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For further information, see CMDT Part 35-34: Loiasis

Key Features

Essentials of Diagnosis

  • Subcutaneous swellings; adult worms migrating across the eye

  • Encephalitis, which may be brought on by treatment

  • Microfilariae in the blood

General Considerations

  • This chronic filarial disease is caused by infection with Loa loa

  • Transmitted by chrysops flies, which bite during the day

  • Over 6–12 months after infection, larvae develop into adult worms, which migrate through subcutaneous tissues, including the subconjunctiva (leading to the term "eye worm")

  • Adults can live for up to 17 years


  • The infection occurs in humans and monkeys in rainforest areas of West and central Africa

  • An estimated 3–13 million persons are infected

Clinical Findings

Symptoms and Signs

  • Many infected persons are asymptomatic, but they may have high levels of microfilaremia and eosinophilia

  • Transient subcutaneous swellings (Calabar swellings)

    • Develop in symptomatic persons

    • Are nonerythematous, up to 20 cm in diameter

    • May be preceded by local pain or pruritus

    • Usually resolve after 2–4 days but occasionally persist for several weeks

    • Commonly seen around joints and may recur at the same or different sites

  • Visitors from nonendemic areas are more likely to have allergic-type reactions, including pruritus, urticaria, and angioedema

  • Adult worms may be seen migrating across the eye, with either no symptoms or conjunctivitis with pain and edema

Differential Diagnosis

  • Dracunculiasis

  • Cutaneous larva migrans

  • Gnathostomiasis

  • Myiasis

  • Filariasis

  • Onchocerciasis (river blindness)

  • Bacterial pyoderma

  • Cysticercosis (with ophthalmic involvement)


Laboratory Tests

  • Obtain blood samples during the day to identify microfilariae in blood

  • Failure to find microfilariae does not rule out the diagnosis

  • Identification of a migrating eye worm is also diagnostic

  • Serologic tests

    • May be helpful in persons from nonendemic areas who may be acutely ill without detectable microfilaremia

    • However, usefulness for residents of endemic areas is limited because most of them will have positive results

  • Molecular methods, including field-friendly LAMP assays, are available to rule out loiasis before administration of ivermectin for the control of other filarial infections



  • Diethylcarbamazine

    • Treatment of choice

    • Eliminates microfilariae and has some activity against adult worms

    • Dosage: 8–10 mg/kg/day for 21 days

    • Repeat courses may be needed

    • Mild side effects include fever, pruritus, arthralgias, nausea, diarrhea, and Calabar swellings

    • Coadministration of antihistamines or corticosteroids can lessen side effects

  • Ivermectin

    • Highly active against microfilariae but not adult worms

    • Entails a high risk of severe reactions

    • To reduce the risk, pretreatment with corticosteroids and antihistamines and escalating dosage of diethylcarbamazine have been used, but this strategy does not prevent encephalitis

  • Strategies to treat patients with high parasite loads include

    • No treatment


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