**FILARIAL NEMATODES – WUCHERERIA BANCROFTI **
- Wuchereria bancrofti = thread‑like filarial nematode that causes lymphatic filariasis (bancroftian filariasis)
- Transmission = bite of infected Culex, Anopheles or Aedes mosquito
Morphology
- Adult male: 40‑50 mm long, 0.1 mm thick, curved tail, 5‑6 pairs of caudal papillae, single testis, no uterus
- Adult female: 60‑100 mm long, 0.2 mm thick, gravid uterus filled with sheathed microfilariae, vulva near posterior end
- Microfilaria (mf): sheathed, 244‑296 µm long, 7‑8 µm wide, nuclei extending to tip of tail, tail tip free of nuclei (key for W. bancrofti)
Life cycle (step‑wise)
- Adult worms live in human lymphatics → females release microfilariae into peripheral blood (mainly night‑time)
- Mosquito takes a blood meal → ingests sheathed microfilariae
- In mosquito midgut → microfilariae shed sheath, develop to L1 → L2 → infective L3 in thoracic muscles (≈10‑14 days)
- Infective L3 migrate to mosquito proboscis → transmitted to another human during next bite
- L3 enter skin → migrate to lymphatic vessels → mature to adult worms (≈6‑12 months) → cycle repeats
Memory rhyme: “Night worms in blood, mosquito bite, L3 takes flight, lymph node home, grow and roam.”
Pathogenesis (step‑wise)
- Adult worms lodge in lymphatic vessels and nodes → mechanical obstruction of lymph flow
- Worm movement & secretions → local inflammation, endothelial damage, lymphangitis episodes
- Repeated inflammation → lymphatic dilation, fibrosis, valve damage
- Impaired drainage → chronic lymphedema → elephantiasis (skin thickening, hyperkeratosis)
- Acute attacks (adenolymphangitis) may be precipitated by bacterial superinfection (Streptococcus, Staphylococcus)
Clinical features
- Many infections are asymptomatic (carrier state)
- Acute adenolymphangitis: fever, pain, swelling of limb, tender lymph nodes, often with itching
- Chronic lymphedema of lower limbs, scrotum (hydrocele), breast, arm → elephantiasis in long‑standing disease
- Hydrocele is common in males due to scrotal lymphatic blockage
Complications
- Secondary bacterial cellulitis, lymphangitis, elephantiasis with disfigurement
- Psychological and social stigma, reduced productivity
Laboratory diagnosis
- Blood smear: night‑time (or after 10 pm) thick/thin smear, Giemsa or Wright stain, look for sheathed microfilariae with tail tip free of nuclei
- Filariasis test strip (FTS) / antigen detection: circulating filarial antigen (CFA) test for adult worm antigen, useful in day‑time screening
- Serology: ELISA for IgG4 antibodies (research/epidemiology)
- Ultrasound (Doppler): “filarial dance sign” – moving adult worms in lymphatics (non‑invasive)
- PCR: species‑specific DNA detection in blood (reference lab)
Management
- Mass drug administration (MDA) for endemic areas: Diethylcarbamazine (DEC) 6 mg/kg single dose + Albendazole 400 mg (or Ivermectin + Albendazole where onchocerciasis co‑exists)
- Acute attacks: NSAIDs for pain, antihistamines for itching, antibiotics (e.g., amoxicillin‑clavulanate) for secondary bacterial infection
- Chronic lymphedema care: meticulous hygiene, limb elevation, compression therapy, physiotherapy, surgery (debulking) for severe elephantiasis
- Vector control: insecticide‑treated nets, indoor residual spraying, environmental management to eliminate mosquito breeding sites
Prevention & control
- Use of mosquito nets and repellents, especially at night
- Community‑wide MDA programs (WHO goal: eliminate LF as public health problem)
- Health education on early detection and hygiene to prevent secondary infections
Key exam points to remember
- Sheathed microfilaria, nuclei do NOT reach tail tip → distinguishes W. bancrofti from B. malayi (nuclei reach tip)
- Night‑time periodicity → blood collection between 10 pm‑2 am for microscopy
- Adult worms reside in lymphatics → main cause of lymphedema/elephantiasis
- DEC is drug of choice; ivermectin used where onchocerciasis is co‑endemic
- “Filarial dance sign” on ultrasound is pathognomonic for live adult worms.