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Unsettling finding from CDC investigation: Tapeworms can transmit cancer cells to humans

This common parasite, known as the hymenolepis nana or the dwarf tapeworm, may carry cancer cells that may take root in people with weakened immune systems. The malignant cells are pictured here. (Centers for Disease Control and Prevention)

Scientists from the Centers for Disease Control and Prevention have identified a puzzling and unsettling new cancer-like condition in a 41-year-old man, who is believed to have become ill through a common stomach bug.

The case — the first known transmission of cancer cells from a parasite to a human — involves an HIV patient from Colombia who developed multiple, large tumors in various parts of his body. Local doctors biopsied those tumors and found that the cells acted like cancer cells in their destructiveness but were strange in other respects. For one, they were about 10 times smaller than normal human cancer cells. The doctors contacted the CDC for help.

Atis Muehlenbachs, an agency pathologist in the special unit that investigates unexplained mystery illnesses and deaths, wasn't sure what to make of the cell samples when he and his team received them in 2013. The cells' growth pattern was cancer-like, they noticed, with overcrowding and a high rate of multiplication. But the cells also fused together, which is rare for human cells.

One early theory, Muehlenbachs said in an interview Wednesday, was that they could be a new type of infectious organism. But after performing dozens of tests, the team discovered the cells contained DNA snippets of a dwarf tapeworm, Hymenolepis nana. That analysis was verified by a researcher and tapeworm expert at the Natural History Museum in London.

"In the initial months, we wondered if this was a weird human cancer or some unusual, bizarre emerging protozoa-amoeba-like infection," he recalled. "Discovering these cells had tapeworm DNA was a big surprise — a really big surprise."

The CDC researchers, who published their findings Wednesday in the New England Journal of Medicine, now think the Colombian man may have ingested some microscopic tapeworm eggs, most likely in food contaminated by mouse droppings, insects or human feces. Because of the man's compromised immune system, the tapeworms multiplied rapidly inside his gastrointestinal tract and the cells invaded other parts of his body. It's unclear whether the cells in the tapeworm eggs exhibited cancer-like properties before they entered the man's body or whether some interaction between the parasite and his body then caused them to become cancerous.

[5 important things you need to know about tapeworm cancer in humans]

"This is the first time we've seen parasite-derived cancer cells spreading within an individual,"  Muehlenbachs said. "This is a very unusual, very unique illness."

The case study is worrisome for numerous reasons.

We know that many creatures, such as various sea animals, are susceptible to cancer, while others, like elephants, are almost immune to it. But until now, scientists had not believed that any human parasite could harbor cancer cells or transfer them to people.

We've also known that certain parasitic infections may put people at higher risk for specific cancers — such as flatworms for bile duct cancer or a river creature called schistosoma haematobium for bladder cancer, according to the American Cancer Society. But the reason was wholly different, namely a hyper-immune response that may weaken an individual.

The presence of cancerous cells in tapeworms also raises numerous questions about where the mutant cells originated — from something in the environment? — and whether other organisms that live inside or on people that could transmit cancer cells. In recent years, many scientists have emphasized that the human body's ecosystem is only made up of 10 percent human cells but 90 percent microbial cells.

"We didn’t believe that cells from a human parasite could become malignant and then invade human tissue. … It's just very unusual that the parasite’s cells became cancerous inside a human and then invaded into human tissue," Bobbi Pritt, director of clinical parasitology at the Mayo Clinic, said in an interview.

Yet the idea of tapeworms also being vulnerable to cancer makes some sense, Pritt said, as "every living animal is made up of cells that divide and could become cancerous."

Cancer is generally not considered to be a transmittable disease, although there have been very rare cases of humans passing on malignant cells to other humans through organ transplants or from mother to fetus during pregnancy. There are also some animal species -- such as Tasmanian devils and domestic dogs -- that are known to have transmissible cancer cells circulating within their populations. The CDC does not believe there is any risk of the tapeworm cancer cells being spread directly from one person to another.

It's unclear how common this type of tapeworm cancer illness is in humans, but experts like Pritt believe it's likely that there are more cases out there: "H. nana is a very common tapeworm infection in humans, and therefore I would expect there to be other cases like the one described…that were misdiagnosed or went undetected."

Matthew B. Laurens, an associate professor at the University of Maryland School of Medicine who specializes in infectious diseases, said the findings highlight our need to strengthen cancer diagnosis and data collection in developing countries.

“We could just be scratching the surface of something that could be very important,” Laurens said.

Peter D. Olson, a researcher at the Natural History Museum in London who helped the CDC interpret its results, said that the finding represents "an enormous advance in our knowledge and raises questions about the conditions under which cells may become cancerous."

The Colombian man came to the attention of researchers at the Pontifical Bolivarian University in his home town of Medellin when he sought care because of fatigue, fever, a cough and weight loss.

The patient had been living with HIV for at least seven years and had not been adhering to therapy. A CT scan showed tumors ranging in size from 0.4 to 4.4 centimeters in his neck, lymph nodes, lungs and liver. Stool samples showed tapeworms inside his body.

Tapeworm infections are very common worldwide, especially in the developing world, with an estimated 75 million people infected with H. nana at any given time. Most do not show symptoms and clear the parasites quickly. But in people with compromised immune systems, tapeworms tend to thrive and can live in their hosts for years.

By the time the CDC researchers figured out what was going on, the man's condition had deteriorated and he was in hospice care. He died 72 hours later, without any opportunity for treatment. His official cause of death was HIV/AIDS, with cancer a contributor to his weakened state.

Muehlenbachs said further study was limited by the patient's death and the fact that researchers have been unable to grow tapeworm cancer cells in the lab.

Muehlenbachs said he's not sure the current arsenal of treatments could have helped given the nature of the man's tumors. He said traditional drugs to treat tapeworm infection -- which target the whole organism at the larval stage - might not have been effective against tapeworm cancer cells, and it's also unclear whether chemotherapy for normal human cancer cells could have helped shrink the tumors.

In fact, the pathologist is cautious about calling the Colombian's illness "cancer" because those cells were different from normal human cancer cells, even though they behaved similarly. Instead, Muehlenbachs referred to it as "an infection with parasite-derived cancer which causes a cancer-like illness."

He said the CDC investigation's findings are so unexpected that we lack the precise terminology to describe it: "Can you say a worm has cancer? That’s a philosophical question how you define this."

While he believes this type of case is rare, no one knows for sure. Muehlenbachs said further investigation is needed to determine whether it is limited to tapeworms or whether the situation is worse — that there's some "underlying biological phenomena" that might lead to transmissible cancer cells developing in other creatures that can pass them along to humans.

This post has been updated.

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