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.
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
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.
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:
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.
Negative Impacts: injures humans (causes disease in humans ); causes or carries domestic animal disease
The research done leads to the conclusion that there is no known economic benefit to humans from Trichinella spiralis.
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%.
Ecosystem Impact: parasite
Species Used as Host:
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.
Animal Foods: body fluids
Primary Diet: carnivore (Eats body fluids)
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.
Biogeographic Regions: nearctic ; palearctic ; oriental ; ethiopian ; neotropical
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.
Habitat Regions: temperate ; tropical ; polar ; terrestrial
Terrestrial Biomes: tundra ; taiga ; desert or dune ; savanna or grassland ; chaparral ; forest ; rainforest ; scrub forest ; mountains
Other Habitat Features: urban ; suburban ; agricultural
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.
Range length: 1.4 to 3.2 mm.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry
Sexual Dimorphism: female larger; sexes shaped differently
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.
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.
Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate); sexual ; fertilization (Internal ); ovoviviparous
Parental Investment: pre-fertilization (Provisioning); pre-hatching/birth (Protecting: Female)
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)
