Lymphatic filariasis (see Overview of Filarial Nematode Infections) is infection with any of 3 species of Filarioidea. Acute symptoms include fever, lymphadenitis, lymphangitis, funiculitis, and epididymitis. Chronic symptoms include abscesses, hyperkeratosis, polyarthritis, hydroceles, lymphedema, and elephantiasis. Tropical pulmonary eosinophilia with bronchospasm, fever, and pulmonary infiltrates is another manifestation of infection. Diagnosis is by detection of microfilariae in blood, ultrasound visualization of adult worms, or serologic testing. Treatment is with diethylcarbamazine; antibiotics are used for complicating bacterial cellulitis.
Lymphatic filariasis is caused by Wuchereria bancrofti, Brugia malayi, and B. timori. Transmission is by mosquitoes. Infective larvae from the mosquito migrate to the lymphatics, where they develop into threadlike adult worms within 6 to 12 mo. Females are 80 to 100 mm long; males are about 40 mm long. Gravid adult females produce microfilariae that circulate in blood.
Bancroftian filariasis is present in tropical and subtropical areas of Africa, Asia, the Pacific, and the Americas, including Haiti. Brugian filariasis is endemic in South and Southeast Asia. Current estimates suggest that about 120 million people are infected.
Symptoms and Signs
Infection can result in microfilaremia without overt clinical manifestations. Symptoms and signs are caused primarily by adult worms. Microfilaremia gradually disappears after people leave the endemic area.
Acute inflammatory filariasis consists of 4- to 7-day episodes (often recurrent) of fever and inflammation of lymph nodes with lymphangitis (termed acute adenolymphangitis [ADL]) or acute epididymitis and spermatic cord inflammation. Localized involvement of a limb may cause an abscess that drains externally and leaves a scar. ADL is often associated with secondary bacterial infections. ADL episodes usually precede onset of chronic disease by ≥ 2 decades. Acute filariasis is more severe in previously unexposed immigrants to endemic areas than in native residents.
Chronic filarial disease develops insidiously after many years. In most patients, asymptomatic lymphatic dilation occurs, but chronic inflammatory responses to adult worms and secondary bacterial infections may result in chronic lymphedema of the affected body area. Increased local susceptibility to bacterial and fungal infections further contributes to its development. Chronic pitting lymphedema of a lower extremity can progress to elephantiasis (chronic lymphatic obstruction). W. bancrofti can cause hydrocele and scrotal elephantiasis. Other forms of chronic filarial disease are caused by disruption of lymphatic vessels or aberrant drainage of lymph fluid, leading to chyluria and chyloceles.
Extralymphatic signs include chronic microscopic hematuria and proteinuria and mild polyarthritis, all presumed to result from immune complex deposition.
Tropical pulmonary eosinophilia (TPE) is an uncommon manifestation with recurrent bronchospasm, transitory lung infiltrates, low-grade fever, and marked eosinophilia. It is most likely due to hypersensitivity reactions to microfilariae. Chronic TPE can lead to pulmonary fibrosis.
Microscopic detection of microfilariae in blood establishes the diagnosis. Filtered or centrifuged concentrates of blood are more sensitive than thick blood films. Blood samples must be obtained when microfilaremia peaks—at night where W. bancrofti is endemic, but during the day in many Pacific islands where B. malayi and B. timori occur. Viable adult worms can be visualized in dilated lymphatics by ultrasonography; their movement has been called the filarial dance.
Several blood tests are available:
Patients with active filarial infection typically have elevated levels of antifilarial IgG4 in the blood. However, there is substantial antigenic cross-reactivity between filariae and other helminths, and a positive serologic test does not distinguish between past and current infection.
Diethylcarbamazine (DEC) kills microfilariae and a variable proportion of adult worms. In the US, DEC is available only from the Centers of Disease Control and Prevention (CDC) after laboratory confirmation of filariasis. DEC 2 mg/kg po tid for 12 days is typically given; 6 mg/kg po once is an alternative. The regimens reduce microfilaremia similarly, but the effect of the 1-day course on adult worms is less certain. Adverse effects with DEC are usually limited and depend on the number of microfilariae in the blood. The most common are dizziness, nausea, fever, headache, and pain in muscles or joints, which are thought to be related to release of filarial antigens. Before treatment, patients should be assessed for coinfection with Loa loa and Onchocerca volvulus because DEC can cause serious reactions in patients with those infections. A single dose of albendazole 400 mg po plus either ivermectin (200 mcg/kg po) in areas where onchocerciasis is co-endemic or DEC (6 mg/kg) in areas without onchocerciasis and loiasis rapidly reduces microfilaremia levels, but ivermectin does not kill adult worms. A number of drug combinations and regimens have been used in mass treatment programs.
Also, doxycycline has been given long-term (eg, 100 mg po bid for 4 to 8 wk). Doxycycline kills Wolbachia endosymbiont bacteria within filaria, leading to death of the worms.
Acute attacks of ADL usually resolve spontaneously, although antibiotics may be required to control secondary bacterial infections.
Chronic lymphedema requires meticulous skin care, including use of systemic antibiotics to treat secondary bacterial infections; these antibiotics may slow or prevent progression to elephantiasis. Whether DEC therapy prevents or lessens chronic lymphedema remains controversial. Conservative measures such as elastic bandaging of the affected limb reduce swelling. Surgical decompression using nodal-venous shunts to improve lymphatic drainage offers some long-term benefit in extreme cases of elephantiasis. Massive hydroceles can also be managed surgically.
TPE responds to DEC 2 mg/kg po tid for 12 to 21 days, but relapses occur in up to 25% of patients and require additional courses of therapy.
Avoiding mosquito bites in endemic areas is the best protection (eg, by using diethyltoluamide [DEET], permethrin-impregnated clothing, and bed nets). Chemoprophylaxis with DEC or combinations of antifilarial drugs (ivermectin/albendazole or ivermectin/DEC) can suppress microfilaremia and thereby reduce transmission of the parasite by mosquitoes in endemic communities. DEC has even been used as an additive to table salt in some endemic areas.
Last full review/revision September 2013 by Richard D. Pearson, MD
Content last modified October 2013