Loiasis is a filarial nematode infection with Loa loa. Symptoms include localized angioedema (Calabar swellings) in skin and subconjunctival migration of adult worms. Diagnosis is by detecting microfilariae in peripheral blood or seeing worms migrating across the eye. Treatment is with diethylcarbamazine.
(See also Approach to Parasitic Infections and Overview of Filarial Nematode Infections.)
Loiasis is confined to the rain forest belt of western and central Africa. Humans are the only known natural reservoir for this parasite.
Loa loa microfilariae are transmitted by day-biting tabanid flies (Chrysops [deer fly or horse fly]). Microfilariae mature to adult worms in the subcutaneous tissues of the human host; females are 40 to 70 mm long, and males are 30 to 34 mm long. The adults produce microfilariae. Adults migrate in subcutaneous tissues and under the conjunctiva of the eye, and microfilariae circulate in blood. Flies become infected when they ingest blood from a human host during the day (when microfilaremia levels are the highest).
Image from the Centers for Disease Control and Prevention, Global Health, Division of Parasitic Diseases and Malaria.
Occasionally, infection causes cardiomyopathy, nephropathy, or encephalitis. Eosinophilia is common but nonspecific.
Symptoms and Signs of Loiasis
Most infected people are asymptomatic. Infection can produce areas of angioedema (Calabar swellings) that develop anywhere on the body but predominantly on the extremities; they are presumed to reflect hypersensitivity reactions to allergens released by migrating adult worms. In native residents, swellings usually last 1 to 3 days but are more frequent and severe in visitors. Worms may also migrate subconjunctivally across the eyes. This migration may be unsettling, but residual eye damage is uncommon.
Nephropathy generally manifests as proteinuria with or without mild hematuria and is believed to be due to immune complex deposition.
Encephalopathy is usually mild, with vague central nervous system (CNS) symptoms.
Diagnosis of Loiasis
Observation of an adult worm subconjunctivally crossing the eye
Identification of an adult worm removed from the eye or skin
Identification and quantification of microfilariae in blood by microscopy or quantitative polymerase chain reaction (PCR)
Loiasis should be suspected in immigrants or travelers who have a history of exposure in an endemic area and who present with eye worms, Calabar swellings, or unexplained peripheral eosinophilia.
Occasionally, the diagnosis of loiasis is confirmed by observing an adult worm migrating under the conjunctiva or by identifying a worm after it is removed from the eye or skin.
Microscopic detection of microfilariae in peripheral blood establishes the diagnosis; the number of microfilariae per milliliter of blood should be quantified. Blood samples should be drawn between 10 AM and 2 PM, when microfilaremia levels are the highest.
Many serologic tests for antibodies do not differentiate Loa loa from other filarial nematode infections. Loa-specific antibody tests have been developed, but they are not widely available in the United States. Also, a positive serologic test does not distinguish between past and current infection. A quantitative real-time PCR (qPCR) to confirm the diagnosis and quantify the microfilarial burden is available at the Laboratory of Parasitic Diseases, National Institutes of Health.
People from endemic regions of Africa should be checked for Loa loaLoa loa microfilariae/mL blood are at risk of potentially fatal encephalopathy, caused by the release of antigens from dying microfilariae.
Treatment of Loiasis
Diethylcarbamazine
Treatment of loiasis is complicated. Diethylcarbamazine (DEC) is the only drug that kills microfilariae and adult worms. In the United States, it is available only from the Centers for Disease Control and Prevention (CDC) after laboratory confirmation of loiasis; clinicians should seek expert advice before they initiate treatment, and they should do the following before initiating treatment with DEC:
Measure the number of microfilariae in the blood, because using DEC to treat heavy infections ( ≥ 8000 microfilariae/mL blood) can result in potentially fatal encephalopathy
Exclude coinfection with onchocerciasis because DEC can cause a severe hypersensitivity reaction (Mazzotti reaction) and worsen eye and skin disease in patients with onchocerciasis
Clinicians should seek expert assistance when measuring the number of microfilariae and thus determining the severity of the infection.
Treatment of light infection
Patients with symptomatic loiasis and < 8000 microfilariae/mL blood are given DEC 2.7 to 3.3 mg/kg 3 times a day for 21 days.
Treatment of heavy infection
Prevention of Loiasis
Diethylcarbamazine (DEC) 300 mg orally once a week can be used to prevent loiasis in long-term travelers to endemic regions.
Key Points
Humans are the only known natural reservoir for Loa loa, which is transmitted by day-biting tabanid flies.
Most infected people are asymptomatic, but some have areas of angioedema (Calabar swellings), which occur mainly on the extremities, or report worms moving across their eyes.
Diagnose by microscopic examination of peripheral blood drawn between 10 AM and 2 PM, when microfilaremia levels are the highest; confirm and measure the concentration of microfilariae by quantitative polymerase chain reaction (qPCR).
Occasionally, diagnosis of loiasis is confirmed by observing an adult worm migrating under the conjunctiva or by identifying a worm after it is surgically removed from the eye or skin.
Diethylcarbamazine, which should not be administered to patients concurrently infected with Onchocerca volvulus, is the only drug that kills microfilariae and adult worms; in the United States, it is available only from the Centers for Disease Control and Prevention.
Seek expert assistance in measuring the number of microfilariae and determining the severity of the infection, and seek expert advice before initiating treatment.
