Loiasis (Loa loa infection)
Loiasis is a filarial nematode infection caused by Loa loa, transmitted by the bite of the deer fly, horse fly, or mango fly (Chrysops species), producing migratory subcutaneous (Calabar) swellings and a worm visibly migrating across the conjunctiva, treated with diethylcarbamazine after excluding a high microfilarial load.
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What this article covers
- Overview
Filarial nematode infection: loiasis is caused by Loa loa, a tissue-dwelling filarial nematode transmitted to humans through the bite of an infected Chrysops fly (deer fly, horse fly, or mango fly).
- Epidemiology
Endemic to the rainforest belt of West and Central Africa, corresponding to the habitat range of the Chrysops fly vector.
- Risk factors
Residence or extended travel in the West or Central African rainforest belt, where Chrysops flies are endemic.
- Pathophysiology
Inoculation by fly bite: infective larvae are deposited in the skin during a Chrysops fly blood meal.
- Clinical features
Migratory subcutaneous (Calabar) swelling: a localized, non-erythematous or mildly erythematous area of soft tissue swelling that appears and resolves over days, then recurs at a different site…
- Differential diagnosis
Lymphatic filariasis: another filarial infection, but transmitted by mosquito rather than Chrysops fly, and produces lymphedema and elephantiasis from lymphatic obstruction rather than migratory skin swelling.
- Diagnosis
Loiasis is confirmed by identifying the parasite: a worm seen crossing the conjunctiva or removed from a Calabar swelling, or microfilariae identified on a blood smear, in a patient with a compatible travel history.
- Management
First, quantify the microfilarial load on a peripheral blood smear to stratify treatment.
- Complications
Encephalopathy from antifilarial treatment: the most serious complication, precipitated by rapid microfilarial death from diethylcarbamazine or ivermectin in a patient with a high pretreatment…
- Prognosis
Favorable with appropriate, load-stratified treatment: correctly sequenced therapy (load reduction first when indicated, then diethylcarbamazine) clears infection with a low risk of severe adverse events.
- Prevention
Avoid Chrysops fly bites in endemic areas: apply insect repellent and wear protective clothing (long sleeves and pants) during outdoor activity in the West or Central African rainforest belt.
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Related articles
- Lymphatic filariasis
Lymphatic filariasis is caused by the filarial nematodes Wuchereria bancrofti or Brugia malayi, transmitted by mosquito bite, producing lymphatic inflammation and obstruction that leads to lymphedema and elephantiasis, confirmed by identifying microfilariae or adult worms and treated with diethylcarbamazine.
- Onchocerciasis (river blindness)
Onchocerciasis, also called river blindness, is a filarial nematode infection caused by Onchocerca volvulus and transmitted by the bite of the female black fly, producing chronic pruritic skin disease (onchodermatitis with skin atrophy and depigmentation) and subcutaneous nodules containing adult worms, and standing as a major cause of blindness in endemic rural communities; treatment is ivermectin, repeated every 6-12 months because it kills microfilariae but not adult worms, with careful screening for coendemic Loa loa before dosing.
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