出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/02/12 02:44:09」(JST)
| Ascaris suum | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Nematoda |
| Class: | Secernentea |
| Order: | Ascaridida |
| Family: | Ascarididae |
| Genus: | Ascaris |
| Species: | A. suum |
| Binomial name | |
| Ascaris suum (Goeze, 1782) |
|
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, Ascaris suum and Ascaris lumbricoides, with different hosts, experiments have show that cross infection between humans and pigs is possible. Researchers have argued that they are the same species. [1] There is an association between ascariasis in Denmark and contact to pigs and pig manure. [2]
Ascaris suum is distributed worldwide and grows up to 40 centimetres (16 in) in length. Ascaris infections are treated with ascaricides. Ascaris suum is in the family Ascarididae, and is one of the oldest association to mankind.
The life cycle can be direct or indirect with paratenic hosts. The pig ingests the egg with an L2 larva inside. The larvae undergo hepatic migration, moult to the L3 form, and then migrate to the lungs. Larvae enter the alveoli, are coughed up and then swallowed. This process is known as hepato-tracheal migration. They then moult two times and become adults.
Paratenic hosts ingest the eggs and the L2 larvae remains in the tissues of the paratenic host until a pig eats it. These may include beetles and earthworms, as well as large to jumbo sized chicken eggs from at-risk fowl.
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.
Males are about 15–31 centimetres (6–12 in) long, and 2–4 millimetres (0.08–0.16 in) wide. The posterior end is curved toward the ventral side with a pointed tail. They have simple spicules that measure 2.0–3.5 mm (0.08–0.14 in) long.
Females are larger than males, measuring in at 20–49 cm (8–19 in) long and 3–6 mm (0.12–0.24 in) in diameter. From the anterior end, the vulva occupy about one-third in body length. Females lay 200,000 eggs per day, and the uteri can contain up to 27 million eggs at a time. When the parasite infect a pig, female stop producing eggs after two to three weeks, then resume egg production when a male worm is present. Fertilized eggs are ovoid, ranging from 45 µm to 75 µm length and 35 µm 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 bile when the eggs are passed in faeces. Before insemination, female would deposit unfertilized eggs that are narrower and longer than normal fertilized eggs, ranging from 88 µm to 94 µm in length, and 44 µm diameter. Only proteinaceous layer can be seen in unfertilized eggs, because after fertilization the vitalline, chitinous, and lipid layers form.[3]
Infection of Ascaris suum occurs when its eggs, containing a third stage juvenile, are swallowed. The eggs are ingested via contaminated food and water. This contamination occurs when the host defecates in soil that is near habitations. The eggs remain viable anywhere from months to as many as four years, even in freezing conditions. The eggs are able to survive on their own for so long because they are resistant to strong chemicals, low temperatures, and dehydration. Their strong viability aids in the success of the parasite, and is due to the lipid layer of the eggshell that contains ascarosides. With this type of longevity with these eggs it is almost impossible to prevent reinfection once the soil has been contaminated.[3]
Ascaris suum egg
When juvenile worms are hatched, little damage is done by their penetration into the intestinal mucosa. Symptoms can be difficult to diagnose and confused with other diseases. Juveniles get into the respiratory system by breaking out of lung capillaries and causing small hemorrhages. Heavy infections cause small pools of blood to accumulate and causes edema. This, along with the accumulation of white blood cells and dead epithelium, causes congestion of the air pathways and is called Ascaris pneumonitis. With this condition the lung can become diseased and when bacterial infections become involved, it can lead to death.[3]
Clinical signs include coughing, rapid shallow breathing called "thumps", unthriftiness, colic, weight loss or reduced weight gain.
Diagnoses by finding eggs (not in paratenic hosts) in the faeces by fecal floatation and/or clinical signs.
Ascaris infections are treated with ascaricides.
In Canada in 1970, a postgraduate student tainted his roommates' food with Ascaris suum. Four of the victims became seriously ill; two of these suffered acute respiratory failure.[4][5]
A 273 megabase draft genome for Ascaris suum was published in 2011 in Nature.[6]
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