Tuli’s lump: Making a rare diagnosis

Tuli had a lump. It was about the size of a ping-pong ball, firm, and sitting just underneath the skin over the back of his neck. It didn’t seem to be bothering him, and it hadn’t grown or changed in the three months since it had been spotted. Even so, it couldn’t be ignored.

Tuli is a 12-year-old male striped hyena, and tends to be a bit nervous. Examining the lump would require anesthesia so that the veterinary staff could get their hands on him. When the time came, the veterinary team at Harter Veterinary Medical Center anesthetized him and gave him a full physical exam (as well as a dental cleaning, while they had the chance), then collected some small tissue samples (biopsies) from the lump. Tuli awoke from anesthesia with no complications.

The biopsies from Tuli’s mass were sent to our Disease Investigations team for evaluation.

There are all kinds of things that could look like a lump under the skin of a hyena – things like infections, benign growths, or more insidious malignant tumors – and it’s a pathologist’s job to make the final assessment. To do this, we rely heavily on microscopic examination of the tissues (histopathology). Tuli’s biopsies were processed for histopathology (check out Liver Let Dye for details on this procedure), and the slides ultimately ended up on my desk.

As a veterinary pathology resident in my final year of residency training, it would be my job to try to figure out what this mass was. My diagnosis would decide what the next step for Tuli would be.

Tuli’s mass turned out to be fairly complicated, and routine histopathology was not enough to make a definitive diagnosis.

Luckily, we also have some special stains and antibodies (for a process called immunohistochemistry, or IHC) at our disposal to help us make more detailed assessments. These techniques helped me to learn more about the structural and chemical properties of cells in the mass, and ultimately brought me to a final and very unusual diagnosis: it was a glomus tumor!

Glomus tumors come from miniscule structures in the skin called “glomus bodies” that form small connections between blood vessels. The body automatically controls these vascular shunts in response to temperature, so that more blood can be sent to the skin surface (to promote cooling) or kept away from the surface (to retain heat). The connections are regulated by small cuffs of modified muscle cells, and glomus tumors arise from uncontrolled proliferation of these muscle cells.

This type of tumor is very uncommon in human beings and extremely rare in domestic animals. It has never been reported before in a hyena.

This diagnosis was good news for Tuli, since glomus tumors are almost always benign.

The clinical veterinarians performed a follow-up procedure to remove the mass in its entirety, and I evaluated it microscopically to confirm that it had been completely removed. (We worry about this, because if a few tumor cells are accidentally left behind in the patient, a whole new tumor can grow.) I also did some additional testing to confirm the original diagnosis of a glomus tumor.

This unusual tumor highlights some of the unique challenges of veterinary pathology – especially in a zoo setting.

Assessing prognosis is one of our major goals with tumor biopsies, but this can be difficult to do in challenging cases like this. We try to base our assessments on the latest available literature, from studies that generally involve human or domestic animal cases.

One obvious limitation for us is the fact that tumors can behave differently in different species – and there’s just not that much literature out there about tumors in striped hyenas! Since Tuli’s tumor is the first of its kind that we’ve seen in this species, it’s difficult to be completely definitive about prognosis.

Based on what we know about glomus tumors and their behavior in various species, however, I believe Tuli has a very good prognosis. There has been no sign of tumor regrowth in the several months since Tuli’s surgery, but his keepers will continue to monitor him.