Tissue & Blood Helminths
Microbiology · Parasitology · lean revision notes
Tissue & Blood Helminths
Helminths that complete part of their life cycle in human tissue, lymphatics, or blood — as opposed to luminal gut dwellers. This cluster (filaria, Taenia solium cysticercosis, Echinococcus, Schistosoma) is among the highest-yield parasitology blocks in NEET PG because of distinctive vectors, diagnostic gold-standards, and "drug-of-choice" recall. Peripheral eosinophilia is the unifying clue — tissue-invasive worms provoke it, luminal worms usually do not.
Classification & big-picture map
Tissue/blood helminths span both major helminth phyla:
| Worm | Phylum / class | Disease | Vector / source | Diagnostic stage |
|---|---|---|---|---|
| Wuchereria bancrofti | Nematode (filaria) | Lymphatic filariasis | Culex mosquito | Microfilaria in night blood |
| Brugia malayi | Nematode (filaria) | Lymphatic filariasis | Mansonia mosquito | Microfilaria |
| Loa loa | Nematode (filaria) | Loiasis (Calabar swelling) | Chrysops (mango/deer fly) | Day blood microfilaria |
| Onchocerca volvulus | Nematode (filaria) | River blindness | Simulium (black fly) | Skin snip microfilaria |
| Taenia solium (larva) | Cestode | Cysticercosis / NCC | Eggs (faeco-oral) | Cysts on imaging, serology |
| Echinococcus granulosus | Cestode | Hydatid cyst | Dog (definitive), sheep | Imaging + serology (Casoni obsolete) |
| Schistosoma spp. | Trematode | Schistosomiasis (bilharzia) | Snail → cercaria (skin) | Eggs in urine/stool |
| Trichinella spiralis | Nematode | Trichinellosis | Undercooked pork | Encysted larva in muscle |
High-yield: Tissue-invasive helminths cause eosinophilia; intestinal lumen-dwellers (e.g. adult Taenia, Ascaris adult, pinworm) cause little or none. The migratory/larval phase is what drives the eosinophil count. A returning traveller or rural patient with marked eosinophilia → think filaria, schistosome, or larval cestode.
1. Lymphatic filariasis — Wuchereria bancrofti
Etiology & life cycle
W. bancrofti causes ~90% of lymphatic filariasis; man is the only definitive host (no animal reservoir). Transmitted by Culex quinquefasciatus. Infective L3 larvae enter via the bite, migrate to lymphatics, mature into adult worms in lymph nodes/vessels, and produce microfilariae that circulate in blood with nocturnal periodicity (peak 10 pm–2 am) — synchronised to the night-biting Culex. Adult worm lifespan is 5–10 years.
Pathophysiology
Living adults cause lymphatic dilatation and dysfunction; dying/dead worms trigger granulomatous inflammation, lymphangitis, and eventual fibrosis → lymphatic obstruction. Wolbachia (endosymbiotic bacteria within the worm) drives much of the inflammatory response — the rationale for doxycycline as adjunct therapy.
Clinical spectrum
- Asymptomatic microfilaraemia — commonest; reservoir for transmission.
- Acute — filarial fever, lymphangitis (retrograde, distal-to-proximal — opposite of bacterial), lymphadenitis, epididymo-orchitis, funiculitis.
- Chronic — lymphoedema → elephantiasis (legs, scrotum, breast), hydrocele (commonest chronic sign of bancroftian filariasis), chyluria.
- Tropical pulmonary eosinophilia (TPE) — hypersensitivity to trapped microfilariae: nocturnal cough/wheeze, very high IgE, eosinophilia >3000/µL, microfilariae absent from blood (cleared in lungs). Responds dramatically to DEC.
| Feature | Bancroftian | Brugian (B. malayi) |
|---|---|---|
| Vector | Culex | Mansonia |
| Periodicity | Nocturnal | Nocturnal (sub-periodic in some) |
| Genital involvement | Common (hydrocele) | Rare |
| Limb oedema | Whole leg | Usually below knee |
| Sheath | Sheathed mf | Sheathed mf |
Diagnosis
- Gold standard / investigation of choice for active infection: thick blood smear (Giemsa) for microfilariae collected at night (~midnight).
- Best overall / antigen test: filarial circulating antigen (ICT card / Og4C3 ELISA) — detects adult worm antigen, can be done any time of day, useful when mf absent.
- Sheathed microfilaria with tail nuclei not extending to tip = W. bancrofti; nuclei up to the tip (two terminal nuclei) = B. malayi.
- USG scrotum: "filarial dance sign" — live adult worms moving in dilated lymphatics.
- Eosinophilia; raised IgE in TPE.
Management (drug of choice)
Flow: bite → L3 → lymphatics → adult → microfilaraemia → chronic lymphoedema.
- DEC (diethylcarbamazine) is the DOC — microfilaricidal and partially macrofilaricidal. Standard 12-day course or single-dose.
- Mass drug administration (MDA) for elimination: DEC + albendazole (± ivermectin). In areas co-endemic for onchocerciasis/loiasis, DEC is avoided (severe reactions) and ivermectin + albendazole used.
- Doxycycline (anti-Wolbachia) gives true macrofilaricidal effect.
- Chronic lymphoedema: limb hygiene, elevation; hydrocelectomy for hydrocele.
High-yield: DEC can precipitate the Mazzotti reaction in onchocerciasis and dangerous encephalopathy in heavy Loa loa loads — hence ivermectin-based regimens in co-endemic Africa. For Onchocerca, ivermectin is DOC; for Loa loa, DEC is DOC.
2. Cysticercosis & Neurocysticercosis — Taenia solium
Two diseases, one worm
- Taeniasis = adult tapeworm in gut from eating undercooked pork containing cysticercus cellulosae. Human is definitive host.
- Cysticercosis = human becomes accidental intermediate host by ingesting T. solium eggs (faeco-oral, autoinfection). Larvae disseminate to tissues — brain, muscle, eye, subcutis.
High-yield: You get the tapeworm from eating the larva (pork); you get cysticercosis from eating the egg. Taenia saginata (beef tapeworm) does NOT cause cysticercosis — only T. solium does. This egg-vs-larva distinction is a perennial exam favourite.
Neurocysticercosis (NCC)
Commonest cause of acquired epilepsy / new-onset seizures in the developing world (India endemic). Stages: vesicular → colloidal vesicular → granular nodular → calcified.
- Investigation of choice: MRI brain (cyst + scolex = pathognomonic "dot-in-hole"; CT best shows calcification). Serology: EITB (enzyme-linked immunoelectrotransfer blot) is most specific.
- Solitary cysticercus granuloma is the typical Indian presentation.
Differentiating T. solium vs T. saginata
| Feature | T. solium (pork) | T. saginata (beef) |
|---|---|---|
| Scolex | 4 suckers + rostellum with hooks (armed) | 4 suckers, no hooks (unarmed) |
| Gravid uterine branches | <13 (7–13) | >15 (15–30) |
| Cysticercosis in man | Yes | No |
| Eggs | Indistinguishable from saginata | Indistinguishable |
Management
- NCC: albendazole (DOC) ± praziquantel + corticosteroids (to control inflammation when cysts die) + antiepileptics. Do NOT treat single calcified (dead) lesions with cysticidals. Ophthalmoscopy before therapy — treating ocular cysts causes vision loss.
- Intestinal taeniasis: praziquantel single dose (or niclosamide).
High-yield: Always exclude ocular cysticercosis before giving albendazole/praziquantel for NCC — killing an intra-ocular cyst can blind the patient.
3. Hydatid disease — Echinococcus granulosus
Life cycle
Dog = definitive host (adult worm, smallest tapeworm, 3 segments). Sheep = usual intermediate host; man is an accidental, dead-end intermediate host. Humans ingest eggs from dog faeces → oncosphere penetrates gut → portal blood → liver (right lobe, ~65%, commonest) → lung (~25%) → other organs.
Pathology
Forms a unilocular hydatid cyst with three layers: outer pericyst (host fibrous reaction), middle acellular laminated membrane (ectocyst), inner germinal layer (endocyst) producing brood capsules, protoscolices, and daughter cysts. Hydatid sand = free protoscolices + hooklets in cyst fluid.
| Echinococcus species | Cyst type | Definitive host |
|---|---|---|
| E. granulosus | Unilocular hydatid (liver) | Dog |
| E. multilocularis | Alveolar (invasive, mimics malignancy) | Fox |
Clinical features & complications
- Slow-growing hepatic mass, RUQ discomfort; often incidental.
- Anaphylaxis on cyst rupture (spillage of antigenic fluid) — the most feared complication; spillage also causes secondary peritoneal seeding.
- Biliary rupture, infection, mass effect.
Diagnosis
- Investigation of choice: USG / CT abdomen — daughter cysts, "water-lily sign" (detached endocyst), membrane, septations. Gharbi/WHO classification grades cysts.
- Serology: ELISA / indirect haemagglutination for Echinococcus antibody (Arc-5, antigen B).
- Casoni intradermal test — historical, now obsolete (poor sensitivity/specificity), but still examinable as the classic eponym.
- Avoid percutaneous diagnostic aspiration (anaphylaxis/seeding) — though therapeutic PAIR is used with cover.
High-yield: Casoni test = hydatid is a pure recall fact even though it is no longer used clinically. Eosinophilia is often mild/absent unless the cyst leaks.
Management
Approach → PAIR or surgery + scolicidal + albendazole cover:
- Albendazole peri-procedure (start before, continue after) to reduce recurrence.
- PAIR (Puncture–Aspiration–Injection of scolicidal–Reaspiration) for selected cysts.
- Surgery (pericystectomy) for large/complicated cysts; instil scolicidal (hypertonic saline / cetrimide) and meticulously avoid spillage.
- Inoperable/disseminated → long-term albendazole ± praziquantel.
4. Schistosomiasis (Bilharzia) — Schistosoma
Life cycle (the only "blood fluke"; sexes separate)
Freshwater snail = intermediate host. Cercariae penetrate intact human skin (swimmer's itch) → become schistosomulae → migrate via lungs → mature in portal/perivesical venous plexus where male and female pair (female lies in the male's gynaecophoric canal). Eggs are released and provoke granulomatous disease.
| Species | Eggs/spine | Disease | Egg found in |
|---|---|---|---|
| S. haematobium | Terminal spine | Urinary; bladder squamous cell carcinoma | Urine |
| S. mansoni | Lateral spine | Intestinal/hepatic | Stool |
| S. japonicum | Small lateral knob | Intestinal/hepatic (severe) | Stool |
Clinical features & pathology
- Cercarial dermatitis at entry; Katayama fever (acute serum-sickness-like — fever, urticaria, eosinophilia) weeks later.
- Chronic: egg granulomas. S. mansoni/japonicum → periportal "pipe-stem" (Symmers') fibrosis → presinusoidal portal hypertension with preserved liver function. S. haematobium → haematuria, bladder fibrosis, obstructive uropathy, and squamous cell carcinoma of bladder.
High-yield: S. haematobium + terminal-spined eggs in urine + chronic haematuria → risk of squamous cell carcinoma of the bladder (classic association). "Pipe-stem fibrosis" of the liver = S. mansoni / japonicum.
Diagnosis & treatment
- Investigation of choice: demonstrate eggs — terminal-spine in urine (haematobium, collect midday/after exercise), lateral-spine in stool/rectal snip (mansoni/japonicum).
- Serology/antigen (CCA) where eggs scanty.
- DOC for all species: praziquantel. (Acts only on adults; may need repeat after maturation. Add steroids in severe Katayama fever / neuroschistosomiasis.)
Mnemonics & eponyms
- Filaria nuclei: bancrofti — nuclei Blow (stop Before) the tail tip; Brugia malayi — 2 terminal nuclei at the tip.
- Vectors: "Culex carries Common (bancrofti); Mansonia carries Malayi."
- Eponym roll-call: Casoni → hydatid; Katayama → acute schistosomiasis; Calabar swelling → Loa loa; Mazzotti reaction → onchocerciasis (DEC); Symmers' pipe-stem → schistosomal liver fibrosis.
- Hooks rule: "soliuM has hooks (arMed), saginata is safe (unarmed)" — and only solium causes cysticercosis.
Recently asked / exam angle
- Microfilaria identification from blood-smear images — distinguishing W. bancrofti (sheathed, nuclei not to tip) from B. malayi (two terminal nuclei) is a frequent image-based question.
- Nocturnal periodicity & midnight blood sampling for filaria — tied to Culex biting habit.
- DOC matching: DEC (filaria/Loa loa), ivermectin (Onchocerca), praziquantel (schistosoma/intestinal taeniasis), albendazole (NCC, hydatid cover) — a classic single-best-answer cluster.
- Egg vs larva distinction for T. solium taeniasis vs cysticercosis.
- Casoni test = hydatid; water-lily sign; hydatid sand composition.
- S. haematobium → SCC bladder; spine direction vs specimen (urine vs stool).
- TPE: high IgE + eosinophilia + absent blood microfilariae responding to DEC.
- Wolbachia → doxycycline as macrofilaricidal adjunct — newer, increasingly tested.
- Eosinophilia as discriminator between tissue (high) and luminal (low) helminths.
Rapid revision
- Nocturnal-periodicity microfilariae + Culex = Wuchereria bancrofti; sample blood at midnight.
- DEC is DOC for lymphatic filariasis; ivermectin for Onchocerca; DEC for Loa loa.
- Tropical pulmonary eosinophilia = high IgE, eosinophilia, microfilariae absent from blood; responds to DEC.
- Doxycycline kills Wolbachia → true macrofilaricidal effect.
- Hydrocele = commonest chronic sign of bancroftian filariasis; filarial dance sign on USG.
- T. solium — eat larva (pork) → tapeworm; eat egg → cysticercosis; T. saginata never causes cysticercosis.
- NCC = commonest cause of acquired epilepsy in endemic areas; MRI (scolex "dot-in-hole"); treat with albendazole + steroids after excluding ocular cysts.
- Echinococcus granulosus: dog = definitive host, liver right lobe commonest; Casoni test obsolete, USG/CT is IOC; cyst rupture → anaphylaxis.
- Hydatid sand = protoscolices + hooklets; treat with albendazole + PAIR/surgery, avoid spillage.
- Schistosoma cercariae penetrate skin via snail-infested freshwater; praziquantel is DOC for all species.
- S. haematobium (terminal spine, urine) → SCC bladder; S. mansoni/japonicum (lateral spine, stool) → pipe-stem liver fibrosis.
- Eosinophilia flags tissue/blood helminths; lumen-dwelling adults provoke little eosinophilia.