DurgicalDurgical
EN
Open the app

Microbiology · Nematodes

Onchocerca volvulus !

  • Parasite
  • Exam priority: medium

A worm spread by a blackfly of fast rivers; it damages skin and eyes and can blind. Colombia was the first country in the world to eliminate it.

Updated 2026-11-01

Also called: river blindness, onchocerciasis

At a glance

Shape
Adults coiled in subcutaneous nodules
Family
Onchocercidae
Type
Filarial nematode (tissue: skin and eye)
Stain
Giemsa or haematoxylin-eosin on skin biopsy: unsheathed microfilariae
Size
Female several tens of centimetres long; unsheathed microfilaria

Morphology

  • Adults coiled inside fibrous nodules over bony prominences.
  • Unsheathed microfilariae in the dermis, with no daily periodicity.
  • Rarely seen in blood, urine or sputum.

Transmission

  • Repeated bites of blackflies (Simulium) that breed in fast-flowing rivers.

Reservoir: Humans

Vector: Blackflies of the genus Simulium, which breed in fast-flowing rivers and streams.

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. The blackfly (Simulium) leaves L3 larvae in the skin while biting.
  2. The larvae mature in subcutaneous tissue and the adults lie coiled in nodules.
  3. Adults live about 15 years; the female produces microfilariae for about 9 years.
  4. Microfilariae roam the dermis and can invade the eye.
  5. Another fly swallows them while biting; in its thoracic muscles they become L3 and move to the proboscis.

Virulence factors

  • Dying microfilariae trigger inflammation, partly through Wolbachia antigens.
  • Skin: intense itch, papular dermatitis, lichenification and depigmentation ('leopard skin').
  • Eye: sclerosing keratitis and chorioretinitis leading to blindness.
  • Diethylcarbamazine provokes a violent inflammatory reaction (Mazzotti reaction).

Diseases it causes

Clinical picture

  • Intense itch and scratch lesions.
  • Painless subcutaneous nodules over the iliac crests, ribs or head.
  • Depigmentation on the shins and loss of skin elasticity.
  • Falling visual acuity up to blindness.

Diagnosis

  • Bloodless superficial skin snips in saline: microfilariae emerge within 30–60 minutes.
  • Snips can be taken at any hour because there is no periodicity.
  • Biopsy or excision of a nodule containing adults.
  • Slit lamp: microfilariae in the anterior chamber.

Treatment

Ivermectin (kills microfilariae and is the basis of mass drug administration); doxycycline targets Wolbachia and weakens adults; moxidectin is another option. Diethylcarbamazine is avoided because of the Mazzotti reaction.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Mass ivermectin administration with high, sustained coverage.
  • Vector control in breeding rivers.
  • Covering clothing and repellents in transmission areas.

Test yourself

Think of the answer, then tap the question to see it.

Which insect transmits Onchocerca volvulus?

The blackfly of the genus Simulium, which breeds in fast-flowing rivers.

How is onchocerciasis diagnosed?

With superficial skin snips in saline to watch microfilariae emerge, or with a nodule biopsy.

Which country was the first in the world verified free of onchocerciasis, and when?

Colombia, in 2013.

Why is diethylcarbamazine avoided in onchocerciasis?

Mass killing of microfilariae triggers a severe inflammatory reaction (Mazzotti reaction) that can damage the eye.

Add to my cards (4)

← Back to the group: Nematodes

Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Onchocerciasis (CDC, 2024); Onchocerciasis (WHO, 2025); Colombia is first country in the world to eliminate river blindness (PAHO, 2013).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.