Ascaris suum

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Ascaris suum
Ascaris-suum.jpg
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, 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. [1] Ascariasis is associated with contact to pigs and pig manure in Denmark. [2]

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.

Lifecycle[edit]

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 hepatotracheal migration. They then moult two times and become adults.

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.

Morphology[edit]

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 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. Females lay 200,000 eggs per day, and their uteri can contain up to 27 million eggs at a time. When the parasite infects a pig, the female stops producing eggs after two to three weeks, then resumes egg production when a male worm is present. 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. Before insemination, females 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 vitalline, chitinous, and lipid layers form.[3]

Epidemiology[edit]

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 type of longevity in these eggs, prevention of reinfection once the soil has been contaminated is almost impossible.[3]

Pathogenesis[edit]

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 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.[3]

Diagnosis and treatment[edit]

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[edit]

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.[4][5]

Genetics[edit]

A 273-megabase draft genome for A. suum was published in 2011.[6]

References[edit]

  1. ^ http://www.parasitesandvectors.com/content/5/1/42
  2. ^ http://jcm.asm.org/content/43/3/1142.full
  3. ^ a b c Larry S. Roberts & John Janovy, Jr. (2008). Foundations of Parasitology (8th ed.). McGraw-Hill. ISBN 978-0-07-131103-8. 
  4. ^ James A. Phills, A. John Harrold, Gabriel V. Whiteman & Lewis Perelmutter (1972). "Pulmonary infiltrates, asthma and eosinophilia due to Ascaris suum infestation in man" (PDF). New England Journal of Medicine 286: 965–970. doi:10.1056/NEJM197205042861802. PMID 5062734. 
  5. ^ "Risk Assessment for Food Terrorism and Other Food Safety Concerns". Food and Drug Administration Center for Food Safety and Applied Nutrition. October 13, 2003. Archived from the original on May 27, 2009. Retrieved February 15, 2012. 
  6. ^ Aaron R. Jex, Shiping Liu, Bo Li, Neil D. Young, Ross S. Hall et al. (2011). "Ascaris suum draft genome" (PDF). Nature 479 (7374): 529–533. doi:10.1038/nature10553. PMID 22031327.