Parasitology Quiz: Life Cycle of Intestinal Worms

Ascaris lumbricoides (Round Worm)

• Habitat: Small intestine of man (Definitive host)
• Morphology:
  • Adult worms: Long, creamy or pink in color, male is smaller and thinner than female
  • Male: 20 cm in length, ventrally curved posterior end
  • Female: 25 cm in length, straight posterior end
• Egg:
  • Fertilized egg: Oval, brown, medium size, course albuminous mamillated layer, immature (contains one cell stage)
  • Unfertilized egg: Elongated or irregular in shape, contain refractile granules, thin shell with weak mamillated layer
  • Decorticated egg: Mamillated layer is lost
• Life cycle:
  • Fertilized females lay immature fertilized eggs (Diagnostic stage) in feces (200,000 egg/female/day)
  • Immature eggs develop in soil into embryonated eggs (Infective stage) in 2 weeks
  • Embryonated eggs are ingested with contaminated food, water, or hands
  • Eggs hatch in small intestine, liberating larvae
  • Larvae penetrate intestinal wall, enter circulation, and migrate to lung, trachea, and finally back to small intestine
• Pathogenesis and clinical picture:
  • Migration stage: Migrating larvae in lung may lead to allergic pneumonitis
  • Intestinal stage: Adult worms may cause abdominal pain, nausea, vomiting, colic, or intestinal obstruction
• Diagnosis:
  • Clinical diagnosis: Clinical picture
  • Laboratory diagnosis: Finding eggs in feces, larvae in sputum, or accidental expulsion of worms in vomits or stools

Schistosoma spp. (Blood Fluke)

• Habitat: Vesical and pelvic plexus of veins (Schistosoma haematobium) or mesenteric plexus of veins (Schistosoma mansoni)
• Morphology:
  • Cercariae (Infective stage): Develop in snail, penetrate human skin, and enter blood circulation
  • Schistosomulae (Immature worms): Migrate to liver, mature, and pair up
• Life cycle:
  • Cercariae penetrate human skin, enter blood circulation, and migrate to liver
  • Male and female worms pair up, and female lays eggs in smaller capillaries
  • Eggs penetrate blood capillary walls, reach urinary bladder (Schistosoma haematobium) or intestine (Schistosoma mansoni)
  • Eggs exit with urine or feces, and reach fresh water to complete the life cycle
• Pathogenesis and clinical picture:
  • Stage of invasion: Penetration of cercariae leads to itching and dermatitis
  • Stage of migration: Immature worms in lung and liver cause allergic manifestations, fever, cough, and haemoptysis
  • Liver becomes tender and enlarged
• Diagnosis:
  • Clinical diagnosis: Clinical picture
  • Laboratory diagnosis: Examination of stool or urine for eggs

Hymenolepis nana (Dwarf Tapeworm)

• Pathogenesis and clinical picture:
  • Light infection: Asymptomatic
  • Intestinal disturbance: Abdominal pain, diarrhea
  • Severe infections: Insomnia, convulsions
• Diagnosis:
  • Clinical diagnosis: Clinical picture
  • Laboratory diagnosis: Examination of stool for eggs
• Treatment: Praziquantel, Niclosamide (Yomesan)
• Prevention and control:
  • Treatment of patients
  • Proper hygiene to prevent autoinfection
  • Protection of food and drink from contamination with infected human feces

General Characteristics of Nematodes

• Bilaterally symmetrical, unsegmented, elongated, and cylindrical
• Separate sexes, males are smaller than females
• Body cavity contains body fluid, digestive and reproductive systems
• Nematodes infecting man can be divided into Intestinal and Tissue nematodes

This quiz covers the life cycle of intestinal worms, including the characteristics of unfertilized and decorticated eggs, and the role of the definitive host.