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Cx. quinquefasciatus is often more abundant during parts of the year when water stagnates from lack of rain. In the Nile River Delta of Egypt, Cx. molestus, a name applied to the autogenous variant of Cx. pipiens, is the primary vector. In rural settings, nocturnally periodic bancroftian and Brugian filariases are transmitted by Anopheles mosquitoes, which are also nocturnally active. Often, the same Anopheles species that transmit W. ban-crofti or B. malayi in an area also are responsible for local malaria transmission (e.g., An. darlingi in South America and An. gambiae and An. funestus in parts of West and East Africa, respectively, for Bancroftian filari-asis; and An. sinensis in rice-growing areas of China, for Brugian filariasis).

Nocturnally periodic Brugian filariasis occurs in rural parts of southern India, Malaysia, the Philippines, and Indonesia; there the nocturnally active Mansonia annulif-era and Ma. uniformis, and also Anopheles species, are vectors near rice fields and open swamps. The nocturnally subperiodic form occurs in swamp forest areas of

Southeast Asia and Indonesia, involving Ma. bonneae and Ma. dives, which are nocturnally active but also feed during the day within the swamp forests. All of these Mansonia species are associated with particular kinds of plants, where the larvae and pupae attach to their submerged roots and stems. In the Pacific region, where many island groups are endemic for W. banerofti, the primary vectors are day-biting Aedes and Ochlerotatus species, but nocturnal biters also are involved, and the form of the parasite is diurnally subperiodic. An. barbirostris is the vector of B. timori.

The endemicity of mosquito-borne filariasis depends on a high and steady rate of transmission of infective-stage larvae in the human population. In endemic areas, the inoculation rate (parasite transfer rate) can range as high as hundreds of infective bites and thousands of larval inoculations per person per year. The estimated number of L3 larvae transmitted per person per year is called the annual transmission potential. With each infective bite, only a few L3 larvae actually enter the skin.

These larvae must then develop further and migrate to a person's lymphatic system, where mature male and female nematodes mate and initiate microfilarial production. Accumulation of thousands of infective bites over months or years eventually results in an infection of mature worms in a human, who normally then will have a microfilaremia and possibly chronic disease. Most microfilariae entering the circulatory system are never ingested by a mosquito, while those that are ingested become infective-stage larvae only in a competent vector and only if the individual mosquitoes survive the extrinsic incubation period. Furthermore, many infective-stage larvae fail to reach a new human host or fail to mature if they do. The inefficiency of transmission and parasite perpetuation is compensated by the prodigious production of microfilariae and the long life of adult worms.

Historical Perspective

Association of infection with filarial nematodes and lymphatic filariasis was first established in the late 1800s. Our understanding of the natural history of lymphatic filariasis is related intimately to the initial discovery of a link between human pathogens and insect vectors. During 1877—1878, Patrick Manson, working in China as a medical officer for the Chinese Imperial Customs Service, conducted experiments on the development of filarial nematodes. Manson had already discovered that the microfilariae occurred in the peripheral blood only at night. He speculated that this was timed to coincide with the night-time biting activity of mosquitoes. After feeding mosquitoes (Cx. quinquefasciatus) on his gardener, who had a microfilaremia, and then dissecting the mosquitoes on successive days, he found that the worms developed within the mosquitoes into longer, different forms. He speculated that the mosquito functioned as a kind of "nurse" for the filarial worms, so that when a mosquito died on the water after laying an egg raft, the worms entered the water and were later infective to a person drinking it. At that time, it was not known that mosquitoes could bite more than once during their lives, so the principle of transmission by bite was not established. Yet, the idea that mosquitoes could function as intermediate hosts for a human pathogen was founded through Manson's experiments.

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