In unusual circumstances it appears that a person can catch cancer as if it were a contagious disease.

That is the implication of a case reported by scientists from the Centers for Disease Control and Prevention (CDC).

The CDC came to that conclusion after being brought in to consult on a perplexing case of a 41-year-old man who sought medical attention in Medellín, Colombia, for a cough along with fatigue, fever, and weight loss.

The anonymous man at the center of this strange episode was HIV-positive, leaving him with a compromised immune system. He was also infected, as many people in developing countries are, with dwarf tapeworms.

Initial testing revealed that the patient had tumors in his lungs and lymph nodes. The biopsy revealed cells that acted like cancer but were smaller than any known human cancer cells.

DNA sequencing revealed that they belonged to a tapeworm, which puzzled researchers because they didn’t look like a tapeworm and were invading organs outside the gastrointestinal tract.

It turns out the cells were cancer, but they were the tapeworm’s cancer, which had spread to its host.

“We were amazed when we found this new type of disease — tapeworms growing inside a person essentially getting cancer that spreads to the person, causing tumors,” said Dr. Atis Muehlenbachs, Ph.D., a staff pathologist in the CDC’s Infectious Diseases Pathology Branch and lead author of the study.

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Outsmarting the Immune System

Bizarre as the case is, it doesn’t completely upend what we know about cancer.

In order to grow, cancer has to trick the immune system into thinking it’s friend not foe. Parasites like tapeworms have also evolved to slip through the cracks of the human immune system.

The HIV-positive patient’s immune system was ill-equipped to “clear these foreign cells once they started taking root,” Muehlenbachs told Healthline.

His immune system’s failings had also allowed tapeworms to thrive in his intestines.

It’s the first known case of a person catching cancer from another species, Muehlenbachs said. But it’s not the only case of cancer being acquired.

A handful of viruses, including human papillomavirus, Epstein-Barr virus, and human T-cell lymphotropic virus 1, seem to lay the groundwork for cancer to develop. But they don’t introduce the cancer themselves, as the tapeworm in the CDC study did.

“The only thing I can think of that is pretty similar is the phenomenon of donor transmitted cancers in transplants,” said Dr. Alfred Neugut, Ph.D., a professor of cancer research at Columbia University Medical Center.

In a small fraction of cases, a donor organ not known to be cancerous can introduce the disease into the recipient. Transplant patients also have weakened immune systems, which can allow the cancer to grow.

“The donated liver or lung or kidney contains previously unrecognized cancer cells, which then have a chance to proliferate in the new host because of the immunosuppressive drugs the recipient takes,” Neugut said.

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Unanswered Questions

It took the international team three years to sketch this bizarre chain of events. Three days after they did, the patient died.

There may not have been much the doctors could have done for him anyway. Efforts to treat the man’s tapeworms hadn’t killed the cancer. Known chemotherapy drugs have never been tested on tapeworms.

The circumstances — tapeworm infection and weakened immune system — though bizarre are not altogether unlikely to repeat themselves. The dwarf tapeworm is the most common type, infecting 75 million people worldwide. In some countries, 1 in 4 children carries the parasite.

HIV infection is also widespread in many of the same places.

“This is a rare disease, but we do not know how rare it is,” Muehlenbachs said.

The CDC wants doctors in areas where dwarf tapeworm and HIV infections are common to be aware that patients could potentially be harboring tapeworm cancers.

But the finding may also prompt more research into whether there are other ways for humans to catch cancer from common parasites.

“I think the study does raise questions: Could cancer cells be derived from other parasites in humans or animals?” Muehlenbachs said.

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