[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Microbiology](https://durgical.com/en/microbiology/) › [Nematodes](https://durgical.com/en/microbiology/groups/nematodes/) › Onchocerca volvulus

[Microbiology](https://durgical.com/en/microbiology/) · [Nematodes](https://durgical.com/en/microbiology/groups/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

## Key points

- Vector: Simulium blackfly of fast rivers.
- Skin (itch, nodules, depigmentation) and eye (river blindness).
- Diagnosis by skin snips, at any hour.
- Mass ivermectin; avoid diethylcarbamazine (Mazzotti reaction).
- Colombia: first country in the world verified free, 2013.

## 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

- The blackfly (Simulium) leaves L3 larvae in the skin while biting.
- The larvae mature in subcutaneous tissue and the adults lie coiled in nodules.
- Adults live about 15 years; the female produces microfilariae for about 9 years.
- Microfilariae roam the dermis and can invade the eye.
- 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

- Onchocerciasis: chronic dermatitis and river blindness [Onchocerciasis (river blindness) →](https://durgical.com/en/conditions/onchocerciasis/)

## 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:

- [Ivermectin](https://durgical.com/en/pharmacology/ivermectin/)
- [Doxycycline](https://durgical.com/en/pharmacology/doxycycline/)

## Prevention

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

## In Colombia

In 2013 WHO verified Colombia as the first country in the world free of onchocerciasis. The last focus was Naicioná, in López de Micay (Cauca), where ivermectin was given twice a year for 12 years and transmission stopped in 2007. In the Americas, transmission remains only on the Brazil–Venezuela border.

## Memory aid

Fast river, black fly, dim eye: and in Colombia, López de Micay was the last river where the parasite's light went out.

## 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)](https://durgical.app/estudiar/?add=/en/microbiology/onchocerca-volvulus/)

## Related

- [Wuchereria bancrofti / Brugia malayi](https://durgical.com/en/microbiology/wuchereria-bancrofti-brugia-malayi/)
- [Mansonella ozzardi / Mansonella perstans](https://durgical.com/en/microbiology/mansonella-ozzardi-mansonella-perstans/)

## Related conditions

- [Onchocerciasis (river blindness)](https://durgical.com/en/conditions/onchocerciasis/)

## What the colours mean

**Gram-positive**
Thick peptidoglycan wall: stains purple.

**Gram-negative**
Thin wall with an outer membrane: stains pink.

**Acid-fast**
Lipid-rich wall (mycobacteria): Ziehl–Neelsen stain.

[← Back to the group: Nematodes](https://durgical.com/en/microbiology/groups/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](https://www.cdc.gov/dpdx/onchocerciasis/index.html) (CDC, 2024); [Onchocerciasis](https://www.who.int/news-room/fact-sheets/detail/onchocerciasis) (WHO, 2025); [Colombia is first country in the world to eliminate river blindness](https://www.paho.org/en/news/29-7-2013-colombia-first-country-world-eliminate-river-blindness) (PAHO, 2013).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/microbiology/onchocerca-volvulus/