ONCHOCERCA

Onchocerca is a genus of roundworm. It contains one human parasite - Onchocerca volvulus - which is responsible for the neglected disease Onchocerciasis, also known as "River Blindness" because the infected humans tend to live near rivers where host black flies live. Over 40 million people are infected in Africa, Central America, and South America.

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

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.

GEOGRAPHIC DISTRIBUTION

Among the agents of lymphatic filariasis, Wuchereria bancrofti is encountered in tropical areas worldwide; Brugia malayi is limited to Asia; and Brugia timori is restricted to some islands of Indonesia.  The agent of river blindness, Onchocerca volvulus, occurs mainly in Africa, with additional foci in Latin America and the Middle East.  Among the other species, Loa loa and Mansonella streptocerca are found in Africa; Mansonella perstans occurs in both Africa and South America; and Mansonella ozzardi occurs only ins the Americas, from Mexico south to South America and in the Caribbean. 

CLINICAL FEATURES

Most infections are probably asymptomatic, as indicated by serologic surveys.  Manifestations of disease include fever, chills, sweating, myalgias, fatigue, hepatosplenomegaly, and hemolytic anemia.  Symptoms typically occur after an incubation period of 1 to 4 weeks, and can last several weeks.  The disease is more severe in patients who are immunosuppressed, splenectomized, and/or elderly.  Infections caused by B. divergens tend to be more severe (frequently fatal if not appropriately treated) than those due to B. microti, where clinical recovery usually occurs.

LABORATORY DIAGNOSIS

Identification of microfilariae by microscopic examination is the most practical diagnostic procedure. Examination of blood samples will allow identification of microfilariae of Wuchereria bancrofti, Brugia malayi, Brugia timori, Loa loa, Mansonella perstans, and M. ozzardi.  It is important to time the blood collection with the known periodicity of the microfilariae.  The blood sample can be a thick smear, stained with Giemsa or hematoxylin and eosin.  For increased sensitivity, concentration techniques can be used.  These include centrifugation of the blood sample lyzed in 2% formalin (Knott's technique), or filtration through a Nucleopore® membrane. Examination of skin snips will identify microfilariae of Onchocerca volvulus and Mansonella streptocerca.  Skin snips can be obtained using a corneal-scleral punch, or more simply a scalpel and needle.  The sample must be allowed to incubate for 30 minutes to 2 hours in saline or culture medium, and then examined for microfilariae that would have migrated from the tissue to the liquid phase of the specimen.

For more information view the source:Center for Disease Control

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