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Filaria are nematodes (roundworms) which are widespread in nature, especially in tropical areas. The adults produce microscopic microfilariae which can be transmitted to humans by biting insects (mosquitos or blackflies).
In the lymphatic filariases, the microfilariae migrate to the lymphatics where they mature to adults and cause obstruction (elephantiasis).
Lymphatic filariases caused by Wuchereria bancroftii occurs primarily in Africa and that caused by Brugia malayi occurs in South and East Asia. Treatment with diethylcarbamazine or ivermectin is effective in all but the most advanced stages.
In onchocerciasis, adults mature in subcutaneous nodules and release new microfilariae which may migrate to the eye and cause blindness (African River Blindness). Ivermectin use has dramatically improved the outlook for control and treatment of onchocerciasis in Africa.
Detection of the microfilariae of the lymphatic filariases (not onchocerciasis) in the peripheral blood by stained smears and/or a concentration technique
Positive results are provided with the genus and species of the microfilariae, if identifiable.
Onchocerciasis cannot be diagnosed using blood (a "skin-snip" examination is necessary).
Microfilariae may be seen in malaria or differential count peripheral blood smears, but concentration techniques are more efficient.
Certain of the microfilariae have a nocturnal periodicity, and the blood specimen is best drawn at night, between 10 p.m. and 2 a.m.
If positive, organism is identified.
Grove DI: Tissue nematodes (trichinosis, dracunculiasis, filariasis). In Principles and Practice of Infectious Diseases, Fourth edition. Edited by GL Mandell, RG Douglas Jr, JE Bennett. New York, Churchill Livingstone, 1995, pp 2531-2537