Often I am called out to see a horse that has a lump. A lump is a very vague term that can mean different things to different people. Any swelling that is not normal is a lump. The term does not refer to size. A lump can be the size of a pea or the size of a pumpkin. It does not refer to location. A lump can be on the jaw or the hoof. Any time an unusual swelling is seen anywhere on the horse, it needs to be seen by a veterinarian. It is difficult to impossible to diagnose what a lump over the phone, though many owners want me to.
Most unusual swellings that are called lumps have in tacked skin over them. If there is a wound nearby or a puncture hole in the middle of the swelling, then usually the lump is an abscess with or without a foreign body. The lump with no sign of a wound or drainage is more likely to be a cyst (water filled swelling), a hematoma (blood filled swelling) or a tumor (abnormal cell expansion). We use the term “tumor” to imply “cancerous” but actually the word is Latin for, you guessed it, “lump”.
Recently, I was called out to look at a lump on the side of the shoulder of a 5-year-old Saddle Bred mare. On examination, the lump was approximately 2” X 3” in diameter. It moved freely under the skin and seemed to be attached to the muscle under the skin called the subcutaneous trunci muscle. This muscle is unique to horses and helps them “giggle” flies of their sides. Normally, it is very unwise to make a diagnosis without a biopsy. A biopsy is looked at under a microscope by a veterinary pathologist to confirm the type of cells involved and the chances of recurrence. Sometimes a veterinarian will stick a needle into a lump to see what comes out. Depending on what fluid is present in the lump, a presumptive diagnosis can be made, but only a biopsy can confirm it. The owner indicated that the lump seemed to be getting bigger. This was important information because it meant that it was probably not a hematoma, since they get smaller. Tumors or cysts will get bigger over time. I decided that the best course of action would be to dissect down to the lump through the skin and take it out.
After sedating the mare and injecting the skin with lidocaine to numb it, I clipped and scrubbed the skin for sterile surgery. I cut through the skin and found the lump to be deep to the truncimuscle. I continued to dissect down next to the lump using my gloved finger to guide me based on the feel of the tissue. The best surgical instrument known is the human finger. You can tell what is tumor and what is not by the subtle difference in firmness between tumor and normal tissue. After removing the lump and suturing the muscle and skin back together, I turned to examine the lump more closely now that it was on my surgical tray. This is actually the most exciting part of a surgery. We get to see what this thing might be. I cut the lump in half. It immediately oozed a brownish tan fluid with white clots. The lining of the tumor was a smooth tissue and inside the tumor was a large rolled up wad of hair. This was a tumor known as a teratoma. Teratomas are an abnormally located clump of cells that grow encased in the inside of the tumor. In this case an abnormal clump of skin cells with hair were developing in a little “cocoon” deep inside the body. They can be made up of any type of cell. They will sometimes have bone and fat tissue as well. They are usually benign (they do not spread if removed). Because the tumor produced hair that could not really go anywhere, it became impacted inside the center of the tumor and irritated the tissue. The pus like fluid was an attempt by the body to dissolve the hair. If I had put a needle in this tumor first, I would have falsely assumed it was an abscess and probably would have lanced it open, causing the irritated tissue to become infected and not removing the abnormal cells.
I put the horse on antibiotics and Bute for pain and swelling and will return in two weeks to remove the sutures.