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Taenia solium

By Ashley Chung

Geographic Range

Taenia solium, known as the pork tapeworm, is found throughout the world. It is particularly prevalent in developing countries where pigs are raised in poor sanitary conditions. In the Western Hemisphere, it is mostly found in South and Central America. Canada, United States, Argentina and Uruguay are four countries from this region that seems to have eradicated the tapeworm, although cases of Taenia solium infection in people have been appearing recently. This reappearance has been attributed to the growing number of immigrants from countries with tapeworm transmission, who are hosts to the T. solium. The tapeworm is also widely found throughout countries in Africa and Asia; however, the number of infections is low in populations of Muslim and Jewish cultures where eating pork is forbidden. (Schantz, 2002)

Habitat

The pork tapeworm has several different habitats depending on the stage in its life cycle. The preadult tapeworm and adult tapeworm can be found in the small intestine of a human host. The proglottid segments full of eggs are found in the host feces, and in the external environment where the feces are released. Unfortunately, there is an inadequate amount of research that has been conducted on the subject of eggs in the external environment. As a result it is difficult to assess the type of habitat the eggs favor. However, temperature is known to effect egg survival. If the habitat is colder than 10 degrees Celsius or above room temperature, the eggs can easily perish.

The next stage of the tapeworm is the oncosphere, and this stage takes place inside the pig intermediate host. The habitat of the oncosphere is the gut and tissue of the pig host and continues its life stage in the muscle and brain of the pig in the cysticercus stage. The cysticercus form is also capable of surviving in a human host, living in the muscles and the brain. (Pawlowski, 2002; Sciutto, et al., 2000)

Physical Description

The morphology of the adult pork tapeworm is divided into three parts: scolex, neck, and strobila. The scolex is the head of the tapeworm, positioned at the anterior end of the organism. The scolex acts as an attachment device with four suckers, hooks and rostellum used to attach itself to the intestine of the host. The neck is an elongated region between the scolex and the stroblia. The stroblia contains the bulk of the systems of the tapeworm and has an average length of 2-3 m. It consists of multiple segments called proglottids. As a monoecious species, every single proglottid contains both the male and female reproductive systems. The proglottids mature sexually as they progress in the posterior direction of the strobila. The pork tapeworm does not have a digestive system, but is composed of the following systems: tegument, nervous, osmoregulatory and muscular.

Taenia solium egg's have a fragile outer shell that can be shed when the egg exits the host's body, leaving the oncosphere larva exposed to the external environment. The oncosphere larva is 30 um in diameter and is also called the hexacanth larva since it has six hooks. The larva is a solid mass of cells and surrounded by a protective cover called the embryophore. This cover protects the oncosphere from harsh conditions when the larva is exposed to the environment. The oncosphere develops into the cysticercus form, converting from a solid larva to a vesicle with an opalescent fluid. In the cysticercus form the scolex is discernible, but is invaginated at this stage. The larva has an outer and inner layer and in between these layers the first signs of organ system differentiation is seen.

The genus Taenia has 20 species and Taenia solium is often confused with Taenia saginata, the beef tapeworm. At the egg stage these two species are indistinguishable, and the differentiation between the two species occur at the adult stage. Taenia solium has a scolex composed of hooks, when compared to Taenia saginata. Taenia solium is also smaller in size and missing the vaginal sphincter. Taenia solium also has three lobes in its ovary compared to two in Taenia saginata. (Pawlowski, 2002; Sciutto, et al., 2000)

  • Range length
    2 to 10 m
    6.56 to 32.81 ft

Development

Taenia solium has six stages in its life cycle: preadult tapeworm, adult tapeworm, egg, oncosphere, postoncospheral form and cysticercus. The preadult and adult tapeworm occur in the definitive host, the human. These two stages can only occur when the definitive host consumes pork infected by the cysticercus stage because the adult must mature in the definitive host's intestine. The adult tapeworm is the reproductive stage in the life cycle and a single sexually mature proglottid contains an average of 40,000 eggs. These eggs are shed through the host's feces and then consumed by a pig, the intermediate host. The pig ingests the human fecal matter under poor sanitation conditions. The egg sheds its outer shell and becomes the oncosphere larva. The egg may also shed its outer shell while being expelled from the definitive host. In this situation, it is the oncosphere that is consumed by the pig. The larva enters the intermediate host's circulation system and is carried to muscle cells, internal organs and brain. From these locations, in the intermediate host, the larva grows into the postoncospheral stage. This stage outlines the transition between the oncosphere larva and the cysticercus larva. The larva begins the transformation into an adult when it reaches the human's intestine. This transfer of host occurs when a human eats undercooked pork meat infected by cysticercus. This transformation into an adult takes an average of 2 months before the tapeworm can become sexually mature. (Pawlowski, 2002)

Reproduction

Taenia solium is a monoecious species that holds both female and male reproductive systems inside a single proglottid. (Pawlowski, 2002)

Each segment in the pork tapeworm has its own set of male and female reproductive systems. The female reproductive system is composed of a vagina, seminal receptacle, ovary, oviduct, vitelline gland, Mehlis glands and uterus. An unfertilized egg from the ovary is carried through an oviduct, where it meets the sperm and becomes fertilized. The sperm created in one of the 150-200 testes in the proglottid, swims through the vas deferens and into the vagina through the genital pore. The genital pore is also connected to the vagina that leads the sperm to the unfertilized egg in the oviduct. After fertilization, the vitelline gland and Mehlis gland secrete a substance that surrounds the zygote. It is believed that these secretions form into the embryophore. The zygote continues its development in the uterus until the adult tapeworm is ready to expel its eggs into the external environment. While this process is self-fertilization, there have been reports of cross fertilization between different individuals. (Maravilla, et al., 2003; Pawlowski, 2002; Roberts and Janovy, 2009; Sciutto, et al., 2000)

  • Average number of offspring
    300,000 eggs/day

Lifespan/Longevity

It is believed that Taenia solium can survive in the intestine of its definitive host up to 25 years. However, there is controversy as to the truth of this number and other researchers claim that their life span is less than 5 years. Eggs can survive for 8 months, however, environmental conditions usually do not let the eggs survive to the next stage in the life cycle. Although there has been little research, temperature is the most important environmental factor for the eggs. (Garcia, et al., 2003; Hoberg, 2002; Pawlowski, 2002; Roberts and Janovy, 2009)

  • Range lifespan
    Status: captivity
    25 (high) years
  • Average lifespan
    Status: captivity
    5 years

Behavior

Taenia solium is different than other species in the genus Taenia in that it can use its definitive host (human) as an intermediate. The oncosphere larva can infect a human host through external autoinfection, where eggs expelled through fecal matter are ingested orally. Internal autoinfection, where a larva can infect the host without being expelled through fecal matter, have yet to be observed. However, researchers are open to the possibility of its occurrence.

The pork tapeworm is a mobile species. In its larval stage it penetrates the intermediate host's gut to reach the circulatory system and migrate throughout the body. The adult tapeworm is often fixed by the scolex to the definitive host's intestine. However, this dynamic can also be temporary because the tapeworm is known to migrate up and down the small intestine depending on ingested food, pH and digestive enzymes. (Pawlowski, 2002; Roberts and Janovy, 2009; Sciutoo, et al., 2007)

Taenia solium is a solitary parasite. In most observations only a single parasite infects an individual host. However, there are possibilities for competition between multiple tapeworms infecting the same host. If a host is infected by multiple pork tapeworms, the tapeworms have greater difficulty reaching sexually maturity. (Conlan, et al., 2009)

Communication and Perception

There has not been a specific discovery of the methods of communication between different individuals in this species. Even the ways tapeworms perceive different environments are unclear. However, there is a nerve ganglia in the scolex and a sensory system linked to touch and chemical stimuli. (Roberts and Janovy, 2009)

Food Habits

Taenia solium like all other Cestoda do not have a digestive system. As a result, they use their tegument to absorb their needed nutrients from the host they are infecting. Their tegument along the entire body is equipped with structures similar to microvilli in certain organisms. These structures called microtriches can be used to increase the surface area of the worm's tegument, allowing for a greater amount of nutrient absorption from the host. (Roberts and Janovy, 2009)

Predation

Taenia solium does not have any known predators.

Ecosystem Roles

Taenia solium is a parasitic tapeworm that infects pigs and humans. The cysticerci stage in the life cycle can cause medical and veterinary problems in its pig host. Porcine cysticercosis, the infection of cysticerci in pigs, have been found to be mostly asymptomatic. This is because most pigs are killed before the cysticerci enter the degenerative stage, which is known to cause symptoms. (Garcia, et al., 2003)

Species Used as Host

Economic Importance for Humans: Positive

There are no positive benefits for humans.

Economic Importance for Humans: Negative

Taenia solium is one of the most dangerous tapeworms to humans, because humans can act as a definitive and intermediate host. The condition where a human is used as a definitive host for an adult tape worm is called taeniasis. This condition is usually asymptomatic in humans and nonfatal. At times there are a few mild symptoms such as abdominal pain, altered appetite, diarrhea, constipation, hunger pains and weight loss.

The fatal infection for humans is cysticercosis, where the human acts as an intermediate hosts for the cysticerci. This stage can occur through external autoinfection of the host. Although there is no evidence for internal autoinfection, it is a possible way for the human to be infected with the oncosphere. In this condition, the cysticerci can infect any organ in the body and the extent of the symptoms caused by cysticercosis depend largely on the location of infection. The most common areas of infection are subcutaneous connective tissues, the eye and the brain. Infection to subcutaneous connective tissues rarely present symptoms and is nonfatal. Infection to the eye can cause blindness in the host. The most dangerous area of infection is in the central nervous system, termed neurocysticercosis. When cysticerci migrate to the brain they can cause epilepsy, intracranial hypertension and can mimic a brain tumor to the host. The most dangerous stage in cysticercosis is when the cysticercus larva dies. The larva's degeneration can cause an inflammatory response in the body that could be fatal to the host.

All conditions caused by Taenia solium cause huge economic losses for countries affected by this species. It has been estimated that in Mexico alone, $15 million US dollars are spent per year in hospital expenses for neurocysticercosis. This tapeworm also results in severe losses for the meat industry. In Mexico, half of the swine industry is estimated to be lost to porcine cysticercosis and the economic losses are about $43 million per year. (Garcia, et al., 2003; Roberts and Janovy, 2009)

Conservation Status

Taenia solium is not an endangered species. Researchers are constantly trying to find ways to control this species as it is a major parasite in the human population.

Contributors

Ashley Chung (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

Glossary

Antarctica

lives on Antarctica, the southernmost continent which sits astride the southern pole.

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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agricultural

living in landscapes dominated by human agriculture.

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

causes or carries domestic animal disease

either directly causes, or indirectly transmits, a disease to a domestic animal

chemical

uses smells or other chemicals to communicate

cosmopolitan

having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.

diapause

a period of time when growth or development is suspended in insects and other invertebrates, it can usually only be ended the appropriate environmental stimulus.

fertilization

union of egg and spermatozoan

internal fertilization

fertilization takes place within the female's body

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oceanic islands

islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

polar

the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.

sessile

non-motile; permanently attached at the base.

Attached to substratum and moving little or not at all. Synapomorphy of the Anthozoa

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

References

Conlan, J., K. Vongxay, S. Fenwick, S. Blacksell, R. Thompson. 2009. Does interspecific competition have a moderating effect on Taenia solium transmission dynamics in Southeast Asia?. Trends in Parsitology, 25 (9): 398-403.

Garcia, H., A. Gonzalez, C. Evans, R. Gilman. 2003. Taenia solium cysticercosis. Lancet, 362: 547-556.

Hoberg, E. 2002. Taenia tapeworms: their biology, evolution and socioeconomic significance. Microbes and Infection, 4: 859-866.

Maravilla, P., V. Souza, A. Valera, M. Romero-Valdovinos, Y. Lopez-Vidal, J. Dominguez-Alpizar, J. Ambrosio, S. Kawa, A. Fisser. 2003. Detection of genetic variation in Taenia solium. The Journal of Parasitology, 89 (6): 1250-1254.

Pawlowski, Z. 2002. Basic biology and transmission. Pp. 1-15 in G Singh, ed. Taenia Solium cysticercosis: from basic to clinical science. Chandigarh, India: CABI Publishing.

Roberts, L., J. Janovy. 2009. Gerald D. Schmidt & Larry S. Roberts' Foundations of Parasitology. New York, NY: McGraw-Hill.

Schantz, P. 2002. Taenia solium cysticercosis: an overview of global distribution and transmission. Pp. 63-73 in G Singh, ed. Taenia Solium cysticercosis: from basic to clinical science. Chandigarh, India: CABI Publishing.

Sciutoo, E., A. Chavarria, G. Fragoso, A. Fleury, C. Larralde. 2007. The immune response in Taenia solium cysticercosis: protection and injury. Parasite Immunology, 29: 621-636.

Sciutto, E., G. Fragoso, A. Fleury, J. Laclette, J. Sotelo, A. Aluja, L. Vargas, C. Larralde. 2000. Taenia solium disease in humans and pigs: An ancient parasitosis disease rooted in developing countries and emerging as a major health problem of global dimensions. Microbes and Infection, 2 (15): 1875-1890.

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  • This scientific name is not yet recognized in our classification database.