Loiasis, called African eye worm by most people, is caused by the parasitic worm Loa loa. It is passed on to humans through the repeated bites of deerflies (also known as mango flies or mangrove flies) of the genus Chrysops. The flies that pass on the parasite breed in certain rain forests of West and Central Africa. Infection with the parasite can also cause repeated episodes of itchy swellings of the body known as Calabar swellings. Knowing whether someone has a Loa loa infection has become more important in Africa because the presence of people with Loa loa infection has limited programs to control or eliminate onchocerciasis (river blindness) and lymphatic filariasis (elephantiasis). There may be more than 29 million people who are at risk of getting loaisis in affected areas of Central and West Africa.

Loa loa parasites are found in West and Central Africa. Ten countries have areas where there are high rates of infection (i.e., where more than 40% of the people who live in that area report that they have had eye worm in the past). An estimated 14.4 million people live in these areas of high rates of infection. Another 15.2 live in areas where 20–40% of people report that they have had eye worm in the past.

The people most at risk for loiasis are those who live in the certain rain forests in West and Central Africa. The deerflies that pass the parasite to humans usually bite during the day and are more common during the rainy season. They are attracted by the movement of people and by smoke from wood fires. Rubber plantations are areas where more deerflies may be found. The flies do not typically enter homes, but they might be attracted to homes that are well lit.

Travelers are more likely to become infected if they are in areas where they are bitten by deerflies for many months, though occasionally they get infected even if they are in an affected area for less than 30 days.

Your risk of infection depends on the number of bites received, the number of infected deerflies in the area you visit, and the length of your stay in the area.

The deerflies (genus Chrysops) that pass Loa loa on to humans bite during the day. If a deerfly eats infected blood from an infected human, the larvae (non-adult parasites) will infect cells in its abdomen. After 7–12 days the larvae develop the ability to infect humans. Then the larvae move to the mouth parts of the fly. When the deerfly breaks a human’s skin to eat blood, the larvae enter the wound and begin moving through the person’s body.

It takes about five months for larvae to become adult worms inside the human body. Larvae can become adults only inside the human body. The adult worms live between layers of connective tissue (e.g., ligaments, tendons) under the skin and between the thin layers of tissue that cover muscles (fascia). Fertilized females can make thousands of microfilariae a day. The microfilaria then move into the lymph vessels of the body (the lymph vessels contain the blood cells that fight infection). Eventually they move into the lungs where they spend most of their time. These microfilariae enter the blood from time to time, usually around midday. It takes five or more months for microfilariae to be found in the blood after someone is infected with Loa loa. The microfilariae can live up to one year in the human body. If they are not consumed in a blood meal by a deerfly they will die. Adult worms may live up to 17 years in the human body and can continue to make new microfilariae for much of this time.

Most people with loiasis do not have any symptoms. People who get infected while visiting areas with loiasis but do not come from areas where loiasis is found (travelers) are more likely to have symptoms. The most common manifestations of the disease are Calabar swellings and eye worm. Calabar swellings are localized, non-tender swellings usually found on the arms and legs and near joints. Itching can occur around the area of swelling or can occur all over the body. Eye worm is the visible movement of the adult worm across the surface of the eye. Eye worm can cause eye congestion, itching, pain, and light sensitivity. Although eye worm can be scary, it lasts less than one week (often just hours) and usually causes very little damage to the eye. People with loiasis can have itching all over the body (even when they do not have Calabar swellings), hives, muscle pains, joint pains, and tiredness. Sometimes adult worms can be seen moving under the skin. High numbers of blood cells called eosinophils are sometimes found on blood counts. Some people who are infected for many years may develop kidney damage though development of permanent kidney damage is not common. Other rare manifestations include painful swellings of lymph glands, scrotal swellings, inflammation of parts of the lungs, fluid collections around the lung, and scarring of heart muscle.

In people who have been bitten by the flies that carry Loa loa in areas where Loa loa is known to exist, the diagnosis can be made in the following ways:

• Identification of the adult worm by a microbiologist or pathologist after its removal from under the skin or eye
• Identification of an adult worm in the eye by a health care provider
• Identification of the microfilariae on a blood smear made from blood taken from the patient between 10AM and 2PM
• Identification of antibodies against L. loa on specialized blood test

Diagnosis of loiasis can be difficult, especially in light infections where there are very few microfilariae in the blood. The specialized blood test is not widely available in the United States. A positive antibody blood test in someone with no symptoms means only that the person was infected sometime in his/her life. It does not mean that the person still has living parasites in his/her body.

Decisions about treatment of loiasis can be difficult and often require advice from an expert in infectious diseases or tropical medicine. Although surgical removal of adult worms moving under the skin or across the eye can be done to relieve anxiety, loiasis is not cured by surgery alone. There are two medications that can be used to treat the infection and manage the symptoms. The treatment of choice is diethylcarbamazine (DEC), which kills the microfilariae and adult worms. Albendazole is sometimes used in patients who are not cured with multiple DEC treatments. It is thought to kill adult worms. Certain people with heavy infections are at risk of brain inflammation when treated with DEC. This can cause coma or sometimes death. People with heavy infections need to be treated by experienced specialists. Sometimes, other medical conditions need to be addressed first in order to make it safer to use DEC. Sometimes treatment is not recommended.

There are no programs to control or eliminate loiasis in affected areas. Your risk of infection may be less in areas where communities receive regular treatment for onchocerciasis or lymphatic filariasis.

There are no vaccines that protect you from loiasis. If you are going to be in an area with loiasis for a long period of time, diethylcarbamazine (DEC)—300mg taken once a week—can reduce your risk of infection. Avoiding areas where the deerflies are found, such as muddy, shaded areas along rivers or around wood fires, may also reduce your risk of infection. You may reduce your risk of bites by using insect repellants that contain DEET (N,N-Diethyl-meta-toluamide) and wearing long sleeves and long pants during the day, which is when deerflies bite. Treating your clothes with permethrin may also help.

What is loiasis?

Loiasis is an infection caused by the parasitic worm Loa loa.

What are the signs and symptoms?

Many people do not develop any symptoms, and symptoms usually do not show up for many months after infection. If you have loiasis, you may have itchy, non-painful swellings of the body that come and go. The swellings can show up anywhere though they are more common near joints. You may develop an eye worm that crawls across the surface of your eye. Sometimes you may see a worm that crawls under your skin. Less common symptoms include itching all over your body, muscle pain, joint pain, and fatigue. Blood tests may show high counts of eosinophils, which are a type of blood cell that can be associated with parasitic infections.

How do you get infected?

You get infected by being bitten by an infected deerfly of the genus Chrysops. Deerflies become infected when they eat blood from an infected person. Travelers are more likely to become infected if they are in areas where they are bitten by deerflies for many months, though occasionally they get infected even if they are in the area for less than 30 days.

Where in the world can one get infected?

The parasite is passed from deerflies to humans in certain rain forests of West and Central Africa. The deerflies usually bite during the day and are more common during the rainy season. They are attracted by the movement of people and by smoke from wood fires. Rubber plantations may be an area where many deerflies are found. Your health care provider can help you determine if you are going to or have been to an area where loiasis is found.

Can loiasis be spread person to person?

No. If you are infected you cannot pass the infection on to other people.

How is it diagnosed?

If you have traveled to an area where infected flies are found and you develop symptoms, your health care provider may order a blood smear. Blood is put on a microscope slide and examined for the larvae of the parasite. Additionally, loaisis can be diagnosed by a health care provider if they see an adult worm on the surface of your eye. Sometimes, if your health care provider is concerned that you may be infected but your tests are normal, he or she may order a specialized blood test.

What is the treatment?

Sometimes treatment of loiasis can be dangerous, so your health care provider may want to talk to an expert in tropical medicine before giving you any medication to treat loiasis. If you have an eye worm, the worm can be surgically removed to provide immediate relief while your health care provider determines if it is safe to treat you with medication to kill the parasite. Removing the worm from your eye does not cure the infection, as the parasite is often found in other parts of your body. A medication called diethylcarbamazine, or DEC, can be used to kill the parasites in your body. Because there is a small risk of serious side effects related to killing the parasites, your health care provider will do tests to be sure that it is safe to treat you. Sometimes people need special treatments before it is safe to give medication and sometimes treatment with medications is not recommended.

How can I prevent loiasis?

Avoid insect bites by wearing long pants and long-sleeved shirts and avoid the smoke of wood fires to reduce your risk of infection. If you are going to stay in an area affected by Loa loa in West and Central Africa for a long period of time, you can reduce your risk of getting loiasis by taking DEC weekly. You should consult a tropical medicine expert to determine whether using DEC to prevent loiasis is right for you. It can take time to get DEC, so do not wait until the last minute.