Hymenolepis diminuta

Hymenolepis diminuta lives in all temperate zones throughout the world. (Andreassen, et al., 1999; Arai, 1980)

The adult worm lives in the intestine of its host, normally rats but also sometimes dogs and humans. Hymenolepis diminuta passes through the required intermediate arthropod host as a juvenile. Only when the intermediate host is injested by the definitive host will H. diminuta mature. The intermediate arthropod host is normally a grain beetle, and injestion of the intermediate host into the definative host normally occurs in piles of grain, where both rats and beetles live. (Arai, 1980; Pappas, SEP 2000; Shostak and Smyth, AUG 1998; Sturdevant, 1907; Willis and Poulin, 1999)

Adult Hymenolepis diminuta reach 20 to 60 cm, and up to 90 cm . The cestode has a long cylindrical body with 4 suckers and an apical organ at its scolex with no rostellar hooks. Hymenolepis diminuta, along with all cestodes, lacks any trace of a digestive tract, and it absorbs all required substances through its external covering. Posteriorly directed microtriches cover the cestode's tegument, which add to the surface area of the animal and thus to the amount of nutrients it can absorb.

Hymenolepis diminuta, like all other cestodes, has three body sections, a scolex (head), neck, and a strobilus, which is the rest of the cestodes body. The strobilus is divided into many sections called proglottids, each with male and female sexual organs. These are the defining characteristics of cestodes. (Arai, 1980; Deines, et al., JUL 1999; Pappas, SEP 2000; Roberts and Janovy, 2000)

The life cycle of H. diminuta involves rodents (rats primarily) as the definitive host and beetles (flour and grain beetles, Tribolium spp. and Tenebrio spp., respectively) as the intermediate host. The tapeworm's eggs are passed in the rat's feces, and beetles are infected when they eat the eggs; the metacestode stage in the beetle is called a cysticercoid. The rat is infected when it eats an infected beetle. Once the egg is digested by the intermediate host arthropod, the oncosphere, a fully developed larva contained in the egg, is released into the intermediate host. Its function is to migrate to the haemocoel (body cavity) of the insect where it will grow and differentiate into an encapsulated juvenile worm. This stage, the cysticercoid, is dormant and resides in the insect until both are eaten by a foraging rat. Within the definitive hosts' stomach and intestine, the larval worm responds to chemical signals and excysts. It then invades the intestinal lumen, evaginates its scolex, attaches to the intestinal mucosa and develops into an adult.

Once the adult H. diminuta is embedded in the host, it can produce over 250,000 eggs per day. Thus, over a period of slightly over a year, a single tapeworm could produce a hundred million eggs and if all these eggs reached maturity, it would be equal to 20 tons of tapeworm tissue. There is an extremely low chance for each egg to reach reproductive maturity and that is why H. diminuta lays so many eggs. (Andreassen, et al., 1999; Arai, 1980; Ohio State University, 2001; Pappas, et al., 1999; University of Aberdeen, 1997)

Hymenolepis diminuta has both male and female reproductive organs in the same individual. Each segment has one complete set of male and female sex organs. As the segments move toward the posterior end of the strobilus, first the male organs mature, and produce sperm that are stored until the maturation of the ovary. Once the adult H. diminuta is embedded in the host, it can produce over 250,000 eggs per day. Thus, over a period of slightly over a year, a single tapeworm could produce a hundred million eggs and if all these eggs reached maturity, it would be equal to 20 tons of tapeworm tissue. There is an extremely low chance for each egg to reach reproductive maturity and that is why H. diminuta lays so many eggs.

Recent studies have been done on the temperature tolerance of Hymenolepis diminuta eggs. The tapeworm's eggs survived at higher and lower temperatures and for longer periods of time than did adult beetles, indicating that the thermal tolerance of the eggs does not limit the parasite's distribution. (Andreassen, et al., 1999; Arai, 1980; Ohio State University, 2001; Pappas and Barley, APR 1999)

A behavior modification of H. diminuta is the cestode's movement. The cestode has two types of muscle portions in the strobilus, the contractile myofibril and the noncontractile myocyton. The myocytons contain a nucleus, rough endoplasmic reticulum, free ribosomes, a vesicular Golgi apparatus, few mitochondria and abundant glycogen. Lipid is stored in them as well. The myofibril contain actin and myosin fibrils which do the contractions and make movement of H. diminuta possible. The contractile portions of the muscle cells are arranged in discrete bundles and propagation of contractions down the body make movement possible. The internal musculature of the scolex is complex, making the scolex extraordinarily mobile. The scolex has three distinct muscle types. The peripheral myofibers similar to those previously described, tentacle retractor muscles, and tentacle bulb muscles. The bulb muscles are obliquely striated and have numerous motor end plates. Thus the muscular system of H. diminuta is complex. (Andreassen, et al., 1999; Arai, 1980; Pappas, SEP 2000; Roberts and Janovy, 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)

Hymenolepis diminuta has no digestive tract, all nutrients needed must be absorbed by the tegument, which is the external covering of the cestode. The cestode is covered in tiny posteriorly directed microtriches which increase the absorbtive area of the tegument. The glycocalyx found on the surface membrane of the microtiches is a layer of carbohydrate-containing macromolecules. Interaction between the glycocalyx and certain molecules has been reported to enhance amylase activity in H. diminuta, inhibit the host trypsin, chymotrypsin, and pancreatic lipase and increase the absorption of cations and adsorbtion of bile salts. (Arai, 1980; Roberts and Janovy, 2000)

There is an extremely low chance for each egg to reach reproductive maturity and that is why H. diminuta lays so many eggs.

The adult worm lives in the intestine of its host, normally rats but also sometimes dogs and humans. Hymenolepis diminuta passes through the required intermediate arthropod host as a juvenile. Only when the intermediate host is injested by the definitive host will H. diminuta mature. The intermediate arthropod host is normally a grain beetle, and injestion of the intermediate host into the definative host normally occurs in piles of grain, where both rats and beetles live.

Hymenolepis diminuta is a cestode which sometimes causes infection in humans. Human infection results from eating such foods as dried fruits and precooked breakfast cereals in which the infected grain insects, themselves infected from eating rat or mouse droppings, are present. Some symptoms of infestation in humans include, enteritis, anorexia, headaches, anal pruritus, abdominal distress and small gut irritation. Hymenolepiasis is the term for a human to be infected with either H. diminuta or Hymenolepis nana, a dwarf sister species very closely related to H. diminuta. (Andreassen, et al., 1999; Arai, 1980; Roberts and Janovy, 2000)

Female rats parasitized with Hymenolepsis diminuta were quicker to retreive their young than non-infected rats. The rat's fitness, or contribution to the future gene pool is increased by investing more time in its present offspring since it's future survival is not known. (Willis and Poulin, 1999)