ONCHOCERCIASIS IN Tropics
Onchocerciasis is perhaps the most studied filarial infection in Nigeria. It is estimated that 7-10 million Nigerians are infected with onchocerca volvulus, approximately 40 million are at risk of disease (WHO,1996), and 120,000 cases of onchocerciasis related blindness (WHO,1987), with many thousand suffering from disabling complications of the disease (Anosike , onwuliri , 1995). New foci of onchocerciasis are still be discovered and therefore its distribution could be far more expensive than has be assumed.
In southeastern Nigeria, there are pockets of endemic foci, although there is gross under reporting of the scourge. The most significant area in the sub region is the hilly and undulating Udi Enugu- Okigwe axis from where some rivers or their tributaries, supporting blackfly vector breeding, have their origin. These include rivers such as:
1) Oji river
2) Ajali river
3) Mamu river
4) Adada river
5) Imo River (the biggest of them.)
Unfortunately, studies in the subregion have be largely cross- sectional. There has not been any comprehensive study on all aspects of infection: parasitological, clinical and epidemiological.
LIFE CYCLE Of Onchocerciasis
The life of the parasite can be traced through the black fly and the human hosts in the following steps.
A simulium female black fly takes a blood meal on infected human hosts. And ingests microfilaria.
(a) The microfilaria enters the gut and thoracic fight muscles of the black fly. Progressing into the first larval stages (JI).
(b) The larvae mature into the second larval stage (J2.). And move to the proboscis and into the saliva in its third larval stage (J3.).Maturation takes about 7 days.
(c) The black fly takes another blood meal. Passing the larvae into the next human host’s blood.
(d) The larvae migrate to the subcutaneous tissue and undergo two more molts. They form nodules as they mature into adult worms over six to twelve months.
(e) After maturing. Adult male worms mate with female worms in the subcutaneous tissue to produce between 700 and 1.500 microfilaria per day.
(f) The microfilaria migrates to the skin during the day and the black flies only feed in the day, so the parasite is in a prime position for the female fly to ingest it. Black flies take blood meals to ingest this microfilaria to restart the cycle.
In the 1970s, when investigations of onchocerciasis in the endemic villages and districts of West Africa began, Scientist made astonishing and disturbing discoveries. More than 60 percent of the savanna population carried the parasite; 10 percent of the adult population and half of the males over 40 years of age were blind, 30 percent of the people were visually impaired.
And early signs of onchocerciasis were common among children (WHO 1973, 1987, 1995a). Scientists revealed the huge socioeconomic consequences of the high infection rates they had found. As village blindness reached epidemic proportion, it left too few able-bodied people to work in fields. Food shortages and economic collapse forced residents to abandon homelands in fertile river valleys.
The socioeconomic consequences of onchocerciasis are profound O.volvulus -induced blindness is associated with a life expectancy 10 years shorter than that of no blinded persons in the same area (Kirkwood , Smith , Marshall , etal.,1983). Microfilarial burden is the single most significant parameter associated with mortality. (Little , Breitling , Basanez , et al., 2004) Consequences of onchocerciasis extend beyond the individual and affect family, community, and country .The observation of young boys holding sticks to guide blind, unproductive men in their twenties and thirties attests to why entire villages, within otherwise fully arable river zones, become economically nonviable and have been deserted when blindness rates reach about 10%.
Onchocerciasis in the Tropics and Its Socio-Economic Toll