Water Level Regulation and Control of Schistosomiasis Transmission: A Case Study in Oyan Reservoir, Ogun State, Nigeria

By Ofoezie, I. E.; Asaolu, S. O. | Bulletin of the World Health Organization, September-October 1997 | Go to article overview

Water Level Regulation and Control of Schistosomiasis Transmission: A Case Study in Oyan Reservoir, Ogun State, Nigeria


Ofoezie, I. E., Asaolu, S. O., Bulletin of the World Health Organization


Introduction

The association between schistosomiasis transmission and water resources development projects such as reservoirs, irrigation schemes, etc. is well established (1-6). Earlier attempts to develop suitable engineering methods to control transmission were hampered by many factors, including the misconception that schistosomiasis is an unavoidable accompaniment of water development projects (7) and a shift in emphasis from control of transmission to morbidity control (8). This shift was due to important advances that simplified the diagnosis of schistosomiasis (9, 10), and to availability of safe and effective oral drugs for population-based chemotherapy (11, 12). However, the inevitability of reinfection has meant that mass drug treatment can only temporarily reduce the prevalence to a level that corresponds to the site-specific environmental risk factors (13-15). This failure of chemotherapy intervention to sustain the benefits of disease control has revived interest in transmission control, but within the context of an integrated control approach (16).

Water level fluctuation has long been recognized as an effective control method against snails and snail eggs, and against mosquito larvae (17-19). Widescale application was, however, constrained by limited understanding of the basic relationship between such control and water release patterns, and the effect of this interplay on reservoir drawdown, human water contact patterns, and schistosomiasis transmission during different seasons of the year.

To obtain information that could enhance our understanding of the foregoing, and hence facilitate transmission control by water level fluctuation, we examined the effect of water release patterns on water level fluctuation and disease transmission in a typical man-made reservoir. The investigation was carried out in Oyan Reservoir, a medium-size artificial lake in south-west Nigeria, over a period of 34 months (August 1990 to May 1993). As this reservoir is typical of many other small and medium-size reservoirs in the schistosomiasis endemic areas of Africa, our results and follow-up studies will have a bearing on many cases concerning recommended water management regimens.

Materials and methods

Study area and reservoir The Oyan Reservoir is located at lat. 7 [degrees]14'N, long. 3 [degrees] 13'E near Abeokuta, the capital city of Ogun State, Nigeria (see 20 for a detailed description of the area). The reservoir was established in 1984 with, lying adjacent, two resettlement communities named Abule-titun and Ibaro. An outbreak of urinary schistosomiasis occurred in the two communities within 4 years of the reservoir being established, probably as a result of the impoundment (20).

The dam has an embankment crest length of 1044m, a height of 30.4m, four spillway gates (each 15m wide and 7m high), and three outlet valves (each 1.8m in diameter). The reservoir has a surface area of 40[km.sup.2], a gross storage capacity of 270 million M3, and a dead storage capacity of 16 million [m.sup.3].

The reservoir is used for breeding fish, sprinkler irrigation, and water supply to downstream end-user Water Boards in Abeokuta and Lagos metropolitan cities. Water discharge is by a regulated opening of the gates and valves. The gates and valves are usually operated at 8.00 a.m. when the percentage opening for each day is set and the water level is read off an automated gauge. Water discharge during each opening period was read off an operating chart provided by the construction engineers.

Transmission studies The transmission of urinary schistosomiasis was assessed by the presence, number, and the rate of Schistosoma infection of Bulinus globosus, the local snail intermediate host; by the human water contact pattern; and by the incidence and intensity of infection among schoolchildren in the two communities. The detailed methods used to carry out these investigations are described below. …

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