''Ascaris'' is a

nematode The nematodes ( or ; ; ), roundworms or eelworms constitute the phylum Nematoda. Species in the phylum inhabit a broad range of environments. Most species are free-living, feeding on microorganisms, but many are parasitic. Parasitic worms (h ...

genus of parasitic worms known as the "small intestinal roundworms". One species, ''

Ascaris lumbricoides ''Ascaris lumbricoides'' is a large parasitic worm, parasitic Nematoda, roundworm of the genus ''Ascaris.'' It is the most common parasitic worm in humans. An estimated 807 million–1.2 billion people are infected with ''Ascaris lumbricoides'' ...

'', affects humans and causes the disease

ascariasis Ascariasis is a disease caused by the parasitic roundworm ''Ascaris lumbricoides''. Infections have no symptoms in more than 85% of cases, especially if the number of worms is small. Symptoms increase with the number of worms present and may in ...

. Another species, ''

Ascaris suum ''Ascaris suum'', also known as the large roundworm of pig, is a parasitic nematode that causes ascariasis in pigs. While roundworms in pigs and humans are today considered as two species (''A. suum'' and '' A. lumbricoides'') with different ho ...

'', typically infects

pigs The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

. Other ascarid genera infect other animals, such as '' Parascaris equorum'', the equine roundworm, and '' Toxocara'' and '' Toxascaris'', which infect dogs and cats. Their eggs are deposited in feces and soil. Plants with the eggs on them infect any organism that consumes them. ''A. lumbricoides'' is the largest intestinal roundworm and is the most common helminth infection of humans worldwide. Infestation can cause morbidity by compromising nutritional status, affecting cognitive processes, inducing tissue reactions such as granuloma to larval stages, and by causing intestinal obstruction, which can be fatal.

Morphology and Anatomy

* Adult: cylindrical shape, creamy white or pinkish in color * Male: average ; more slender than the female * Female: average The body is long, cylindrical, and fusiform (pointed at both the ends). The body wall is composed of

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

, epidermis and musculature. There is a pseudocoelom. The digestive tract is complete with prominent muscular

pharynx The pharynx (: pharynges) is the part of the throat behind the human mouth, mouth and nasal cavity, and above the esophagus and trachea (the tubes going down to the stomach and the lungs respectively). It is found in vertebrates and invertebrates ...

. Respiration is by simple diffusion across body wall. The nervous system consists of a nerve ring and many longitudinal nerve cord. They are dioecious and have separate reproductive systems consisting of thread like gonads and genital ducts that open outside by apertures. Fertilization is internal and development is mostly indirect.

Sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

is well marked. Externally, males are much shorter than females and males also have a curved posterior end, unlike females. Internally, in males, all the digestive, reproductive systems open in a common chamber- cloaca whereas in females, there is a separate anus for digestive tract and female genital pore for female reproductive system.

Defense mechanism

As part of the parasite defense strategy, ''Ascaris'' roundworms secrete a series of inhibitors to target digestive and immune-related host proteases, which include

pepsin Pepsin is an endopeptidase that breaks down proteins into smaller peptides and amino acids. It is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food. Pe ...

trypsin Trypsin is an enzyme in the first section of the small intestine that starts the digestion of protein molecules by cutting long chains of amino acids into smaller pieces. It is a serine protease from the PA clan superfamily, found in the dig ...

, chymotrypsin/ elastase, cathepsins, and metallocarboxypeptidases (MCPs). ''Ascaris'' species inhibit MCPs by releasing an enzyme known as ''Ascaris'' carboxypeptidase inhibitor (ACI). This enzyme binds to the active site of MCP and blocks the cleavage of its own proteins by the host MCP. Similarly, they inhibit trypsin by releasing the protein Ascaris Trypsin Inhibitor
pdb 1ATA
.

History

''Ascaris'' has been present in humans for at least several thousand years, as evidenced by ''Ascaris'' eggs found in paleofeces and in the intestines of mummified humans. ''A. lumbricoides'' was originally called ''Lumbricus teres'' and was first described in detail by Edward Tyson in 1683. The genus ''Ascaris'' was originally described as the genus for ''Ascaris lumbricoides'' by

Carl Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...

in 1758. The morphologically similar ''Ascaris suum'' was described from pigs by Johann August Ephraim Goeze in 1782.

References

{{Authority control Ascaridomorpha Chromadorea genera Taxa named by Carl Linnaeus

Morphology and Anatomy

Defense mechanism

History

See also

References