Human whipworm

Trichuris trichiura

ORDER Stichosomida

FAMILY Trichuridae

TAXONOMY

Trichuris trichiusa Linnaes, 1771.

OTHER COMMON NAMES English: Whipworm, threadworm.

PHYSICAL CHARACTERISTICS

Exceptionally thin for nematodes. They derive their name from the characteristic "whip-like" shape. Adults are 1.2-2.0 in (30-50 mm) in length, with a thread-like anterior end that becomes thicker at the posterior end. Both sexes have two distinct body regions. Males are 1.2-1.8 in (30-45 mm) in length, while adult females are 1.4-2.0 in (35-50 mm) in length. Females are very attenuated on the anterior three-fifths of the body, and become greatly expanded in the posterior two-fifths. Males are similarly shaped, but the swollen posterior is less pronounced. For most of the body's length, there is an area designated as the bacillary band, which is a combination of hypodermal and glandular tissues. The glandular tissue opens up to the exterior through cuticular pores. They have a mouth with a simple opening, and do not have lips. The buccal cavity is tiny and is provided with a minute spear. The esophagus is very long, occupying about two-thirds of the body length and consists of a thin-walled tube surrounded by large, unicellular glands, the stitchocytes. The entire structure is called a sticho-some. The anterior end of the esophagus is somewhat muscular. The transition from the anterior, filiform portion of the esophagus and the posterior, stout portion is sudden. Both sexes have a single gonad, and the anus is near the tip of the tale. Males have a single spicule that is surrounded by a spiny spicule sheath. The ejaculatory duct joins the intestine anterior to the cloaca. In the female, the vulva is near the junction of the esophagus and the intestine. The uterus contains many un-embryonated eggs. The excretory system is absent. The ventral surface of the esophageal region bears a wide band of minute pores, leading to underlying glandular and non-glandular cells.

DISTRIBUTION

Found worldwide, but are most commonly in Europe, followed by Asia, Africa, and South America. (Specific distribution map not available.)

HABITAT

Most often found in temperate and tropical areas, especially in moist areas. The final hosts are humans, where they primarily inhabit the colon.

BEHAVIOR

After 10-14 days in soil, their eggs become infective. For the host to become infective, the eggs must be swallowed. Upon entering the host's body, larvae hatch in the small intestine where, over a short period of time, they grow and molt at various points along the intestines, including the lumen and ileoce-cal area. Adults eventually bury their thin, thread-like anterior half into the mucosa of the large intestine, and then feed on tissue secretions, but not blood. They complete maturation at this time with their large posterior end breaking out of the mucosa and protruding. Adults are characterized by a lack of a tissue migration phase. They can reach the lungs by way of the lymph and blood systems. In the lungs, the larvae break out of the pulmonary capillaries into the air sacs, ascend into the throat, and descend to the small intestine again, where they grow. From the time of ingestion of the eggs to the development of mature worms is approximately three months. The prepatent period in the final host is 4-12 weeks.

FEEDING ECOLOGY AND DIET

As parasites, they feed off of their human hosts, primarily receiving nutrients from tissue secretions mostly within the intestines; it does not feed on blood.

REPRODUCTIVE BIOLOGY

The reproductive system is found at the esophago-intestine region. Both males and females have single reflexed gonads. The male has only one spicule. The eggs are operculate, and the females are oviparous. In early larval development, there are two rows of cells. When they infect a host, the eggs are "sticky." Eggs have, smooth outer shells. Adult females can lay eggs for up to five years. Egg production is estimated at 1,000-7,000 per day following copulation, and may contain up to 46,000 eggs at any one time. The eggs can be expelled with the feces of the host. Embryonation is completed in about 21 days in soil at about 86°F (30°C), where it is moist and shady. Adults can live for several years, so large numbers can accumulate in humans.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

Occasionally cause the condition eosinophilia, with the cecum and colon being the most commonly infected sites in the host. The particular disease is trichuriasis, or "whip worm" infection. In the United States, whipworm infection is rare overall, but is less rare in the rural southeast, where more than 2.2 million people are infected. Internationally, human whipworm infection is common in less-developed countries, with about 300-500 million people infected worldwide, but with some estimates as high as one billion people. Human whipworm infection is rarely fatal, but rectal prolapse may occur in heavily infected hosts. Fewer than 100 worms rarely cause clinical symptoms. Only hosts with very heavy infections become symptomatic. For those with heavy infection, common symptoms include abdominal discomfort, anemia, abdominal tenderness, secondary bacterial infections, and diarrhea; for more severe cases, symptoms include rectal prolapse, insomnia, nervousness, loss of appetite, vomiting, urticaria (skin rash), prolonged diarrhea, constipation, and flatulence. The infection is usually diagnosed by observing eggs in the patient's feces, and on occasion the presence of larval or adult worms in the feces. ♦

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