Hymenolepis nana

Hymenolepis nana can be found throughout the world, but is usually most common in temperate zones. It is the most common cestode of humans and is found wherever there are human inhabitants. (Roberts and Janovy Jr., 2000)

Hymenolepis nana can be found wherever humans and rodents live. They have been found in almost all types of terrestrial biomes. (Roberts and Janovy Jr., 2000)

The adults of the dwarf tapeworm are 25 to 40 mm in length and 1 mm in width (Lapage, 1951). This tapeworm is transparent, and has a long slender neck with segments wider than they are long. The genital pores are unilateral, or on the side of the segment. Each segment contains a single proglottid, which contains a single set of reproductive organs. On the scolex, a retractable rostellum with 20 to 30 hooks can be found (Roberts and Janovy, 2000). The scolex also has four suckers, or a tetrad.

The cysticercoid has a tail, which is made of longitudinal fibers and is spade shaped with the rest of the worm still inside the cyst (Roberts and Janovy, 2000). The eggs of H. nana are round or slightly oval at about 40-60 micrometers X 30-50 micrometers with 4-8 polar filaments spread out between the inner and outer membranes (Ghaffar, 2001). Unlike other taeniid eggs, the eggs of H. nana do not have a striated appearance (Roberts and Janovy, 2000). (Ghaffar, 2001; Lapage, 1951; Roberts and Janovy Jr., 2000)

A gravid proglottid contains fertilized eggs, which are sometimes expelled with the feces (Cameron, 1956). However, most of the time, the egg settles in the microvilli of the small intestine, hatch, and the larvae can develop to sexual maturity without ever leaving the host (Olsen, 1974). An intermediate host is optional; H. nana can go through its life cycle with only one host or can also go through the normal two-host cycle (Roberts and Janovy, 2000). The life cycle can be described as: 1)eggs are ingested by definitive hosts 2)eggs hatch in the duodenum, releasing oncospheres and lie in the lymph channels of the villi 3)oncospheres develops into a cysticercoid, which has a tail and a well formed scolex, and it attaches to the small intestine and matures into an adult 4a)gravid proglottids then release and pass out through feces along with eggs 4b)or eggs can hatch and infect original host and start cycle over or 5)eggs can be ingested by insects or rodents (Roberts and Janovy, 2000). (Cameron, 1956; Olsen, 1974; Roberts and Janovy Jr., 2000)

Hymenolepis nana, like all tapeworms, contain both male and female reproductive structures in each proglottid (Roberts and Janovy, 2000). This means that the dwarf tapeworm like other tapeworms is hermaphroditic (Cameron, 1956). Each segment contains 3 testes and a single ovary. When a proglottid becomes old and unable to absorb any more nutrition, it is released and is passed through the host's digestive tract (Roberts and Janovy, 2000). This gravid proglottid contains the fertilized eggs, which are sometimes expelled with the feces (Cameron, 1956). However, most of the time, the egg may also settle in the microvilli of the small intestine, hatch, and the larvae can develop to sexual maturity without ever leaving the host (Olsen, 1974). (Cameron, 1956; Olsen, 1974; Roberts and Janovy Jr., 2000)

H. nana does not have a digestive system and each body segment has its own reproductive structures (Roberts and Janovy, 2000). Usually after it is ingested as an egg, it will lodge itself in the intestinal wall and then in five to six days, the cysticercoid emerges into the lumen of the small intestine, sheds it tail and becomes a mature tapeworm (Roberts and Janovy, 2000). (Roberts and Janovy Jr., 2000)

Cestodes in general have sensory organs in the scolex, which are attached to longitudinal nerves extending down the body. The nerves are attached to organs and the cestodes can detect tactile stimulation. (Brusca and Brusca, 2003)

The dwarf tapeworm like all other tapeworms lacks a digestive system and feeds by absorption on nutrients in the intestinal lumen (Cameron, 1956). They have non-specific carbohydrate requirements and it seems like they will absorb whatever is being passed through the intestine at that time (Cameron, 1956). When it becomes an adult, it will attach to the intestinal walls with its suckers and toothed rostellum and have its segments reaching out into the intestinal space to absorb food (Roberts and Janovy, 2000). (Cameron, 1956; Roberts and Janovy Jr., 2000)

These animals are probably not preyed on directly but are ingested. Egg and larval mortality are high due to not reaching a suitable host.

An intermediate host is optional; H. nana can go through its life cycle with only one host or can also go through the normal two-host cycle (Roberts and Janovy, 2000). Some of the hosts that this tapeworm can be found in are humans, rodents and insects (Roberts and Janovy, 2000). The insect hosts of H. nana could be fleas, flour beetles, and other copraphagous (dung eating) insects. (Olsen, 1974; Roberts and Janovy Jr., 2000)

Hymenolepis nana is the most common cestode parasite of humans in the world (Roberts and Janovy, 2000). It lodges itself in the intestines and absorbs nutrients from the intestinal lumen (Cameron, 1956). In human adults, the tapeworm is more of a nuisance than a health problem, but in small children, many H. nana can be dangerous. Usually it is the larva of this tapeworm that causes the most problem in children (Lapage, 1951). The larva will burrow into the walls of the intestine, if there are enough tapeworms in the child, severe damage can be inflicted. This is done by absorbing all the nutrition from the food the child eats (Lapage, 1951). Usually a single tapeworm will not cause any danger, but in small children, many tapeworms can become a problem (Lapage, 1951). Hymenolepis nana usually will not cause deaths unless in extreme circumstances and usually in young children or in people who have weakened immune systems. In some parts of the world, individuals that are heavily infected are a result of internal autoinfection (Olsen, 1974). (Cameron, 1956; Lapage, 1951; Olsen, 1974; Roberts and Janovy Jr., 2000)