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Helminths - Complete NEET PG Guide

Classification of Medically Important Helminths

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PART 1 - NEMATODES (Roundworms)

1. Ascaris lumbricoides (Common Roundworm)

• Transmission: Ingestion of embryonated eggs from fecally contaminated soil/food
• Site: Small intestine; larvae migrate through lungs (Loeffler's syndrome = pulmonary eosinophilia with transient infiltrates)
• Diagnosis: Stool exam for ova & parasites (eggs)
• Treatment (DOC): Albendazole 400 mg single dose OR Mebendazole OR Pyrantel pamoate
• Key NEET point: Largest intestinal nematode; causes Loeffler's syndrome during larval migration; surgical emergency if large worm bolus

2. Enterobius vermicularis (Pinworm / Threadworm)

• Transmission: Feco-oral; self-contamination (ingestion of eggs)
• Site: Caecum and colon; female migrates to perianal region at night to lay eggs
• Diagnosis: "Scotch tape test" (adhesive tape pressed to perianal skin, microscopy for eggs) - do NOT use stool exam (eggs not found in stool)
• Treatment (DOC): Mebendazole 100 mg single dose OR Pyrantel pamoate 11 mg/kg single dose; repeat dose after 2 weeks (to kill newly hatched larvae); treat entire household
• Key NEET point: Most common helminthic infection in temperate/developed countries; nocturnal perianal pruritis; Scotch tape test

3. Trichuris trichiura (Whipworm)

• Transmission: Ingestion of eggs from contaminated soil
• Site: Caecum and colon
• Diagnosis: Stool O&P for characteristic barrel-shaped eggs with polar plugs
• Treatment (DOC): Mebendazole OR Albendazole
• Key NEET point: Can cause rectal prolapse in heavy infections; barrel-shaped eggs with bipolar plugs = pathognomonic

4. Hookworms - Ancylostoma duodenale & Necator americanus

• Transmission: Larvae in soil penetrate skin (filariform larvae) then migrate through lungs
• Site: Small intestine; attach to mucosa and suck blood
• Diagnosis: Stool O&P (eggs); Occult blood in stool
• Complications: Iron deficiency anemia (most important cause of helminth-related anemia); hypoalbuminemia
• Treatment (DOC): Albendazole 400 mg single dose OR Mebendazole OR Pyrantel pamoate
• Special: Ancylostoma is more pathogenic (loses more blood per worm); can cause cutaneous larva migrans
• Key NEET point: A. duodenale = Old World hookworm; N. americanus = New World; Loeffler's syndrome during lung migration; causes hypochromic microcytic anemia

5. Strongyloides stercoralis (Threadworm)

• Transmission: Larvae penetrate skin (like hookworm); unique = autoinfection possible
• Diagnosis: Stool exam / sputum / bronchial lavage for larvae (not eggs); serology
• Treatment (DOC): Ivermectin (preferred); Albendazole as alternative
• Key NEET point: Only nematode capable of autoinfection and hyperinfection (deadly in immunocompromised - disseminated strongyloidiasis); DOC = Ivermectin

6. Trichinella spiralis (Trichinosis)

• Transmission: Eating undercooked pork or other infected meat
• Site: Adults in small intestine 1-4 months; larvae encyst in muscle tissue (especially diaphragm, masseter, intercostal)
• Diagnosis: Serology + muscle biopsy showing encysted larvae
• Clinical: Diarrhea, periorbital edema, myalgia, eosinophilia - classic triad
• Treatment (DOC): Albendazole OR Mebendazole + corticosteroids for severe infection (myocarditis, CNS involvement)
• Key NEET point: Periorbital edema + myalgia + eosinophilia + pork ingestion = Trichinosis

7. Cutaneous Larva Migrans (Creeping Eruption)

• Cause: Ancylostoma braziliense (dog/cat hookworm); larvae in contaminated sand/soil
• Site: Skin (serpiginous/creeping track)
• Treatment (DOC): Albendazole 400 mg once daily x 7 days OR Ivermectin (single dose); Thiabendazole topically (alternative)

8. Visceral Larva Migrans (Toxocariasis)

• Cause: Toxocara canis (dogs)
• Treatment (DOC): Albendazole; Mebendazole as alternative

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PART 2 - FILARIAE

• Wuchereria bancrofti: nocturnal periodicity of microfilariae; chyluria, hydrocele, lymphedema
• Diagnosis: Night blood smear; daytime collection for Loa loa and O. volvulus
• Onchocerca = "River blindness" - DOC is Ivermectin (DEC causes Mazzotti reaction - severe allergic reaction); transmitted by Simulium black fly near fast-flowing rivers
• DEC is the DOC for most filarial infections EXCEPT Onchocerca

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PART 3 - TREMATODES (Flukes)

• ALL flukes use Praziquantel EXCEPT Fasciola hepatica = Triclabendazole (this is a classic MCQ)
• Schistosoma: intermediate host = snail; cercariae penetrate skin in water; S. haematobium causes squamous cell carcinoma of bladder
• Schistosoma eggs: S. haematobium = terminal spine; S. mansoni = lateral spine; S. japonicum = small lateral knob

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PART 4 - CESTODES (Tapeworms)

• T. solium causes BOTH taeniasis (intestinal) AND cysticercosis (tissue, especially brain = NCC)
• Neurocysticercosis: commonest cause of adult onset seizures in India; ring-enhancing lesion on CT/MRI; DOC = Albendazole + steroids
• Hydatid cyst (Echinococcus): sheep dog cycle; "daughter cysts" on USS; NEVER aspirate blindly (risk of anaphylaxis); PAIR (Puncture-Aspiration-Injection-Reaspiration) + Albendazole
• Diphyllobothrium latum = fish tapeworm; can cause B12 deficiency (competes for B12)
• Niclosamide = non-absorbable, acts on intestinal tapeworms only; mechanism = uncouples oxidative phosphorylation in mitochondria

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PART 5 - ANTHELMINTIC DRUGS (Pharmacology - High Yield)

Benzimidazoles (Albendazole & Mebendazole)

• Mechanism: Bind to beta-tubulin → inhibit tubulin polymerization → disrupt microtubule formation → impair glucose uptake and cause worm immobilization/death; also inhibit fumarate reductase
• Albendazole = broad-spectrum DOC; systemic infections (hydatid, NCC, visceral larva migrans, cutaneous larva migrans); absorbed better with fatty food; undergoes first-pass metabolism to active sulfoxide metabolite
• Mebendazole = poorly absorbed (acts locally in gut); better for intestinal infections; NOT recommended in pregnancy (teratogenic in animals)
• Spectrum: Ascaris, hookworm, pinworm, whipworm, Trichinella, Strongyloides, hydatid, cysticercosis

Ivermectin

• Mechanism: Potentiates GABA-mediated neurotransmission → hyperpolarization of nerve/muscle cells → paralysis of parasite; also disrupts parasite surface/tegument
• DOC for: Strongyloides stercoralis, Onchocerca volvulus, cutaneous larva migrans (alternative), head lice, scabies
• Note: Does NOT cross blood-brain barrier in humans (safe); Mazzotti reaction can occur with Loa loa if microfilarial load is high

Praziquantel

• Mechanism: Causes influx of Ca2+ → spastic paralysis of worm; disrupts tegument → worm killed by host immune system
• DOC for: ALL trematodes (except Fasciola) and ALL cestodes (except hydatid/cysticercosis where albendazole preferred)
• Given in 1-3 doses (very convenient)

Diethylcarbamazine (DEC)

• Mechanism: Immobilizes microfilariae; alters parasite surface so host immune system can kill them
• DOC for: Wuchereria bancrofti, Brugia malayi, Loa loa (filariasis)
• Contraindicated in Onchocerciasis (causes severe Mazzotti reaction)

Pyrantel Pamoate

• Mechanism: Depolarizing neuromuscular blockade → spastic paralysis of worm; inhibits acetylcholinesterase
• DOC for: Pinworm, Ascaris, hookworm (alternative)
• NOT effective against: Trichuris (whipworm), Strongyloides
• Note: Antagonistic with piperazine (avoid combination)

Niclosamide

• Mechanism: Uncouples oxidative phosphorylation in tapeworm mitochondria → loss of ATP → death
• Used for: Intestinal tapeworms (T. saginata, T. solium adult - NOT cysticercosis because not absorbed); small intestinal flukes
• Key: Not absorbed, works only in intestinal lumen

Triclabendazole

• DOC for Fasciola hepatica (sheep liver fluke) - the ONE fluke NOT responding to praziquantel
• Also used for Paragonimus (but praziquantel is preferred)

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HIGH YIELD MCQ SUMMARY TABLE

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CLASSIC MCQ TRAPS

1. Fasciola hepatica = Triclabendazole (NOT praziquantel) - asked every year
2. Onchocerca = Ivermectin (DEC causes Mazzotti reaction - dangerous)
3. Strongyloides = Ivermectin DOC (NOT albendazole)
4. Pinworm diagnosis = Scotch tape test (NOT stool O&P)
5. Neurocysticercosis = Albendazole DOC (NOT praziquantel as first choice)
6. Hydatid = Albendazole + PAIR; never blindly aspirate
7. Niclosamide works only on intestinal worms (not absorbed) - so useless for NCC
8. B12 deficiency = Diphyllobothrium latum (fish tapeworm)
9. Squamous cell carcinoma of bladder = S. haematobium
10. Autoinfection = only Strongyloides stercoralis

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