than the word testicle in the English language. However, the term testicle has been used almost exclusively in LORI. Although the author defers to that term in this entry, future entries in LORI will employ the terms testis and testes as some viewers have suggested.

In the main image below this paragraph, the relationships between the intra-pelvic and inguinal sections of the tract have been maintained by removing these sections intact and photographing them as they floated under water.  The image was then transposed into a "ghost" pelvis, showing the relative position of a hand during per rectum palpation. Although it is possible to palpate all of these structures per rectum, including the internal inguinal openings (rings), the accessory glands are so flaccid that they can barely be defined in most cases. During sexual excitation however, they fill with accessory fluid and enlarge greatly. With the exception of the bulbourethral glands, this allows them to be become palpable.

The inset image shows some of the same structures, well defined in a neonate.

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Notes:

1. The umbilical arteries descend from the aorta on either side of the bladder. In the inset image, the arteries have kinked the lateral aspects of the bladder giving the appearance of two separate vesicular (bladder) structures.

2. In the inset image, the paper-thin cranial margin of the genital fold (the membrane between the ampullae) has been drawn cranially by the author to stabilize structures within the fetal pelvis. The same structure in a mature stallion is shown in the main image.

The accessory genital glands of the horse are the ampullae, vesicular glands, the prostate, and bulbourethral glands.

Recent data on the histology and function of the ampullae of stallions are lacking but findings in Jacks (male donkeys) suggest that the ampullae of stallions have both secretory and smooth muscle functions. Although ampullae produce or store alkaline phosphatase in stallions, the ampullae (at least in donkeys) also secrete various unidentified mucopolysaccharides. That secretory function qualify the ampullae as accessory glands and not merely as storage units for sperm. Interestingly, the ampullae are large structures in donkeys and clearly have the capacity to store sperm but as shown below, the ampullae of stallions have narrow lumens at rest. However,  if their histology is similar to that of donkeys, their walls contain elastic fibers, allowing them to stretch and indeed, to act as sperm reservoirs as is commonly asserted.

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As shown above, the vesicular glands lie dorsal to the ampullae. As shown in this LORI entry, they open into the colliculus seminalis together with the ampullae, via the common ejaculatory duct. The vesicular glands are responsible for the secretion of large amounts of gel, characteristic of an equine ejaculate. Still referred to as the seminal vesicles (< L. sacs that contain semen or seed i.e. sperm) by some, they are now preferentially referred to as vesicular glands because of their sac-like architecture and presumably because they contain neither semen nor seeds; only a component of semen. Interestingly, the same glands in bulls are not sac like at all and certainly do not qualify for "sac" terminology but they too only secrete a fraction of semen, yet remain known as seminal vesicles. Given time, that name will probably change to make more sense as well.

Although the function of seminal plasma have been fairly well studied, relatively little information is available on the functions of the individual secretions of the accessory glands.

The sponge-like (see above) lobes of the prostate gland crosses the pelvic urethra just caudal to the neck of the bladder and widens out on either side, rather like a bow-tie. As a student, the author used this mental image to remember the comparative anatomy of prostate glands between species. The stallion is a regal animal, befitting a bow tie. By comparison, the appearance of the (body) of the prostate is rather unspectacular in bulls! Silly perhaps, but useful..

The prostate secretion is carried through through to the colliculus seminalis via 15 to 20 prostatic
ducts. These are shown in the LORI entry on the colliculus seminalis. The prostate produces a watery, alkaline secretion that neutralizes the acidity of fluid from the ampullae and testicles.

The bulbourethral glands are partially covered especially by the bulboglandularis muscle (part of the ischiourethralis muscle that runs between the ischium and the base of the bladder).  They produce a thin watery secretion that is thought to be important in flushing urine and bacteria from the urethra before ejaculation. The bulbourethral glands are usually not palpable per rectum because of their muscular covering. That muscular covering is evident in this old, but still valuable specimen:

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The image below shows the cremaster muscle (from the recent dissection) mainly attached to the lateral surface of the parietal vaginal tunic. The cremaster is a slip of the internal oblique muscle on each side yet is is different from the rest of that muscle bed; unique  in both its physiology and histology. The cremaster muscle contains smooth muscle fibers as well as striated muscle and and responds to environmental temperature changes autonomously, drawing the testicles towards the warmth of the body or releasing them to a cooler environment. This allows the temperature of the testicles to remain between 30 and 33 deg C; optimal for spermatogenesis. At the body temperature of 37.8 to 38.4 deg. C. spermatogenesis would be completely disrupted.

Note: some anatomists include the cremaster muscle as part of the spermatic cord, others not.

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In the image below, a testicle at left is shown covered by its (parietal) vaginal tunic. On the right side of the image, the parietal vaginal tunic has been removed completely showing the testicle in its visceral vaginal tunic, a tunic that is tightly adherent to the testicle. Functionally therefore, the visceral vaginal tunic is the outermost covering of the testicle itself.

The word vagina originates from the Latin term for sheath; as in the sheath of a sword. In this context however, the vagina is a not a sheath as commonly conceived but a pouch that originates as a ventral extension of the peritoneum itself. This is the parietal vaginal tunic. Within this pouch, lies another pouch; attached firmly to the outside of the testicle. It is none other than the visceral vaginal tunic mentioned in the previous paragraph.. The visceral and parietal tunics are separated from one another by a space i.e. the vaginal cavity. It is important to realize that the vaginal cavity is continuous with the peritoneal cavity. In fact, intestines can move through an inguinal canal and into a vaginal cavity, resulting in an inguinal hernia. Also, septic peritoneal fluid can extend into the vaginal cavities as well. This fluid may cause orchitis or insulate the testicles, thereby disrupting spermatogenesis. 

Although the vaginal cavity separates the testicle from the parietal vaginal tunic, the testicle (within the visceral vaginal tunic) is attached to the parietal tunic along a margin that runs distal to proximal on the dorso-caudal aspect of the testicle in horses. In other species, the same connection exists but as shown in another LORI entry its orientation varies (see bottom of that entry). The double walled membrane that connects the visceral and parietal tunics is the mesotestes and within or just adjacent to the mesotestes, run blood vessels, the epididymis and the ductus deferens.

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Although the cremaster plays an indispensable role in thermoregulation in spermatogenesis, the vascularization of the testicle is important as well. Both the arterial supply and venous drainage are highly contorted, providing increased area for heat exchange between the cooler venous blood and arterial blood which is initially at body temperature. This cooling mechanism results in a lower overall temperature of blood supply to the testicles. 

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When the function of the gubernaculum has been fulfilled and the testicle has descended into the scrotum, it matures into two ligamentous sections. They are the proper ligament of the testes (the proximal ligament) and the ligament of the tail of the epididymis (the distal ligament of the testes). The author finds these terms confusing because they both relate to the epididymis. Both ligaments arise from the same structure (the gubernaculum) so it would seem logical to simplify terminology by renaming them the proximal and distal ligaments of the epididymis (the testicle being taken as the most proximal object). At risk of invoking the ire of anatomists, the author has labeled these structures according to that assertion.

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As the urethra curves over the ischial arch, its covering changes from the urethralis muscle to the bulbospongiousis muscle and it adopts a name from human anatomy where the area is known as the bulb of the penis. That is because in humans, the corpus spongiosum penis resembles a narrow bulb in cross section. Whether this is also the case in horses, or any other animal for that matter, is a subject for fertile imaginations. Suffice to say, it is that area of the penis that is immediately below the anus.

In the image below, one is reminded that the retractor penis muscles also enter the picture here because they emerge just ventral to the anus, a little distance from where they originate, on the first few coccygeal vertebrae. They join together ventral to the anus and run along the ventral aspect of the penis for most of its length. When the author inspected this specimen closely, it was apparent that the two main bellies of the retractor muscles split into several smaller bellies towards the distal half of the penis.

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As the bulbospongiosus muscle contracts, large volumes of blood are forced down the corpus spongiosum penis (corpus spongiousum urethra in some texts) causing a "tooth paste tube" effect on the urethra, expelling its contents as the ejaculate. Unlike the corpus cavernosum penis, the corpus spongiosum is not a dead-end structure and the substantial volume of blood forced down the sides of the urethra must have a large reservoir into which blood can flow before it is finally drained by the external pudendal veins. This task is fulfilled by a group of venous sinuses dorsal to the penis. As shown in another LORI entry, those sinuses can be quite large.  The venous sinus in this stallion was not spectacular.

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In the image above, one can also see the two main dorsal nerves of the penis. Injury to the same nerves (sensory in function) is of major importance in bulls with hematoma penis. Less emphasis is placed on that problem in stallions because stallions do not sustain injury to the dorsal nerves as a result of rupture of the corpus cavernosum. Nevertheless, stallions do sustain kicks to the dorsal surface of the penis, so the potential for damage to those nerves exists in stallions as well.

The image below shows the erectile tissue of the penis i.e. the corpus cavernosum penis with its internal supporting trabecular framework. The control of erection and ejaculation is complex but generally speaking, the corpus cavernosum fills with blood under parasympathetic stimulation, causing erection. Then, under sympathetic stimulation, ejaculation occurs through the mechanism described above. However, immediately after ejaculation, the sympathetic system (certainly in some animals) has an anti-erectile effect on the penis. Why? Perhaps it is essential to retract the penis as soon as possible to prevent injuries after copulation.

Although it is sometimes stated that the pudendal nerves only carry sympathetic fibers, it has been shown that parasympathetic fibers travel in the pudendal nerves as well.

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During ejaculation, the blood pressure wave in the corpus spongiosum caused by contraction of the bulbospongiosus muscles approaches the glans penis. From the image below it can be seen that the corpus spongiosum around the urethra is continuous within the glans, both above and below the urethral process. This causes the glans to swell into a massive mushroom shaped structure containing perhaps ten times the volume that is does at rest. The function of this expanded structure is probably to pump the ejaculate into the uterus and to prevent its back flow into the vagina. Blood begins to leave the glans rapidly, even as the stallion is dismounting, draining into the dorsal venous sinuses.

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Selected references:

Abou-Elhamd et al. 2012. Histological and Histochemical Studies on the Ampulla of the Deferent Duct of donkey (Equus asinus). J. Advanced Veterinary Research. 2:261-270

Auer J and Stick J. 2006. Eds: Equine surgery Third edition.ISBN 13: 978-1-4160-0123-2

Budras, K-D et al 2003. Anatomy of the Horse: An Illustrated Text. Fourth edition. Hannover : Schlüter, Anat Rec (Hoboken). 2008 Jul;291(7):790-6. doi: 10.1002/ar.20711.

Davies Morel, M.C.G Equine Reproductive Physiology, Breeding and Stud Management. 3rd Edition. Cabi. ISBN-10: 1845934504

Kayalioglu, G.1. et al 2008 Morphology and innervation of the human cremaster muscle in relation to its function. Anat Rec (Hoboken). 291:790-796

McDonnell, S. M. 1992 Physiology and Dysfunction. Stallion management. Ejaculation. Physiology and dysfunction. Vet.Clinics of North America. 8: 57-70

Pozora, M.A. and McDonnell, S.M. 2004. Color Doppler ultrasound evaluation of testicular blood flow in stallions. Theriogenology 61: 799–810

Samper, J.C. 2009. Equine Breeding Management and Artificial Insemination. Second edition. Saunders. ISBN: 9781416052340

Smith, J.A. 1974 Biopsy and the testicular artery of the horse. Equine Vet J. 6: 81-83

Szekeres,L. and Papp, J.G. 1994, Pharmacology of smooth muscle. Springer-Verlag ISBN-13: 978-3-642-78922-9

Turner, R.M.O. and McDonnell, S.M. 2003. Alkaline phosphatase in stallion semen: characterization and clinical applications Theriogenology. 60: 1–10

Voermans, M.I. 2006.The complex blood supply to the equine testis as a cause of failure in laparoscopic castration. Equine Vet J. 2006 Jan;38(1):35-9.

Penile amputation

Keywords: surgery, amputation, equine, stallion, penis

Penile amputation is used to treat penile neoplasia, especially squamous cell carcinoma. Numerous other indications for amputation exist, amoungst them, priapism. Priapism is morbid engorgement of the penis or penile "paralysis". In some cases, priapism may be an adverse side effect of phenothiazine-type tranquilizer but most often, the cause is unknown. This was such a case.  

The amputation operation is shown in the following images. All images measure 1239 x 847px; click to enlarge them.

Under general anesthesia, the site is surgically prepared, the urethra catheterized, and a tourniquet is placed around the base of the penis for hemostasis.

 The penis is then pulled caudally and a triangular incision is made on the ventral aspect of the penis. The  base of the triangle lies towards the glans penis.

The triangle of skin is removed and as shown below, dissection continues to the level of the  catheter in the urethra, exposing the urinary catheter.

An adequate amount of skin is retained and folded towards the base of the penis, to expose the corpus cavernosum penis surrounded by its tough, tunica albuginea. The next step is to appose the upper and lower sections of the tunica albuginea (seen below) to close the corpus cavernosum. Closure should be secure to afford excellent hemostasis. Absorbable suture material is used.

In the image below, the first step in closure of the corpus cavernosum has been completed.

Here, apposition has almost beencompleted. 

With the corpus cavernosum closed (see below), the skin of the penis and the urethral mucosa are apposed using non-capillary absorbable sutures. 

After starting apposition of the urethral mucosa and skin ventrally (at the apex of the original triangular incision) the circumference of the unattached skin is lifted as shown below. This causes that the corpus cavernosum penis to drops within the operative site and skin to be drawn upwards, towards the tip of the penis. This provide adequate skin for apposition to the urethral mucosa.

The urethral mucosa is then apposed to the skin on the dorsal surface of the penis. In the image below, only the first suture in this series is in place.

Below, circumferential apposition of the skin and urethra is almost complete.

A view of the new mucocutaneous junction at the end of the penis. The "V" shaped apposition on the ventral surface f the penis will help to prevent stranguria in the event of post-operative swelling in this area. The urethral catheter and tourniquet are still in place.

The completed phallectomy. Enough of the penis has been removed so that it is no longer visible on external inspection.

Morbid engorgement of the penis 

Keywords: penis, engorgement, equine, stallion paralysis

Morbid engorgement of the penis or penile "paralysis".

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Although there is a common association between the use of a phenothiazine tranquillizers and penile paralysis in horses some cases are associated with trauma and in others (such as this case) there is no known etiology.
Although the term priapism is sometimes used to describe this condition, it is probably inaccurate because priapism is a state of persistent erection rather than one of passive engorgement.

As shown in this image on the right side, phallectomy (penile amputation) was performed in this horse to prevent trauma in this stallion. Amputation is considered to be an extreme measure and not always necessary; some cases may resolve through the short-term application of truss supports as shown elsewhere in LORI. In some cases, permanently affected stallions can even provide fertile ejaculates using pharmacological stimulation (ex copula ejaculation) with imipramine and xylazine.

Penile injury

Keywords: stallion, equine, penis, injury, Gunn

An approximately five year old 500 kg Australian Stock Horse stallion was presented having been found in the paddock adjoining an “in season” mare, with a very large and swollen penis and prepuce.

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There was also an obvious wound on the left dorsal mid-shaft of the penis. The stallion appeared to have jumped into the paddock adjoining the mare, and attempted to serve her through the fence.

Initial treatment was cold hydrotherapy for approximately 20 minutes. When examined approximately three hours later, the stallion was bright, alert and responsive with obvious balanoposthitis resulting in paraphimosis. Clinical examination was otherwise unremarkable. The stallion was sedated with 20 mg romifidine and 20 mg butorphanol, and 750 mg flunixin meglumine was administered intravenously. Procaine penicillin (10,500,000 IU) was administered intramuscularly. The penis was extended with manual traction, palpated, examined visually, and scanned ultrasonographically. This suggested that the penile tunica albuginea was intact but as shown below (note cursors) there was substantial edema between the tunica albuginea and the skin.

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There was an obvious full-thickness wound on the left dorso-lateral aspect of the internal preputial lamina at the level of the internal preputial ring. The wound was left to heal by second intention. Massage was instigated to decrease the edema and assist in returning the penis to the prepuce. The penis was cleaned with water and very dilute  iodine surgical scrub, rinsed and dried. Emollient antibiotic ointment was applied to the swollen area.

The “underpants support mechanism” was applied to this horse, similar to other described by Gunn. A.J. et al 2013). He was hand walked four times daily and treatment was similar to that in the cases described above. The support was deviated to the side twice daily for hydrotherapy and application of antibiotic ointment. The support was changed daily for three days. On the fourth day, the stallion was able to retract his penis and prepuce into the preputial cavity. The support was removed as it was no longer required.

Daily hydrotherapy of the penis and prepuce, and antibiotic ointment application continued for seven days. Then stallion was then able obtain an erection and after erection, to retract his penis into the preputial cavity. The wound had granulated, epithelialization was occurring, and contraction of the area was evident.

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The stallion was exposed to mares daily to induce frequent erections for a period of two weeks. The penile wound had healed substantially by that time (see inset) allowing the stallion to serve a mare successfully. The mare became pregnant.

Copyright: Dr Allan. BSc.Agric, BVM&S, MANZCVS (horse med; repro), DACT, MRCVS
Lecturer; School of Animal and Veterinary Science Charles Sturt University, Wagga Wagga, NSW 2678, Australia. (algunn@csu.edu.au).

Reference: Gunn. A.J. et al 2013. Balanoposthitis and paraphimosis in the stallion. A novel support for an inflamed penis and prepuce Clinical Theriogenology • Volume 5:45-55 • March 2013

This material has been reproduced courtesy of Dr R.Youngquist, Editor.Clinical Theriogenology

An innovative method for suspending the penis

Keywords: underwear, penis, injury, stallion, equine

Injuries to the penis are common in stallions. These are most often sustained during breeding or teasing Usually a sophisticated sling and suspensory system is not available. Instead, a pair of men's underwear can be used suspended through the hind legs and over the flanks using baling twine or a bandage. The basic method is shown here. Additional details are described in the accompanying reference.

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With appropriate modifications to prevent abrasions, the system can even used for extended periods of time.

Copyright: Dr A. Gunn. BSc.Agric, BVM&S, MANZCVS (horse med; repro), DACT, MRCVS
Lecturer; School of Animal and Veterinary Science Charles Sturt University, Wagga Wagga, NSW 2678, Australia. (algunn@csu.edu.au).

Photographic credit: Katrina Partidge http://www.katrinapartridgephotography.com

Reference: Gunn. A.J. et al 2013. Balanoposthitis and paraphimosis in the stallion. A novel support for an inflamed penis and prepuce Clinical Theriogenology • Volume 5:45-55 • March 2013

This material has been reproduced courtesy of Dr R.Youngquist, Editor.Clinical Theriogenology

Penile injury

Keywords: penis, injury, stallion, Monroe

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This stallion ran  into a steel fence post, traumatizing its lower abdomen (see scrape marks) and inguinal area. Initially no other damage was evident but ovet the next few days, this prolapse developed.

Annotation on treatment and recovery pending. See these other entries in LORI for approaches to treating these injuries:

Kicking injuries to the penis

&

Approaches to penile prolapse in a stallion

Note: Low resolution image. Copyright: Dr Brian Monroe (lvh@dragnet.com.au)

 Approaches to penile prolapse in a stallion

Keywords: stallion, equine, penis, injury

A mature Icelandic stallion sustained an injury to his penis that resulted  in partial prolapse. The stallion had not been in contact with mares so this was not the result of a kicking injury (a common cause of penile injuries in stallions). Instead,  it was thought that he had been traumatized while attempting to tease mares across a paddock fence.

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Because trauma to the penis was not severe, it was though sufficient to apply a purse-string suture around the prepuce to retain the penis; similar to the principle occasionally used in bulls with penile prolapse. As will become evident, this was an error in judgement by the author. The purse-string suture was difficult to place because the circumference for suture placement was not as easy to define as that in a bull.  Nevertheless, the penis was well retained after suture placement. An aperture of approximately 2.5 cm was left for the passage of urine. Zinc oxide ointment was applied to prevent excoriation because the stallion would not be able to exteriorize his penis during urination.

Appropriate tetanus prophylaxis and antibiotic treatment was used.

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Within 12 hours the penis had escaped the restraint of the purse-string suture. Fortunately this was discovered soon after that had occurred, so strangulation of the penis was not yet evident.

The purse-string suture was removed immediately....

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 ..... and the penis was suspended in a conventional sling (below) used for penile injuries in stallion. 

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The method used for suspending the sling was conventional as well. It is shown below.

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Soon after the application of this sling, it was noticed that pouch of that sling was not large enough to contain the prolapsing penis in this animal. This was replaced by the larger pouch seen below. Zinc oxide treatment continued to prevent excoriation because the pouch remained damp during during the support period.

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Recovery was uneventful. 

This cases illustrates the caution that should be exercised if  the application of a purse string suture is contemplated. It also shows that a suspensory  system and several sizes of pouches should be on hand for penile injuries in stallions. 

A kicking injury after natural service

Keywords: injury, penis, stallion

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This amalgum of two images shows the appearance of a stallion's penis and prepuce after a typical injury sustained during breeding. This topic is described in some detail elsewhere in LORI. Click here to reach that entry.

Kicking injuries to the penis

Keywords: stallion, equine, injury, penis

Damage to the penis of a stallion sustained by a kick from a mare during breeding. This is all too common and can be prevented by appropriate teasing and mare restraint. Alternatively and preferably one should use a phantom and AI.

On no account should inexperienced stallions be turned out into a group of mares to breed them, especially stallions that have been trained to serve a phantom or have been hand-bred to mares.

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In the lower image in this amalgum, note how the penis has curved to face caudally. This is due to damage of a major venous plexus on the dorsal aspect of the penis, forming a hematoma that induces this curvature of the penis. 

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This plexus is shown in the following image. An overview of the anatomy of a stallion's penis viewed from the lateral aspect and in the inset, from the dorsal aspect.

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Of particular interest here is the massive venous plexus that lies dorsal to the penis. Its function is unknown but it may serve as a temporary reservoir for blood as it is drained from the penis after an erection. As shown in the previous image, this plexus is often damaged when stallions are kicked on the dorsal aspect of their partially erect penises during mounting or dismounting.

A major priority in these cases is to catheterize the urethra to ensure normal urine flow. Also, as shown below, zinc oxide ointment should also be applied to protect the penile mucosa from urine scalding after the catheter has been removed.

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Hydrotherapy is a valuable part of treatment as well:

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When injuries such as this occur, the stallion often cannot be used again for the rest of the breeding season.

These images show the penis and prepuce of a three-year-old Appaloosa stallion. He stood in a pasture with a number of mares and was kicked whilst trying to serve one of them. Treatment included tetanus prophylaxis, antibiotics and pressure bandage placement on his penis.

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The top two images in this amalgum show the state of the penis at presentation. The stallion was able to withdraw the penis into his prepuce but there was severe peri-penile edema. At lower left the penis is shown compressed with "Vetwrap" after being catheterized.  In this case, the wrap was only left in place for a period of approximately 3 hours and as shown at lower right, improvement was substantial. Some of the edema had however, been driven caudally and covered the scrotum. Examination of the scrotum using ultrasound suggested that there was very little effusion into the tunica vaginalis itself. However, local inflammation and insulation the testicles would probably have compromised spermatogenesis anyway. Owners should always be warned of this potential.

The image below shows some options for penile suspension after kicking injuries.

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The first is a crude truss made from netting, suspended with loops of surgical rubber tubing; two cranially and a single loop caudally. The single loop is passed up through the hind legs with tubing on either side of the tail. That loop is then joined to both of the cranial loops which are passed dorsally on the flanks of the stallion.  The inset shows how a similar effect can be achieved with a simple belly bandage.  Whichever system is used, it should allow for frequent bandage changes and access to the penis.

In general, treatment of penile kicking injuries should include catheterization of the urethra, compression of the penis and re-bandaging every few hours, suspension of the penis to prevent gravitational exacerbation of edema, tetanus prophylaxis, analgesia and prophylactic antibiotics.

It is important to realize that the dorsal nerves of  the penis can be damaged by kicking injuries as well. In such cases, sensitivity to the glans penis is lost and as is the case in bulls, the stallion may be unable to detect the vulva for intromission. Should this be suspected, a client can be convinced of the fact (after preliminary testing!) by placing a hemostat on the stallion's glans penis (personal communication, Dr John Cavalieri,  john.cavalieri@jcu.edu.au.).

Precancerous lesions and squamous cell carcinomas on the penis

Keywords: precancerous, squamous cell carcinoma, penis, stallion, equine, neoplasia

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Squamous cell carcinomas (SCC) are common neoplasms on the penises of older horses. They do not respond well to most forms of treatment and tend to recur. Their etiology is unsure. In most animals there is a general acceptance that ultraviolet light is important in the pathogenesis of  SCC but for obvious reasons, that is unlikely in in these cases. In both humans and animals, has been speculated that chronic irritation caused by accumulations of smegma in the prepuce may induce tumor formation. However current data suggests a more tangible role for papilloma virus (EcPV2).

In both cases pictured above the lesions were characterized histologically as precancerous that could, in time, develop into squamous cell carcinomas.  The penis at lower right was that of an aged gelding. The gelding was treated using cryotherapy. The penis at upper left was that of an intact miniature stallion, photographed in 2007.  The miniature pony was not treated.

Seven years later, in 2014, the miniature stallion was presented again. At this time, the precancerous lesion had indeed become cancerous. Several foci of SCC can be seen on the glans penis.

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The large cancerous and erosive lesion seen at right, communicated with the original focus of the SCC shown at left. The stallion was euthanized.

References:

Bogaert, L. et al. 2012. EcPV2 DNA in equine genital squamous cell carcinomas and normal genital mucosa. Vet. Microbiology. 158: 33-41

Knight, C.G. et al. 2011. Equine penile squamous cell carcinomas are associated with the presence of equine Papillomavirus Type 2 DNA sequences. Vet. Path. 48: 1190-1194

Lange, C.E. 2013. EcPV2 DNA in equine papillomas and in situ and invasive squamous cell carcinomas supports papillomavirus etiology. Vet. Path. 50: 686-692

Van den Top, J.G.G. et al. 2011. Penile and preputial squamous cell carcinoma in the horse and proposal of a classification system. Equine Vet. Ed. 23: 636-648

Habronemiasis

Keywords: Habronema, penis, stallion

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Habronemiasis on the penis of a Quarter Horse stallion. This is due to infestation by the fly-borne larvae of Habronema muscae, H. microstoma and Draschia megastoma. The lesions these larvae cause are intensely pruritic and may occasionally cause hemosemen. Habronemiasis should be differentiated from squamous cell carcinoma by biopsy. A diagnosis is based on the presence of larvae in scrapings of the lesion.  Oral ivermectin Rx is an effective treatment for habronemiasis when two treatments are given one month apart.

Note: Habronema spp. lay their eggs in manure piles and their larvae are ingested by the larvae of the housefly (Musca domestica) or Stable fly (Stomoxys calcitrans) which also develop in manure. The habronema larvae then emerge from adult houseflies or stable flies as they alight on horses. Fly control is therefore of great importance in the control of habronemiasis.