Filariasis

Filariasis

Filariasis (philariasis) is a parasitic disease (usually an infectious tropical disease) that is caused by thread-like nematodes (roundworms) belonging to the superfamily Filarioidea, also known as "filariae". These are transmitted from host to host by blood-feeding arthropods, mainly black flies and mosquitoes.

Eight known filarial nematodes use humans as their definitive hosts. These are divided into three groups according to the niche within the body they occupy: 'lymphatic filariasis', 'subcutaneous filariasis', and 'serous cavity filariasis'.

• Lymphatic filariasis is caused by the worms Wuchereria bancrofti, Brugia malayi, and Brugia timori. These worms occupy the lymphatic system, including the lymph nodes, and in chronic cases these worms lead to the disease elephantiasis.
• Subcutaneous filariasis is caused by Loa loa (the eye worm), Mansonella streptocerca, and Onchocerca volvulus. These worms occupy the subcutaneous layer of the skin, in the fat layer. L. loa causes Loa loa filariasis while O. volvulus causes river blindness.
• Serous cavity filariasis is caused by the worms Mansonella perstans and Mansonella ozzardi, which occupy the serous cavity of the abdomen.

The adult worms, which usually stay in one tissue, release early larvae forms known as microfilariae into the host's bloodstream. These circulating microfilariae can be taken up with a blood meal by the arthropod vector; in the vector they develop into infective larvae that can be transmitted to a new host.

Individuals infected by filarial worms may be described as either "microfilaraemic" or "amicrofilaraemic", depending on whether or not microfilaria can be found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilaria in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood.