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BIOLOGY

CAUSAL AGENTS

Filariasis is caused by nematodes (roundworms) that inhabit the lymphatics and subcutaneous tissues. Eight main species infect humans. Three of these are responsible for most of the morbidity due to filariasis: Wuchereria bancrofti and Brugia malayi cause lymphatic filariasis, and Onchocerca volvulus causes onchocerciasis (river blindness). The other five species are Loa loa, Mansonella perstans, M. streptocerca, M. ozzardi, and Brugia timori. (The last species also causes lymphatic filariasis.)

LIFE CYCLE

Infective larvae are transmitted by infected biting arthropods during a blood meal. The larvae migrate to the appropriate site of the host’s body, where they develop into microfilariae-producing adults. The adults dwell in various human tissues where they can live for several years. The agents of lymphatic filariasis reside in lymphatic vessels and lymph nodes; Onchocerca volvulus in nodules in subcutaneous tissues; Loa loa in subcutaneous tissues, where it migrates actively; Brugia malayi in lymphatics, as with Wuchereria bancrofti; Mansonella streptocerca in the dermis and subcutaneous tissue; Mansonella ozzardi apparently in the subcutaneous tissues; and M. perstans in body cavities and the surrounding tissues. The female worms produce microfilariae which circulate in the blood, except for those of Onchocerca volvulus and Mansonella streptocerca, which are found in the skin, and O. volvulus which invade the eye. The microfilariae infect biting arthropods (mosquitoes for the agents of lymphatic filariasis; blackflies [Simulium] for Onchocerca volvulus; midges for Mansonella perstans and M. streptocerca; and both midges and blackflies for Mansonella ozzardi; and deerflies [Chrysops] for Loa loa). Inside the arthropod, the microfilariae develop in 1 to 2 weeks into infective filariform (third-stage) larvae. During a subsequent blood meal by the insect, the larvae infect the vertebrate host. They migrate to the appropriate site of the host’s body, where they develop into adults, a slow process than can require up to 18 months in the case of Onchocerca.

LIFE CYCLE OF ONCHOCERCA-VOLVULUS

O volvulus LifeCycle

During a blood meal, an infected blackfly (genus Simulium) introduces third-stage filarial larvae onto the skin of the human host, where they penetrate into the bite wound . In subcutaneous tissues the larvae develop into adult filariae, which commonly reside in nodules in subcutaneous connective tissues . Adults can live in the nodules for approximately 15 years. Some nodules may contain numerous male and female worms. Females measure 33 to 50 cm in length and 270 to 400 um in diameter, while males measure 19 to 42 mm by 130 to 210 um. In the subcutaneous nodules, the female worms are capable of producing microfilariae for approximately 9 years. The microfilariae, measuring 220 to 360 um by 5 to 9 μm and unsheathed, have a life span that may reach 2 years. They are occasionally found in peripheral blood, urine, and sputum but are typically found in the skin and in the lymphatics of connective tissues . A blackfly ingests the microfilariae during a blood meal . After ingestion, the microfilariae migrate from the blackfly’s midgut through the hemocoel to the thoracic muscles . There the microfilariae develop into first-stage larvae and subsequently into third-stage infective larvae. The third-stage infective larvae migrate to the blackfly’s proboscis and can infect another human when the fly takes a blood meal.

For more information view the source: Center for Disease Control

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