Filarial worms are nematodes that dwell in the subcutaneous tissues and the lymphatics. Eight filarial species infect humans (Table 228-1); 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 228-1). 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.
TABLE 228-1Characteristics of the Filariae |Favorite Table|Download (.pdf) TABLE 228-1 Characteristics of the Filariae
|Organism ||Periodicity ||Distribution ||Vector ||Location of Adult ||Microfilarial Location ||Sheath |
|Wuchereria bancrofti ||Nocturnal ||Cosmopolitan areas worldwide, including South America, Africa, southern Asia, Papua New Guinea, China, Indonesia ||Culex, Anopheles (mosquitoes) ||Lymphatic tissue ||Blood ||+ |
| ||Subperiodic ||Eastern Pacific ||Aedes (mosquitoes) ||Lymphatic tissue ||Blood ||+ |
|Brugia malayi ||Nocturnal ||Southeast Asia, Indonesia, India ||Mansonia, Anopheles (mosquitoes) ||Lymphatic tissue ||Blood ||+ |
| ||Subperiodic ||Indonesia, Southeast Asia ||Coquillettidia, Mansonia (mosquitoes) ||Lymphatic tissue ||Blood ||+ |
|Brugia timori ||Nocturnal ||Indonesia ||Anopheles (mosquitoes) ||Lymphatic tissue ||Blood ||+ |
|Loa loa ||Diurnal ||West and Central Africa ||Chrysops (deerflies) ||Subcutaneous tissue ||Blood ||+ |
|Onchocerca volvulus ||None ||South and Central America, Africa ||Simulium (blackflies) ||Subcutaneous tissue ||Skin, eye ||– |
|Mansonella ozzardi ||None ||South and Central America ||Culicoides (midges) ||Undetermined site ||Blood ||– |
| ||None ||Caribbean ||Simulium (blackflies) ||Undetermined site ||Blood ||– |
|Mansonella perstans ||None ||South and Central America, Africa ||Culicoides (midges) ||Body cavities, mesentery, perirenal tissue ||Blood ||– |
|Mansonella streptocerca ||None ||West and Central Africa ||Culicoides (midges) ||Subcutaneous tissue ||Skin ||– |
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.