Onchocerciasis

Etiology & Transmission - Worm's Wanderings

• Causative Agent: Onchocerca volvulus (filarial nematode).
• Vector: Female Simulium blackflies.
  • Transmit infective L3 larvae during a blood meal.
• Transmission Cycle:
  • Blackfly ingests microfilariae from an infected human.
  • Larvae develop to L3 stage in the fly (1-3 weeks).
  • Infective L3 larvae are deposited on human skin during a subsequent bite, entering via the bite wound.
• Worm's Wanderings:
  • Adult worms reside in subcutaneous nodules (onchocercomata).
  • They release millions of microfilariae (mf) that migrate throughout the skin and eyes, causing pathology.

⭐ Vector (Simulium blackfly) breeds in fast-flowing, oxygen-rich rivers, linking the disease to riverine areas.

Skin Manifestations - Itch, Lumps, Leopard Spots

• Pruritus (Itch): Intense, often earliest symptom; leads to excoriations.
  • May have "crawling sensation".
• Onchocercomata (Nodules): Firm, non-tender subcutaneous lumps with adult worms.
  • Typically over bony prominences (e.g., iliac crests, ribs).
• Depigmentation ("Leopard Skin"): Characteristic spotted loss of pigment, common on shins.
• Chronic Changes:
  • "Lizard skin": Thin, dry, atrophic, wrinkled skin.
  • "Hanging groin": Loss of skin elasticity.
• Sowda: Localized, hyperpigmented, lichenified, intensely itchy dermatitis. (Hyper-reactive form)

⭐ "Sowda" is a chronic, hyperpigmented, lichenified onchodermatitis, representing a hyper-reactive immune response, typically seen in Yemen and Sudan.

Ocular & Systemic Impact - The Blinding Path

• Ocular Manifestations ("River Blindness"):
  • Microfilariae (mf) invade all eye structures.
  • Anterior Segment:
    • Live/dead mf in anterior chamber (AC).
    • "Snowflake" (punctate) keratitis → progresses to sclerosing keratitis (corneal opacity).
    • Iridocyclitis, secondary glaucoma, complicated cataracts.
  • Posterior Segment:
    • Chorioretinitis (retinal pigment epithelium [RPE] changes, "leopard skin" fundus).
    • Optic neuritis & subsequent optic atrophy.
  • Outcome: Progressive, often irreversible blindness.
• Systemic Involvement:
  • Severe Dermatological: Chronic onchodermatitis; Sowda (hyper-reactive onchodermatitis in hypoendemic areas).
  • Lymphatic System: Lymphadenopathy (e.g., "hanging groin"), chronic lymphedema.
  • General Debilitation: Musculoskeletal pain, weight loss/cachexia.
  • Neurological Links: Potential association with Nodding syndrome & other forms of epilepsy.

⭐ Onchocerciasis is the second leading infectious cause of blindness worldwide.

Diagnosis & Management - Spot & Stop the Worm

• Diagnosis:
  • Clinical: Chronic pruritus, "Sowda" (papular dermatitis), "leopard skin" (atrophy), onchocercomata (nodules), ocular signs ("river blindness").
  • Gold Standard: Skin snips (2-3mg, bloodless) for microfilariae.
  • Supportive: PCR on skin (↑sensitivity); slit-lamp for eye microfilariae; nodule exam (adult worms).
• Management:
  • Ivermectin: (📌 Dose: 150 mcg/kg) PO, single dose, repeat q 6-12 months. Potent microfilaricidal.
  • Doxycycline: 100-200 mg/day PO x 4-6 wks. Targets Wolbachia, sterilizes adults, slow macrofilaricidal, ↓Mazzotti.
  • Nodulectomy: Reduces worm burden, esp. head/neck.

⭐ Mazzotti reaction (post-DEC: fever, rash, pruritus) is diagnostic but severe. Ivermectin causes milder reactions. DEC is contraindicated.

High‑Yield Points - ⚡ Biggest Takeaways

• Caused by Onchocerca volvulus, transmitted by Simulium blackfly bites near fast-flowing rivers.
• Leads to river blindness from ocular microfilarial invasion and chronic inflammation.
• Skin signs: intense itching, papular dermatitis, lichenification (Sowda), leopard skin, onchocercomata (nodules).
• Diagnosis: skin snip microscopy for microfilariae is gold standard.
• Treatment: Ivermectin (microfilaricidal); Doxycycline targets Wolbachia symbionts, reducing adult worm fertility.
• Mazzotti reaction: post-treatment hypersensitivity (fever, rash), historically with DEC, less with Ivermectin_._