Threadworm

Strongyloides stercoralis

ORDER Rhabditida

FAMILY

Strongyloididae

TAXONOMY

Anguilla stercoralis (Bavay in Normond, 1876), In Toulon, France, on host Homo sapiens, but host from Cochin China.

OTHER COMMON NAMES English: Dwarf threadworm.

PHYSICAL CHARACTERISTICS

An important parasite of humans, primates, dogs, and other animals, being one of the smallest to inhabit the human body. Females measure from 0.079-0.098 in (2.0-2.5 mm) in length; males slightly smaller. Size of the eggs is about 0.00158 in (40 pm) in length by 0.00118 in (30 pm) in width. Both sexes have small buccal cavities and a long esophagus. Females have a cylindrical pharynx with no posterior bulb swelling. Both free-living male and female worms have a prominent rhabdiform pharynx.

DISTRIBUTION

Occur all over the world, but exist in large numbers in Southeast Asia and South America, and in Eastern Europe and in the Mediterranean region. In the United States, they are rare, al though are more prevalent in the rural southeast. (Specific distribution map not available.)

HABITAT

Found primarily in temperate climates, and to a lesser degree in tropical climates. Female worms are found in the superficial tissues of the small intestines of vertebrates, mainly humans and dogs. Males are believed not to be parasitic, but free living in the soil.

BEHAVIOR

Heterogonic lifecycle, i.e., a parasitic generation is interspersed with a free-living one. There can be multiple cycles of each phase. Females are more invasive than males. This parasite generally replicates within its definitive hosts. Females lay partially embryonated eggs that are released into the host's sub-mocosa or the intestinal lumen. Once in the external environment (after passing through the host's feces), the larvae either remain as infective stages or develop to free-living adults.

FEEDING ECOLOGY AND DIET

As parasites, they feed off of their hosts, taking nutrients, mostly mucosa from the intestine, from various locations within the body as they travel through the circulatory system, lungs, stomach, and small intestines. As free living, they feed on organic matter, bacteria, and other nutrients in the soil.

REPRODUCTIVE BIOLOGY

Females parthenogenetic; males exist, but are only rarely found, and then only in feces. Free-living males and females mate, produce more larvae, and some of these larvae will develop into free-living larvae, while others will develop into parasitic juveniles.

CONSERVATION STATUS Not listed by the IUCN.

SIGNIFICANCE TO HUMANS

Septicemia is caused by the infection of this worm into the human body. More than 70 million people are infected, mostly in the temperate regions of the world. ♦

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