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Filariasis is caused by an infection with Wuchereria spp. roundworms that parasitize humans in the adult stage and also many animals. Filarial worms may develop in mosquitoes, resulting in Lymphatic Filariasis. Both Bancroftian Filariasis and Brugian Filariasis occurs in two distinct forms: "Nocturnally periodic form" in which the microfilariae are in the peripheral blood only a night and "Subperiodic form" where they are present during the day and night (Service 2008). The two forms exhibit varying degrees of periodicity.
There are many vectors of Filariasis among the mosquito genera Anopheles, Mansonia, Coquillettdia and Culex. Also some Culicoides spp. are vectors of filarial parasites to humans (See Matheson 1950 for a long list).
Mansonella perstans of West and Central Africa is vectored by Culicoides milnei and C. austeni and probably also C. grahamsii. Breeding is in rotting stumps of bananas. Mansonella perstans also occurs in Central and South America where the vectors are other species of Culicoides.
Mansonella streptocerca is a species found in Central and West Africa. The principal vector is Culicoides grahamii, but C. milnei and C. austeni are also suspected (Service 2008).
Wuchereria bancrofti incites most cases of filarial infection in humans. It occurs over much of the tropics and subtropics of South American, central and southern Africa, and Asia and the South Pacific. Matheson (1950) reported that the adult worms live together, frequently coiled up in various parts of the lymphatic system. The females discharge their embryos in the lymph channels from which they gain access to the blood stream. The embryos are called "microfilariae." There is a periodicity in the appearance of the microfilariae, the maximum nocturnal abundance occurring between 10 PM and 2 AM, while in daytime they concentrate in the pulmonary vessels, heart capillaries and kidneys. In the Pacific area there is also a nonperiodic strain, the microfilariae being present in the blood stram of infected humans at all times during the day as well as the night.
When a mosquito obtains blood infected with microfilariae, the embryos escape from their sheaths and bore through the intestinal wall. After 24 hours most have migrated to the thoracic muscles where each worm undergoes further development without an increase in numbers. Then from 11-20 days the larval development is complete and the parasites migrate forward to the mosquito's proboscis. Later they end up in the hemocele of the labium from which they are set to pass to a new host. When the mosquito takes blood the worms escape from the labium and bore directly through the human's skin. Afterwards the larvae reach the lymphatics where they become sexually mature and new generations of microfilariae enter the blood stream (Matheson 1950). The mosquito is important in the development and transfer of the roundworm. Temperature and humidity determine whether a mosquito becomes infected, as was demonstrated by Basu & Rao (1939). They found although almost 100 percent infection will occur at 80-deg. F., and Relative humidity of over 90 percent, but at temperatures under 60-deg. F. and low humidity infection rarely occurs. In cases where infection does occur at the lower temperature the developmental period in the mosquito was much prolonged.
There are many different species of mosquito that can act as intermediate hosts in the developmental cycle of Wuchereria. In 1950 Matheson listed the following species, but noted that more species are certainly involved. The acceleration of world trade in the 21st Century can also be expected to distribute species to different world sites.
Infection with filarial worms in humans does not always cause an apparent disease expression. But there may be marked changes in the lymphatic system that cause serious health problems among which are lymphangitis, adenitis and elephantiasis. As of 2016 there are no known effective treatments other than mosquito control for Filariasis. Avoidance of geographic areas where the disease is prevalent, such as the Marquesas Islands of the southern Pacific, is a precautionary measure.
The principal species of Flavivirus involved in Filariasis were listed by Service (2008) as shown in the following table:
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Service (2008) reported that Wuchereria bancrofti occurs in tropical areas of the world as the most common filarial human infection. Bancroftian Filariasis is is found mostly in urban areas with no animal reservoir hosts.
In the nocturnal periodic form Culex quinquefasciatus breeds in polluted water in Asia, South America and Africa vectors the nocturnal periodic form. Adult mosquitoes bite during nighttime and subsequently rest in dwellings. Although Cx. quinquefasciatus is an efficient vector in Africa, Anopheles gambiae and An. funestus are the main vectors in the western portion of that continent. Various other mosquito species transmit the virus in Asia and New Guinea (e.g., Anopheles spp, Mansonia uniformis and Culex annulirostris). Aedes poicilius is the main vector in the Philippines, which bites at dusk. Their larvae develop in the leaf axils of bananas and coco yams.
Only the diurnal subperiodic form exists in Polynesia where the main vector is Aedes polynesiensis that bites during daytime. Their larvae develop in natural containers, coconut shells, crab holes and various human made receptacles. Aedes pseudoscutellaris in Fiji oviposits in tree holes and bamboo stumps with larval development being in crab holes. In New Caledonia Aedes vigilax is a daytime biter, and their larvae develop in pools of standing water.
In Thailand the nocturnal subperiodic form involves the Aedes niveus complex, which breed primarily in bamboo.
Occurring through most of Asia the nocturnal periodic form is primarily a disease in rural areas without any known animal reservoirs. The vectors, which bite both during the day or night, include Anopheles and Mansonia mosquitoes that bite mainly during the night (e.g., Mansonia annulifera of India and Mansonia uniformis elsewhere breeding in permanent water). In Malaysia, Indonesia, Thailand and the Philippines this form is vectored by Mansonia (e.g., Mansonia dives, Mansonia bonneae and Mansonia annulifera). Coquillettidia crassipes is active in the Philippines also. Reservoir hosts are wild simians, and humans become infected when encountering them in forests (Service 2008).
Service (2008) listed a number of ways to control the disease, all involving avoidance of the vector mosquitoes. He emphasized that it is more difficult to protect against the culicine mosquitoes than the anophelines because many species are outdoor biters during the day. Insecticidal control is not very effective against culicines. Therefore, control of larvae is the most effective approach, which involves the application of insecticides.
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Dobson, M. 2001. Lymphatic Filariasis: The Quest to Eliminate a 4,000-Year-Old Disease. Hollis Pub. Co., Hollis, New Hampshire.
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Herald, Budapest. 978 p.
Muller, R. 2002. Worms and Human Diseases. 2nd ed., CABI, Wallingford, England.
Sasa, M. 1976. Human Filariasis: a Global Survey of Epidemiology & Control. Univ. of Tokya Press.
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World Health Organization. 2005. Global programme to eliminate lymphatic Filariasis. Weekly Epidemiol. Rec. 80: 202-12.
Zagaria, N. & L. Savioli. 2002. Elimination of lymphatic filariasis: a public health challenge. Ann. Trop Med. & Parasit. 96(suppl. 2): 3-13.