Trichina worm

Trichinella spiralis

ORDER

Stichosomida

FAMILY

Trichinellidae

TAXONOMY

Trichinella spiralis Owens, 1835.

OTHER COMMON NAMES English: Pork worm.

PHYSICAL CHARACTERISTICS

Classified as animal (mammal) parasites and the causal organism of the disease trichinosis, they are small roundworms that live mainly in rats and other small mammals such as pigs that pick up the worm while rooting for food. Adults have a length of 0.055-0.158 in (1.4-4.0 mm), with males measuring 0.055-0.063 in (l.4-1.6 mm) in length and females 0.118-0.158 in (3.0-4.0 mm) in length. Males and females have distinct features. Females possess a uterus and vulva. The vulva is located near the middle of the esophagus, which is about one-third the length of the body. Males have a single gonad, but no copula-tory spicule, and have an ejaculatory duct. Structures identifiable on both sexes include the muscular esophagus, stichosome, and intestine. Stichosomes are formed by a single short row of stichocytes, following a short muscular esophagus. The color of the external surface of the adult is translucent and white. Both sexes are more slender at the anterior than at the posterior, but do not have two distinct sections. The anus is nearly terminal and has a large papilla on each side of it.

DISTRIBUTION

They are found worldwide, especially in South America and Africa. (Specific distribution map not available.)

HABITAT

Found more commonly in temperate rather than tropical regions. Unlike many parasites that show a high degree of host specificity, they can be found in many species of carnivores and omnivores, and in virtually all mammals. They are found attached to or buried in the mucosa (mucous membrane) of the duodenum of their host.

BEHAVIOR

Have no stages outside a host, which is unusual for helminth parasites. Their lifecycle begins when viable encysted larvae are eaten with the flesh of any meat-eating animal. When these infectious cysts are ingested, the juveniles are freed from the hard cysts by the dissolving action of stomach acids. The liberated juveniles pass to the duodenum where they go through four rapid molting periods within 27-29 hours and then mature within 1-2 days. Adults live in the gastrointestinal tract of their host. Mature males fertilize the females and then die. Mature females will penetrate the mucosa of the intestine and, within a few days, will begin laying pre-juveniles into lymph vessels. Females die after producing the juveniles. The new pre-juveniles are then carried to the lymphatic and mesenteric veins, and are found throughout the arterial circulation 7-25 days after the initial infection. The pre-juveniles move in the hepatoportal system through the liver, and then to the heart, lungs, and the arterial system, which distributes them throughout the body. They are transported to striated muscles, where they penetrate individual fibers. Within the muscles, the immature worms curl into a ball and encyst. Juvenile cysts can grow to 0.12 in (3 mm) in diameter. These newly infective juveniles are now ready to be eaten by another host (which eats the infected muscle of the previous host). If another host does not eat the previous host, then juveniles may remain viable for many years, for example, up to 25 years in humans and 11 years in pigs. Their lifecycle is considered to be complete when a definitive host ingests the intermediate host. There is general agreement among scientists that there are four juvenile stages of the worm, but there is distinct disagreement as to whether or not nematode development occurs within the cyst.

FEEDING ECOLOGY AND DIET As parasites, they feed off of their hosts.

REPRODUCTIVE BIOLOGY

The single female uterus is filled with developing eggs in its posterior portion, while the anterior region contains fully developed hatching juveniles. The size of the larvae is about 0.00394 in (100 pm) in length by 0.000394 in (10 pm) in width. After impregnating the female, the male dies. The females subsequently increase to their maximum size and burrow deeper into the mucosa. Females are ovoviviparous (very thinly shelled or shell-less eggs are developed within the female and juveniles hatch before leaving the uterus). A female produces about 1,500 young over a period of 4-6 weeks, after which the female dies. Both males and females live for only a short time in the intestinal epithelium of a wide variety of mammals. The juveniles then migrate into the intestinal lymphatic and mesen-teric venules, on to the heart and lungs, and eventually into the arterial circulatory system. Concurrently, they molt three times in order to reach adulthood. When reaching striated muscle, they encyst, remaining viable for more than five years within the cysts. The termination of their lifecycle is reached at this time, and the larvae must wait for their host to be devoured in order to continue other stages.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

Trichinosis is a zoonotic infection associated with the colonization of worms in muscles. It is often found in humans because of the consumption of uncooked or insufficiently cooked pork products (though other animals are also potential sources). Trichinella is the third most common worm that infects humans. They cause nausea, dysentery, puffy eyes, and colic. They also cause pain and more severe problems such as edema, cardiac and pulmonary problems, deafness, delirium, muscle pain, muted reflexes, nervous disorders, and pneumonia. Their natural hosts are flesh-eating animals, especially humans, pigs, rats, and other mammals. Humans are considered accidental hosts because, under normal conditions, the parasite ends its cycle; that is, no other animals eat humans in order to transfer their larvae to other hosts. But concern for trichinosis is not as great today with improved pork production practices. Still, an estimated 5-6 million human infections are present at any one time in North America (about 2% of the population). ♦

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