The vector-borne filariases encompass a diverse range of parasitic infections, including lymphatic filariasis, onchocerciasis, mansonellosis, and loiasis. For some, such as lymphatic filariasis and onchocerciasis, large-scale World Health Organization (WHO)-supported elimination campaigns are now underway. Others, such as mansonellosis, remain neglected with limited large-scale, well-controlled clinical and epidemiologic data available. This chapter will review the common vector-borne filariases of humans. Dirofilariasis and dracunculiasis, diseases of humans also caused by filarial nematodes, are covered in Chapter 127: Tissue Nematodes chapter of this textbook.
Lymphatic filariasis is a chronic, often debilitating parasitic disease caused by infection with the filarial parasites Wuchereria bancrofti, Brugia malayi, and B. timori. It is estimated that there are currently about 68 million lymphatic filariasis cases, including 36 million people harboring active infection and 36 million living with chronic morbidity subsequent to active disease.1,2 It is estimated that 856 million people in 72 endemic countries of the tropics and subtropics of Africa, Asia, Oceania, the Caribbean, and parts of South America are at risk for infection.1 In 1993, the International Task Force for Disease Eradication identified lymphatic filariasis as one of six “eradicable” or “potentially eradicable” infectious diseases.3 In 1997, the World Health Assembly (Resolution 50.29)4 targeted lymphatic filariasis for global elimination as a public health problem by 2020. In response, in 2000, WHO launched the Global Programme to Eliminate Lymphatic Filariasis (GPELF), which aims to stop the spread of infection and alleviate suffering in affected persons.5 Almost half of all endemic countries had initiated national control programs by the end of 2004 leading to a dramatic reduction in the prevalence of microfilaremia in the participating countries.6
Human infection with W. bancrofti occurs when infective larvae penetrate the skin during the bite of a mosquito vector and migrate to the nearest lymphatic vessel. Over the course of several months, they develop into thread-like adult worms (the males are approximately 40 by 0.1 mm and the females 100 by 0.25 mm in size). The average reproductive life span of the adult worms has been estimated at 5 years.7 Fertilized female worms produce sheathed microfilariae, which are released into the bloodstream. Circulating microfilariae are ingested by the appropriate mosquito vector intermediate host during a blood meal; they develop within the insect over the course of several weeks into infective larvae, completing the parasite life cycle.8
In most areas of the world, microfilariae are detectable in the peripheral blood only at night (nocturnal periodicity) based on the biting times of the Anopheles and Culex mosquito vectors. However, where Aedes mosquitoes are the primary vector, diurnal subperiodicity (microfilarial counts maximal during the day; parts of Polynesia) or nocturnal subperiodicity (microfilaria circulating throughout the day, with only slight increase in microfilaremia at night; some regions of South East ...