Filarial worms are nematodes that dwell in the subcutaneous tissues and the lymphatics. Eight filarial species infect humans (Table 218-1); 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 218-1). 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 Rickettsia-like endosymbiont Wolbachia has been found intracellularly in all stages of Brugia, Wuchereria, Mansonella, and Onchocerca and has become a target for antifilarial chemotherapy.
Table 218-1 Characteristics of the Filariae
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Table 218-1 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||+|
|B. 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||−|
|M. perstans||None||South and Central America, Africa||Culicoides (midges)||Body cavities, mesentery, perirenal tissue||Blood||−|
|M. 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 diseases 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 anopheline or aedean mosquitoes in rural areas.
Brugian filariasis due to B. malayi occurs primarily in eastern India, Indonesia, Malaysia, and the Philippines. B. malayi also has two forms distinguished by the periodicity of microfilaremia. The more common nocturnal form is transmitted in areas of coastal rice fields, while the subperiodic form is found in forests. B. malayi naturally infects cats as well as humans. The distribution of B. timori is limited to the islands of southeastern Indonesia.
The principal pathologic changes result from inflammatory damage to the lymphatics, which is typically caused by adult worms and not by microfilariae. Adult worms live in afferent lymphatics or sinuses of lymph nodes and cause lymphatic dilatation and thickening of the vessel walls. The infiltration of plasma cells, eosinophils, and macrophages in and around the infected vessels, along with endothelial and connective tissue proliferation, leads to tortuosity of the lymphatics and damaged or incompetent lymph valves. Lymphedema and chronic stasis changes with hard or brawny edema develop in the overlying skin. These consequences of filarial infection are due both to the direct effects of the worms and to the host's inflammatory response to the parasite. Inflammatory responses are believed to cause the granulomatous and proliferative processes that precede total lymphatic obstruction. It is thought that the lymphatic vessel remains patent as long as the worm remains viable and that the death of the worm leads to enhanced granulomatous reaction and fibrosis. Lymphatic obstruction results, and, despite collateralization of the lymphatics, lymphatic function is compromised.
The most common presentations of the lymphatic filariases are asymptomatic (or subclinical) microfilaremia, hydrocele (Fig. 218-1), acute adenolymphangitis (ADL), and chronic lymphatic disease. In areas where W. bancrofti or B. malayi is endemic, the overwhelming majority of infected individuals have few overt clinical manifestations of filarial infection despite large numbers of circulating microfilariae in the peripheral blood. Although they may be clinically asymptomatic, virtually all persons with W. bancrofti or B. malayi microfilaremia have some degree of subclinical disease that includes microscopic hematuria and/or proteinuria, dilated (and tortuous) lymphatics ...