Filarial worms are nematodes that dwell in the subcutaneous tissues and the lymphatics. Eight filarial species infect humans (Table 258-1); of these, four—Wuchereria bancrofti, Brugia malayi, Onchocerca volvulus, and Loa loa—are responsible for most serious filarial infections. Filarial parasites, which infect an estimated 170 million persons worldwide, are transmitted by specific species of mosquitoes or other arthropods and have a complex life cycle, including infective larval stages carried by insects and adult worms that reside in either lymphatic or subcutaneous tissues of humans. The offspring of adults are microfilariae, which, depending on their species, are 200–250 μm long and 5–7 μm wide, may or may not be enveloped in a loose sheath, and either circulate in the blood or migrate through the skin (Table 258-1). To complete the life cycle, microfilariae are ingested by the arthropod vector and develop over 1–2 weeks into new infective larvae. Adult worms live for many years, whereas microfilariae survive for 3–36 months. The bacterial endosymbiont Wolbachia has been found intracellularly in all stages of Brugia, Wuchereria, Mansonella, and Onchocerca species and has become a target for antifilarial chemotherapy.
Usually, infection is established only with repeated, prolonged exposures to infective larvae. Since the clinical manifestations of filarial diseases develop relatively slowly, these infections should be considered to induce chronic infections with possible long-term debilitating effects. In terms of the nature, severity, and timing of clinical manifestations, patients with filarial infections who are native to endemic areas and have lifelong exposure may differ significantly from those who are travelers or who have recently moved to these areas. Characteristically, filarial disease is more acute and intense in newly exposed individuals than in natives of endemic areas.
Lymphatic filariasis is caused by W. bancrofti, B. malayi, or B. timori. The threadlike adult parasites reside in lymphatic channels or lymph nodes, where they may remain viable for more than two decades.
W. bancrofti, the most widely distributed filarial parasite of humans, affects an estimated 110 million people and is found throughout the tropics and subtropics, including Asia and the Pacific Islands, Africa, areas of South America, and the Caribbean basin. Humans are the only definitive host for the parasite. Generally, the subperiodic form is found only in the Pacific Islands; elsewhere, W. bancrofti is nocturnally periodic. Nocturnally periodic forms of microfilariae are scarce in peripheral blood by day and increase at night, whereas subperiodic forms are present in peripheral blood at all times and reach maximal levels in the afternoon. Natural vectors for W. bancrofti are Culex fatigans mosquitoes in urban settings and Anopheles or Aedes mosquitoes in rural areas.
TABLE 258-1Characteristics of the Filariae