Ascaris lumbricoides - MegaMicro

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Ascaris lumbricoides

Microbiology > Parasitology > Nematodes
Ascaris lumbricoides
Structure:  Helminth, nematode (roundworm): largest nematode parasitizing the human intestine.   Adult female 20-35 cm and adult male 15-30 cm in length.

• The most common helminthic infection worldwide, with an estimated 1 billion people infected.
• Highest prevalence in tropical/subtropical regions and areas with inadequate sanitation, especially where human feces are used for fertilizer.  Historically, infections were reported in rural areas of southeastern USA.
• No animal reservoir, however, a nearly identical species is seen in pigs (A. suum)  and may infect humans.

• Fecal-oral transmission, with infection following ingestion of food or water contaminated with Ascaris eggs.
• Following ingestion of infective eggs, a larval worm is released which penetrates the intestinal mucosa, enters the bloodstream and is transported to the lungs.  Larvae will further mature in the lungs where they are coughed up and swallowed.  Mature adult worms will then establish infection in the small intestine where they produce eggs that will be passed into the stool.  
• Fertilized eggs become infectious after 2-3 weeks in the soil.  Ascaris eggs are also very hardy and may persist in feces/soil for months to years.

Disease Manifestations:
• Many infections are asymptomatic.
• An infection with many larvae may cause an eosinophilic pneumonitis (Loeffler’s syndrome).
• Heavy adult worm burdens may result in intestinal blockage, perforation, migration into the bile duct, liver and gallbladder resulting in severe tissue damage, as well as stunted growth in children due to malabsorption.

Laboratory Diagnosis:
• Microscopic identification of eggs in a stool sample (characteristically knobby-coated, bile stained, 55-75 um long and 35-50 um wide).
• Occasionally, adult worms are passed in the feces, which can be quite dramatic due to their large size, and/or larvae may be seen in sputum or gastric aspirates.

Differential Diagnosis: other intestinal nematodes, bowel obstruction, pancreatitis, malabsorption, biliary obstruction

• Albendazole or mebendazole, alternatively, pyrantel pamoate or Ivermectin

Prevention and Control:
• Good personal hygiene and improved sanitation, avoid using human feces as fertilizer.

Related concepts
1. Ascaris lumbricoides
2. Nematode
3. Loeffler’s syndrome

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