A horse's penis is a more complicated bit of equipment than many people realise, and issues can arise as a result of the unknown and unattended details of equine penile anatomy. This is especially true for geldings, whose libidos no longer stimulate the penile shaft to extend beyond the confines of the sheath with any frequency. Failure of handlers to clean the sheath can lead to painful topical infections.
Treat an infected sheath in the same manner as prevention: with a thorough cleaning. Fill a bucket with warm water and add a mild antibacterial soap until it easily makes suds. It is important that the liquid is not only antibacterial, but a lubricant that does not hurt the horse.
Bring a stallion near a mare to stimulate his penis to engorge and "let down." If you are cleaning a gelding, grasp the shaft through the sheath, behind the glans -- the bulbous portion at the end of the penis -- and slide the sheath back to expose the glans and adjacent shaft.
Soap down the glans and shaft gently. When the horse becomes comfortable, gently lift the ventral portion of the sheath and check for accumulations of dirt and grease (smegma). Wash away the smegma. Insert a finger under the ventral sheath and manually remove any smegma that remains, then rewash with the soapy solution.
Regrasp the penis below the glans, and insert a soapy finger into the end of the urtethra, feeling just inside the urethra and upward. There is a small cavity there that can also accumulate smegma that hardens into what is called a "bean," and causes the horse to experience pain during urination. Sweep a finger through the cavity to clear any smegma, then rewash the glans, soaking the orifice of the urethra.
Administer an antibiotic if inflammation is encountered during the cleaning that is swollen, feels hot to the touch and causes the horse to flinch. Administer 15mg/kg of penicillin G procaine, intramuscular, every 12 hours for three days.
Stallions with sheath infections can be rowdy. Take special care not to let anyone behind the stallion while its sheath is being cleaned.