Simulium and Culicoides

At a glance

Shape

Simulium: small, stout, humpbacked dark fly; Culicoides: minute midge with spotted wings

Family

Simuliidae (black flies); Ceratopogonidae (biting midges)

Type

Blood-feeding dipteran insects (biological vectors)

Size

Simulium a few millimetres; Culicoides about 1–3 mm, small enough to pass through ordinary screens

Morphology

• Simulium has short, blade-like mouthparts that cut the skin and feed from a small pool of blood.
• Culicoides females have short piercing mouthparts; wing spot patterns help identify species.
• Simulium larvae cling to rocks and plants in running water with a posterior sucker and filter food with head fans.

Transmission

• Only females bite, because they need blood for their eggs.
• Black flies bite by day near fast-flowing rivers and streams where they breed.
• Biting midges bite mainly at dawn and dusk, often in swarms, near damp soil, mangroves and rotting vegetation.
• Filarial larvae develop inside the insect to the infective third stage and enter the skin at the next bite.

Reservoir: Humans infected with Onchocerca or Mansonella microfilariae; Animals, sloths and primates in the Oropouche virus sylvatic cycle

Vector: Simulium transmits Onchocerca volvulus; Culicoides transmits Mansonella perstans, M. ozzardi and M. streptocerca, and Oropouche virus. In parts of South America Simulium also carries M. ozzardi.

Isolation precautions

What the care team adds to standard precautions.

• Standard

Life cycle

1. Simulium females lay eggs on wet rocks or plants in well-oxygenated, fast-flowing water.
2. Simulium larvae attach to submerged surfaces and filter food from the current.
3. They pupate in a silk cocoon under water; the adult rises to the surface in an air bubble.
4. Culicoides lay eggs in moist mud, manure, tree holes or rotting plants, not in open water.
5. Culicoides larvae are worm-like and live in the wet substrate; they then pupate there.
6. In the fly, ingested microfilariae reach the thoracic muscles, become L3 larvae and move to the mouthparts.

Virulence factors

• Painful or itchy bites; swarms can make outdoor work and life hard.
• Bite reactions range from small itchy papules to large local reactions and papular urticaria in children.
• Black fly breeding in rivers keeps onchocerciasis close to riverside villages: hence the name river blindness.

Diseases it causes

• Onchocerciasis (river blindness), carried by Simulium Onchocerciasis (river blindness) →
• Mansonellosis (Mansonella perstans, M. ozzardi, M. streptocerca), carried mainly by Culicoides
• Oropouche fever, carried by Culicoides midges Oropouche fever and other emerging arboviruses →
• Insect-bite reactions and papular urticaria

Clinical picture

• Itchy papules or small bleeding points after black fly bites, often on the head, neck and ankles.
• Clusters of tiny itchy papules after midge bites at dusk.
• The diseases they carry appear months later (onchocerciasis, mansonellosis) or days later (Oropouche fever).

Diagnosis

• Entomology: identifying adults by wing pattern and body shape, and larvae in rivers (Simulium).
• Dissection or PCR of flies to detect filarial larvae, used to certify that transmission has stopped.
• Patient: skin snips for Onchocerca microfilariae; blood smears for M. perstans and M. ozzardi.

Treatment

Does not apply to the insect. Bite reactions are managed with topical corticosteroids and oral antihistamines; the filarial infections they carry are treated with antiparasitic drugs such as ivermectin.

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

In Pharmacology:

• Ivermectin
• Hydrocortisone
• Cetirizine

Prevention

• Long clothes and repellents during the day near rivers (black flies) and at dusk (midges).
• Larviciding rivers where Simulium breeds, as done in onchocerciasis programmes.
• Fine-mesh screens and bed nets, as midges pass through ordinary screens.
• Mass ivermectin treatment of communities to stop the cycle through the fly.

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Connections

← Back to the group: Arthropod vectors

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); CDC - DPDx - Mansonellosis (CDC, 2024); Onchocerciasis (WHO, 2025); Vector-borne diseases (WHO, 2024).

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Representative model, for educational purposes only.