| Ascaris lumbricoides | |
|---|---|
 |
|
| An adult female Ascaris worm. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Nematoda |
| Class: | Secernentea |
| Order: | Ascaridida |
| Family: | Ascarididae |
| Genus: | Ascaris |
| Species: | A. lumbricoides |
| Binomial name | |
| Ascaris lumbricoides Linnaeus, 1758 |
|
Ascaris lumbricoides is the member of the Ascaris family responsible for the disease ascariasis.
It can reach a length of up to 35 cm.[1]
Contents |
Life cycle
Ascaris lumbricoides, or "roundworm", infections in humans occur when an ingested infective egg releases a larval worm that penetrates the wall of the duodenum and enters the bloodstream. From here, it is carried to the liver and heart, and enters pulmonary circulation to break free in the alveoli, where it grows and molts. In 3 weeks, the larvae pass from the respiratory system to be coughed up, swallowed, and thus returned to the small intestine, where they mature to adult male and female worms. Fertilization can now occur and the female produces as many as 200,000 eggs per day for a year. These fertilized eggs become infectious after 2 weeks in soil; they can persist in soil for 10 centuries or more.[2]
The eggs have a lipid layer, containing ascarocides and it makes them resistant to the effects of acids and alkalis as well as other unpleasant chemicals. This resilience helps to explain why this nematode is such a ubiquitous parasite.[3]
Morphology
Ascaris lumbricoides is characterized by its great size. Males are 2–4 mm in diameter and 15–31 cm long. The males' posterior end is curved ventrally and has a bluntly pointed tail. Females are 3–6 mm wide and 20–49 cm long. The vulva is located in the anterior end and accounts for about a third of its body length. Uteri may contain up to 27 million eggs at a time with 200,000 being laid per day. Fertilized eggs are oval to round in shape and are 45-75 micro meters long and 35-50 micro meters wide with a thick other shell. Unfertilized eggs measure 88-94 micro meters long and 44 micro meters wide.[4]
Epidemiology
Approximately 1.27 billion people or about a quarter of the world population are infected. In the United States there is a reported prevalence of 0.8% of the total population as of 1987. Ascaris lumbricoides eggs are extremely resistant to strong chemicals, desiccation, and low temperatures. The eggs can remain viable in the soil for several months or even years.[4]
Infections
Infections with these parasites are more common where sanitation is poor[5] and human feces are used as fertilizer.
Prevention of this infection centers around education, not using human feces as fertilizer, and cleanliness, especially among those who handle food.
Infections happen when a human swallows water or food contaminated with unhatched juveniles. The juveniles hatch in the duodenum (1st section of small intestine). They then penetrate the mucosa and submucosa and enter venules or lymphatics. Next they pass through the right heart and into pulmonary circulation. They then break out of the capillaries and enter the air spaces. Acute tissue reaction occurs when several worms get lost during this migration and accumulate in other organs of the body. The juveniles migrate from the lung up the respiratory tract to the pharynx where they are swallowed. They begin producing eggs within 60–65 days of being swallowed. These are produced within the small intestine where the juveniles mature. It might seem odd that the worms end up in the same place where they began. One hypothesis to account for this behavior is that the migration mimics an intermediate host, which would be required for juveniles of an ancestral form to develop to the third stage. Another possibility is that tissue migration enables faster growth and larger size, which increases reproductive capacity.[6]
More than 1 billion people are affected by this infection.[2] Infections can be treated with drugs called ascaricides.
Diagnosis and Treatment
Most diagnoses are made by identifying the appearance of the worm or eggs in feces. Due to the large quantity of eggs laid physicians can diagnose using only one or two fecal smears.
The treatment of choice is Mebendazole. The drug functions by binding to tubulin in the worms' intestinal cells and body wall muscles. Nitazoxanide and ivermectin can also be used.[4]
References
- ^ "eMedicine - Ascaris Lumbricoides : Article by Aaron Laskey". http://www.emedicine.com/EMERG/topic840.htm. Retrieved 2008-02-03.
- ^ a b Murray, Patrick R.; Rosenthal, Ken S.; Pfaller, Michael A. Medical Microbiology, Fifth Edition. United States: Elsevier Mosby, 2005
- ^ Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals, Greenwood Press.
- ^ a b c Roberts, Larry S.; Janovy, John Jr. Foundations of Parasitology, Eight Edition. United States: McGraw-Hill, 2009
- ^ "DPDx - Ascariasis". http://www.dpd.cdc.gov/dpdx/html/Ascariasis.htm. Retrieved 2008-02-03.
- ^ Read, A.F.; Skorping, A. 1995. The Evolution of Tissue Migration by Parasitic Nematode Larvae. Parasitology 111:359-371
External links
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
