Overview

Brief Summary

A number of Trichinella nematode (roundworm) species infect humans and cause trichinellosis (trichinosis). In addition to the classical agent T. spiralis (found worldwide in many carnivorous and omnivorous animals), several other species of Trichinella are now recognized, including T. pseudospiralis (from mammals and birds worldwide), T. nativa (from Arctic bears and walruses), T. nelsoni (from African predators and scavengers), T. britovi (from carnivores, pigs, and horses of temperate Europe and western Asia and northern and western Africa ), T. murelli (from bears and horses in North America), and T. papuae (from wild and domestic pigs and saltwater crocodiles in Papua New Guinea and Thailand). Trichinella zimbabwensis is found in crocodiles and monitor lizards in Africa but there are no known associations of this species with human disease.

Trichinellosis occurs worldwide, but is most common in parts of Europe and the United States. Adult worms and encysted larvae develop within a single vertebrate host and an infected animal serves as a definitive host and potential intermediate host. A second host is required to perpetuate the life cycle. The domestic cycle most often involved pigs and anthropophilic rodents, but other domestic animals such as horses can be involved. In the sylvatic cycle, the range of infected animals is great, but animals most often associated as sources of human infection are bear, moose, and wild boar.

Trichinellosis is caused by the ingestion of undercooked meat containing encysted larvae (except for T. pseudospiralis and T. papuae, which do not encyst) of Trichinella species. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms. Females are 2.2 mm in length; males 1.2 mm. The life span in the small bowel is about four weeks. After 1 week, the females release larvae that migrate to striated muscles, where they encyst. Diagnosis is usually made based on clinical symptoms and is confirmed by serology or identification of encysted or non-encysted larvae in biopsy or autopsy specimens.

Gottstein et al. (2009) reviewed the epidemiology, diagnosis, treatment, and control of trichinellosis.

(Centers for Disease Control Parasites and Health website; Gottstein et al. 2009)

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Distribution

Trichinella pseudospiralis thrives in many areas around the world. First isolated from the skeletal muscles of a raccoon in Russia, its wide geographic range today is attributed to its main hosts, which are birds. The parasite has spread widely with the migration of birds. Trichinella pseudospiralis is limited by temperature, and does not survive in extreme climates.

Biogeographic Regions: nearctic ; palearctic (Native ); oriental ; ethiopian ; neotropical ; australian

Other Geographic Terms: cosmopolitan

  • Smyth, J. 1962. Introduction to Animal Parasitology. London: Cambridge University Press.
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Physical Description

Morphology

Both adult males and females are of nearly uniform diameter and measure around 2.46 mm in length. Females are larger than males. They gradually increase in thickness posteriorly. The head of the worm is tapered and round. Adults possess a mouth stylet which is used to cut host tissues and vessels. Trichinella species have larvae that are made up of two definite layers separated by membrane. The inner layer has a large amount of very fine fibrils, arranged parallel to the circumference of the larvae. There is also a thin external membrane present.

Average length: 2.46 mm.

Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry

Sexual Dimorphism: female larger; sexes shaped differently

  • Barnes, R. 1987. Invertebrate Zoology. Orlando, Florida: Dryden Press.
  • Gould, S. 1970. Trichinosis in Man and Animals. Springfield, Illinois: Charles C. Thomas Publisher.
  • Lee, D. 1965. The Physiology of Nematodes. San Fransisco: W.H. Freeman and Company.
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Ecology

Habitat

Trichinella pseudospiralis lives on terrestrial animals. The primary hosts for these worms are birds, and they are commonly seen on captive American kestrels. They have a wide distribution that is limited by temperature. This parasite is seen in domestic environments as well as temperate, torrid, and frigid zones.

Habitat Regions: temperate ; tropical ; terrestrial

Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains

Other Habitat Features: urban ; suburban ; agricultural

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Trophic Strategy

Trichinella pseudospiralis has the same intermediate and definitive host. It spends its entire life in that one host. The juvenile parasite feeds inside the muscles of its host. Using its stylets it cuts open the host cells and feeds on the intracellular material that flows out. The Trichinella repeatedly protrude and retract the stylet in order to get through the host tissue. This process releases cell contents and other fluids that, together with the products of histolysis, are ingested by active pumping of the pharynx. The adults living in the intestine have a stylet as well. However, the adult Trichinella feed on the intestinal mucosa of the host rather than on its intestinal contents.

Animal Foods: body fluids

Primary Diet: carnivore (Eats body fluids)

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Associations

Trichinella pseudospiralis lives on terrestrial animals. The primary hosts for these worms are birds.

Ecosystem Impact: parasite

Species Used as Host:

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These parasites are probably not preyed on directly. However, larval mortality is high since most larvae do not reach the proper host species.

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Life History and Behavior

Behavior

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

Communication Channels: tactile ; chemical

Other Communication Modes: pheromones

Perception Channels: tactile ; chemical

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Life Cycle

The fertilized egg develops into a coeloblastula, which is followed by gastrulation, and by the formation of embryos. These embryos fill the uterus of the adult female worm by the 5th or 6th day after copulation. As many as 1500 larvae can be deposited by one worm. Once the mother has deposited the larvae, they penetrate the mucosa and are carried to the voluntary muscles, especially those of the diaphragm, jaws, tongue, larynx and eye. After their migration, other species of Trichinella, such as T. spiralis, form cysts and remains in the host muscle. Trichinella pseudospiralis is different because it does not induce capsule formation in host muscles. The larva is passed to other hosts by an animal eating the flesh of another infected animal or its feces.

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Reproduction

Trichinella species are unique in that all stages of their life cycle occur with in a single host. Copulation occurs around 30 to 40 hours after the host has been infected with the parasite. The ovum is fertilized within the female's body by the male sperm. The fertilized egg develops into a coeloblastula, which is followed by gastrulation, and by the formation of embryos. These embryos fill the uterus of the adult female worm by the 5th or 6th day after copulation. As many as 1500 larvae can be deposited by one worm.

Range number of offspring: 1500 (high) .

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); oviparous

Parental Investment: pre-fertilization (Provisioning)

  • Gould, S. 1970. Trichinosis in Man and Animals. Springfield, Illinois: Charles C. Thomas Publisher.
  • Smyth, J. 1962. Introduction to Animal Parasitology. London: Cambridge University Press.
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Conservation

Conservation Status

US Federal List: no special status

CITES: no special status

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Relevance to Humans and Ecosystems

Benefits

Since the main hosts of Trichinella pseudospiralis are birds, their parasitism has an effect on humans. Endangered species, as well as birds that are bred in captivity are threatened. Natural infections with the nematode are known to cause severe debilitation and death in Cooper's hawks. Trichinella pseudospiralis also causes behavioral changes and decreases the reproductive success of captive American kestrels. Infected pairs of the birds produce significantly fewer offspring than their uninfected counterparts, which can cause a great amount of economic loss for breeders.

Not all of the effects of Trichinella pseudospiralis are seen in both sexes, during all periods of the host's life cycle, or during all times of the day. Both sexes were observed to fly much less and they scratched more frequently. Infected females exhibited an increase in the frequency of aggressive displays in the late morning, which suggests that they may be less receptive to mating than uninfected females. Males seemed to feel the pains of hunger stress more than the infected females (Saumier et. al 1991).

Negative Impacts: causes or carries domestic animal disease

  • Saumier, M., M. Rau, D. Bird. 1991. Behavioral changes in breeding American kestrels infected with Trichinella pseudospiralis. Bird-Parasite Interactions, Ecology, Evolution, and Behavior: 290-313.
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