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Cancer cells from a tapeworm form tumors in patient with HIV

A rare infection caused by a common parasite

Wikimedia Commons/Tomas De la Rosa

For the first time, cancer cells originating from a common tapeworm have been found inside a patient with HIV. The cells formed into cancer-like tumors that looked nearly identical to human tumors in the patient's body.

The unique case, detailed by the Centers for Disease Control and Prevention in the New England Journal of Medicine, suggests that a dwarf tapeworm, Hymenolepis nana, may cause tumors in people with weakened immune systems. Though such a scenario is rare, clinicians should consider if a patient with a compromised immune system also presents with tumor cells, the CDC researchers say. A closer look at the cancer cells may help avoid a cancer misdiagnosis and guide doctors toward the best course of treatment.

The tumors looked nearly identical to human tumors

The patient described in NEJM first came to the CDC's attention in 2013. Doctors in Colombia had done biopsies on tumors found in the 41-year-old's lungs and lymph nodes. The tumor cells were puzzling: they grew rapidly like cancer cells, but lab analysis suggested that the cells weren't human. When the CDC got ahold of the biopsy samples, researchers there found an even weirder trait: the cells were quite small.

"The cells were growing like cancer does, but they were remarkably small — too small to really be considered to be human cells," said lead study author Attis Muehlenbachs, in the CDC’s Infectious Diseases Pathology Branch. He said that the cells were about 10 times smaller than human cancer cells. "That didn’t make sense in terms of what we knew."

The cancerous Hymenolepis nana cells (CDC)

Initially, the CDC had no idea what the cells were, suspecting they could be some sort of slime mold. It took years of tests for researchers to find Hymenolepis nana tapeworm DNA in the cells. Unfortunately, doctors didn't have enough time to treat the patient with medication to fight the tapeworm. The patient died from kidney failure just three days after the discovery was made.

Normally, these tapeworms don't cause major problems in their hosts. People all over the world get infected with the Hymenolepis nana tapeworm, either by eating food contaminated with the worm's eggs or by ingesting insects that are carrying the worm. For people with healthy immune systems, the tapeworm usually stays in the gut and doesn't cause any symptoms or health issues. "Most of the time you wouldn’t even know it was there," said Peter Olson, a tapeworm expert at the Natural History Museum in London.

That's why Olson thinks the patient's compromised immune system may be to blame for the tumors. In order for a Hymenolepis nana tapeworm to grow inside a human, the worm's larvae burrow into the lining of the intestines to morph into adult form; it's similar to how a caterpillar transitions into a butterfly — with the human gut serving as the cocoon. While the tapeworm larvae are inside the intestinal lining, the immune system somehow prevents the parasite from invading the rest of the body.

The larvae likely traveled through the patient using his lymphatic system

But in patients with HIV, the compromised immune system isn’t telling the parasites to stay in the gut. "These parasites take cues from the immune system, but in the absence of those cues, things go awry," said Olson.

That means the larvae likely traveled through the patient using the series of vessels that carry fluids throughout the body — this is called the lymphatic system. And traveling with the larvae were tapeworm stem cells, which probably lodged themselves in other parts of the body. This patient was just unlucky enough to have at least one tapeworm whose cells started growing out of control.

Hymenolepis nana tapeworms are fairly common throughout the world, infecting up to 75 million people at any given time. Children are much more likely to get infected, as well as those in developing nations, where sanitation levels may be low. Muehlenbachs hopes that doctors in these regions especially will learn about this particular case study, so that they will keep an eye out for a similar rare event. It's possible that tapeworm medication may get rid of the cancer cells, as well as certain forms of chemotherapy — but doctors will only know for sure if they see such an infection again.

As for now, they can only guess as to how the mechanisms of this tapeworm cell invasion work. "There’s so much we don’t know about this disease," said Muehlenbachs.