H. diminuta - The Rat Tapeworm

   This tapeworm is much longer than H. nana, growing up to 60cm or more in length. This is primarily a parasite of the rat, humans only being infected by accidental ingestion of the insect intermediate host. This species is of more importance as a research model for the study of the biochemistry, physiology, chemotherapy and immunology of tapeworm infections. In addition there are a number of species found in animals, including;

H. carioca - A common non-pathogenic parasite of fowl in the USA.
H. microstoma - A parasite of rodents.
H. lanceolata - A pathogenic parasite of ducks, geese and other anseriform
domestic fowl.
H. coronula- A parasite of anseriform domestic fowl.
H. cantaniana - A parasite of chickens and other galliform domestic fowl.

Morphology

   Apart from their relative sizes, these two parasites of man are very similar, H. nana being up to 4cm in size, the strobila consisting of up to 200 proglottids, whilst H. diminuta grows up to 60cm or more in length and the strobila consists of up to 1000 proglottids. These proglottids are trapezoidal in shape, and are approximately four times as wide as they are long. Each proglottid contains three
round testes, a bi-lobed ovary, a compact vitelline gland and a large uterus opening to a lateral genital pore (as does the cirrus).

   The scolex in both parasites have four suckers and a retractile rostellum which in H. nana is equipped with 20 - 30 hooks (the rostellum is unarmed in H. diminuta). Finally the eggs of the two species both have the characteristic thickened walls of all cestode eggs, but may
easily be differentiated. Those of the yellowish brown H. diminuta eggs are much rounder than colourless H. nana eggs and are larger with 60 - 80µm in diameter. In H. nana the eggs are oval in shape, measuring ~ 40 by 50µm and contains an oncosphere equipped with 3 pairs of embryonic hooks (i.e. a "hexacanth" larvae) and long wavy filaments (absent in H. diminuta ) which lie in the space between the larvae and the egg shell wall. The two species infecting man have rather different lifecycles which will be considered separately here.

H. nana

    This parasite has rats and mice as well as man as the definitive host, and differs from H. diminuta and almost all other tapeworm in that an intermediate host is not required, although fleas and beetles may be used. The embryonated eggs are passed in the faeces where they contaminate soil. If the eggs are ingested by the definitive host the oncosphere is activated and breaks out of the egg and penetrates the gut villus. Here it develops as a cysticoid larvae in about 4 days before rupturing into the gut lumen. Once ruptured, the scolex attaches to the gut mucosa and the parasite develops into the adult tapeworm after about 15 to 20 days. If the insect intermediate hosts are utilised the lifecycle is similar to that of H. diminuta below. In heavy infections eggs liberated by adult worms in the intestine may hatch here rather than passing out of the body, to give autoinfection

H. diminuta

   This parasite as in most tapeworms does require an intermediate host. Embryonated eggs pass out of the body of the definitive host in the faeces and are ingested by the insect intermediate hosts. Many insects may act as intermediate hosts for this parasite, the most common being fleas and beetles such as the flour beetle. When ingested by the intermediate host the oncosphere larvae become activated, break out of the egg shell and penetrate into the insect’s body cavity where they develop into a cysticercoid larvae. For completion of the lifecycle, the infected intermediate host must be eaten by the definitive host. On ingestion, the cysticercoid larva becomes activated, the scolex becomes attached to the gut mucosal wall, and the parasite develops into the adult tapeworm.

An interesting feature of Hymenolepis tapeworms is that they undergo a diurnal migration within the gut, which is associated with the feeding patterns of the host. From about 4pm to 4am few parasites are seen in the lower part of the small intestine, whilst from about 4am to 4pm many parasites are seen in the upper part of the small intestine. This was first observed in H. diminuta and subsequently in other species, and is indicative of a nocturnal feeding pattern by the parasite.

Pathology of Infection.

 
  These parasites are not very pathogenic, usually with asymptomatic infections. In man infected with H. nana there may be a slight irritation of the gut mucosa and slight abdominal pain, and with very heavy infections (>2000 worms) there may also be some diarrhoea. In the bird species there may be enteritis and intestinal obstruction with some species.