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What Is Loiasis?
Loiasis is a parasitic infection caused by the nematode (roundworm) Loa loa, commonly known as the “African eye worm”. The name derives from one of the most striking features of the infection: the worm’s ability to migrate visibly across the surface of the eye.
The infection spreads through the bite of flies in the Chrysops genus. These flies are often called deer flies or mango flies. These flies serve as the biological vector (carrier) of the parasite.
Loiasis is endemic (persistently present) in the forested regions of West and Central Africa, particularly in humid, rainforest environments. It represents a significant public health concern in affected areas.
While the majority of infections are asymptomatic or cause only mild symptoms, the migration of adult worms through the body can lead to considerable discomfort. For example, worms may move across the surface of the eye. Without appropriate treatment, serious complications can also develop.
What Is the Loa Loa Worm? How Do You Get It?
The parasite responsible for loiasis, Loa loa, belongs to the class of filarial nematodes (Filarioidea). Its life cycle involves two principal hosts: the human host and the fly vector.
The Life Cycle of Loa loa
1. Transmission to humans: When a Chrysops fly bites an infected individual, it ingests microfilariae (microscopic larval forms) circulating in the blood. Inside the fly, the microfilariae mature into infective third-stage larvae (L3). When this fly subsequently bites another person, the L3 larvae are deposited into the bloodstream of the new host.
2. Development in the human body: The larvae migrate through the body and develop into adult worms within the subcutaneous tissue (the layer of fat and connective tissue beneath the skin):
- Female worms can grow up to 7 cm in length
- Male worms reach 3–4 cm in length
Once mature, female worms produce microfilariae, tiny larval forms that circulate in the bloodstream with a characteristic diurnal periodicity. This means they are present in peripheral blood during the day but concentrate in the lungs at night.
3. Ocular migration: One of the most distinctive and visually alarming features of loiasis is the ability of adult worms to migrate through the subconjunctival space (beneath the transparent membrane covering the white of the eye). This movement is visible to the naked eye and causes a pronounced sensation of movement or irritation. While deeply distressing for the patient, it generally does not cause permanent damage to vision.
4. Return transmission to the fly: When a Chrysops fly bites an infected individual, it ingests microfilariae from the blood. Within the fly, these larvae pass through successive developmental stages. They ultimately mature into infective L3 larvae, which are ready to be transmitted to the next human host, completing the cycle.
Important Facts About the Parasite
- Adult Loa loa worms can survive for 10–15 years in the human host.
- Microfilariae can circulate for a prolonged period in the bloodstream.
- The infection is not transmitted person-to-person; the Chrysops fly vector is essential.
Endemic Regions
Loiasis is endemic in the following countries and regions:
- Congo River basin
- Nigeria
- Cameroon
- Gabon
- Equatorial Guinea
- Central African Republic
Rural populations living in forested areas are at highest risk because Chrysops flies thrive in the humid, shaded environments of tropical rainforests. Transmission peaks during the rainy season, when fly activity is greatest.
Incubation Period
After infective larvae are introduced into the body by a Chrysops fly bite, approximately 5 months are required for the larvae to develop into mature adult worms capable of causing symptoms.
However, in most cases, symptoms do not appear until years, or even more than a decade, after the initial infective bite. This prolonged incubation makes the diagnosis particularly challenging, especially in travelers who may have left endemic regions long before symptoms develop.
Rarely, symptoms have been reported as early as 4 months after the bite.
Symptoms: How to Know If You Are Infected
The majority of people infected with Loa loa are completely asymptomatic and never develop recognizable symptoms. When clinical manifestations do occur, they can range from mild and transient to more severe and debilitating.
Principal Clinical Manifestations
1. Calabar Swellings (Subcutaneous Migration) The hallmark feature of symptomatic loiasis is the appearance of “Calabar swellings”, localized, temporary swellings caused by the migration of adult worms through the subcutaneous tissue. These swellings:
- Are itchy and painful
- Resolve spontaneously within a few days (typically 1–3 days in individuals from endemic regions, but may be more frequent and severe in travelers and temporary visitors)
- Appear most commonly on the arms and legs, but can occur anywhere on the body
- Are thought to represent an allergic (hypersensitivity) response to substances released by the migrating worm
2. Ocular Migration Adult worms can migrate through the subconjunctival space, which is under the transparent conjunctiva covering the white of the eye. This is one of the most distressing manifestations of loiasis:
- The worm’s movement is visible to the naked eye, which is understandably alarming for the patient.
- Causes itching, irritation, or intense pain in the affected eye
- Does not generally cause permanent damage to vision or the eye
- The worm typically passes through within minutes to hours
Additional Symptoms
Patients with symptomatic loiasis may also report:
- Generalized itching (pruritus)
- Joint pain (arthralgia)
- Muscle aches (myalgia)
- Fever and general malaise
On laboratory testing, two characteristic findings may be present:
- Lymphadenopathy (swollen lymph nodes)
- Eosinophilia (elevated eosinophil count in the blood, a type of white blood cell that is characteristically elevated in parasitic infections)
Complications
Although loiasis is generally not life-threatening, serious complications can occur in some patients, particularly those with high microfilarial loads or those who receive certain treatments:
- Heart involvement: Cardiomyopathy (heart muscle disease).
- Kidney involvement: Nephropathy (kidney disease), including nephrotic syndrome.
- Brain involvement: Encephalitis, or brain inflammation, is the most feared complication. It may occur spontaneously or, more critically, as a treatment-related side effect. This typically happens in patients with very high microfilariae counts who receive ivermectin or diethylcarbamazine.
Critical treatment risk: In patients with very high levels of circulating microfilariae (over 30,000 per mL), treatment with certain antiparasitic drugs, particularly ivermectin, may lead to a severe post-treatment encephalopathy. This condition involves a potentially life-threatening inflammatory reaction in the brain. This is why treatment of loiasis requires specialist assessment and careful pre-treatment microfilarial load measurement.
Diagnosis
Diagnosis of loiasis is based on a combination of clinical history, symptoms, and laboratory investigations.
Key Elements of the Diagnostic Workup
1. Epidemiological history: A history of residence in or travel to an endemic region (Central or West Africa) serves as an essential diagnostic clue. This is especially important because the incubation period can be very long.
2. Blood examination for microfilariae: Detection of microfilariae in peripheral blood is the primary diagnostic test. Because Loa loa microfilariae exhibit diurnal periodicity (they circulate in peripheral blood during the day and concentrate in the lungs at night), blood samples must be collected between 10:00 AM and 2:00 PM for optimal yield. The sample is examined under the microscope to identify the characteristic microfilariae.
3. Serological tests: Blood tests for anti-Loa loa antibodies can be helpful, but their specificity is limited. Cross-reactivity with other filarial infections, for example, is common. They are most useful when microfilariae are undetectable (as in pre-patent or low-burden infections).
4. Direct visualization of the worm: The observation of an adult worm migrating beneath the conjunctiva of the eye or under the skin is pathognomonic (definitively diagnostic) of loiasis. When visible, the worm may be surgically removed at that time.
5. Molecular testing (PCR): Polymerase chain reaction (PCR) tests targeting Loa loa DNA can provide a sensitive and specific diagnosis, particularly in cases with low or absent microfilaremia.
Treatment
The treatment of loiasis is primarily based on antiparasitic chemotherapy. Treatment decisions must be individualized, taking into account the patient’s microfilarial load, symptom severity, and risk of complications.
Antiparasitic Drugs
1. Diethylcarbamazine (DEC): Recommended by the CDC and MSD Manual, DEC is the drug of first choice for loiasis treatment in most countries. It acts against both adult worms and microfilariae.
Potential adverse effects include:
- Fever, headache, and muscle pain (commonly observed during the first days of treatment)
- In patients with high microfilarial loads, severe systemic inflammatory reactions, such as encephalopathy, may occur. Therefore, pre-treatment assessment and close monitoring are required.
2. Ivermectin: The use of ivermectin in loiasis is controversial and potentially dangerous, particularly in patients with high microfilarial counts (>8,000–30,000 microfilariae/mL). It can trigger severe neurological adverse events, including post-treatment encephalopathy. When considered, it is reserved for specific situations under close specialist supervision.
3. Albendazole: Used in particular circumstances, such as a preparatory treatment to reduce microfilarial levels before DEC or in patients who cannot tolerate DEC. Albendazole acts more slowly and does not typically trigger the acute inflammatory reactions seen with DEC or ivermectin, making it a safer option in high-microfilarial-burden patients.
Surgical Removal
Adult worms visible in the eye or under the skin can be physically removed. The procedure typically involves:
- Topical anesthesia to immobilize (partially paralyze) the worm
- Rapid surgical extraction with forceps
The removal must be performed quickly, because the paralysis is incomplete and temporary. If the worm is not extracted promptly, it may retreat back beneath the tissue and become inaccessible.
Prevention
No vaccine exists against loiasis. Prevention relies entirely on behavioral and physical measures to reduce the risk of Chrysops fly bites:
- Insect repellents: Use repellents containing DEET (N,N-diethyl-meta-toluamide) at concentrations of 30–50% on exposed skin; reapply regularly.
- Protective clothing: Wear long-sleeved shirts, long trousers, and socks. This is especially important during daytime hours, when Chrysops flies are most active.
- Permethrin-treated clothing: Applying permethrin (an insecticide) to clothing provides additional protection
- Avoidance of high-risk environments: Limit time in dense rainforest areas, particularly during the rainy season when fly activity is at its peak.
- Awareness for travelers: Healthcare providers should counsel travelers to endemic regions about the risk of loiasis. They should also explain the symptoms to watch for and emphasize the importance of reporting potential exposures upon returning home, even if it’s years later.
Sources and Bibliography
- Centers for Disease Control and Prevention (CDC) – Loiasis
- MSD Manual – Loiasis (Professional Version)
- Molyneux DH, et al. Loiasis: a neglected but complex tropical disease. The Lancet Infectious Diseases. 2018;18(9):983–990.
- Wanji S, et al. Loa loa biology and epidemiology. Parasitology Research. 2019.
- Boussinesq M. Loiasis. Annals of Tropical Medicine and Parasitology. 2006;100(8):715–731.
- Chippaux JP, Bouchite B, Demanou M, Morlais I, Le Goff G. Density and aggressiveness of Chrysops spp., the vector of loiasis. Parasite. 2000;7(1):50–54.
- World Health Organization (WHO). Lymphatic filariasis and loiasis co-endemicity: management challenges.
