Eggs of the dwarf tapeworm (Hymenolepsi nana).
A tapeworm that infected a Colombian man deposited malignant cells inside his body that spread much like an aggressive cancer, researchers have reported in a bizarre, but not unprecedented, case.
“We have a situation where a foreign organism is developing as a tumour rather than developing as an organism,” says Peter Olson, a developmental parasitologist at the Natural History Museum in London. He is part of a team that describes the case in a 4 November report in the New England Journal of Medicine1.
Under a microscope, those samples revealed small odd-shaped cells that, like a cancer, appeared to be invading nearby healthy tissue, the CDC team found. Yet the cells tested negative for human proteins. That was a conundrum: although the US investigators knew about the man’s tapeworm infection, the invading cells did not look like they should belong to a complex, multicellular organism such as a tapeworm.
Tumours from tapeworms
Olson believes that the tumorous tapeworm cells are rogue larvae that burrowed from the stomach into the lymph nodes of immunocompromised people (a healthy immune system would stop this invasion). The larvae are loaded with regenerative stem cells, so instead of turning into an adult tapeworm, they proliferate. “Those stem cells that would normally give rise to a segmented worm don’t, because they’re in the wrong place and have the wrong environmental cues,” says Olson.
Elizabeth Murchison, a molecular geneticist at the University of Cambridge, UK, says that she finds the case astonishing. Although there is no evidence that the proliferative tapeworm cells might be transmitted between humans, Murchison (who studies tumour cells that spread between animals) wonders whether proliferative cells from other parasites could become infectious.
“This paper is tremendously important as it presents the existence of a new type of disease process, which may have previously been overlooked,” she says.