First of all, I’m glad you asked this because I had completely forgotten about this species, Hymenolepis nana. In graduate school, we raised this species by cycling it between mice and flour beetles. Memory may be failing me here, but it seems like we didn’t consider it very significant as a human parasite back then. Reading my textbook from then, which I still have, I find,
Dwarf tapeworm in humans is primarily limited to children in warm climates. It is prevalent throughout India, U.S.S.R., countries bordering on the Mediterranean, all of the countries of Latin America, Hawaii, and some of the islands of the South and Southwest Pacific. It is the common tapeworm in southeastern United States. (Faust, Beaver, & Jung, Animal Agents and Vectors of Human Disease, 4th ed. Lea & Febiger 1975).
Maybe the reason it didn’t lodge as firmly in my memory as other human tapeworms is that it rarely causes any disease or discomfort. Most infected people don’t feel any adverse effects or even suspect they have worms. Although size has nothing to do with my memory of any parasite, this is a remarkably small species as tapeworms go—usually 25–40 mm long and only 1 mm wide. Unlike any other tapeworm, though, H. nana is capable of autoinfection—it can complete its entire life cycle in a human host without the necessity of an intermediate host, and it can thus build up to very high numbers in a person. Its its larval stages (cysticercoids) can reside in the intestinal wall, and in very rare cases, in other organs such as lungs, liver, and lymph nodes.
Adult Hymenolepis nana. For scale, these are only about 1 mm wide at the widest point. (Source: Wikipedia
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H. nana life cycle.
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Now as to your cancer question, I found a remarkable 2015 case study of infection in a 41-year-old man in Medellin, Colombia.
A & B: Tumors composed of H. nana tissue in the patient’s lungs (X-ray and CT scan). C: Solid masses of malignant H. nana cells in lymph node biopsy specimens. D: Malignant H. nana cells in a patient’s lymph node.
Over the next 4 months, the patient remained noncompliant with recommended treatments for both HIV and parasitic worms; he went into renal failure and refused dialysis; and he died, but granted permission for the use of his body and publication of findings.
PCR studies (DNA amplification) of atypical cell masses (tumors) from his lungs and lymph nodes, and assessment of specimens sent to the U.S. Centers for Disease Control,
Invasion of human tissue by abnormal, proliferating, genetically altered tapeworm cells is a novel disease mechanism that links infection and cancer.
They speculate that this patient’s severely compromised immune system, due to his poorly managed HIV, allowed some of the tapeworm’s stem cells to turn malignant and produce these tumors. In other words, the cancerous tumors were tapeworm cells, not human cells. This report is the first time tapeworms have ever been reported turning malignant in a human host, the first time any tapeworm has been known to have cancer, and first report of human disease caused by a parasite’s cancer.
In my literature search (admittedly not exhaustive), I found no cases of any tapeworm species causing human cancer. With my thanks to Gwendolyn Dennis, I’ve added a link (4) to the story of this case from the CDC’s viewpoint. It’s interesting to see it from their side too. Gwendolyn quotes it extensively in her comment below, so scrolling down is an easier way to see it than opening another tab.
Footnotes
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