What you see. The starting point for addressing any equine health related issue is your observation.


Lump, Bump, Growth on Gray Horse, Near Anus or Tail


Hard lumps and bumps occurring on a gray horse, especially around the anus and tail area are melanoma until proven otherwise. Melanoma affects a very high percentage (about 80%) of middle aged to older grays.

A melanoma is a kind of tumor (abnormal cell growth), that can be benign (slow growing or stagnant), or malignant and cancerous (fast growing, or invading other tissues or organs) and it is a uniquely common problem in gray horses. While vets often presume this sort of bump to be melanoma, we cannot confirm this diagnosis without biopsy and microscopic examination of the tissue.

These masses usually don't cause the horse much trouble unless they become very large. That said, occasionally, fatal malignant melanoma can develop in which the tumors spread throughout the body and the vital organs. Of course, other cancer can occur here too (especially in horses with pink skin), so always share your findings with your vet.

  • Code Orange

    Call Your Vet at Their First Available Office Hours
    • If the mass is large, painful or seems to be growing rapidly.
  • Code Yellow

    Contact Your Vet at Your Convenience for an Appointment
    • If this seems to be a slowly changing problem not seeming to cause the horse much difficulty.

your role


What To Do

Monitor the general health of a horse with presumed melanoma. Weight loss can occasionally indicate spread of the tumor internally.

Examine any gray horse regularly for these common bumps. Track their size and talk to your vet about them because treatment might slow their progression. The tail base/anal area is the most common location for melanoma but bumps anywhere are possible. Other common locations are behind the jaw, the sheath, and in the flesh of the lips.

your vet's role


Your vet will examine the mass to confirm that this is indeed melanoma, since other conditions are possible. If there is any question, your vet will biopsy or remove a mass and send it to the laboratory for diagnosis.

In our practice, we remove melanomas that become large enough to cause problems defecating or that repeatedly open and bleed. Whether or not cimetidine and other treatments really changes the course is questionable.
Questions Your Vet Might Ask:
  • When did you first notice this problem?
  • How is your horse's attitude and appetite?
  • Have the number or size of the bumps changed?
  • What are the results of the Whole Horse Exam (WHE)?
  • Can you send a photo of the problem?

Author: Doug Thal DVM Dipl. ABVP