Do Parasites Cause Cancer?

Do parasites cause cancer? It’s a hot debate, and the answer is simply not that simple.

There are confounding and confusing factors.

Some parasites do cause cancer. Dr. Paul Marik explains that certain parasites do indeed cause specific cancers. A small number of helminths are firmly-established human carcinogens. The clearest examples are below. These are real, well-defined parasite–cancer associations, but they are geographically specific, biologically specific, and pathologically specific.  This is a very different claim from saying that parasites are the universal root cause of most or all cancers.

• Schistosoma haematobium → squamous cell carcinoma of the bladder, especially in endemic regions with urinary schistosomiasis

• Opisthorchis viverrini → cholangiocarcinoma in Southeast Asia

• Clonorchis sinensis → cholangiocarcinoma in East Asia

Dr. Lee Merritt has discussed cases cited in Japanese literature, like the 1999 Japanese case report of a 66-year-old woman with sigmoid colon adenocarcinoma and prior exposure to Japanese schistosomiasis. Pathologic examination found “innumerable eggs of Schistosoma japonicum” in her colon, including in the tumor tissue and lymph nodes. The authors specifically discussed a possible relationship between S. japonicum eggs and development of colon cancer. Japanese researchers also reported a 2001 case of a 68-year-old man with rectal cancer and previous S. japonicum infection. Tissue pathology showed many calcified schistosome eggs in the tissue adjacent to the tumor.

Even though eggs were found in the tumor, these cases do not show that the cancerous tumor itself was a parasite egg sac. That may be where some of the confusion comes from.

Dr. Merritt has also recounted the story of Danish pathologist and 1926 Nobel Prize winner Johannes Fibiger. He examined stomach tumors in rats, thought they resembled parasite “egg sacs,” opened them and found nematodes. Fibiger originally interpreted his experiments as demonstrating parasite-induced cancer. His work won the Nobel Prize, but subsequent research showed the lesions were not evidence that cancers generally consist of parasites.

Chronic parasitic infection and the resulting inflammation have long been investigated as possible contributors to carcinogenesis. Dr. Bryan Ardis has said that 70% of autoimmune cases he saw in his own practice were linked to parasites, and that half of the tumors and polyps he encountered were not cancer. He has talked about rheumatoid arthritis, lupus, fibromyalgia, Sjogren’s, and even cancer. He has stated that medicine may be missing an underlying cause.

Ivermectin and Parasites

Part of the confusion arises because some antiparasitic drugs — including ivermectin, mebendazole, fenbendazole, and niclosamide — have also shown anticancer effects. This does not mean that their anticancer activity translates to killing hidden parasites. The pathways through which these drugs affect parasites are entirely different from the pathways through which they may affect cancer cells.