Author(s): Mary Kamb, Sharon Roy

Infectious Agent

Lymphatic filariasis is caused by 3 species of filarial nematodes, Wuchereria bancrofti, Brugia malayi, and B. timori.

Transmission

Infective larvae can be transmitted by both day- and night-biting mosquitoes; vectors include species from several genera, including Aedes, Anopheles, Coquillettidia, Culex, and Mansonia. Transmission occurs in rural, urban, and semiurban settings.

Epidemiology

Once widespread in Africa and Asia, and swaths of Latin America and the Pacific, sustained elimination efforts based on mass drug administration have greatly reduced the global prevalence of parasites that cause lymphatic filariasis. Currently, infections occur in parts of Africa (Egypt, sub-Saharan Africa); Asia (southeast Asia, the Indian subcontinent); and some southwestern Pacific Islands. In the Americas, focal distribution has been reported on the northeastern coast of Brazil, in Guyana, and on the island of Hispaniola (Dominican Republic, Haiti). Most infections in the United States occur in immigrants and refugees.

Because multiple exposures (bites from infected mosquitoes) over time are usually required for infection, travelers are at low risk for this disease. Infections have, however, been documented in long-term travelers (usually people living many months–years in endemic countries) and in visitors to areas with highly efficient mosquito vectors (e.g., Aedes in the Pacific) with as little as 1 month of exposure.

Clinical Presentation

Infective filarial larvae grow into adult worms that inhabit the human lymphatic and subcutaneous tissues. Infections can be asymptomatic (subclinical) or associated with acute and chronic clinical manifestations involving moderate to severe lymphedema of the arm, breast, leg, penis, or scrotum. In people with lymphedema, episodes of acute secondary infections can involve painful swelling of an affected limb, fever, or chills; repeated episodes can hasten the progression of lymphedema to its advanced stage, known as elephantiasis.

Tropical pulmonary eosinophilia (TPE) syndrome is a potentially serious, progressive lung disease characterized by fever and nocturnal cough, wheezing, or both, which results from immune hyper-responsiveness to microfilariae in the pulmonary capillaries. Most cases of TPE have been reported in long-term residents from Asia, often men aged 20–40 years.

Diagnosis

In symptomatic people with plausible exposure based on epidemiology, diagnosis can be made by microscopic detection of characteristic microfilariae on a thick blood film made from an appropriately timed sample collection. Microfilariae are usually not detected in patients with TPE.

Filarial antibody tests that detect elevated IgG and IgG4 can be useful, especially when microfilariae are not detected. Serologic assays are available through the National Institutes of Health (301-496-5398) or Centers for Disease Control and Prevention Dpx (404-718-4745; parasites@cdc.gov). PCR tests exist in some research settings but are not yet commercially available. Ultrasound and lymphoscintigraphy can be used to detect presence of motile adult worms in lymphatic vessels, known as filarial dance sign.

Treatment

Diethylcarbamazine (DEC) is the drug of choice for lymphatic filariasis, regardless of the causative parasite species. Although no longer approved for use in the United States by the US Food and Drug Administration and not commercially available, DEC can be obtained under an investigational new drug protocol from the CDC Drug Service (M–F, 8 a.m.–4:30 p.m. Eastern, 404-639-3670; after hours/weekends/holidays, 770-488-7100; drugservice@cdc.gov).

Before initiating DEC treatment for lymphatic filariasis, first rule out co-infection with Onchocerca volvulus in at-risk patients (see Sec. 5, Part 3, Ch. 17, Onchocerciasis / River Blindness). DEC use is contraindicated in patients with O. volvulus infection due to the potential for causing a severe allergic response (Mazzotti reaction) that especially affects the eyes and skin. In addition, DEC must be used with extreme caution in patients with loiasis (Loa loa infection) due to possible life-threatening side effects in people with high circulating microfilariae loads.

People with lymphedema and hydrocele can benefit from lymphedema management and, in the case of hydrocele, surgical repair. Evidence suggests that a 4–8-week course of doxycycline (200 mg daily) can both sterilize adult worms and improve lymphatic pathologic features.

Prevention

No vaccines or drugs to prevent infection are available. Travelers should avoid mosquito bites (see Sec. 4, Ch. 6, Mosquitoes, Ticks & Other Arthropods).

CDC website: Lymphatic filariasis