Filariasis Diagnostics

Lymphatic filariasis (Wuchereria bancrofti, Brugia malayi, B. timori), loiasis (Loa loa) and onchocerciasis (Onchocerca volvulus) are diagnosed by detecting microfilariae in blood or skin, circulating filarial antigen, antibody tests and molecular methods.

For lymphatic filariasis, the thick blood film taken at night (22:00 to 02:00 hours, nocturnal periodicity) shows sheathed microfilariae, with concentration methods (Knott technique, membrane filtration of 1 to 5 mL of blood) increasing sensitivity.

Circulating filarial antigen by immunochromatographic card test (W. bancrofti only, sensitivity 96 to 100%, specificity 100%) can be done at any time of the day. Loiasis is diagnosed by daytime blood films (10:00 to 14:00 hours) and by eye worm.

Onchocerciasis is diagnosed by skin snips (microfilariae emerging into saline or water after 30 minutes to 24 hours), nodule biopsy, slit lamp examination of the eye and serology (Ov16 antibody).

Lymphatic filariasis affects about 50 million people, with 860 million at risk in 44 countries, and causes lymphoedema, elephantiasis, hydrocele, chyluria and tropical pulmonary eosinophilia. Onchocerciasis affects about 20 million, with 1 million blind or visually impaired (the second cause of infectious blindness), mostly in sub-Saharan Africa.

Loiasis affects 10 to 15 million in Central and West Africa, and microfilaraemia over 30 000 in each mL carries a risk of severe reaction to ivermectin and diethylcarbamazine.

Indications. Lymphoedema, hydrocele, scrotal swelling, chyluria or recurrent adenolymphangitis in a person from or living in an endemic area. Eosinophilia, nocturnal cough and wheeze (tropical pulmonary eosinophilia). Itchy skin, nodules, depigmentation or eye disease in an endemic area (onchocerciasis). Calabar swellings, eye worm or itching in West or Central Africa (loiasis).

Unexplained eosinophilia in a returned traveller or migrant. Mass drug administration surveys. Loa loa screening before ivermectin in co-endemic areas.

Specimen. Blood collected at night (lymphatic filariasis, Brugia) or at midday (loiasis) in EDTA or finger prick thick films (60 microlitres) stained with Giemsa; venous blood 1 to 5 mL for filtration or Knott concentration; serum for antigen and antibody; skin snips (1 to 2 mg) from the iliac crest, buttock, calf and shoulder, placed in saline; nodule excision; ultrasound of the scrotum or lymphatics (filarial dance sign) and slit lamp of the eye.

Reference values. No microfilariae in blood or skin. Circulating filarial antigen and filarial antibody negative. Eosinophils up to 0.5 times 10 to the power 9/L.

Interpretation.

• Sheathed microfilariae with pale staining nuclei not reaching the tail tip (W. bancrofti) or with two separate terminal nuclei (Brugia) in a night blood film: lymphatic filariasis. Loa loa microfilariae are sheathed with nuclei extending to the tip, seen in daytime blood. Mansonella species are unsheathed and usually non-pathogenic.
• Positive circulating filarial antigen test: active W. bancrofti infection (adult worms present), including in amicrofilaraemic people. Antigen persists for 1 to 3 years after treatment.
• Positive antibody (Bm14, Wb123) test: exposure or infection, which does not separate past from current disease, but is used for children and surveillance after elimination.
• Skin snip positive for microfilariae (unsheathed, tail free of nuclei): onchocerciasis, with density counted for severity. Ov16 serology is positive in 80 to 90% and can be used for mapping.
• Eosinophilia over 3 times 10 to the power 9/L, raised IgE, restrictive and obstructive lung pattern, and diffuse chest radiograph infiltrates in a person from an endemic area: tropical pulmonary eosinophilia, with microfilariae usually absent from blood and high antifilarial antibody titres.
• Ultrasound showing moving adult worms (filarial dance sign) in scrotal lymphatics or breast: active infection.
• Chyluria (milky urine with triglyceride) and lymphoscintigraphy abnormalities: lymphatic damage.
• Loa loa microfilaria density over 8000 in each mL or 30 000 in each mL: risk of severe encephalopathy after ivermectin or diethylcarbamazine, so alternative strategies (albendazole, apheresis, expert input) are needed.
• Co-infection with Loa loa and onchocerciasis is common, and counts direct treatment choice.

Critical values and findings needing immediate action.

• Loa loa microfilaraemia over 30 000 in each mL: avoid ivermectin and diethylcarbamazine because of risk of fatal encephalopathy; refer for specialist management.
• Confirmed onchocerciasis with eye disease (sclerosing keratitis, uveitis, optic atrophy): ophthalmology review before and during treatment.
• Acute adenolymphangitis with fever and limb swelling: antibiotics, limb hygiene and exclusion of lymphatic obstruction.
• Tropical pulmonary eosinophilia with respiratory failure: diethylcarbamazine and corticosteroids as the picture requires.
• Mazzotti reaction (fever, pruritus, hypotension) after treatment of onchocerciasis: supportive care, corticosteroids and antihistamines.
• Severe lymphoedema with ulceration and secondary infection: antibiotics and skin care.

Interferences and pitfalls.

• Night blood sampling is difficult in practice, and microfilarial counts vary with time of day and between individuals; the antigen test removes this limitation for W. bancrofti, but not for Brugia.
• Low density infections and prior treatment give negative films, and PCR and antigen are more sensitive.
• Cross-reactions occur between filarial antigens and Loa loa on W. bancrofti antigen cards at high Loa loa loads, and between filarial antibodies and other helminths (Strongyloides, Ascaris), so serology cannot be used alone.
• Skin snips can be negative in early or low intensity onchocerciasis and after repeated ivermectin, and Ov16 serology or PCR (O-150) adds sensitivity.
• Drug treatment (diethylcarbamazine, ivermectin, albendazole, doxycycline for Wolbachia) causes reactions from dying parasites, which must be anticipated.
• Eosinophilia can be absent in established chronic disease, and is common in many other helminth infections.
• Lymphoedema in endemic areas also arises from podoconiosis, which is non-infectious, so both are considered.
• Hydrocele and lymphoedema persist after cure and need surgical and hygiene care.
• Mass drug administration programmes are monitored by antigen surveys (transmission assessment surveys) and require community compliance.

Special groups.

• Children: early lymphatic damage by subclinical ultrasound changes; antigen and antibody tests are used in school surveys.
• Pregnancy: diethylcarbamazine and albendazole are generally deferred until after delivery.
• Travellers: short exposures cause symptoms of acute filariasis, eosinophilia and angio oedema (Calabar swellings in loiasis), with negative microfilariae common.
• Co-infection with HIV, tuberculosis and malaria: altered immune responses.
• Regions: lymphatic filariasis in South Asia, Southeast Asia, Africa, the Pacific and parts of the Americas; onchocerciasis in sub-Saharan Africa (with foci in Yemen and Latin America); loiasis in Central and West African rainforest zones.

Follow up tests. Full blood count with eosinophils, total IgE and antifilarial antibody, urinalysis for chyluria and haematuria, scrotal and limb ultrasound, lymphoscintigraphy, chest radiograph and spirometry in tropical pulmonary eosinophilia, slit lamp and fundus examination in onchocerciasis, quantification of Loa loa microfilariae before ivermectin,

stool and blood tests for co-infections, HIV test, repeat circulating filarial antigen at 12 months after treatment, and Wolbachia targeted therapy planning.