MAST CELL TUMORS IN PIGS: by Beth A. Valentine, DVM, PhD

When Barbara Baker noticed that Lord had developed a lump on his right cheek, she didn’t give it much thought. It was about the size of a cherry, and was not growing, bleeding, changing shape or color, or otherwise bothering the pig. It was only when Dr. Lawhorn, who was attending the potbellied pig symposium in Florida, visited her home and examined Lord’s lump that she became concerned. Dr. Lawhorn advised Barbara to have the lump removed and sent for pathologic examination. A few weeks later the surgery was performed. It was easy for the veterinary pathologist to identify the characteristic features of a mast cell tumor, but it was almost impossible for the pathologist, or anyone else for that matter, to offer an idea of what might happen now that the tumor was removed. We just don’t know enough about mast cell tumors in pigs to be able to predict their behavior.

When Barbara asked if I would be willing to write an article on mast cell tumors in pigs, I went to my files to see what I could find. There is precious little written about mast cell tumors in pigs, so at least I didn’t have to wade through a lot of material! In this article I will try to summarize what we do know about mast cell tumors in pigs and in other species.

First off, let me explain what a mast cell is. Mast cells are normal residents of the areas of the body that are exposed to what can be considered an “external” environment. This includes the skin, sinuses, the lining of the airways from the nose to the lungs, and the lining of the stomach and intestines. Mast cells are identified by their characteristic granules. The purpose of mast cells is to release the contents of their granules during infections. The granules of mast cells contain a variety of substances, many of which make blood vessels “leaky,” which helps other inflammatory cells to get to the site of an infection to try to clear it up. This normal role is often overlooked in discussions of mast cells, because mast cells are so often involved in allergic responses. Allergies can be considered a form of “healthy” inflammation gone awry. One of the substances that mast cells produce and release is histamine, which is the basis for the use of antihistamines in allergic conditions. Histamines cause vessels to be leaky, which leads to fluid accumulation and tissue swelling, and often cause an itching sensation in the area.

Tumors of mast cells, called either mast cell tumors or mastocytomas, are common in dogs, cats, and ferrets, and are much less common in horses, cattle, pigs and people. We know a lot about the behavior of mast cell tumors in dogs, cats, ferrets, and horses, but very little about their behavior in cattle and pigs. Such tumors are very uncommon in people, usually occurring in children, where they often regress on their own. In dogs, mast cell tumors are graded 1, 2 and 3, with grades 1 and 2 being relatively benign and generally cured by wide surgical excision, and grade 3 being malignant and likely to recur and/or spread. Mast cell tumors in cats are most often benign, although there is a less common malignant variant. Mast cell tumors in ferrets and horses are considered benign tumors. In cats, the most common mast cell neoplastic processes are skin tumors (that can be multiple), and an internal mastocytosis syndrome in which neoplastic mast cells infiltrate multiple internal organs. In the cat, these appear to be two distinct disorders.

Adding to the confusion is that there is a condition called mastocytosis, in which mast cells are increased throughout the body, creating havoc by producing their inflammation-promoting substances. In some cases the mast cells themselves are not thought to be neoplastic, but in others they are. I should define the term neoplastic here. Neo means new, and plastic means growth, so a neoplasm is a growth of new tissue. Neoplasia can be benign, meaning that the tumor does not invade or spread. Malignant tumors invade and/or spread through the body, causing what is often called cancer. The term cancer comes from the Latin word for crab, because the spread of cancer can resemble the spreading legs of a crab. And, to make matters worse, there is also a condition in which neoplastic mast cells circulate in the blood, known as mast cell leukemia.

The literature regarding mast cell disorders in pigs includes mast cell tumors, mastocytosis, and mast cell leukemia. Mast cell tumors of the skin or of internal organs are the most common mast cell problems in pigs, and have even been reported in young meat pigs examined at slaughterhouses. These tumors are not common in young pigs, though, with one study citing only 5 cases of mast cell tumors of the skin in 664 million pigs examined.

Barbara was able to find me pathology reports from three other potbellied pigs with mast cell tumors. Two were single tumors, similar to Lord’s, and another pig had multiple mast cell tumors in the skin. The ages recorded were around 7 years of age. The pig with multiple tumors had re-growth of three of the tumors following removal. So far, I have not seen any reports of internal mast cell tumors in potbellied pigs. But, given findings in other breeds of pigs, I suspect that this is a possibility. The behavior of mast cells tumors is often described as unpredictable, usually because we simply don’t know enough to be able to predict it. This is certainly the case with mast cell tumors in pigs. From what I can gather from the literature to date, mast cell tumors in pigs appear to be more similar to cats than to other species. That is, tumors that form in the skin may be multiple and can recur following incomplete excision, but they tend to stay in the skin. Tumors that form internally are not often associated with skin tumors. I hasten to add that this is still speculation, based on a very small number of reported cases of mast cell neoplasia in pigs. We still have a lot to learn about mast cell tumors in pet pigs.

So, what should you do if your pig develops a skin lump? My best advice is to have it examined by a veterinarian and removed as soon as possible. A mast cell tumor that is completely removed early on is far less likely to recur or to spread than is one that has been there for awhile. The veterinary surgeon will aim to remove the tumor as widely as possible, and removing a small tumor is much preferred to waiting for it to grow to a really worrisome size. Always insist on sending the sample for pathologic evaluation.

It is only by removing tumors and having them identified as mast cell tumors, and then following up on the pigs to see what happens, that we will be able to learn about the behavior of mast cell tumors in potbellied pigs. The efforts of people like Barbara Baker and of organizations like the Duchess Fund are to be lauded. Only by continuing to gather and examine data on disorders of potbellied pigs will we be able to learn more about these vexing tumors, as well as other problems that plague potbellied pigs.

Beth A. Valentine, DVM, PhD, is a board-certified veterinary pathologist on the faculty of the College of Veterinary Medicine at Oregon State University, Corvallis, OR.