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Microbiology · Other parasites of interest

Loa loa !

  • Parasite
  • For completeness

An African filarial worm that travels under the skin and sometimes crosses the eye. Day-biting flies spread it.

Updated 2026-11-01

Also called: African eye worm, loiasis

At a glance

Type
Filarial nematode
Stain
Giemsa or haematoxylin-eosin on blood: sheathed microfilaria with nuclei to the tip of the tail
Size
Adults several centimetres long in the subcutaneous tissue

Morphology

  • Sheathed microfilaria in peripheral blood, with nuclei reaching the tip of the tail.
  • Thread-like adults that migrate through the subcutis and under the conjunctiva.

Transmission

  • Bite of Chrysops flies, which bite by day in the rainforest.
  • Not spread from person to person.

Reservoir: Humans

Vector: Chrysops flies (C. silacea, C. dimidiata), which bite by day.

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. The Chrysops fly deposits infective larvae on the skin while biting.
  2. The larvae mature into adults in the subcutis over several months.
  3. Adults migrate under the skin and conjunctiva and produce sheathed microfilariae.
  4. Microfilariae circulate in blood by day (diurnal periodicity) and stay in the lungs at night.
  5. A new fly swallows them; in its muscles they become infective larvae that reach the proboscis.

Virulence factors

  • Migrating adults cause localised hypersensitivity reactions (Calabar swellings).
  • Very high microfilarial loads can cause encephalopathy when killed suddenly by certain drugs.

Diseases it causes

  • Loiasis: Calabar swellings and subconjunctival migration of the worm

Clinical picture

  • Many infections are asymptomatic.
  • Calabar swellings: transient swellings, often of the wrist or forearm, that come and go.
  • Visible passage of the adult worm under the conjunctiva (eye worm).
  • Marked eosinophilia, especially in travellers.

Diagnosis

  • Giemsa-stained smear of blood drawn around midday (about 10 AM to 2 PM).
  • Concentration techniques (Knott, filtration) for higher sensitivity.
  • Identification of the adult removed from the conjunctiva; Calabar swellings support the diagnosis.

Treatment

Anthelmintics: diethylcarbamazine is the drug of choice; albendazole is used in some cases. With high microfilaraemia, diethylcarbamazine and ivermectin can cause encephalopathy, so the load is measured before treatment. An adult in the eye can be removed surgically.

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

In Pharmacology:

Prevention

  • Long sleeves and repellent against the day-biting flies.
  • Screening for Loa loa before mass ivermectin campaigns in parts of Africa where it overlaps with onchocerciasis.

Test yourself

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

When should blood be drawn to look for Loa loa microfilariae?

Around midday, roughly 10 AM to 2 PM, because their periodicity is diurnal.

What are Calabar swellings?

Transient migratory swellings, often of the forearm, caused by the reaction to adult worms moving under the skin.

Why is microfilaraemia measured before treating loiasis?

Because with high loads, mass killing of microfilariae by diethylcarbamazine or ivermectin can cause severe encephalopathy.

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

Sources: CDC - DPDx - Loiasis (CDC, 2024); About Loiasis | Filarial Worms | CDC (CDC, 2024).

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Representative model, for educational purposes only.