An HIV-positive Colombian man has died of cancer that began inside his tapeworm. His tumors were not made up of his own cells, but those of the parasitic worm that invaded his gut. This rare and unique situation is thought to be the first known to medicine whereby tumors have developed as a result of parasite-derived cancerous cells having spread and taken hold throughout a human host.

Described in the New England Journal of Medicine, the patient was a 41-year-old man who, back in early 2013, had been suffering fatigue, a fever, cough and weight loss for several months. He was diagnosed with HIV seven years earlier but did not take his medication. Consequently, his white blood cell (CD4) count was dangerously low and blood samples were loaded with virus particles. Fecal analysis revealed that he was also infected with the dwarf tapeworm Hymenolepis nana.

The patient was then given a CT scan which revealed that his lungs were riddled with tumors, ranging in size from 0.4 to 4.4 centimeters (0.16 to 1.7 inches) in size. His liver and adrenal glands were also infected. He was given biopsies at the time and sent home with both anti-HIV and anti-tapeworm drugs, but his condition worsened so further samples were taken and sent off to the CDC for analysis.

This is where things got a little bit weird. The cells were clearly cancerous – they were invasive, grew rapidly and in a crowded, disordered manner, and all looked the same. But they were tiny, roughly ten times smaller than would have been expected; far too small to be considered a human cell. Baffled, scientists subjected them to a bounty of tests, which eventually revealed the tumor cells contained H. nana DNA. Unfortunately, it was too late for the patient, who died just 72 hours after the discovery was made.

Analysis of the tumor cells' worm DNA when compared with a reference genome of an otherwise normal worm also revealed something quite striking. "Three of the identified genes that were 'broken,' or mutated, have already been implicated in other forms of human cancer," study author Dr Peter Olson told IFLScience. "If they mutate in humans, they can lead to a cell becoming cancerous. That's never really been shown, that sort of commonality between becoming malignant in a human cell line and an invertebrate cell line."

Cancerous cells removed from the patient. Image credit: CDC

This case has completely gob-smacked scientists and doctors alike. H. nana is the most common human tapeworm, infecting around 75 million people worldwide, yet nothing like this has ever been described before. Although cancerous cells naturally circulate in certain animals, like dogs, cancer isn’t really transmissible between humans – although agents that can cause cancer, like human papillomavirus and the trematode worm Schistosoma haematobium can be transmitted between people.

Normally, people infected with this worm are symptomless and their immune system gets rid of the worm over time. But this man was immunocompromised by HIV, allowing the parasitic cells to grow uncontrollably, thus presenting the opportunity for cancer-causing mutations to arise from mistakes in cell division.

While this case seems to be unique, given the abundance of H. nana worldwide, and HIV, it seems plausible that there have been other cases that have gone undrecognized. "But the report is not that there is a new health risk that people should be worried about," said Olson. "The importance is awareness. It's a conundrum for clinicians when they look at these tumors. There's nothing anatomically to suggest they could be from a worm. But now this report is published, doctors should hopefully look out for this possibility."