Ascaris

MICROBIOLOGY

• Ascaris lumbricoides is the largest intestinal nematode (nemato from Greek, thread) or roundworm, a human pathogen and helminth or worm-like parasite.
• Soil-transmitted helminths (STH) also include Strongyloides stercoralis, Trichuris trichiura (whipworm), Ancylostoma duodenale (hookworm)and Necator americanus (hookworm).[2]
  • A. lumbricoides ova are excreted in stool, are hardy, can sustain freezing, and live up to 6 years in moist soil. Eggs are rounded and may have a thick shell with an external mammillated (bumpy) layer or may appear smooth and lack the mammilated layer.[11]
  • Ova develop in warm, humid soil over a period of 3 weeks before becoming infectious; thus, they are known as ’soil-transmitted’ helminths.
  • Life cycle- Humans ingest infective ova that hatch and release larvae. Larvae invade the intestinal mucosa and travel from the portal to systemic circulation to the lungs in about 4 days.
  • Larvae penetrate alveoli, ascend the trachea, and are reingested, then mature in the intestines and live for 10 to 24 months.[11]
  • Adult worms can live in the lumen of the small intestine for up to 2 years; adult female worms can produce 200,000 ova/day.

Epidemiology:

• Ascariasis is endemic in warm, humid areas, especially where human excreta is used as fertilizer or contaminated wastewater for irrigation.[3]
• Infection occurs most commonly in younger individuals but affects all ages.
  
  • Heavy infections can cause malnutrition, especially in children.[5]
• Transmission is from ingesting soil, food, or water contaminated with eggs.
• Ascariasis due to A. suum is a swine-associated zoonosis.