''Ascaris suum'', also known as the large roundworm of pig, is a parasitic

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 ...

that causes

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 ...

in pigs. While roundworms in pigs and humans are today considered as two species (''A. suum'' and '' A. lumbricoides'') with different hosts, cross-infection between humans and pigs is possible; some researchers have thus argued they are the same species. Ascariasis is associated with contact to pigs and pig manure in

Denmark Denmark is a Nordic countries, Nordic country in Northern Europe. It is the metropole and most populous constituent of the Kingdom of Denmark,, . also known as the Danish Realm, a constitutionally unitary state that includes the Autonomous a ...

. ''A. suum'' is distributed worldwide and grows up to in length. ''Ascaris'' infections are treated with ascaricides. ''A. suum'' is in the family Ascarididae, and is one of the oldest associations to mankind.

Life cycle

Pigs get infected with ''A. suum'' by ingesting infectious parasite eggs that are present in the environment. The

larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...

e of Ascaris complete two moults within the egg; therefore, the larvae emerging from the egg is not a second-stage larva (L2) as was previously presumed, but rather a third stage larva (L3) covered by a loosened L2

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 ...

. The larvae hatch from the egg inside the pig's

intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...

and subsequently start their migration through the body. First, they penetrate the intestinal wall at

caecum The cecum ( caecum, ; plural ceca or caeca, ) is a pouch within the peritoneum that is considered to be the beginning of the large intestine. It is typically located on the right side of the body (the same side of the body as the appendix, ...

/ colon and use the mesenterial blood veins to migrate to the

liver The liver is a major metabolic organ (anatomy), organ exclusively found in vertebrates, which performs many essential biological Function (biology), functions such as detoxification of the organism, and the Protein biosynthesis, synthesis of var ...

. After burrowing their way through the liver tissue, they again use the efferent blood stream to carry them to the

lungs The lungs are the primary organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the backbone on either side of the heart. Their function in the respiratory syste ...

. There, they get stuck in the capillaries surrounding the lungs and they penetrate the lung alveoli. It takes approximately 7 days to reach the lungs. Once the larvae are inside the lung, they migrate up the respiratory tree and are eventually

cough A cough is a sudden expulsion of air through the large breathing passages which can help clear them of fluids, irritants, foreign particles and Microorganism, microbes. As a protective reflex, coughing can be repetitive with the cough reflex fol ...

ed up and swallowed by the host to reach the

small intestine The small intestine or small bowel is an organ (anatomy), organ in the human gastrointestinal tract, gastrointestinal tract where most of the #Absorption, absorption of nutrients from food takes place. It lies between the stomach and large intes ...

again as soon as 10 days after infection. There, the larvae undergo their first molt inside the host to reach the L4 stage by day 14 post-infection. Around day 25 post infection, they develop into the L5 stage. Worms reach adulthood 6 weeks after infection, and when both female and male worms are present in the same host, fertilized eggs are produced and secreted by the female worm. These eggs are then excreted together with the

faeces Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...

. After an incubation period, infective stage larvae develop in the eggs and are ready to cause infection in a new host. Paratenic hosts ingest the eggs and the L3 larvae remain in the tissues of the paratenic host until a pig eats them. These may include

beetles Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 40 ...

and

earthworms An earthworm is a soil-dwelling terrestrial animal, terrestrial invertebrate that belongs to the phylum Annelida. The term is the common name for the largest members of the class (biology), class (or subclass (biology), subclass, depending on ...

, as well as large to jumbo chicken eggs from at-risk

fowl Fowl are birds belonging to one of two biological orders, namely the gamefowl or landfowl ( Galliformes) and the waterfowl ( Anseriformes). Anatomical and molecular similarities suggest these two groups are close evolutionary relatives; toget ...

Epidemiology

''Ascaris'' eggs can remain infective for years in the soil, even in a temperate climate. The migration of ''A. suum'' larvae through the lungs may cause secondary bacterial infections and result in acute respiratory symptoms in pigs.

Morphology

Males are about long, and wide. The posterior end is curved toward the ventral side with a pointed tail. They have simple spicules that measure long. Females are larger than males, measuring long and in diameter. From the

anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

end, the

vulva In mammals, the vulva (: vulvas or vulvae) comprises mostly external, visible structures of the female sex organ, genitalia leading into the interior of the female reproductive tract. For humans, it includes the mons pubis, labia majora, lab ...

occupies about one-third of the body length. In addition to their large size, these species also have the three prominent lips. Each lip contains a dentigerous ridge, and no interlabia or alae. Females can lay up to 200,000 eggs per day, and their uteri can contain up to 27 million eggs at a time. Fertilized eggs are ovoid, ranging from 45 to 75 μm length and 35 to 50 μm in diameter. The uterine wall contributes to the lumpy and thick outer layer of the egg. The mammillated layer is stained golden-brown by the host's

bile Bile (from Latin ''bilis''), also known as gall, is a yellow-green/misty green fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is primarily composed of water, is pro ...

when the eggs are passed in faeces. Females can also deposit unfertilized eggs that are narrower and longer than normal fertilized eggs, ranging from 88 to 94 μm in length, and 44 μm diameter. Only the proteinaceous layer can be seen in unfertilized eggs, because after fertilization, the vitelline, chitinous, and lipid layers form.

Incidents and outbreaks

In Canada in 1970, a postgraduate student tainted his roommates' food with ''A. suum''. Four of the victims became seriously ill; two of these suffered acute respiratory failure.

Genetics

A 273-megabase draft genome for ''A. suum'' was published in 2011.

References

* {{Taxonbar, from=Q2062253 Ascaridomorpha Parasitic nematodes of mammals Swine diseases Veterinary helminthology Nematodes described in 1782