What are intestinal nematodes and how do the four differ?
Nematodes are unsegmented roundworms. Of the intestinal ones, the WHO groups Ascaris lumbricoides (roundworm), Trichuris trichiura (whipworm) and the hookworms (Necator americanus, Ancylostoma duodenale) as soil-transmitted helminths (STH) — about 1.5 billion people, 24% of the world's population, are infected. They are addressed together because they need similar diagnostic procedures and respond to the same medicines. Strongyloides stercoralis is a soil-transmitted nematode with peculiar features, and Enterobius vermicularis (pinworm) is spread person to person without any soil phase. Hookworm has its own page: hookworm.
| Feature | Ascaris | Enterobius | Trichuris | Strongyloides |
|---|---|---|---|---|
| Common name | Giant roundworm | Pinworm / threadworm | Whipworm | Threadworm |
| How acquired | Swallowing embryonated eggs | Swallowing eggs from contaminated fingers, bedding or dust | Swallowing embryonated eggs | Skin penetration by filariform larvae |
| Habitat | Small intestine | Caecum and appendix | Terminal ileum, caecum, colon (anterior end in mucosa) | Small-intestinal mucosa |
| Lung phase | Yes (Loeffler syndrome) | No | Not part of the cycle described | Yes |
| Autoinfection | No | Yes (fingers) | No | Yes (internal; perianal) |
| Diagnostic stage | Eggs in stool | Eggs on perianal skin (tape test) | Eggs in stool | Rhabditiform larvae in stool |
| Drug of choice | Albendazole | Albendazole, mebendazole or pyrantel | Mebendazole (or albendazole) | Ivermectin |
What is the life cycle and clinical picture of Ascaris lumbricoides?
Ascariasis follows ingestion of embryonated eggs from stool-contaminated soil, food or water. Eggs hatch in the duodenum; larvae penetrate the intestinal mucosa, travel by the portal system to the liver and reach the lungs in about 7–10 days, break into the alveoli, ascend the bronchial tree, are coughed up and swallowed, and mature into adults in the small intestine over 2–3 weeks. The adult female is 20–30 cm long and males are 15–20 cm, a female lays up to 200,000 eggs a day, and the adult lives about one year. Eggs become infective after 2–8 weeks in warm, moist, shaded soil and may survive for years.
| Phase | Features |
|---|---|
| Larval migration (lungs) | Transient eosinophilic pneumonitis (Loeffler syndrome): cough, wheeze, dyspnoea, fever, haemoptysis, peripheral eosinophilia |
| Intestinal phase | Most are asymptomatic; abdominal pain, bloating, nausea, anorexia; chronic malnutrition and growth retardation in children |
| Complications (adult worms) | Intestinal obstruction, volvulus, intussusception, appendicitis; biliary colic, cholangitis, cholecystitis, pancreatitis when worms enter the biliary or pancreatic ducts |
- Diagnosis: stool microscopy — the egg is large, oval, brown and trilayered with a mammillated (knobbly) outer coat. Eggs are absent in the first 3–4 weeks, so early infection may be missed; look for eosinophilia or larvae in sputum.
- Radiology: a plain abdominal film may show a 'whirlpool' or 'railway track' pattern; ultrasound or CT shows worms in the biliary tree, and ERCP can both diagnose and remove them.
- Epidemiology: mostly children aged 5–15 years in tropical areas with poor sanitation; adult worms do not multiply in humans, so infection resolves only if reinfection stops.
How does Enterobius vermicularis cause perianal itching?
The pinworm lives in the caecum and appendix. Females are 8–13 mm long (males 2–5 mm) and live about 3 months. A gravid female migrates out of the colon at night to the perianal skin to lay about 11,000 eggs and then dies. The eggs cause local inflammation and intense itching (pruritus ani); scratching contaminates the fingers and under the nails, which re-infect the mouth (autoinfection) and spread eggs to bedding, clothing and other people. Eggs become infective within 4–6 hours of being laid. Swallowed eggs hatch in the duodenum and the larvae mature in the caecum over 15–40 days.
| Point | Detail |
|---|---|
| Commonest symptom | Perianal itching, worse at night; sleep disturbance, irritability |
| Other effects | Vulvovaginitis and urinary tract infection in girls; rarely salpingitis, oophoritis and peritoneal inflammation; worms are found in the appendix, but a causal role in appendicitis is controversial |
| Asymptomatic | About 30–50% of infected people |
| Diagnosis | Cellophane (adhesive) tape test on the perianal skin, early morning before washing. One test is only about 50% sensitive, rising to 90% after 3 days and 99% after 5 days. Stool examination is not useful; no serology exists |
| Treatment | Single dose of mebendazole 100 mg, albendazole 400 mg or pyrantel pamoate 11 mg/kg (maximum 1 g), repeated after 14 days; treat all household members together; wash bedding and clothes |
| Pregnancy | Avoid drug treatment in the first trimester; mebendazole is the drug of choice after that |
What are the features of Trichuris trichiura (whipworm)?
Whipworm is acquired by faeco-oral transmission of embryonated eggs. Larvae hatch in the small intestine and move to the large intestine, where the anterior end of the worm is embedded in the mucosa, damaging cells and recruiting eosinophils, lymphocytes and plasma cells. The worm lives mostly in the terminal ileum and caecum (sometimes the whole colon and rectum) for 1–4 years. Eggs leave in the faeces unembryonated and become infective after 2–4 weeks in soil. Young boys are most affected, as they play outside and exhibit pica.
- Mild infection: usually asymptomatic. Heavier infection: abdominal pain, painful passage of stool, mucus, diarrhoea or constipation, nocturnal stools.
- Heavy infection in children: rectal prolapse, anaemia, growth deficiency and impaired cognition (through iron deficiency and poor nutrition); Trichuris dysentery syndrome.
- Diagnosis: eggs in stool — barrel-shaped, thick-shelled, with a polar plug at each end. WHO recommends the Kato-Katz method to count eggs per gram. There is a pre-patent period of about 3 months.
- Endoscopy: the classic finding is 'coconut cake rectum' — adult worms dangling from inflamed mucosa.
- Treatment: mebendazole 100 mg twice daily for 3 days is first-line; albendazole 200–400 mg twice daily for 3 days is the alternative; ivermectin is less effective. Whipworm is more resistant than other worms — single-dose cure rates as low as 28–36% have been reported.
Why is Strongyloides stercoralis dangerous — autoinfection and hyperinfection?
Strongyloides is the only human helminth that can complete its life cycle both inside the host and in the soil. Filariform (L3) larvae penetrate intact skin, travel in blood to the lungs, ascend the trachea, are swallowed and mature into adult females in the small-intestinal mucosa. The females reproduce by parthenogenesis; eggs hatch in the gut and rhabditiform larvae are passed in the stool. In soil they either become infective filariform larvae directly or develop into free-living adults that reproduce sexually.
| Point | Detail |
|---|---|
| Autoinfection | Rhabditiform larvae mature to filariform inside the gut and re-invade through the bowel mucosa or perianal skin; infection can last decades |
| Hyperinfection / dissemination | Intensified autoinfection in the immunosuppressed, above all after corticosteroids; also HTLV-1, diabetes, alcoholism. Larvae reach the lungs, CNS and other organs; gram-negative sepsis and meningitis from bacterial translocation. Mortality about 70% to nearly 100% |
| HIV | Not clearly a risk factor for hyperinfection |
| Skin sign | Larva currens — 'creeping' skin eruption, a form of cutaneous larva migrans |
| Eosinophilia | Common in chronic infection but often absent in hyperinfection |
| Diagnosis | Larvae (not eggs) in stool. A single direct stool smear detects only about 21%; Baermann concentration about 72%; agar-plate culture up to 89%; PCR and serology help |
| Treatment | Ivermectin 200 µg/kg/day for 2 days (first-line); albendazole 400 mg twice daily for 3–7 days (second-line). Hyperinfection: ivermectin daily for at least 2 weeks plus antibiotics and reduction of immunosuppression |
| Safety point | Screen for loiasis before ivermectin in Loa loa endemic areas (risk of encephalopathy) |
Which drug treats which intestinal nematode?
| Worm | Preferred regimen | Notes |
|---|---|---|
| Ascaris | Albendazole 400 mg single dose; alternatives mebendazole (100 mg twice daily for 3 days or 500 mg once), ivermectin 150–200 µg/kg | Pyrantel pamoate 11 mg/kg (maximum 1 g) preferred in first-trimester pregnancy; defer during the lung phase |
| Enterobius | Mebendazole 100 mg, albendazole 400 mg or pyrantel 11 mg/kg, single dose, repeat in 14 days | Treat the whole household |
| Trichuris | Mebendazole 100 mg twice daily for 3 days (first-line); albendazole 200–400 mg twice daily for 3 days | Hardest to cure; ivermectin less effective |
| Strongyloides | Ivermectin 200 µg/kg/day for 2 days | Albendazole is second-line; thiabendazole is out of favour because of adverse effects |
| Mass deworming (STH programmes) | WHO-recommended albendazole 400 mg or mebendazole 500 mg | Safe enough for non-medical staff such as teachers to give |
Albendazole and mebendazole act by inhibiting tubulin polymerisation, which causes loss of cytoplasmic microtubules in the worm. Ivermectin binds glutamate-gated chloride channels of invertebrate nerve and muscle cells, causing hyperpolarisation, paralysis and death of the parasite.