No matter how beneficial they might be given the right circumstances, tapeworms are creepy as fuck. Here we have a parasitic organism whose entire face consists of a hooked mouth, which it uses to attach to the intestinal wall (or sometimes much worse) of a host. This is where the tapeworm stays and grows, reaching lengths of up to 100 feet in some animals.
It turns out that one very common tapeworm species comes with an additional, potentially quite deadly capability. As described in a report published in the New England Journal of Medicine last week, a tapeworm of the Hymenolepis nana species has been found to produce malignant cancer cells within its unwitting host.
This was revealed in the case study of a severely immunocompromised man with a long-untreated HIV infection who had appeared at a clinic in Medellín, Colombia with ongoing symptoms of fatigue, fever, cough, and weight loss. He was found to have a CD4 cell count of 28 per cubic-milliliter, which indicated that his immune system had more or less declined to nothing (the lower end of the normal CD4 count range is 500 per cubic-milliliter).
Tests revealed the presence of H. nana eggs in the man's stool, but he was in rough shape generally, with computed tomographic imaging revealing nodules on his lungs, liver, and adrenal glands, along with widespread lymphadenopathy (swollen or enlarged lymph nodes). Biopsies revealed that the cells in question (those comprising the nodules) were highly unusual. They were really small, for one thing, occupying less space than a human red blood cell, but they were also undifferentiated. That is, they didn't much look like normal human tissue cells, but more so like stem cells. This is usually bad news, cancer-wise.
"This case posed a diagnostic conundrum," the case study notes. "The proliferative cells had overt features of a malignant process—they invaded adjacent tissue, had a crowded and disordered growth pattern, and were monomorphic, with morphologic features that are characteristic of stem cells—but the small cell size suggested infection with an unfamiliar, possibly unicellular, eukaryotic organism."
"Human disease caused by parasite-derived cancer cells is a novel finding."
The general idea is that the H. nana tapeworms, a very common variety known to exist mostly in children, manage to spread beyond the intestines in some hosts. In a highly immunocompromised patient, they may lodge themselves in places like the lungs and proliferate and grow. Growth means undifferentiated cells (e.g. stem cells) and this is where mutations begin to appear and accumulate among the tapeworm cells.
The result is a "malignant transformation," which is the process by which cells become cancerous. In this case, however, the newly unleashed cancer cells didn't originate with human cells but, rather, the tapeworm cells. It's an unfortunate twist on the parasite-host relationship and one that hints at much deeper fear: transmissible cancer.
"In humans, cancer cells are infrequently transmitted through organ transplantation or from mother to fetus during pregnancy," the study notes. "Human disease caused by parasite-derived cancer cells is a novel finding. Multicellular parasites that live in host tissue generally possess cellular mechanisms for host tissue invasion and immune evasion; these mechanisms could potentially be co-opted during malignant transformation within the host."
What happened in this one case can safely assumed to be quite rare. For tapeworms to proliferate and mutations to accumulate requires the help of a depleted immune system. It's likely, however, that in regions where both HIV and H. nana infections are both common, tapeworm-born cancers may be routinely misdiagnosed as typical human cancers.
The patient in question died four several months later. It was too late—the cancer had gained too much traction and his immune system had lost too much to recover in time. At the very least, his case may help ensure the same doesn't happen to others.