Hymenolepis microstoma
Encyclopedia
Hymenolepis microstoma, also known as the rodent tapeworm, is an intestinal dwelling parasite. Adult worms live in the bile duct
and small intestine
s of mice and rats, and larvae metamorphose in the haemocoel of beetles. It belongs to the genus Hymenolepis
; tapeworms that cause hymenolepiasis
. H. microstoma is prevalent in rodents worldwide, but rarely infects humans.
. Adults live in the bile duct and small intestine of rodents such as mice (Mus musculus), and larvae infect grain beetles such as Tribolium spp., in which they metamorphose from larvae into juvenile worms. Worms vary from 4 to 30cm in length, depending on the age and number of worms within the host. Adults have completely lost their mouth and intestine. Instead they use their skin (tegument
) to absorb nutrients directly from the host gut.
Hymenolepis species and other tapeworms often exhibit a 'crowding effect' in which the total biomass of the worms stays more or less constant, regardless of the intensity of infection. Thus low intensity infections result in larger worms and high intensity infections produce smaller worms. Under laboratory conditions, H. microstoma adult infections in mice are typically limited to approximately 12 worms.
larvae in approximately 7-10 days. The larvae can remain in the cysticercoid stage in the beetle as long as the lifespan of the adult beetle (up to 3 years), although any age-related decrease in viability has not been studied. When ingested, the eggs develop into cysticercoids. Rodents can become infected when they eat arthropods, such as flour beetles (Tribolium ssp). Humans, especially children, can ingest the arthropods as well and therefore become infected via the same mechanism. Rodents, especially rats, are definitive hosts and natural reservoirs of H. microstoma. As the definitive host (rats) eats an infected arthropod, cysticercoids present in the body cavity transform into the adult worm. Juvenile worms establish in the bile duct of mice after approximately 3 days movement within the upper gastrointestinal tract
. Once established in the bile duct, the worms then mature sexually and begin producing eggs within approximately 1 week.
Eggs are released with mouse faeces and thus dispersal is passive - through the movement and defecation of mice. Adult worm infections in mice held under laboratory conditions persist for 6-12 months.
Worms reproduce sexually via the cross fertilization of segments, each of which contains a complete complement of male and female reproductive organs (hermaphroditic). Shelled embryos develop in the ovaries through spiral cleavage, to become infective larvae with 3 pairs of hooks. When ingested by beetles, these larvae use their hooks and secretory glands to penetrate the gut of the beetle and enter the haemocoel where they undergo complete metamorphosis into cysticercoid larvae, replete with an adult scolex, ready for establishment in the final host.
(segment formation).
Neoblasts divide in the neck region and become incorporated into new segments where they eventually differentiate into the reproductive organs and other elements of the body.
The diploid chromosome number of H. microstoma is 12 and the total genome size
has been estimated by the Sanger Institute
to be 1.4 megabases (with GC-content
of ~35%). These values are similar to the genomes of the fox tapeworm Echinococcus multilocularis
and the pig tapeworm Taenia solium. All three species belonging to the tapeworm order Cyclophyllidea
. Genome sizes outside of this order are presently unknown.
and monogeneans, evolved from a single major lineage of free-living flatworm ancestors. The switch from a free-living to a parasitic lifestyle in the common ancestor of the parasitic flatworms involved a fundamental change in their tegument, which is found in all contemporary groups.
Early-branching tapeworm groups are found in bony (e.g. teleost) and cartilaginous fishes (e.g. shark
s and rays) and have entirely aquatic life cycles involving arthropod (e.g. copepod
s) first intermediate hosts and vertebrate (fish) final hosts. Tetrapod
hosts (including mice and humans) were acquired later in tapeworm evolution and eventually part-aquatic life cycles led to the evolution of fully terrestrial
life cycles, albeit still involving an arthropod intermediate host and a vertebrate definitive host.
Hymenolepis microstoma is a member of the Cyclophyllidea, one of the youngest and most species-rich group of tapeworms.
, H. microstoma, H. nana
) have been maintained as laboratory models for studying tapeworm biology since the 1950s. They can be readily maintained in vivo
in rodent and beetle hosts, which makes them useful for teaching and research purposes. They can also be grown in culture (in vitro
), giving easy manipulation of the life cycle..
are among the existing symptoms in a few of the reported cases. Since data regarding praziquantel
treatment of H. microstoma is sparse, scientists have recommended that every case and treatment of H. microstoma be reported for development of protocols and parasitological purposes.
Bile duct
A bile duct is any of a number of long tube-like structures that carry bile.Bile, required for the digestion of food, is excreted by the liver into passages that carry bile toward the hepatic duct, which joins with the cystic duct to form the common bile duct, which opens into the intestine.The...
and small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
s of mice and rats, and larvae metamorphose in the haemocoel of beetles. It belongs to the genus Hymenolepis
Hymenolepis
Hymenolepis is a genus of cyclophyllid tapeworms responsible for hymenolepiasis.Species include:*Hymenolepis nana*Hymenolepis diminuta*Hymenolepis microstoma- History :...
; tapeworms that cause hymenolepiasis
Hymenolepiasis
Hymenolepiasis is infestation by one of two species of tapeworm:* Hymenolepis nana* Hymenolepis diminutaAlternative names are:* Dwarf tapeworm infection* Rat tapeworm-Causes, incidence, and risk factors:...
. H. microstoma is prevalent in rodents worldwide, but rarely infects humans.
Ecology
Hymenolepis microstoma is an obligate parasiteObligate parasite
An obligate parasite is a parasitic organism that cannot complete its life cycle without dependence on its host.-See also:*Obligate intracellular parasite*Parasitism*Parasitic plant*Facultative parasite...
. Adults live in the bile duct and small intestine of rodents such as mice (Mus musculus), and larvae infect grain beetles such as Tribolium spp., in which they metamorphose from larvae into juvenile worms. Worms vary from 4 to 30cm in length, depending on the age and number of worms within the host. Adults have completely lost their mouth and intestine. Instead they use their skin (tegument
Tegument
Tegument may refer to:* Integumentary system* Tegument * Viral tegument...
) to absorb nutrients directly from the host gut.
Hymenolepis species and other tapeworms often exhibit a 'crowding effect' in which the total biomass of the worms stays more or less constant, regardless of the intensity of infection. Thus low intensity infections result in larger worms and high intensity infections produce smaller worms. Under laboratory conditions, H. microstoma adult infections in mice are typically limited to approximately 12 worms.
Life cycle
The cycle begins as arthropods become intermediate hosts by ingesting the parasite eggs. Oncospheral larvae are released from the eggs and use hooks and secreted enzymes to penetrate the gut of the beetles and enter the haemocoel. In the haemocoel the larvae undergo complete cellular reorganization (i.e. metamorphosis), transforming into cysticercoidCysticercoid
A cysticercoid is the larval stage of certain tapeworms, similar in appearance to a cysticercus, but having the scolex filling completely the enclosing cyst. In tapeworm infestations, cysticercoids can be seen in free form as well as enclosed by cysts in biological tissues such as the intestinal...
larvae in approximately 7-10 days. The larvae can remain in the cysticercoid stage in the beetle as long as the lifespan of the adult beetle (up to 3 years), although any age-related decrease in viability has not been studied. When ingested, the eggs develop into cysticercoids. Rodents can become infected when they eat arthropods, such as flour beetles (Tribolium ssp). Humans, especially children, can ingest the arthropods as well and therefore become infected via the same mechanism. Rodents, especially rats, are definitive hosts and natural reservoirs of H. microstoma. As the definitive host (rats) eats an infected arthropod, cysticercoids present in the body cavity transform into the adult worm. Juvenile worms establish in the bile duct of mice after approximately 3 days movement within the upper gastrointestinal tract
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
. Once established in the bile duct, the worms then mature sexually and begin producing eggs within approximately 1 week.
Eggs are released with mouse faeces and thus dispersal is passive - through the movement and defecation of mice. Adult worm infections in mice held under laboratory conditions persist for 6-12 months.
Worms reproduce sexually via the cross fertilization of segments, each of which contains a complete complement of male and female reproductive organs (hermaphroditic). Shelled embryos develop in the ovaries through spiral cleavage, to become infective larvae with 3 pairs of hooks. When ingested by beetles, these larvae use their hooks and secretory glands to penetrate the gut of the beetle and enter the haemocoel where they undergo complete metamorphosis into cysticercoid larvae, replete with an adult scolex, ready for establishment in the final host.
Cytology
Like all flatworms (phylum Platyhelminthes), H. microstoma maintains totipotent stem cells (called neoblasts in flatworms) throughout its life cycle. These are located in the neck region of the adult worms and are responsible for the continual production of new organs during the process of strobilationStrobilation
Strobilation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths...
(segment formation).
Neoblasts divide in the neck region and become incorporated into new segments where they eventually differentiate into the reproductive organs and other elements of the body.
The diploid chromosome number of H. microstoma is 12 and the total genome size
Genome size
Genome size is the total amount of DNA contained within one copy of a single genome. It is typically measured in terms of mass in picograms or less frequently in Daltons or as the total number of nucleotide base pairs typically in megabases . One picogram equals 978 megabases...
has been estimated by the Sanger Institute
Sanger Institute
The Wellcome Trust Sanger Institute is a non-profit, British genomics and genetics research institute, primarily funded by the Wellcome Trust....
to be 1.4 megabases (with GC-content
GC-content
In molecular biology and genetics, GC-content is the percentage of nitrogenous bases on a DNA molecule that are either guanine or cytosine . This may refer to a specific fragment of DNA or RNA, or that of the whole genome...
of ~35%). These values are similar to the genomes of the fox tapeworm Echinococcus multilocularis
Echinococcus multilocularis
Echinococcus multilocularis is a cyclophyllid tapeworm that, along with some other members of the Echinococcus genus , produces the disease known as echinococcosis in certain terrestrial mammals, including wolves, foxes, jackals, coyotes, domestic dogs and humans. Unlike E. granulosus, E...
and the pig tapeworm Taenia solium. All three species belonging to the tapeworm order Cyclophyllidea
Cyclophyllidea
Tapeworms of the order Cyclophyllidea are the most important cestode parasites of humans and domesticated animals. All have multiple proglottid "segments," and all have four suckers on their scolex , though some may have other structures as well...
. Genome sizes outside of this order are presently unknown.
Evolution
The parasitic flatworms, which includes tapeworms, flukesTrematoda
Trematoda is a class within the phylum Platyhelminthes that contains two groups of parasitic flatworms, commonly referred to as "flukes".-Taxonomy and biodiversity:...
and monogeneans, evolved from a single major lineage of free-living flatworm ancestors. The switch from a free-living to a parasitic lifestyle in the common ancestor of the parasitic flatworms involved a fundamental change in their tegument, which is found in all contemporary groups.
Early-branching tapeworm groups are found in bony (e.g. teleost) and cartilaginous fishes (e.g. shark
Shark
Sharks are a type of fish with a full cartilaginous skeleton and a highly streamlined body. The earliest known sharks date from more than 420 million years ago....
s and rays) and have entirely aquatic life cycles involving arthropod (e.g. copepod
Copepod
Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Some species are planktonic , some are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s) first intermediate hosts and vertebrate (fish) final hosts. Tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
hosts (including mice and humans) were acquired later in tapeworm evolution and eventually part-aquatic life cycles led to the evolution of fully terrestrial
Terrestrial
Terrestrial refers to things related to land or the planet Earth.Terrestrial may also refer to:* Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on or near the ground, as opposed to in trees etc.** A fishing fly that simulates the...
life cycles, albeit still involving an arthropod intermediate host and a vertebrate definitive host.
Hymenolepis microstoma is a member of the Cyclophyllidea, one of the youngest and most species-rich group of tapeworms.
Research impact
Most of our understanding of the basic biology of tapeworms, such as their anatomy, physiology and ultrastructure, stems from work on this genus. Species in the genus Hymenolepis (e.g. H. diminutaHymenolepis diminuta
Hymenolepis diminuta, also known as rat tapeworm, is a species of Hymenolepis tapeworm that causes hymenolepiasis. It has slightly bigger eggs and proglottids than H. nana and infects mammals using insects as intermediate hosts...
, H. microstoma, H. nana
Hymenolepis nana
Dwarf tapeworm is a cosmopolitan species that is one of the most common cestodes of humans in the world, especially among children. It can be found throughout the world, but is usually most common in temperate zones...
) have been maintained as laboratory models for studying tapeworm biology since the 1950s. They can be readily maintained in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
in rodent and beetle hosts, which makes them useful for teaching and research purposes. They can also be grown in culture (in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
), giving easy manipulation of the life cycle..
Infection and treatment
H. microstoma primarily infects rodents, and is only very rarely found in humans. Human H. microstoma infection is often asymptomatic, but abdominal pain, irritability, itching, and eosinophiliaEosinophilia
Eosinophilia is a condition in which the eosinophil count in the peripheral blood exceeds 0.45×109/L . A marked increase in non-blood tissue eosinophil count noticed upon histopathologic examination is diagnostic for tissue eosinophilia. Several causes are known, with the most common being...
are among the existing symptoms in a few of the reported cases. Since data regarding praziquantel
Praziquantel
Praziquantel is an anthelmintic effective against flatworms. Praziquantel is not licensed for use in humans in the UK; it is, however, available as a veterinary anthelmintic, and is available for use in humans on a named-patient basis....
treatment of H. microstoma is sparse, scientists have recommended that every case and treatment of H. microstoma be reported for development of protocols and parasitological purposes.