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Loiasis is a nematode disease vectored to humans by flies of the family Tabanidae. Occurance of the disease is primarily in forested areas of Nigeria, Ghana, Angola, Sudan, Uganda, Guinea, Congo and the Cameroons. The microfilariae are diurnally periodic and migrate to different parts the body at different times of the day and night. When they are situated just below the skin they can be transferred to tabanid flies that bite during the daytime. Service listed the principal vectors as Chrysops silaceus, C. dimidiatus, C. distinctipennis and C. longicornis.
Following a blood meal some of the ingested microfilariae that manage to survive digestion penetrate the intestinal wall and migrate to the abdomen or thorax and head (Service 2008). After two molts they develop into a 3rd stage larva, which is only 2 mm. in length. These move to the thorax and head and after 7-15 days amass in the fly's proboscis. When a vector feeds on humans as many as 200 stage-3 larvae can be left on the skin. Service (2008) noted that most of these die, but some pass through the wounds made by the biting fly or even through skin abrasions. They migrate to connective tissue where they mature in about three months. Several months later the microfilariae can be found in the peripheral blood.
Reservoir hosts may involve forest simians that are bitten by Chrysops centurionis and C. langi that are active at twilight or nighttime. However, transmission to humans is uncertain.
The Tabanidae are extremely difficult to control, especially in areas where aquatic habitats cannot be practically reduced, such as swamps and lakes. Nevertheless, . Control measures have involved draining swampland where the flies breed. Insecticidal control is also plagued by the development of resistance and difficulties in locating principal breeding areas. Service (2008) noted that some control could be achieved with attractant traps for adults and the use of repellents.
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