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Filariasis (Philariasis) is a parasitic and infectious tropical disease, that is caused by thread-like filarial nematode worms in the superfamily Filarioidea, also known as “filariae”. There are 9 known filarial nematodes which use humans as the definitive host. These are divided into 3 groups according to the niche within the body that 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 African eye worm), Mansonella streptocerca, Onchocerca volvulus, and Dracunculus medinensis (the guinea worm). These worms occupy the subcutaneous layer of the skin, the fat layer. Serous Cavity Filariasis is caused by the worms Mansonella perstans and Mansonella ozzardi, which occupy the serous cavity of the abdomen. In all cases, the transmitting vectors are either blood sucking insects (fly or mosquito) or Copepodcrustaceans in the case of Dracunculus medinensis.
Human filarial nematode worms have a complicated life cycle, which primarily consists of five stages. After the male and female worm mate, the female gives birth to live microfilariae by the thousands. The microfilariae are taken up by thevector insect (intermediate host) during a blood meal. In the intermediate host, the microfilariae molt and develop into 3rd stage (infective) larvae. Upon taking another blood meal the vector insect injects the infectious larvae into the dermis layer of our skin. After approximately one year the larvae molt through 2 more stages, maturing into the adult worm.
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.
The most spectacular symptom of lymphatic filariasis is elephantiasis-edema with thickening of the skin and underlying tissues-which was the first disease discovered to be transmitted by mosquito bites[ Elephantiasis results when the parasites lodge in the lymphatic system.
Elephantiasis affects mainly the lower extremities, while the ears, mucus membranes, and amputation stumps are affected less frequently. However, different species of filarial worms tend to affect different parts of the body:Wuchereria bancrofti can affect the legs, arms, vulva, breasts, and scrotum (causing hydrocele formation) while Brugia timori rarely affects the genitals[Interestingly, those who develop the chronic stages of elephantiasis are usually amicrofilaraemic, and often have adverse immunlogical reactions to the microfilaria as well as the adult worm
The subcutaneous worms present with skin rashes, urticarial papules, and arthritis, as well as hyper- and hypopigmentation macules. Onchocerca volvulus manifests itself in the eyes causing “river blindness” (onchocerciasis), the 2nd leading cause of blindness in the world[Serous cavity filariasis presents with symptoms similar to subcutaneous filariasis, in addition to abdominal pain because these worms are also deep tissue dwellers.