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Loiasis (Loa loa infection)

Amy D Klion, MD
Section Editor
Peter F Weller, MD, MACP
Deputy Editor
Elinor L Baron, MD, DTMH


Loiasis, also known as African eye worm, is caused by the filarial nematode Loa loa. Loiasis is transmitted by the bite of the Chrysops fly; west and central Africa are endemic regions. Manifestations of infection include transient localized subcutaneous swellings (known as Calabar swellings) and migration of the adult worm across the subconjunctiva of the eye.

The epidemiology, clinical features, diagnosis, and treatment of L. loa will be reviewed here. Other filarial infections, including onchocerciasis, lymphatic filariasis, and Mansonella infection are discussed separately. (See "Onchocerciasis" and "Epidemiology, pathogenesis, and clinical manifestations of lymphatic filariasis" and "Diagnosis, treatment, and prevention of lymphatic filariasis".)


Loiasis is transmitted by biting deerflies (Chrysops), which breed in the high-canopied rainforest of west and central Africa, including the coastal plains of northern Angola, southeastern Benin, Cameroon, Central African Republic, Chad, Republic of the Congo, Equatorial Guinea, Gabon, Nigeria, Sudan, the Democratic Republic of Congo, and Uganda.

It is estimated that between 3 and 13 million people are infected [1]. Infection is occult in a large proportion of patients; therefore, in many areas, the epidemiology of loiasis has not been clearly defined. In endemic regions, the probability of infection increases with age; the proportion of infected individuals varies depending on vector abundance, which in turn is dependent on local ecology [2].

Large-scale mapping of loiasis in 11 potentially endemic countries has been developed using a Rapid Assessment Procedure for Loiasis (RAPLOA), based on community questionnaires documenting the prevalence of a history of eye worm. RAPLOA surveys have helped identify specific regions in 10 African countries where loiasis is endemic, including regions where the prevalence of infection exceeds 40 percent (figure 1) [3,4]. This is important given potential complications arising from ivermectin treatment of onchocerciasis in those with concomitant loiasis.

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Literature review current through: Nov 2017. | This topic last updated: Nov 27, 2017.
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