What is Loiasis?

What is Loa Loa (Loiasis)?

Loa loa is a filarial parasitic worm that causes loiasis, a chronic infection affecting more than 20 million people in the rainforest and adjacent savannah regions of central and west Africa, transmitted by blood-sucking Chrysops flies (deer flies). 1

Transmission and Geographic Distribution

• Loa loa is transmitted exclusively through the bite of Chrysops dimidiata and Chrysops silacea flies, which breed in forest and swamp areas of west and central Africa 2, 3
• The endemic region extends from southwest Benin to west Ethiopia, and from north Angola to central Chad 1
• The infection occurs only in travelers to or residents of these endemic African regions 4, 5

Clinical Manifestations

Common Presentations

• Calabar swelling: Transient, migratory subcutaneous edema that is painful and pruritic, caused by adult worm movement through tissues 1, 3
• Eye worm migration: Visible migration of the adult worm (3-7 cm in length) through the subconjunctival space of the eye, causing discomfort 1, 6, 5
• Pruritus (itching) is a frequent symptom 1, 3
• Fever, asthenia (fatigue), and arthralgia may occur 3, 5

Serious Complications

• Multi-organ system involvement can occur, including cardiac, respiratory, neurological, ophthalmic, renal, gastrointestinal, and dermatological manifestations 1
• High microfilarial loads are associated with renal, cardiac, and neurological sequelae and increased risk of death 6, 5
• The population attributable fraction of death is estimated at 14.5% in highly endemic regions 6

Diagnostic Categories and Clinical Significance

Three Critical Infection Categories

Patients must be classified into one of three categories to guide safe treatment and prevent fatal complications: 1

1. Occult infection: Indirect signs (eye worm, Calabar swelling) without detectable microfilaremia in blood 1

2. Microfilaremic infection: Detectable microfilariae in blood but below critical thresholds 1

3. Hypermicrofilaremic infection: Microfilarial load >8,000 microfilariae/mL, with severe risk when >30,000/mL 1

Diagnostic Challenges

• Only about one-third of infected individuals show detectable microfilaremia on thick smear microscopy, which is the primary diagnostic method in endemic settings 1
• Daytime blood microscopy using 20 mL citrated blood is required for Loa loa screening, as microfilariae circulate during daytime hours 7
• Eosinophilia is commonly present (found in 22 of 26 patients in one series) 3
• Filarial serology can support diagnosis but lacks specificity 3

Treatment-Related Mortality Risk

Critical Treatment Contraindications

The most important clinical consideration is that rapid-acting antifilarial drugs (diethylcarbamazine and ivermectin) can cause fatal encephalopathy in patients with high Loa loa microfilarial loads. 1

• Risk of severe neurological adverse events is high when microfilarial load exceeds 8,000 microfilariae/mL 1
• The reaction becomes more severe if the load exceeds 30,000 microfilariae/mL 1
• Encephalitis is the most severe adverse event and can be fatal 1

Pre-Treatment Screening Requirements

• Mandatory screening for Loa loa infection before administering ivermectin or diethylcarbamazine (DEC) in anyone who has traveled to endemic regions 7
• Failure to screen can result in severe neurological adverse events, including encephalitis and death 7
• Determination of microfilarial count is essential if Loa loa is detected 7

Treatment Options

Primary Treatment Agents

The three main drugs recommended for loiasis treatment are: 1

1. Diethylcarbamazine (DEC): Gold standard in non-endemic countries, typically dosed incrementally 1, 2
2. Ivermectin: Alternative treatment, particularly when DEC cannot be used 1
3. Albendazole: Alternative agent based on microfilarial levels 1

Treatment Selection Based on Microfilarial Load

• The common microfilarial threshold of 8,000 microfilariae/mL dictates treatment strategy adjustments 1
• Adjunctive treatments such as corticosteroids (prednisolone) and antihistamines are used to mitigate inflammatory side effects 1, 7, 4
• When using DEC with microfilaremia present, prednisolone is usually given alongside to reduce inflammatory reactions 7

Public Health Impact

Disease Burden

• Morbidity measured by disability-adjusted life-years lost might be as high as 400 per 100,000 residents in endemic areas 6
• At least 10 million residents of central and west Africa are thought to have loiasis 5
• The disease burden is substantial and previously underestimated 6, 5

Impact on Other Disease Control Programs

Loiasis poses a major obstacle to the elimination of onchocerciasis (river blindness) and lymphatic filariasis in co-endemic areas. 1, 5

• Millions of people in co-endemic areas are left untreated with standard community-wide ivermectin treatment due to risk of fatal complications 1
• High refusal rates for ivermectin occur in co-endemic areas due to fear of adverse events 1
• This has contributed to sustained transmission and reintroduction of onchocerciasis into previously eliminated areas 1

Key Clinical Pitfalls

• Never assume loiasis is benign: While historically regarded as relatively benign, it causes significant morbidity, mortality, and complications 6, 5
• Never treat with ivermectin or DEC without screening for microfilarial load: This can be fatal in hypermicrofilaremic patients 1, 7
• Never use DEC in patients co-infected with onchocerciasis: This is absolutely contraindicated 1, 7
• Always obtain daytime blood samples: Loa loa microfilariae circulate during daytime, unlike other filariae 7
• Treatment relapses can occur (noted in 8 of 26 cases in one series), requiring follow-up 3