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(Loa loa Filariasis; Calabar Swellings)

By Richard D. Pearson, MD, Emeritus Professor of Medicine, University of Virginia School of Medicine

Loiasis is infection of tissues under the skin with the roundworm Loa loa.

  • Itchy swellings appear, mainly on the arms and legs.

  • Sometimes worms travel under the clear membrane that covers the eyes.

  • Doctors diagnose the infection by identifying the worm in a sample of blood.

  • The only drug that kills the worm is diethylcarbamazine.

Loiasis is a filarial worm infection that occurs only in rain forest areas of western and central Africa.

Transmission of loiasis

Loiasis is transmitted when an infected African deer fly bites a person and deposits larvae of the worm on the skin. The larvae enter through the bite wound and mature in the tissues under the skin (subcutaneous tissues). Adult worms may be about 1 to 3 inches (3 to 7 centimeters) long. The adults produce immature worm larvae (called microfilariae) that circulate in the bloodstream during the day and stay in the lungs at night. The infection is spread when the infected person is bitten by a fly during the day, when microfilariae are in the bloodstream. The fly then transmits larvae of the worm when it bites another person.

Symptoms of Loiasis

Itchy swellings (Calabar swellings) develop mainly on the arms and legs but can develop anywhere on the body. They are thought to be an allergic reaction to substances released by the worms. Swellings usually last 1 to 3 days in residents of affected areas, but in travelers to these areas, swellings occur more often and are more severe. Worms may also travel under the clear mucous membrane that covers the eyes (conjunctiva). This movement may be unsettling and make the eyes itch or feel irritated. But it usually causes no permanent damage.

Occasionally, loiasis affects the heart, kidneys, or brain, but symptoms are usually mild. Urine may contain more protein than usual and a little blood.

Diagnosis of Loiasis

  • Examination or testing of a sample of blood

Doctors suspect the infection in people who have typical symptoms if they have traveled to or emigrated from areas where the infection is common.

Doctors diagnose the infection when they identify microfilariae in a sample of blood examined under a microscope. The sample is taken around noon, when the most microfilariae are in the bloodstream.

New diagnostic tests can identify genetic material (DNA) of the parasite in a blood sample.

Prevention of Loiasis

In areas where the infection is common, diethylcarbamazine, given once a week, can help prevent infection.

To reduce the number of deer fly bites, people can

  • Use insect repellents

  • Wear clothing that has been saturated with the insecticide permethrin

  • Wear long-sleeved shirts and long pants

Because the flies bite during the day, mosquito netting over beds does not help.

Treatment of Loiasis

  • Diethylcarbamazine

  • Sometimes also albendazole or a procedure to filter the blood

The only drug that kills microfilariae and adult worms is diethylcarbamazine (a drug used to treat some filarial worm infections). It is taken by mouth for 21 days. Treatment may need to be repeated to eliminate the infection.

Diethylcarbamazine can have serious, sometimes fatal side effects, especially if the infection is severe. Reducing the number of microfilariae in the bloodstream before using the drug can reduce this risk. So before treatment with diethylcarbamazine, doctors determine the number of microfilariae in the blood. When the number is high, they reduce the number of microfilariae by giving people another drug (albendazole) or by doing a procedure using a device that filters the blood.

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