Hymenolepis nana is in the phylum Platyhelminthes (flatworms), the class Cestoda (tapeworms), and the order cyclophilidea. This tapeworm is found in rats, mice, and humans and is the most common tapeworm of humans world-wide with up to 2% of children in southern US infected. Adults are in the intestine of vertebrate host, but larvae can infect insects or the intestinal villi of the vertebrate.
Hymenolepis nana exists as an embryonated egg (embryo present in egg) in the external environment. When this egg is ingested, the oncosphere (first larval stage: embryo with hooks) hatches and a cysticercoid (second larval stage; looks like a banjo or a ghost) develops in villus. The cysticercoid then emerges from villus, and the scolex attaches to the intestine. The adult exists in the small intestine and sheds gravid proglottids containing eggs. These embryonated eggs are then exposed to the external environment through the feces of the host.
This tapeworm is not a huge problem for human adults, but more of a problem for children. The tapeworm robs the children of nutrients that they need to grow and develop. The tapeworm larva burrow into the intestine and can cause malnutrition and severe damage. Hymenolepis nana only causes death in severe cases. Symptoms usually occur due to toxaemia or the immuno response to the waste products of the worm. Infections that have 2,000+ worms can cause inflammation of the small intestine, abdominal pain, diarrhea, loss of appetite, restlessness, irritability, and anal and nasal pruritus (unpleasant sensations accompanied by scratching).