Loa loa filariasis, also known as African eye worm, is a parasitic disease caused by the nematode worm Loa loa. This condition is transmitted by vectors in parts of Africa and is considered a neglected tropical disease.
The Loa loa Parasite: Life Cycle and Transmission
The vectors that transmit the parasite are day-biting tabanid flies of the Chrysops genus, known as deerflies or mango flies. When an infected fly bites a human, it introduces third-stage filarial larvae onto the skin, which enter the body through the bite wound. These larvae develop into adult worms within the subcutaneous tissues.
Adult female worms measure 40 to 70 millimeters in length, while males are 30 to 34 millimeters long. The adult worms can live for up to 17 years in a human host, migrating through tissues and mating. After mating, female worms produce immature larvae called microfilariae, which circulate in the bloodstream during the day.
When a fly bites an infected person, it ingests blood containing microfilariae. Inside the fly, the microfilariae develop into infective larvae over 10 to 12 days. These larvae migrate to the fly’s proboscis, ready to be transmitted to another human during the fly’s next blood meal.
Clinical Manifestations of Loa loa Infection
The clinical signs of a Loa loa infection vary, and many people native to endemic regions remain asymptomatic. The first signs may not appear for months or years after the initial infection. In some cases, the latent period can extend up to two decades.
A widely recognized manifestation is the visible migration of an adult worm across the subconjunctival tissue of the eye. While this “eye worm” phenomenon can be distressing, it does not cause long-term vision damage in most cases. Patients may experience a foreign body sensation, pain, itching, or light sensitivity during the worm’s transit. Worms can also move across the bridge of the nose or under the eyelids.
Another sign is the appearance of Calabar swellings, which are temporary, localized areas of swelling under the skin. These are a hypersensitivity reaction to allergens from migrating adult worms. They can develop anywhere but are most common on the limbs near joints, where they can be painful. Other less specific symptoms include generalized itching, fatigue, and muscle or joint pain.
Epidemiology and Prevention Strategies
Loa loa filariasis is geographically confined to the rainforest and swamp forest ecosystems of West and Central Africa. The disease is endemic in these regions due to the presence of the Chrysops deerfly vectors. People living in or near these forested areas are at the highest risk of infection.
Humans are the only confirmed natural reservoir for the parasite. The prevalence of the disease is estimated by tracking the history of “eye worm” cases in affected countries. Both men and women are affected, though advanced age can be a risk factor for more severe symptoms.
Prevention focuses on avoiding deerfly bites. Personal protective measures include using insect repellents with DEET and wearing long-sleeved shirts and pants during the day. Bed nets are less effective against the day-biting Chrysops flies. Community-level vector control is challenging due to the flies’ breeding habitats.
Diagnosis of Loiasis
Diagnosis is suspected in individuals with a travel history to endemic regions who present with classic symptoms. The most direct confirmation is the visual identification and surgical removal of an adult worm migrating across the eye or under the skin. This procedure is both diagnostic and a partial treatment.
Laboratory diagnosis relies on identifying microfilariae in a blood sample. Because the microfilariae circulate in the blood during the day, samples should be drawn between 10 AM and 2 PM. A microscopic examination of a stained blood smear can then reveal the microfilariae.
Quantifying the number of microfilariae per milliliter of blood, often done with a quantitative polymerase chain reaction (qPCR), helps guide treatment decisions. A high level of eosinophils (a type of white blood cell) is a common but non-specific finding. Antibody tests are also available but may not distinguish between a past or current infection.
Treatment and Management Approaches
Treatment for loiasis focuses on killing the parasites with medication, though surgical removal of a visible adult worm is an option. The primary drug used is diethylcarbamazine (DEC), which kills both microfilariae and adult worms. A standard course of DEC for adults involves taking the medication three times daily for 21 days.
A challenge in managing loiasis is the risk of severe adverse reactions to treatment, especially in patients with a high density of microfilariae. When DEC rapidly kills many microfilariae, it can trigger a systemic inflammatory response. In severe cases, this can lead to life-threatening complications like encephalopathy, a brain disorder.
To mitigate these risks, careful patient assessment before treatment is required. For individuals with very high microfilarial loads, albendazole may be used first to gradually reduce the parasite count before administering DEC. Co-infection with other filarial worms is another consideration. Using ivermectin, a treatment for onchocerciasis (river blindness), can cause severe adverse events in patients who also have a high load of Loa loa microfilariae.