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Ascaris suum
''Ascaris suum'', also known as large roundworm of pigs, 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 hosts, cross infection between humans and pigs is possible, so researchers have argued they are the same species.〔http://www.parasitesandvectors.com/content/5/1/42〕 Ascariasis is associated with contact to pigs and pig manure in Denmark.〔http://jcm.asm.org/content/43/3/1142.full〕
''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 larvae of Ascaris complete two moults within the egg and 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.〔Fagerholm et al., 2000, Differentiation of cuticular strutures during the growth of the third-stage larva of Ascaris suum after emerging from the egg. J Parasitol 86, 421-427.〕 The larvae hatch from the egg inside the intestines and subsequently start their migration through the body of the pig. First they penetrate the intestinal wall at the level of the caecum/colon 〔Murrel et al., 1997, J Parasitol 83, 255-260.〕 and use the mesenterial blood veins to migrate to the liver. After borrowing their way through the liver tissue, they again use the efferent blood stream to carry them to the lungs. 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 coughed up and swallowed by the host to reach the small intestine 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. 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 and earthworms, as well as large to jumbo chicken eggs from at-risk fowl.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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