Hymenolepis nana

Hymenolepis nana is one of the most common tapeworms of rodents, and the most common one in humans. Up to 75 million people are estimated to be carriers, and the prevalence among children is as high as 25% in some areas.

This species is able to modulate the human immune system.

… it is tempting to therapeutically administer further species like H nana, which can generally be left untreated due to their asymptomatic persistence in the host while they are invasive enough to modulate the microenvironment within the intestinal mucosa and have an effect on IBDs. Thus, we suggest that this predominantly indolent human-specific macroparasite may be a candidate to be administered in selected, immunocompetent patients suffering from IBD, even though there are a few reports about severe complications of H. nana infections in HIV patients and malnourished children. [1]

Notwithstanding the potential for immunomodulation and the typically asymptomatic course of infection by H. nana, this species is not suitable for use in therapy because, uniquely among tapeworms, it can complete its life cycle in the human small intestine, without the need for an intermediate host. Such autoinfection can persist for years and lead to a high parasite load, especially in immunocompromised hosts.

There is also one case in which a man died when the H. nana he was hosting developed cancer.

Malignant Transformation of Hymenolepis nana in a Human Host -- Full text | PDF

For more discussion about this case, see this support group thread.

Hymenolepis diminuta is a much safer rodent tapeworm that is widely used in helminthic therapy. In this case, it is the larval form of the helminth - the cysticercoids - that are self-administered.