Nematodes: Ascaris Lumbricoides

Nematoda: General Features

• Ascaris lumbricoides is a common parasitic roundworm that infects humans, primarily in the intestines.
• It is the largest nematode to infect humans, with adult females measuring up to 45 cm long.
• An estimated 1 billion people are infected, with 25% of the world's population affected.

Host and Transmission

• Definitive hosts: humans or pigs
• No intermediate host
• Transmission occurs mainly through ingestion of contaminated food or water, or inhalation of contaminated dust.
• Children playing in contaminated soil can acquire the parasite from their hands.
• Prior infection does not confer protective immunity.

Egg Characteristics

• Eggs can survive for up to 10 years in warm, shaded, and moist conditions.
• Eggs are resistant to chemical water purification methods, but can be removed by filtration and killed by boiling.
• Developing larvae are destroyed by sunlight and desiccation.

Life Cycle

• Females lay eggs in the small intestine, which are then passed out through feces.
• Eggs develop into L1 larvae after 14 days, and L2 larvae after one week.
• Ingestion of contaminated food or water leads to infection.
• L2 larvae penetrate the intestinal wall, enter the portal blood stream, and migrate to the liver, heart, and lungs.
• After several stages, the larvae mature and mate in the small intestine.

Food Habits

• The parasite feeds on semi-digested contents in the gut.
• It can also bite the intestinal mucus membrane and feed on blood and tissue fluids.

Symptoms

• Larval migration can cause hemorrhagic pneumonia, cough, and breathing difficulties.
• Adult parasites in the intestine can cause abdominal discomfort, nausea, and malnutrition.
• In severe cases, the worm mass can block the intestine, leading to fatalities.

Host Immune Response

• Innate immune response involves macrophages, neutrophils, and eosinophils.
• Adaptive immune response involves a Th2 response with high IL-4 production, eosinophilia, and mastocytosis.

Diagnosis

• Stool microscopy: eggs may be seen on direct examination of feces.
• Eosinophilia: found during larval migration through the lungs.
• Imaging: ultrasound or ERCP can detect worms in the abdomen or hepatobiliary/pancreatic ascariasis.

Treatment

• Medications include mebendazole, albendazole, pyrantel pamoate, ivermectin, and piperazine citrate.

Prevention

• Good sanitation is necessary to prevent fecal contamination of soil.
• Limiting the use of human feces as fertilizer can help prevent infection.
• Mass treatments of children with single doses of mebendazole or albendazole can reduce transmission.