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Trichinella spiralis

By Ginger Hartwell

Geographic Range

Trichinella spiralis is prevalent in Mexico, the northern hemisphere, parts of southern Asia, Africa, South America, and the Middle East. The species is also found in other tropical regions. (Olsen, 1974; Roberts and Janvoy, 1996)

Habitat

Trichinella spiralis has an extremely broad host range; almost any species of mammal can become infected. Adult worms live around the columnar epithelial cells of the small intestine and the larvae live in striated muscle cells of the same mammal.

There are three different ecological types of life cycles, the urban cycle, the sylvatic cycle, and the marine cycle. In the urban cycle, rats and pigs serve as hosts and reservoirs of the parasite. Humans can become infected with the worm by eating pork that is not cooked thoroughly. In the sylvatic cycle, predators and scavengers are hosts to T. spiralis. Seals, walruses, whales, and polar bears are all hosts in the marine cycle. (Lapage, 1957; ; Olsen, 1974; Roberts and Janvoy, 1996; Wassom, 1988)

Physical Description

Trichinella spiralis is the smallest known nematode parasite of humans. The males measure about 1.4 mm to 1.6 mm in length and the females are twice the size of the males. The body of the worm is more slender at the anterior then at the posterior end. In females the uterus is contained in the posterior portion of the worm and is filled with the developing eggs. The anterior end of the female contains hatching juveniles.

This nematode has a cuticle with three or more main outer layers made of collagen and other compounds. The outer layers are non-cellular and are secreted by the epidermis. The cuticle layer protects the nematodes so they can invade the digestive tracts of animals.

Nematodes have longitudinal muscles along the body wall. The muscles are obliquely arranged in bands. Dorsal, ventral and longitudinal nerve cords are connected to the main body of the muscle. (Barnes, 1987; Olsen, 1974; Roberts and Janvoy, 1996)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    1.4 to 3.2 mm
    0.06 to 0.13 in

Development

The life cycle for this species begins after ingestion of the first stage juvenile from the intermediate host. The worm molts four times within the first thirty hours and then mates. These larvae exit through the gut wall and enter the blood system through the branches of the hepatic portal vein or through the lymphatic system. They are transported all over the body and take up residence in voluntary muscles by entering individual muscle cells. The larvae grow within the muscles and a covering is created around them causing a cyst. After the cyst is formed the worm cannot migrate any further. The only way this species can continue its life cycle is to be ingested by another host through a predator-prey interaction. When the new host eats the muscle tissue containing the cyst the digestive juices break down the capsule and release the worm. (Lapage, 1957; Olsen, 1974; Read, 1972; Wassom, 1988)

Reproduction

The life cycle for this species begins after ingestion of the first stage juvenile from the intermediate host. The worm molts four times within the first thirty hours and then mates.

Females may produce a phermomone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the female during copulation. Nematode sperm are amoeboid-like and lack flagella.

The female is ovo-viviparous. This means that she produces eggs, but doesn't lay them until they have already hatched in her uterus. She lays her living larvae within the small intestine beginning the fifth or sixth day after infection. (Barnes, 1987; Lapage, 1957; Olsen, 1974; Read, 1972; Wassom, 1988)

  • Parental Investment
  • pre-fertilization
    • provisioning
  • pre-hatching/birth
    • protecting
      • female

Behavior

Trichinella spiralis is a common parasite of carnivorous and omnivorous mammals, including humans. There are three different ecological types of life cycles, the urban cycle, the sylvatic cycle, and the marine cycle. In the urban cycle, rats and pigs serve as hosts and reservoirs of the parasite. Humans can become infected with the worm by eating pork that is not cooked thoroughly. In the sylvatic cycle, predators and scavengers are hosts to T. spiralis. Seals, walruses, whales, and polar bears are all hosts in the marine cycle. (Olsen, 1974; Roberts and Janvoy, 1996)

Communication and Perception

Nematodes in general have papillae, setae and amphids as the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987)

Food Habits

Adults feed in the intestinal epithelium of the host. The juveniles penetrate individual fibers in skeletal muscles and feed there.

Pharyngeal glands and intestinal epithelium produce digestive enzymes to feed on the hosts’ body fluids. Extracellular digestion begins within the lumen and is finished intracellularly. (Barnes, 1987; Roberts and Janvoy, 1996)

  • Animal Foods
  • body fluids

Predation

These parasites are usually not preyed on directly, but are ingested from host to host. Larval mortality is high as most of the parasites do not reach appropriate hosts.

Ecosystem Roles

Humans tend to become infected from infected pigs, however the incidence in pigs is only 0.37% (Lukashenko, 1966). The species can be found more easily in cats with an incidence of 71.23%, rats with 6.43%, or even mice with 3.38%. (Lukashenko, 1966; Roberts and Janvoy, 1996)

Species Used as Host

Economic Importance for Humans: Positive

The research done leads to the conclusion that there is no known economic benefit to humans from Trichinella spiralis.

Economic Importance for Humans: Negative

Humans may know this parasite more commonly by the disease that it causes. This disease is known as trichinosis, trichiniasis, or trichinelliasis. Humans can obtain this parasite by eating meat that is already infected. Generally, a human gets the disease by eating undercooked pork. Raw sausage is a delicacy in many areas of the world, making trichinosis a chronic health problem. Symptoms of this disease may include:

  1. Weakness and muscular twitching, from the juvenile being deposited by the female worm.
  2. Edema around the eyes, intense muscular pain, disturbances in the way the muscles function, difficulty with respiration, and a consistent fever are symptoms that correspond to the juvenile's migration and penetration of muscle tissue.
  3. Face becomes puffy. The swelling of extremities, damage to the heart, nervous system and other organs, puffiness in the face, and pneumonia are symptoms that can occur with the encysting of the worms in the muscle tissue.

A good treatment for ridding the body of this parasite is not known. Treatment with analgesics and corticosteroids merely relieves the symptoms of trichinosis. The incidence of infection has steadily declined throughout the world. Cases in the United States declined from over 400 per year in the 1940's to 30-40 cases per year from 1987-1989. (Lapage, 1957; Read, 1972; Roberts and Janvoy, 1996)

Other Comments

Trichinella spiralis is the world's largest intracellular parasite.

The calcified granules that are created in the muscle of the host are eventually what led to the discovery of this species in 1835. James Paget, who was studying medicine in London, noticed that his scalpels were becoming dull due to gritty particles in the muscle of the cadaver he was working on. He noticed the wormlike nature of them and showed them to the anatomist Richard Owen, who eventually gave them their scientific name. Twenty-five years later they determined that these animals caused disease. (Roberts and Janvoy, 1996)

Contributors

Renee Sherman Mulcrone (editor).

Ginger Hartwell (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.

Glossary

Ethiopian

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

World Map

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.

World Map

Neotropical

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

World Map

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.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

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

chaparral

Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.

chemical

uses smells or other chemicals to communicate

desert or dunes

in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

forest

forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

internal fertilization

fertilization takes place within the female's body

motile

having the capacity to move from one place to another.

mountains

This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.

oriental

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

World Map

ovoviviparous

reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.

parasite

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

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

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.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

scrub forest

scrub forests develop in areas that experience dry seasons.

sedentary

remains in the same area

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

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

taiga

Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.

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.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.

savanna

A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.

tundra

A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

References

Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.

Lapage, G. 1957. Parasitic Animals. Cambridge at the University Press.

Lukashenko, N. 1966. The Epizootology of Trichinellosis. Pp. 371-375 in A Petrov, ed. Contributions to Helminthology. Jerusalem: Israel Program for Scientific Translations Ltd..

Ohio State University, 2001. "Trichinella spiralis (Trichinellosis or trichinosis)" (On-line). Parasites and Parasitological Resources. Accessed October 07, 2004 at http://www.biosci.ohio-state.edu/~parasite/trichinella.html.

Olsen, O. 1974. Animal Parasties: Their Life Cycles and Ecology. University Park Press.

Read, C. 1972. Animal Parasitism. New Jersey: Prentice-Hall, Inc..

Roberts, L., J. Janvoy. 1996. Foundations of Parasitology, 6th edt.. The McGraw-Hill Companies, Inc..

Wassom, D. 1988. Genetic control of Immunity to Parasite Infections: Studies of Trichinella-infected Mice. Pp. 329-346 in P Englund, A Sher, eds. The Biology of Parasitism. MBL Lectures in Biology Vol. 9. New York: Alan R. Liss Inc..

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