Trichinella spiralis

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)

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)

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)

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)

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)

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

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)

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.

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)

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)

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

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)