Ancylostoma, Ascaris and Enterobius

Nematodes: general points

• Roundworms: unsegmented, cylindrical, separate sexes; complete gut.
• Soil-transmitted helminths (Ascaris, hookworms, Trichuris, Strongyloides): eggs or larvae must mature in warm, moist soil contaminated by open defecation; common in Pakistan's rural areas and urban slums, mainly among school-age children.
• Eosinophilia is typical during tissue migration (larval stages), less so with adult worms in the gut lumen.
• Larvae that pass through the lungs: Ascaris, hookworms, Strongyloides ("ASH").

Hookworm (Ancylostoma duodenale and Necator americanus)

Features

Life cycle

1. Eggs passed in faeces onto warm moist soil hatch into rhabditiform larvae, which moult into infective filariform (L3) larvae (5–10 days).
2. Filariform larvae penetrate intact skin (usually bare feet, hands) → itchy rash (ground itch).
3. Carried in the blood to the lungs, break into alveoli, migrate up the airways and are swallowed (cough, wheeze, eosinophilia).
4. Mature in the small intestine (jejunum), attach to the mucosa with teeth or plates, suck blood (anticoagulant secreted), mate; eggs appear in stool about 6–8 weeks after infection. Adults live for years.

Disease

• Iron-deficiency anaemia (microcytic, hypochromic): chronic intestinal blood loss; hookworm is a leading cause of anaemia in children and pregnant women in endemic areas; fatigue, pallor, breathlessness, pica, koilonychia, heart failure in severe cases.
• Hypoalbuminaemia (protein loss) → oedema.
• Growth retardation and impaired cognitive development in children.
• Epigastric pain, diarrhoea.
• Eosinophilia.
• Cutaneous larva migrans ("creeping eruption") is caused by animal hookworms (dog and cat Ancylostoma braziliense) that cannot complete their cycle in humans.

These per-worm figures are approximate teaching values.

Diagnosis and treatment

• Stool microscopy: oval, thin-shelled, colourless eggs with 4–8 cell stage embryo (Ancylostoma and Necator eggs look alike); Kato-Katz method counts eggs per gram (intensity).
• Full blood count: microcytic anaemia, eosinophilia; low ferritin.
• Treatment: albendazole (single dose) or mebendazole; iron supplements; folic acid.
• Prevention: wearing shoes, latrines (stop open defecation), mass deworming.

Ascaris lumbricoides (roundworm)

Features

• The largest intestinal nematode (females up to about 35 cm, males smaller with a curved tail); pinkish-white.
• Eggs: fertilised eggs are round to oval with a thick, mammillated (bumpy), bile-stained brown outer coat; unfertilised eggs are longer. Eggs are very resistant and survive in soil for years.

Life cycle

1. Embryonated eggs (which need about 2–3 weeks in soil to become infective) are swallowed with contaminated food, water, raw vegetables or soil (hands, nails, children playing).
2. Larvae hatch in the small intestine, penetrate the wall, travel through the portal blood to the liver, then the heart and lungs.
3. In the lungs they moult, break into alveoli, are coughed up and swallowed (lung phase about 1–2 weeks after ingestion).
4. Mature into adults in the small intestine (jejunum), where they feed on intestinal contents; eggs appear in stool about 2–3 months after infection. Adults live about 1–2 years. No multiplication in the host: worm burden depends on repeated exposure.

Disease

• Pulmonary (larval) phase: Löffler syndrome (eosinophilic pneumonitis): dry cough, wheeze, fever, transient patchy lung shadows, eosinophilia; larvae sometimes found in sputum.
• Intestinal phase: often asymptomatic; abdominal pain, nausea, malnutrition (steals nutrients, causes lactose and vitamin A malabsorption), growth and cognitive impairment in children.
• Complications (heavy loads, children): intestinal obstruction by a bolus of worms (especially at the ileocaecal valve), volvulus, intussusception, perforation; migrating worms (stimulated by fever or some drugs): biliary ascariasis (biliary colic, cholangitis, obstructive jaundice), acute pancreatitis, appendicitis, liver abscess; worms vomited or passed in stool, or coming out of the nose.

Diagnosis and treatment

• Stool microscopy: characteristic eggs (very numerous, as each female lays about 200,000 eggs per day); adult worms passed or vomited; Kato-Katz for intensity.
• Eosinophilia in the larval phase (eggs not yet in stool).
• Imaging: worms in the bowel or bile ducts on ultrasound, contrast studies ("tram-track" shadows).
• Treatment: albendazole (single dose) or mebendazole; pyrantel pamoate (safe in pregnancy); ivermectin. Obstruction: conservative (nasogastric suction, fluids, then anthelmintic) or surgery.

Enterobius vermicularis (pinworm, threadworm)

Features

• Small white worms (female about 1 cm) with a pointed tail ("pin"); live in the caecum, appendix and colon.
• The commonest helminth infection in children worldwide (including temperate countries), all social classes; common in households, schools and institutions.

Life cycle and transmission

1. Gravid females migrate out of the anus at night and lay eggs on the perianal skin, causing intense itching.
2. Eggs become infective within about 6 hours.
3. Transmission: scratching → eggs under fingernails → hand-to-mouth (autoinfection); eggs on bedding, clothing, toys, and in airborne dust; retroinfection (larvae hatch on perianal skin and crawl back into the rectum).
4. Swallowed eggs hatch in the small intestine; worms mature in the caecum in about 4–6 weeks. No tissue migration, so eosinophilia is usually absent.

Disease

• Nocturnal perianal itching (pruritus ani) → disturbed sleep, irritability, bedwetting, scratching with secondary bacterial infection and eczema.
• Girls: vulvovaginitis, urethritis; rarely salpingitis, peritoneal granulomas.
• Occasionally found in the appendix (appendicitis is debated).

Diagnosis

• Adhesive tape (sellotape, Scotch tape) test or NIH swab: transparent tape pressed on the perianal skin first thing in the morning, before washing or defecation, then stuck on a slide; repeat on three consecutive mornings to increase sensitivity.
• Eggs: colourless, flattened on one side ("D-shaped" or plano-convex), contain a larva.
• Adult worms may be seen at the anus at night or on stool.
• Stool examination is usually negative (eggs are laid outside the gut).

Treatment and prevention

• Mebendazole or albendazole (single dose), or pyrantel pamoate; repeat after 2 weeks (the drugs do not kill eggs; the second dose kills newly hatched worms).
• Treat all household members at the same time.
• Hygiene: short nails, handwashing (especially after the toilet and before meals), morning bathing, washing bedding and night clothes in hot water, avoiding scratching.

Comparison

Control in Pakistan

• School-based deworming with single-dose albendazole or mebendazole (once or twice a year in high-prevalence areas, as recommended by WHO).
• Water, sanitation and hygiene (WASH): latrines, ending open defecation, handwashing, safe water, wearing shoes, washing vegetables.
• Treating anaemia in children and pregnant women (deworming after the first trimester).

Quick revision

• Hookworm: filariform larva penetrates skin → lungs → jejunum; blood loss → iron-deficiency anaemia.
• A. duodenale: teeth, more blood loss; Necator: cutting plates.
• Ascaris: largest; swallowed embryonated eggs → liver → lungs (Löffler, eosinophilia) → intestine; obstruction, biliary ascariasis.
• Enterobius: eggs laid perianally at night; pruritus ani; sellotape test in the morning; D-shaped eggs; autoinfection.
• Lung migration: Ascaris, Strongyloides, Hookworm.
• Treatment: albendazole or mebendazole; for pinworm treat the family and repeat at 2 weeks.
• Prevention: shoes, latrines, handwashing, school deworming.

Written by The House Job from standard microbiology and parasitology references (Levinson's Review of Medical Microbiology and Immunology).

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