FILARIAL NEMATODES – ONCHOCERCA VOLVULUS

Onchocerca volvulus – filarial nematode that produces onchocerciasis (river blindness)

Cause – infection by bite of an infected black‑fly (Simulium species) that deposits infective L3 larvae on the skin

Types – not classified into sub‑types; disease manifests as skin, ocular and systemic forms depending on worm burden and location

Pathogenesis – step‑wise sequence
a. Black‑fly bite → L3 larvae released onto skin surface
b. Larvae penetrate epidermis → enter subcutaneous tissue → migrate to lymphatics
c. Larvae develop into adult worms in fibrous nodules (onchocercomas) under skin
d. Adult female releases microfilariae (MF) into surrounding tissue
e. MF migrate through skin and eye tissues → provoke intense inflammatory response when they die
f. Chronic inflammation → fibrosis, skin atrophy, depigmentation, corneal opacity, cataract → vision loss

Memory rhyme for pathogenesis: “Bite‑Larvae‑Dive‑Nest‑Spawn‑Spread‑Scar”

• Bite (fly) – Larvae enter – Dive (into lymph) – Nest (nodules) – Spawn (MF) – Spread (skin/eye) – Scar (fibrosis)

Morphology – adult worm: long, white, thread‑like, up to 70 cm long, 0.5 mm wide; male smaller, curved tail; female larger, uterus filled with MF; MF: 300–600 µm long, slender, unsheathed

Clinical features –
a. Subcutaneous nodules (onchocercomata) palpable under skin
b. Skin changes – atrophy, hyper‑pigmentation, “leopard skin” depigmentation, intense itching
c. Ocular lesions – punctate keratitis, sclerosing keratitis, cataract, optic neuritis → progressive visual loss, blindness
d. Systemic – low‑grade fever, malaise, lymphadenopathy in heavy infections

Complications – irreversible blindness, severe skin disfigurement, secondary bacterial infections of skin lesions, social stigma

Diagnosis –
a. Clinical examination of nodules and eye findings
b. Skin snip biopsy → microscopic detection of live MF (wet mount)
c. Slit‑lamp examination of eye for MF and inflammatory plaques
d. Serology (ELISA) for antibodies (used in surveys)

Management –
a. Ivermectin single dose 150 µg/kg every 6‑12 months – kills MF, reduces ocular inflammation
b. Albendazole 400 mg twice daily for 21 days – adjunct to reduce adult worm load
c. Surgical excision of large nodules if causing discomfort or for histology
d. Vector control – larvicides in rivers, community education to avoid black‑fly breeding sites

Life cycle – numbered flow

1. Adult worms live in subcutaneous nodules → produce MF
2. MF migrate to skin surface and ocular tissues → become available for uptake
3. Black‑fly takes a blood meal → ingests MF → MF penetrate fly gut, migrate to thoracic muscles
4. In fly, MF develop → L1 → L2 → infective L3 larvae in the fly’s mouthparts
5. Infected fly bites another human → L3 larvae deposited on skin → start new cycle

Memory rhyme for life cycle: “Nodules‑Migrate‑Fly‑Grow‑L3‑Bite”

• Nodules (adult MF source) – Migrate (to skin/eye) – Fly (ingests) – Grow (to L3) – L3 (released) – Bite (new host)