Overview
Nematodes are cylindrical, bilaterally symmetrical worms ranging from the tiny Trichinella spiralis (4–5 mm) to the enormous Dracunculus medinensis (1.2 metres). Ascaris lumbricoides, the round worm, is the most common intestinal nematode infecting humans worldwide.
Ascariasis is especially prevalent in Asian and African nations with poor sanitation, and while often asymptomatic, heavy worm burdens can cause serious complications including intestinal obstruction and Loeffler's pneumonia during larval migration through the lungs.
Learning Objectives
After this lesson you will be able to…- Describe the general characteristics of nematodes
- Describe the morphology of Ascaris lumbricoides, including adult worms and ova
- Explain the life cycle of Ascaris lumbricoides in man and soil
- Discuss the pathogenicity of ascariasis
- Explain the laboratory diagnosis of ascariasis
Clinical Story
Why This MattersA 6-year-old child from a rural community presents with abdominal pain and passes a long, cream-coloured worm in the stool after vomiting. The mother brings the worm to the clinic, and stool microscopy confirms bile-stained, mammilated fertilised ova — a classic presentation of ascariasis requiring anthelmintic treatment and improved sanitation counselling.
Core Concepts
Nematodes have an elongated, cylindrical, unsegmented body with bilateral symmetry, covered by a multi-layered cuticle, and a pseudocoelomic body cavity. Sexes are separate; males are generally smaller with a curved posterior end. The digestive system has a mouth, muscular oesophagus, intestine, rectum and anus.
The adult female measures 200–400 × 3–6 mm and the male 150–300 × 2–4 mm, with three lips (2 ventral, 1 dorsal) at the anterior end. Fertilised ova (75 × 50 µm) are bile-stained, ovoid, with a thick chitinous shell and mammilated albuminous coat; unfertilised ova (90 × 55 µm) are larger, thinner-shelled, and contain disorganised refractile granules.
Gravid females lay eggs that mature in soil within 1–2 weeks. Ingested infective ova hatch in the stomach; larvae penetrate the gut wall, travel via the venous circulation to the lungs, moult twice, ascend the airway to the pharynx, and are swallowed to mature in the small intestine (adult lifespan ~1 year). Heavy infestation causes nutritional deficiency, intestinal/biliary/appendiceal obstruction, and larval migration through the lungs can cause allergic Loeffler's pneumonia.
Laboratory Principle
Diagnosis of ascariasis relies on direct microscopic demonstration of characteristic fertilised or unfertilised ova in a saline or iodine-stained stool wet mount, or occasionally identification of the expelled adult worm itself. The size, shape, bile staining, and mammilated coat of the ova are diagnostic features.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Normal Saline (0.85%) | Isotonic | Wet mount to visualise ova morphology | Room temperature |
| Lugol's Iodine (2%) | 2% | Enhances contrast of ova internal structures | Room temperature, dark bottle |
| Formalin-Ether Concentration Reagent | 10% formalin + ether | Concentrates ova from dilute stool specimens | Room temperature |
Step-by-Step Procedure
Collect a fresh stool sample in a clean, leak-proof container.
Emulsify a small portion of stool in saline on a slide and cover with a coverslip.
Prepare a second mount with iodine to enhance internal ovum structures.
Scan systematically under 10x, confirm ova morphology under 40x.
Report presence, approximate number (egg count if required), and type of ova observed.
Flow Diagram
Quality Control
Reference slides of fertilised and unfertilised Ascaris ova are reviewed periodically to maintain identification accuracy among laboratory staff.
Participation in stool parasitology proficiency testing programmes verifies correct identification of nematode ova across laboratories.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Bile-stained, mammilated fertilised ova seen | Confirms active ascariasis | Treat with albendazole/mebendazole; assess for obstruction |
| Decorticated fertilised ova (no albuminous coat) | Still diagnostic of Ascaris infection | Report as decorticated fertilised ova |
| Adult worm passed in stool or vomitus | Confirms heavy worm burden | Treat and monitor for obstructive complications |
Common Errors & How to Avoid Them
Cause: Unfertilised ova with disorganised granules can be mistaken for artefact.
Prevention: Compare size and shape carefully with reference images; unfertilised ova are larger (90×55 µm) and ovoid.
Cause: Loss of the mammilated coat can make ova less recognisable.
Prevention: Familiarise staff with both corticated and decorticated forms during training.
Cause: Light infections may be missed on a single smear.
Prevention: Use concentration techniques or examine multiple samples if ascariasis is strongly suspected.
Laboratory Tips from the Bench
Bile staining giving a brown colour to the fertilised ovum on saline mount is one of the fastest ways to recognise Ascaris eggs at low power.
Always ask if the patient or parent has seen a worm passed in stool or vomit — this history alone can be diagnostic in endemic settings.
Memory tip: 'Mammy's coat' — the mammilated albuminous coat wraps the fertilised Ascaris ovum, distinguishing it from the smoother decorticated form.
Important Notes
Larval migration through the lungs can cause a transient allergic pneumonitis with cough, wheeze and eosinophilia — always consider ascariasis in unexplained pulmonary eosinophilia in endemic areas.
Heavy Ascaris burden in children can cause intestinal obstruction, and worms may migrate into and obstruct the appendix or bile duct, presenting as surgical emergencies.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePresents with recurrent abdominal pain over several weeks, poor appetite, weight loss and occasional passage of a long worm noted by his mother in the stool.
The combination of a history of passing a worm, bile-stained mammilated ova on stool microscopy, and eosinophilia confirms ascariasis as the cause of this child's symptoms.
- →Ascariasis commonly presents with vague GI symptoms and nutritional deficiency in children
- →Bile-stained mammilated ova on stool microscopy is diagnostic
- →Eosinophilia often accompanies nematode infections due to larval tissue migration
Frequently Asked Questions
Infection occurs by ingesting infective ova from soil contaminated with human faeces, often via unwashed hands, contaminated vegetables, water, or dust.
This migration is part of the parasite's evolved life cycle — larvae must pass through the venous circulation to the heart and lungs, ascend the airway, and be swallowed again before completing maturation in the small intestine.
Heavy infestations can cause intestinal obstruction from entangled worm masses, and worms may migrate into and obstruct the appendix or bile duct, leading to appendicitis, pancreatitis or peritonitis.
Quick Revision
10-Minute ReviewKey Takeaways
- Ascariasis is one of the most common soil-transmitted helminth infections worldwide.
- Diagnosis relies on demonstrating characteristic ova in stool microscopy.
- The larval migration through the lungs can cause allergic Loeffler's pneumonia.
- Heavy worm burdens can cause serious obstructive surgical complications.
- Good sanitation and hand hygiene are key to preventing ascariasis.
- Fertilised and unfertilised ova have distinct, testable morphological differences.
Competency Checklist
Track Your MasteryReferences
- Ananthanarayan R, Paniker CKJ. Textbook of Microbiology. 10th ed. Universities Press.
- Chatterjee KD. Parasitology (Protozoology and Helminthology). 13th ed. CBS Publishers.
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 13th ed. Mosby.