Merck Manual

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Loiasis

(Loa loa Filariasis; African Eye Worm; Calabar Swellings)

By

Richard D. Pearson

, MD, University of Virginia School of Medicine

Last full review/revision Sep 2020| Content last modified Sep 2020
Click here for the Professional Version
Topic Resources

Loiasis is infection of tissues under the skin or under the clear outer membrane that covers the eye (conjunctiva) with the roundworm Loa loa.

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

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

  • Doctors diagnose loiasis by identifying worm larvae (microfilaria) in a sample of blood or seeing an adult worm traveling across the eye.

  • The only drug that kills both adult worms and larvae 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 or horse fly bites a person and deposits larvae of the worm. The larvae enter through the bite wound and mature into adult worms in the tissues under the skin (subcutaneous tissues). The adult worms travel in tissues under the skin and under the clear mucous membrane that covers the eyes. Adult worms may be about 1 to 3 inches (3 to 7 centimeters) long.

The adult worms 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

Most people with loiasis have no symptoms.

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 migrating worms. Swellings usually last 1 to 3 days in residents of affected areas, but in travelers to these areas, swellings may occur more often and be more severe.

The movement of adult worms across the eye 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

  • Examination or testing of a sample of blood

  • Observation of worms traveling under the conjunctiva of the eye

  • Identification of worms removed from the eye or skin

Doctors suspect loiasis in people who have typical symptoms if they have traveled to or emigrated from areas in western and central Africa where the infection occurs.

Doctors diagnose loiasis when they identify microfilariae in a sample of blood examined under a microscope. The sample is taken between 10 AM and 2 PM, when the most microfilariae are in the bloodstream.

Occasionally, doctors diagnose loiasis when they see worms traveling under the conjunctiva of the eye or when they identify worms removed from the eye or skin.

New diagnostic tests can identify genetic material (DNA) of the parasite in a blood sample, but these tests are not widely available.

Prevention

For long-term travelers to endemic regions who have not been previously exposed to Loa loa or Onchocerca volvulus, diethylcarbamazine, given once a week, can help prevent disease.

To reduce the number of deer fly and horse 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

  • Diethylcarbamazine

  • Before diethylcarbamazine is given to people with heavy infections, albendazole or a procedure to filter the blood, which reduces the number of microfilariae and the risk of serious side effects

The only drug that kills microfilariae and adult worms is diethylcarbamazine. 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 diethylcarbamazine 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 (apheresis) to remove microfilariae. This approach reduces the risk of serious side effects.

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