Lymphatic filariasis, onchocerciasis and loiasis

• Lymphatic filariasis (LF), onchocerciasis (river blindness) and loiasis (African eye worm) are three infections caused by long-lived parasitic filarial nematodes (roundworms); all are transmitted by biting insects.
• LF is spread by mosquitoes and damages the lymphatic system, leading years later to painless leg swelling, elephantiasis, breast or genital swelling and hydrocele; it is treated with diethylcarbamazine (DEC), with ivermectin and albendazole used in mass treatment.
• Onchocerciasis is spread by blackflies in fast-flowing water; it causes intense itching, skin changes, nodules and, in the eyes, gradual vision loss and blindness.
• Loiasis is spread by deerflies in the rainforests of West and Central Africa; it causes itchy, passing limb swelling (Calabar swellings) and an adult worm crossing the eye; DEC cures it but is dangerous when microfilarial counts are high.
• The diseases overlap geographically and in their treatment: DEC can blind or severely injure an onchocerciasis patient and can kill a person with high-count loiasis, and ivermectin can also trigger fatal brain illness in high-count loiasis, so the infections must be told apart before treating.

By the end of this page you should be able to:

1. Name the worm, the insect vector and the broad geography of each of the three infections.
2. Trace the full life cycle of a filarial worm from insect bite to adult and back to the insect.
3. Explain why the chronic damage of LF appears years after infection, and why onchocerciasis damages the eye.
4. Take a sensible diagnostic approach to an at-risk traveller or resident for each infection, including the timing of blood films.
5. Choose and dose the correct drug regimen for an individual, stating the specific situations in which a drug is contraindicated or dangerous.
6. Explain how mass drug administration (MDA) differs from individual treatment and which regimen is used for which co-endemic region, with the WHO and CDC sources named.

A filarial worm is a thread-like nematode (roundworm) whose adult lives in human tissue or lymphatic vessels and whose larvae (microfilariae, immature worms) circulate in blood or skin to be picked up by a biting insect. The three infections on this page belong to the group of neglected tropical diseases targeted by the World Health Organization (WHO).

• Lymphatic filariasis (LF), commonly called elephantiasis, is infection with Wuchereria bancrofti (about 90% of cases^[1][3]), Brugia malayi (most of the rest) or Brugia timori. The adult worms live in lymphatic vessels and nodes.
• Onchocerciasis, or river blindness, is infection with Onchocerca volvulus. The adults live in nodules under the skin and the microfilariae live in skin and eye.
• Loiasis, or African eye worm, is infection with Loa loa. The adults migrate through subcutaneous (under-the-skin) tissue and across the surface of the eye; microfilariae circulate in blood.

LF. LF impairs the lymphatic system and can lead to abnormal enlargement of body parts, pain, severe disability and social stigma.^[1] Over 657 million people in 39 countries remain at risk and require preventive chemotherapy.^[1] In 2023 the same population figure was given as living in areas requiring it.^[1] The global baseline estimate is 25 million men with hydrocele and over 15 million people with lymphoedema, and at least 36 million people remain with chronic manifestations.^[1] As of 2018, 51 million people were infected, a 74% decline since 2000, and 871 million people no longer require preventive chemotherapy.^[1] Cumulative delivery since 2000 exceeds 9.7 billion treatments.^[1] Ten countries no longer required preventive chemotherapy on reviewed data, and 21 countries and territories have been acknowledged as eliminating LF as a public health problem.^[1]

Onchocerciasis. More than 99% of infected people live in Africa and Yemen, with the remaining 1% on the Brazil–Venezuela border.^[4] At least 252.3 million people required preventive treatment in 2024; 26 countries reported treating 171.6 million people worldwide in that year, and 25.5 million people lived in areas no longer requiring ivermectin treatment.^[4] An estimated 14.6 million infected people already had skin disease and 1.15 million had vision loss (2017 Global Burden of Disease analysis).^[4] Five countries have been verified by WHO as free of onchocerciasis: Colombia, Ecuador, Mexico, Guatemala and, in 2025, Niger.^[4]

Loiasis. Loiasis occurs in West and Central African rainforests where deerflies breed; it is rare in casual travellers because repeated bites are usually needed, though infection can occur in less than 30 days in an affected area.^[9] Loiasis has impeded eradication of other tropical diseases because the drugs used for onchocerciasis and LF can cause serious side effects in people with Loa loa infection.^[9]

• LF: infection follows bites of infected mosquitoes. There are no simple modifiable personal risk factors; protection is from bite prevention and MDA in endemic areas. People may have no symptoms yet already have lymphatic damage and contribute to transmission.^[1][3]
• Onchocerciasis: repeated bites of infected blackflies (genus Simulium) that breed in fast-flowing rivers and streams; rural residents, long-stay travellers, missionaries, field workers and Peace Corps volunteers are most at risk, and casual tourists rarely become infected.^[4][6]
• Loiasis: repeated bites of infected deerflies in rainforests of West and Central Africa; risk rises with longer stays, more bites and more infected flies, and deerflies bite in the daytime, especially in the rainy season.^[9]

Each filarial worm has the same two-host pattern; what differs is the tissue the adult occupies and the insect that carries it.

1. An insect ingests microfilariae when it bites an infected person. ^[1][4][9]
2. Inside the insect the microfilariae develop into infective larvae over days to weeks. ^[4][9]
3. The larvae enter a new person during a later bite and migrate to their final site (lymphatics for LF, skin and subcutaneous tissue for onchocerciasis and loiasis). ^[1][4]
4. They mature into adults, mate and produce new microfilariae: millions circulating in blood for LF and loiasis, and in skin for onchocerciasis. ^[1][3][4]
5. The cycle continues when an insect bites again. ^[4][9]

LF specifics. Wuchereria bancrofti, responsible for 90% of cases, and the two Brugia species nest in lymphatic vessels and nodes and can live roughly 6 to 8 years, producing millions of microfilariae that circulate in blood.^[1][3] Mosquito vectors are Culex in urban and semi-urban areas, Anopheles in rural areas and Aedes in Pacific islands.^[1]

Onchocerciasis specifics. Onchocerca volvulus adults form nodules under the skin and produce microfilariae that move around the subcutaneous tissue and induce intense inflammatory responses when they die; adult worms in communities need 10 to 15 years of treatment to eliminate transmission, matching the worm's lifespan.^[4]

Loiasis specifics. Loa loa adults migrate through subcutaneous tissue and under the conjunctiva of the eye, and microfilariae circulate in blood.^[7][9] The deerfly, not the mosquito or blackfly, is the vector, and transmission occurs in rainforest Central and West Africa.^[9]

Most infection is asymptomatic even though adult worms commonly produce subclinical lymphatic dilatation and dysfunction.^[3] Chronic lymphoedema and hydrocele develop in about 30% of infected people, usually years after the initial infection has cleared, as adult worms die and damage lymphatics, and as recurrent bacterial infections hasten progression to elephantiasis.^[3] The worms are microfilaricidal targets; the adult worm is responsible for the pathology of lymphedema and hydrocele, and antiparasitic treatment does not reverse established chronic disease.^[2]

Symptoms are caused by microfilariae moving in subcutaneous tissue and triggering inflammation when they die.^[4] In the eye the same dead microfilariae produce keratitis, inflammation and scarring that lead gradually to visual impairment and blindness; permanent blindness is a major outcome, with an estimated 1.15 million infected people having vision loss.^[4] Early childhood exposure to O. volvulus infection is also associated with epilepsy.^[4]

Asymptomatic (subclinical) infection is the commonest presentation; most infected people have no external signs yet have lymphatic damage and cannot be detected without laboratory tests.^[1][3]

Acute. Filarial lymphadenopathy is common in infected children, with adult worms detectable by ultrasound of inguinal, crural and axillary nodes and vessels.^[3] Death of an adult worm triggers acute filarial lymphangitis, an inflammatory response progressing distally (retrograde) along the affected lymphatic vessel, usually in a limb; systemic symptoms such as fever and headache are generally mild.^[3] In post-pubertal males worms favour intrascrotal lymphatics and inflammation presents as funiculitis, epididymitis or orchitis, sometimes with a tender granulomatous nodule.^[3]

Chronic. Lymphoedema (tissue swelling), most often of the legs but also arms, breasts and genitalia; elephantiasis (thickened, warty skin) after recurrent bacterial infection of the limb; hydrocele (scrotal swelling) from lymphatic damage; chyluria (lymph in the urine) from rupture of dilated lymphatics into the renal pelvis; microscopic haematuria and proteinuria.^[1][3]

Tropical pulmonary eosinophilia (TPE) is a serious progressive lung disease with fever, non-productive nocturnal cough and wheezing, marked eosinophilia, high serum IgE and positive antifilarial antibodies, but no peripheral microfilaraemia; it is seen most in long-term residents from Asia, men aged 20 to 40.^[3]

Infection produces eye and skin disease. Symptoms come from microfilariae moving in subcutaneous tissue and the inflammation produced when they die.^[4] Clinical findings include severe itching, disfiguring skin changes and rashes, subcutaneous nodules over the adult worms, and eye lesions that progress to visual impairment and permanent blindness.^[4][6] Many people never develop symptoms.^[6] The disease is often called river blindness because affected communities live near the fast-flowing rivers where blackflies breed.^[6]

Most infected people have no symptoms.^[9] When symptoms occur they include:

• itchy, non-painful swelling (Calabar swelling) that comes and goes, often near joints; ^[7][9]
• an adult worm migrating across the surface of the eye (eye worm), a brief, painless, recurring event that gives the disease its name; ^[9]
• worms visibly crawling under the skin; ^[9]
• less commonly generalised itching, muscle and joint pain and fatigue. ^[9]

Loiasis does not spread from person to person.^[9]

A table comparing the three infections is the most useful framing, because patients in Africa may have more than one and the distinction is a safety matter.

Clinically, LF resembles venous insufficiency, chronic lymphoedema of other causes, filarial (TPE) asthma and, in the acute phase, bacterial cellulitis. Onchocerciasis resembles allergic dermatitis, scabies and leprosy for the skin, and other causes of corneal blindness for the eye. Loiasis in a returning traveller must be distinguished from other causes of eosinophilia and fleeting skin swellings.

LF. The standard definitive diagnosis of active infection is identification of microfilariae in a blood smear by microscopy. Blood should be taken at the peak of microfilarial circulation, most often at night (10 PM to 2 AM), as a thick smear stained with Giemsa or haematoxylin and eosin, with concentration techniques for added sensitivity.^[3] Serologic enzyme immunoassays (antifilarial IgG1 and IgG4) are an alternative; active infection is associated with elevated IgG4.^[3] Circulating W. bancrofti antigen assays are not presently approved by the US Food and Drug Administration, and there is no antigen test for Brugia species.^[3] Ultrasound can visualise adult worms, and tissue biopsy can show adult worms or microfilariae.^[3]

Onchocerciasis. The most common confirmatory method is the skin snip: a thin skin biopsy examined microscopically for immature worms.^[6] Nodules can be excised to examine for adult worms, the eyes examined for larval damage, and blood tested for antibodies.^[6] The CDC guidance for treatment is clinical and based on examination, skin snips or eye findings, rather than a formal score.^[5]

Loiasis. The standard test is a daytime (10 AM to 2 PM) Giemsa-stained thin or thick blood smear, with concentration techniques to improve yield in low-count infection; the time must be adjusted for jet-lag in returning travellers.^[8] Identification of microfilariae confirms infection and the count per millilitre directs treatment.^[7][8] An adult worm seen on the eye or extracted tissue also confirms infection, but a blood smear is still needed before antiparasitic therapy to quantify microfilaraemia.^[8] Antibody tests indicate exposure rather than active infection and are used in research settings; a general filarial antibody screen may exclude infection in smear-negative, highly suspected cases.^[8]

A threshold that changes management in loiasis is the microfilarial count:

• at less than 8,000 microfilariae per mL, DEC (8 to 10 mg per kg per day in 3 divided doses for 21 days) is used; ^[7]
• at 8,000 or more per mL, DEC is started only after apheresis in specialist centres, or after albendazole 200 mg twice daily for 21 days with re-measurement of levels; ^[7]
• some authors use a more conservative cut-off of 2,500 microfilariae per mL. ^[7]

The drug of choice is DEC, which is both microfilaricidal and active against the adult worm.^[2] Infection is rare in the United States, and DEC is no longer FDA-approved; it is supplied by the CDC after confirmed positive laboratory results.^[2]

• DEC 6 mg per kg per day, given either as a single day or a 12-day course; one day is generally as effective as 12 days.^[2]
• Tropical pulmonary eosinophilia warrants a longer course of 14 to 21 days.^[2]
• Contraindication: DEC is contraindicated in patients who may also have onchocerciasis (risk of severe skin and eye exacerbation, the Mazzotti reaction); onchocerciasis must be excluded first in anyone with a consistent exposure history.^[2]
• Caution: DEC should be used with extreme caution, with specialist consultation, where circulating Loa loa microfilarial levels exceed 8,000 per mL. Current sources state this threshold per millilitre (CDC loiasis guidance, Merck Manual); the CDC lymphatic filariasis page prints it as ">8,000/mm3", which reads as a unit slip (8,000/mm3 = 8,000,000/mL). Authorities also differ on how conservative the DEC cut-off should be: the CDC/WHO-derived guidance states that the risk of fatal encephalopathy approaches zero below 8,000 microfilariae/mL (with a conservative 2,500/mL variant), whereas the German Society for Tropical Medicine and Global Health guideline (2024) treats more than 2,000 mf/mL as a relative contraindication to DEC. Neither steroid pre-treatment nor slow dose escalation prevents the possible encephalopathy and renal failure.^{[2] [7]}
• Hydrocele: surgical (hydrocelectomy); patients usually do not improve clinically with DEC.^[2]
• Lymphoedema and elephantiasis: DEC is not indicated; the late chronic phase is unaffected by antiparasitic treatment. Management is hygiene, skin care, exercise, elevation, treatment of wounds and infections, appropriate footwear, complex decongestive physiotherapy and referral to a certified lymphoedema therapist. A 4 to 6-week course of doxycycline 200 mg per day may stabilise or reverse progression, but evidence is limited.^[2]
• Adult worm killing: ivermectin kills microfilariae only. Doxycycline 200 mg per day for 4 to 6 weeks has shown adult worm killing in some studies.^[2]

• Drug of choice: ivermectin, 150 micrograms per kg (mcg per kg) orally as a single dose every 6 months; given annually it reduces blindness and skin symptoms but does not kill adult worms.^[5]
• Higher-than-recommended doses are more likely to cause side effects and may be harmful, and prolonged daily dosing shows no benefit over annual dosing because one dose lowers microfilarial load for a year or more.^[5]
• For patients who will not return to an endemic area, treat every 6 months for as long as evidence of infection persists (itch, microfilariae in skin biopsies, microfilariae on eye examination); every 3 months is an option, and palpable nodules alone are not evidence of active infection requiring further treatment because ivermectin does not kill adults.^[5] Interval wording differs by source: the CDC guidance above uses 6 months (with 3 months as an option), while the Australian ivermectin product information (Stromectol) frames retreatment as 6 to 12 months after the initial dose for river blindness. Follow national and product-specific guidance.
• Ivermectin may sterilise adult female worms; it can cause mild itching from dying microfilariae but does not worsen eye symptoms. Severe reactions in the absence of Loa loa co-infection are rare.^[5]
• Doxycycline (not standard therapy) kills the Wolbachia endosymbiont required by the adult worm: a 6-week course of 200 mg orally daily kills more than 60% of adult females and sterilises 80 to 90% of them at 20 months.^[5] It does not kill microfilariae, so ivermectin is given first (about one week before) for symptom relief; standard protocols give ivermectin 4 to 6 months after doxycycline, so simultaneous treatment safety is unproven.^[5] If 200 mg daily is not tolerated, 100 mg daily is sufficient to sterilise female worms.^[5]
• Do not use: suramin and DEC; suramin has multiple systemic toxicities, and DEC accelerates onchocercal blindness.^[5]
• Loa loa co-infection: do not treat without specialist advice; fatal encephalitic reactions to ivermectin can occur, and doxycycline has only been studied with Loa counts below 8,000 microfilariae per mL.^[5] Follow the MECTIZAN Expert Committee and African Programme for Onchocerciasis Control guidance.^[4]

Treatment is complex and is best undertaken with tropical medicine expertise; sometimes it is best not to treat at all.^[7]

• DEC is the drug of choice (8 to 10 mg per kg per day orally in 3 divided doses for 21 days) because it kills both microfilariae and adult worms, giving quicker resolution.^[7]
• Cure is defined as resolution of symptoms, resolution of eosinophilia and falling antifilarial antibody titres; most achieve this with one or two courses, and some need more DEC or albendazole.^[7]
• Prophylaxis: DEC 300 mg once weekly for long-term travellers to endemic areas.^[7]
• Albendazole 200 mg twice daily for 21 days is used when DEC fails or to lower a high microfilarial load before DEC; its effect is slow and counts must be re-checked.^[7]
• DEC is contraindicated with onchocerciasis because of risk of blindness or severe skin disease.^[7]
• Loa loa lacks Wolbachia, so doxycycline is ineffective.^[7]

Complications to anticipate and prevent: secondary bacterial infection driving progression of lymphoedema to elephantiasis; chyluria; TPE in LF; eye damage and blindness, epilepsy association, skin disease in onchocerciasis; renal and neurological catastrophe in DEC-treated high-count loiasis. ^[3][4][7]

MDA uses WHO-recommended regimens based on local co-endemicity.^[1] The impact depends on regimen efficacy and coverage; the two-drug regimens interrupt transmission when delivered annually for at least 4 to 6 years with effective coverage.^[1]

WHO onchocerciasis guidance. Treat with ivermectin at least once yearly for 10 to 15 years, with a minimum requirement of 80% therapeutic coverage; where O. volvulus co-exists with Loa loa adjust strategies, because treatment of people with high L. loa levels can cause severe adverse events and affected countries should follow MECTIZAN Expert Committee and African Programme guidance.^[4] Strategies include community-directed treatment with ivermectin (CDTI), vector control and onchocerciasis elimination mapping.^[4]

WHO response and targets. World Health Assembly resolution WHA50.29 launched the Global Programme to Eliminate Lymphatic Filariasis (GPELF, 2000), whose two components are stopping transmission by annual treatment and alleviating suffering through an essential care package (treatment of acute attacks, hygiene measures, hydrocele surgery, infection treatment).^[1] The WHO road map for neglected tropical diseases 2021 to 2030 set targets for onchocerciasis elimination by 2030.^[4]

Vector control is a supplemental strategy: insecticide-treated nets (especially with Anopheles), indoor residual spraying and personal protection, and control of blackflies for onchocerciasis. Environmental modification to reduce mosquito breeding is a long-standing approach.^[1][9]

Prevention of Loiasis for travellers follows the same insect-bite prevention: DEET repellent, long sleeves, window screens, avoiding wood smoke which attracts deerflies; long-term residents may use weekly DEC to reduce risk.^[9][7]

Pregnancy.

• LF: morbidity care measures are safe. DEC safety in pregnancy is not addressed in our sources; the clinician should balance any treatment against fetal risk. MDA programmes following WHO guidance allow albendazole use in the second and third trimesters where benefits outweigh risks.^[7]
• Onchocerciasis: the CDC classifies ivermectin as pregnancy category C and WHO excludes pregnant women from MDA programmes using ivermectin; doxycycline is category D and should not be used; treatment should only be given where maternal benefit justifies fetal risk.^[5] The US FDA/WHO nomenclature is not universal: Australian ivermectin product information rates pregnancy safety as not established rather than assigning a category, though the underlying position is the same (avoid unless benefit justifies risk).
• Loiasis: albendazole is category C, with WHO allowing use in MDA in the second and third trimesters where benefits outweigh risks; DEC decisions in pregnancy require expert advice.^[7]

Children.

• LF: CDC guidance covers children older than 18 months for DEC; albendazole is used in MDA campaigns from age one year. ^[2][7]
• Onchocerciasis: ivermectin is unclear under 15 kg body weight; WHO MDA criteria treat children at least 90 cm tall (about 50% of boys by 28 months and girls by 30 months) and some campaigns treat younger children at a reduced dose. ^[5] Doxycycline is contraindicated in children 8 years and under because of tooth discoloration, except for life-threatening infection where benefits outweigh risks (FDA and CDC tetracycline-class labelling). ^[5] The Australian ivermectin product's separate under-5 limit applies to ivermectin for river blindness, not to doxycycline.
• Loiasis: paediatric DEC and albendazole are dosed by body weight as in adults. ^[7]

Older adults, immunocompromised. No special guidance exists in our sources beyond standard caution; however DEC toxicity in those with renal or nervous system disease, and corticosteroid needs, should be assessed individually.

Co-infection is the safety-critical scenario. Onchocerciasis excludes DEC; loiasis at high counts excludes routine DEC or ivermectin; any patient from a region co-endemic for loiasis who is about to receive LF or onchocerciasis treatment should be screened for Loa loa to avoid fatal encephalopathy. ^[2][7][8]

• Diagnosis: the thick blood smear and skin snip are the cheapest confirmatory tests; a daytime (10 AM to 2 PM) smear for loiasis and a night (10 PM to 2 AM) smear for LF; microfilarial counts in loiasis can usually be done by basic microscopy. ^[3][8]
• Treatment: where DEC is unavailable, ivermectin plus albendazole or albendazole alone per WHO's co-endemicity regimens can be given through community programmes. ^[1]
• Lymphoedema care needs no laboratory: daily washing, elevation, exercise and treatment of entry lesions and bacterial infection prevent progression; complex decongestive therapy is ideal but not essential. ^[2]
• Hydrocele needs surgical capacity; without it, patients require referral. ^[2]
• Onchocerciasis community-directed treatment with ivermectin is the backbone where specialist care is unavailable; diagnosed eye disease needs slit-lamp or equivalent assessment. ^[4]
• High-count loiasis cannot safely be treated in basic settings; apheresis and specialist monitoring are required. ^[7]

• Lymphatic filariasis, onchocerciasis and loiasis are filarial nematode infections transmitted respectively by mosquitoes, blackflies and deerflies. ^[1][4][9]
• W. bancrofti causes about 90% of LF; adults live 6 to 8 years in lymphatics and produce millions of microfilariae. ^[1][3]
• Chronic LF (lymphoedema, elephantiasis, hydrocele) develops in about 30% of infected people years after infection. ^[3]
• Onchocerciasis is caused by O. volvulus; microfilariae in skin and eye trigger inflammation when they die, causing itch, skin disease, nodules and vision loss and blindness in 1.15 million people. ^[4]
• Onchocerciasis requires 10 to 15 years of annual ivermectin treatment; the adult worm lives in nodules. ^[4]
• Loiasis is spread by deerflies in Central and West African rainforest; many people are asymptomatic; it presents with transient swelling and a worm crossing the eye. ^[9]
• DEC is the drug of choice for LF (6 mg per kg per day, one or twelve days) and for loiasis (8 to 10 mg per kg per day for 21 days). ^[2][7]
• DEC is contraindicated in onchocerciasis and dangerous with high Loa counts (8,000 or more microfilariae per mL). ^[2][7]
• Ivermectin treats onchocerciasis (150 mcg per kg as a single dose every 6 months) and microfilariae in LF; it is dangerous in high-count loiasis. ^[5][7]
• Doxycycline (200 mg daily for 4 to 6 weeks, or 6 weeks in onchocerciasis) kills adult filarial worms or Wolbachia. ^[2][5]
• LF diagnosis peaks at night (10 PM to 2 AM blood smear); loiasis diagnosis uses a daytime (10 AM to 2 PM) smear. ^[3][8]
• WHO MDA regimens depend on co-endemicity: albendazole alone where loiasis is present; ivermectin plus albendazole where onchocerciasis is present; DEC plus albendazole or triple therapy where neither (or conditions met) is present. ^[1]
• Over 657 million people remain at risk of LF; about 252.3 million required onchocerciasis treatment in 2024. ^[1][4]
• Mass treatment is not a substitute for lymphoedema and hydrocele care; morbidity management is an essential package. ^[1]

Vector-borne disease, mosquitoes and blackflies; neglected tropical diseases; lymphoedema and compression therapy; opportunistic infection in immunocompromised hosts; tropical pulmonary eosinophilia; parasitic infections in travellers; skin disease in migrants.