Filarial worms are parasitic nematodes that dwell within the lymphatics and the subcutaneous tissues. Eight filarial species are associated with human disease, though only 4 cause significant morbidity in children (Table 323-1). These species include the causative agents of lymphatic filariasis—Wuchereria bancrofti, Brugia malayi, and Brugia timori—and Onchocerca volvulus, the causative agent of onchocerciasis. The filarial parasites can be distinguished by their geographic distribution, vector, anatomic location of adult worms, and anatomic location and periodicity of microfilaria detection. Of the human filarial parasites, W bancrofti is, by far, the most important in terms of both morbidity and numbers of people infected. O volvulus remains an important cause of worldwide morbidity, especially in Africa. With elimination of O volvulus from selected transmission zones in the Americas, there is hope of eventual elimination from Africa.
TABLE 323-1EPIDEMIOLOGIC AND BIOLOGIC FEATURES OF THE FILARIAE ||Download (.pdf) TABLE 323-1EPIDEMIOLOGIC AND BIOLOGIC FEATURES OF THE FILARIAE
|Species ||Endemic Areas ||Vector ||Microfilariae ||Adult Worm Location ||Primary Pathology |
|Brugia malayi ||China, India, Malaysia, and some Pacific Island groups ||Mosquitoes, including Anopheles, Aedes, and Mansonia species ||Bloodborne, sheathed, nocturnally periodic or subperiodic ||Lymphatics ||Lymphedema, adenolymphangitis |
|Brugia timori ||Southeastern Indonesia ||Anopheles barbirostris ||Bloodborne, sheathed, nocturnally periodic ||Lymphatics ||Lymphedema, adenolymphangitis |
|Loa loa ||Western and central Africa ||Chrysops species ||Bloodborne, sheathed, diurnally periodic ||Subcutaneous tissues, including subconjunctiva ||Migratory angioedema, conjunctivitis |
|Mansonella ozzardi ||Central and South America, Caribbean ||Culicoides species and Simulium amazonicum ||Skin and bloodborne, unsheathed, nonperiodic ||Thoracic and peritoneal cavities, lymphatics ||Unknown/poorly defined |
|Mansonella perstans ||Sub-Saharan Africa, northern coast of South America, Tunisia, and Algeria ||Culicoides species ||Bloodborne, unsheathed, nonperiodic ||Serous cavities, mesentery, perirenal and retroperitoneal tissues ||Unknown/poorly defined |
|Mansonella streptocerca ||Western and central Africa ||Culicoides grahamii ||Skin, unsheathed, nonperiodic ||Dermis ||Pruritus, rash, hypopigmented macules, adenopathy |
|Onchocerca volvulus ||Sub-Saharan Africa, Latin America, and the Arabian peninsula ||Simulium species ||Skin, unsheathed, nonperiodic ||Subcutaneous nodules ||Subcutaneous nodules, pruritic dermatitis, lymphadenopathy, keratitis, uveitis, chorioretinitis |
|Wuchereria bancrofti ||Sub-Saharan Africa, Southeast Asia, western Pacific, Caribbean, and northern coast of South America ||Mosquitoes, including Anopheles, Aedes, and Culex species ||Bloodborne, sheathed, nocturnally periodic or subperiodic ||Lymphatics, lymph nodes ||Lymphedema, adenolymphangitis, hydrocele |
Each filarial parasite is transmitted by biting arthropods, either mosquitoes or flies, and all go through complex life cycles that include a slow maturation (often 3–24 months) from the infective larval stages carried by the insects to the adult worms that live within the lymphatics and lymph nodes (W bancrofti and Brugia species) or in the subcutaneous tissues (O volvulus, Loa loa, Mansonella streptocerca). Infection occurs when male and female adult worms mate and females produce microfilariae offspring, 200 to 400 mm in length, that either circulate in the blood or migrate to the skin while awaiting ingestion by insect vectors. Productive infection is usually not established unless exposure to infective larvae is intense or prolonged. Maturation of microfilariae ...