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Contributors: Zachary Thomas, Edith Lederman MD, Cristina Thomas MD, Susan Burgin MD
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Loiasis is caused by the filarial parasite Loa loa. The disease is endemic to Central and West Africa. The infection is transmitted to humans by the bite of blood-sucking flies of the genus Chrysops (deer fly, horse fly, mangrove fly). The clinical picture of loiasis differs between those individuals residing in endemic areas compared to visitors to these areas.

In long-term residents of endemic areas, infection can be asymptomatic despite large numbers of microfilariae in the blood.

Because travelers to endemic regions are not immune to L loa, infection is characterized by localized subcutaneous swellings (Calabar swellings) and angioedema, both representing localized allergic reactions to L loa. These swellings are transient, lasting several hours to days, and most commonly occur on the extremities and face. Occasionally, migration of adult worms can be seen within or outside of the Calabar swellings as a moving wormlike structure. Pruritus or pricking sensations from adult worm migration is common in addition to arthralgia, fatigue, and myalgias. If the adult worm migrates under the conjunctiva (seen in 10%-20% of patients), a Calabar swelling can occur periorbitally, and this can be associated with eye pain.

Complications can include meningitis, encephalitis, nephropathy, endomyocardial fibrosis, and rarely, pulmonary involvement. Although loiasis is generally thought to be benign, these complications and an increased risk of mortality are more often seen in patients with high microfilariae levels and those living in endemic areas.


B74.3 – Loiasis

44250009 – Infection caused by Loa loa

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Differential Diagnosis & Pitfalls

  • In regions where onchocerciasis and loiasis are endemic, visitors can present with urticaria, making differentiation between the diagnoses difficult. Coinfection is not uncommon.
  • Cutaneous larva migrans does not cause eye findings, and the lesions are more superficial.
  • In Guinea worm infections, the large worm is easily palpable and often partially protruding from the skin.
  • Gnathostomiasis causes similar subcutaneous swellings, but this infection is endemic in Southeast Asia and Central / South America.

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Last Reviewed:03/22/2022
Last Updated:03/23/2022
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A medical illustration showing key findings of Loiasis : Fatigue, Central Africa, Eye burning, Eyelid edema, West Africa, Arthralgia, Myalgia, Ocular pruritus, Paresthesias, Pruritus, EOS increased
Clinical image of Loiasis - imageId=229728. Click to open in gallery.  caption: 'Marked lymphedema of the left leg.'
Marked lymphedema of the left leg.
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