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.  Ascariasis is associated with contact to pigs and pig manure in Denmark. 
A. suum is distributed worldwide and grows up to 40 cm (16 in) in length. Ascaris infections are treated with ascaricides. A. suum is in the family Ascarididae, and is one of the oldest associations to mankind.
The life cycle can be direct or indirect with paratenic hosts. Pigs get infected by ingesting infectious parasite eggs that are present in the environment. The eggs contain a stage 3 larva (L3). The larvae hatch from the egg inside the intestines of the pig and start their migration through the body of the pig. First they penetrate the intestinal wall (presumably at the level of the caecum/colon) and use the mesenterial blood veins to migrate to the liver. After borrowing their way through the liver tissue, they again use the bloodstream to reach the lungs. There, they get stuck in the capillaries and invade the lung alveoli. It takes approximately 7 days to reach the lungs. Once the larvae are in 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 another molt to the L4 stage. Two weeks later, 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 L2 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.
Males are about 15–31 cm (6–12 in) long, and 2–4 mm (0.1–0.2 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 20–49 cm (8–19 in) long and 3–6 mm (0.12–0.24 in) in diameter. From the anterior end, the vulva 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 bile 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.
Infection with A. suum occurs when its eggs, containing a third-stage juvenile, are swallowed. The eggs are ingested in contaminated food and water when the host defecates in soil near habitations. The eggs remain viable from several 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 longevity, prevention of reinfection once the soil has been contaminated is almost impossible.
When juvenile worms hatch, little damage is done by their penetration through 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 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, and can lead to death.
Diagnosis and treatment
Clinical signs include coughing, rapid shallow breathing called "thumps", unthriftiness, colic, and weight loss or reduced weight gain. Diagnoses are by finding eggs (not in paratenic hosts) in the faeces by fecal floatation and/or clinical signs. Ascaris infections are treated with ascaricides.
Incidents and outbreaks
A 273-megabase draft genome for A. suum was published in 2011.
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