Loiasis: Symptoms, Types, Causes and Treatment

Loiasis, often referred to as "African eye worm," is a neglected tropical disease that is both fascinating and challenging for clinicians and researchers alike. Caused by the filarial nematode Loa loa, this infection is endemic to Central and West Africa and has recently garnered renewed attention due to its significant morbidity and complex treatment landscape. In this comprehensive guide, we break down the symptoms, types, causes, and treatment of loiasis, synthesizing the latest research to help demystify this important public health concern.

Symptoms of Loiasis

Loiasis presents with a mix of classic and sometimes surprising symptoms. While many people associate the disease with the dramatic migration of the "eye worm," its clinical spectrum is much broader, impacting the skin, eyes, and even internal organs. Recognizing these symptoms is crucial for timely diagnosis and management, especially as global travel increases the likelihood of encountering cases outside Africa.

Symptom	Description	Frequency/Significance	Source(s)
Eye Worm	Visible migration of adult worm across eye	Pathognomonic, ~42-54%	2 4 5
Calabar Swelling	Transient, localized angioedema, often pruritic	Common in non-natives, ~42%	1 4 5
Pruritus	Intense itching, often with edema	Very common	1 5
Eosinophilia	Elevated eosinophil count in blood	Frequent finding	1 4 5
Arthralgia	Joint pain	Not uncommon	2
Vision Changes	Temporary visual disturbances	Reported in ~79% with eyeworm	2
Fever/Fatigue	General malaise, low-grade fever	Occasional	5
Atypical Systemic	Cardiac, renal, neurological symptoms	Rare, more in high mf loads	3

Table 1: Key Symptoms

Classic Symptoms: The Signature Signs

The most recognized signs of loiasis are:

• Eye Worm Migration: The dramatic, visible movement of the adult worm across the subconjunctiva of the eye. This is not only distressing but is almost unique to loiasis and thus highly diagnostic. Over 40% of surveyed patients report this symptom, often accompanied by pain and temporary visual disturbances 2 4 5.
• Calabar Swellings: These are transient, localized angioedematous swellings, often on the limbs or face. They can be intensely itchy and are more common in temporary residents or travelers than in lifelong residents of endemic areas. The swelling typically resolves in a few days but can recur 1 4 5.

Pruritus and Allergic Manifestations

Pruritus (itching) is almost universal, sometimes occurring with or without visible swelling. This symptom reflects the body’s hypersensitivity response to the migrating larvae or adult worms 1 5. Eosinophilia—an elevated count of eosinophil white blood cells—is a hallmark laboratory finding and indicates an ongoing allergic or parasitic process 1 4 5.

Additional Symptoms

• Arthralgia and Myalgia: Some patients experience joint and muscle pain, likely due to immune complex deposition or direct worm migration 2.
• Fever and Fatigue: These are less common but can occur, particularly during acute episodes or in those with a high parasite burden 5.

Atypical and Severe Complications

While loiasis is often considered benign, recent studies highlight its potential for serious complications, particularly in those with high microfilarial loads:

• Cardiac, Renal, Neurological, and Dermatological Manifestations: Cases have been reported of renal failure, endomyocardial fibrosis, encephalitis (especially post-treatment), and other organ involvement. These atypical presentations are more likely in long-term residents of endemic regions with heavy infections 1 3 11.
• Systemic Allergic Reactions: In rare cases, severe allergic or inflammatory reactions can occur, especially after anti-filarial treatment 1 11 13.

Types of Loiasis

Loiasis does not present the same way in every individual. Its manifestations vary depending on the person’s background, immune status, and exposure history. Understanding these types helps tailor both diagnosis and management.

Type	Key Features	At-Risk Group	Source(s)
Classic	Eye worm, Calabar swelling, pruritus	Most cases, travelers	1 4 5
Asymptomatic	Microfilaremia without clinical symptoms	Lifelong endemic residents	1 4 6
Hyperresponsive	Marked swelling, high eosinophilia, IgE	Temporary residents/travelers	1
Atypical/Severe	Cardiac, renal, neurological complications	High parasite burden	3

Table 2: Types of Loiasis

Classic Loiasis

This form is what most people think of: the combination of visible eye worm migration and Calabar swellings, often with itching and mild systemic symptoms. Both locals and travelers can exhibit these signs, but they are especially prominent in newcomers to endemic areas 1 4 5.

Asymptomatic Loiasis

Surprisingly, many long-term residents of endemic regions carry the parasite without any obvious symptoms, despite having microfilaremia (circulating larvae in the blood). This tolerance is likely due to immune adaptation over time 1 4 6.

Hyperresponsive or Allergic Loiasis

Non-immune individuals (such as expatriates, tourists, or short-term workers in Africa) are prone to a hyperresponsive syndrome, with pronounced swelling, severe pruritus, high levels of eosinophils, and elevated IgE antibodies. These individuals often have little or no detectable microfilaremia but experience more severe symptoms 1.

Atypical or Severe Loiasis

A subset of patients, particularly those with high microfilarial loads, may develop complications involving the heart, kidneys, or nervous system. These forms are less common but can be life-threatening, especially following treatment 3 11.

Causes of Loiasis

At its core, loiasis is a vector-borne filarial disease with a complex life cycle. Understanding its causes helps inform both strategies for prevention and the challenges in eradicating the disease.

Cause	Mechanism/Agent	Notes	Source(s)
Loa loa Worm	Filarial nematode parasite	Only infects humans	6 8
Chrysops Flies	Biting flies (deerflies/mango flies)	Transmits infective larvae	6 8
Endemic Regions	Central and West Africa, forested zones	>10 million infected	6 8 12
Human Factors	Travel, migration, lack of immunity	Travelers at higher risk	1 4 5

Table 3: Causes of Loiasis

Loa loa: The Parasite

Loiasis is caused by the nematode Loa loa. The adult worm lives in subcutaneous tissues of humans, producing microfilariae that circulate in the blood, especially during the day 6 8.

The Vector: Chrysops Flies

Transmission occurs when a Chrysops fly (commonly known as the deerfly or mango fly) bites a human, injecting infective larvae into the skin. These flies breed in the humid forested and swampy areas of Central and West Africa, explaining the geographical distribution of the disease 6 8.

Epidemiological and Human Risk Factors

• The disease is endemic in forested regions of Cameroon, Gabon, Congo, Nigeria, and neighboring countries 6 8 12.
• Anyone living, working, or traveling in these regions is at risk, though the clinical presentation may vary depending on immune status and duration of exposure 1 4 5.
• As people migrate or travel for business or tourism, cases are increasingly seen outside Africa, complicating diagnosis for non-endemic clinicians 4 5.

Treatment of Loiasis

Treating loiasis can be challenging, requiring a personalized approach based on infection intensity, patient background, and available resources. Therapy aims to clear the infection while minimizing potentially severe adverse reactions.

Treatment	Mechanism/Use	Considerations/Limitations	Source(s)
Diethylcarbamazine	Drug of choice, kills adult/microfilaria	Risk of severe side effects, especially in high microfilarial loads	5 10 11 13
Ivermectin	Effective at lowering microfilaremia	Safer at low parasite loads; caution in high loads	5 9 12 13
Supportive Care	Antihistamines, corticosteroids	To manage allergic reactions	1 11 13
Surgical Removal	For visible adult worms (eye/skin)	Symptom relief, not curative	5
Preventive Strategies	Vector control, travel precautions	No vaccine, personal protection	6 8 12

Table 4: Treatment Options

Medical Therapy: Diethylcarbamazine and Ivermectin

• Diethylcarbamazine (DEC): Traditionally the drug of choice, DEC is effective against both adult worms and microfilariae. However, treatment can trigger severe reactions, including encephalitis and renal complications, particularly in patients with high parasite loads. Multiple courses may be needed, and relapses are not uncommon 5 10 11 13.
• Ivermectin: Increasingly used, especially in areas where loiasis and onchocerciasis overlap. Ivermectin rapidly lowers microfilaremia and is generally well-tolerated at low to moderate parasite loads. However, in patients with very high microfilarial densities, treatment can provoke dangerous systemic reactions (e.g., encephalopathy), so careful screening is required 9 12 13.
• Supportive Medications: Antihistamines and corticosteroids are sometimes used to mitigate allergic and inflammatory reactions during therapy 1 11 13.

Monitoring and Management of Complications

• Eosinophilia and Allergic Reactions: Close monitoring is required during treatment, as post-treatment eosinophil surges and systemic symptoms can occur 1 13.
• Severe Adverse Events: Both DEC and ivermectin can precipitate life-threatening reactions in heavily infected patients. Slow, graded dosing and hospitalization for high-risk individuals are recommended 11 13.

Non-Pharmacological and Preventive Approaches

• Surgical Removal: In cases where the adult worm is visible in the eye or under the skin, surgical extraction can provide immediate relief, though it does not address systemic infection 5.
• Prevention: Currently, there is no vaccine against loiasis. Avoiding Chrysops fly bites through protective clothing, insect repellents, and vector control measures is the best preventive strategy, especially for travelers 6 8 12.
• Community-Level Interventions: Mass ivermectin distribution for onchocerciasis has incidentally reduced loiasis prevalence and intensity in some areas, though caution is needed to prevent severe adverse events in those with high Loa loa microfilaremia 12.

Conclusion

Loiasis is a complex, neglected tropical disease with a broad spectrum of symptoms and potentially serious complications. Its management requires a nuanced understanding of both clinical presentation and the risks associated with treatment. With globalization, awareness of loiasis is more important than ever—not just for health workers in Africa, but for clinicians worldwide.

Key Points:

• Loiasis is caused by the filarial worm Loa loa, transmitted by Chrysops flies in Central and West Africa 6 8.
• Classic symptoms include eye worm migration, Calabar swellings, pruritus, and eosinophilia, but atypical and severe complications can occur 1 2 3 4 5.
• Types of loiasis vary with exposure history and immune status, with non-natives often having more severe allergic manifestations 1 4 5.
• Treatment involves diethylcarbamazine or ivermectin, but both can trigger severe reactions in patients with high parasite loads, necessitating careful monitoring 5 9 10 11 12 13.
• Prevention relies on vector avoidance and, at the community level, careful implementation of mass drug administration in co-endemic areas 6 8 12.

Loiasis is no longer a disease that can be ignored. Its public health impact is now recognized as substantial, and concerted research and control efforts are needed to reduce its burden for both endemic and global populations.