The simulium control scheme at Abuja, Northern Nigeria, and its effect on the prevalence of onchocerciasis in the area.

This account of the conduct of the Similium Control Scheme at Abuja, Northern Nigeria, over the period 1956-66 shows that, by larvicide treatment of some 160 km of rivers with DDT in the early wet season, the Simulium damnosum density has been reduced by about 90% of its precontrol level each year over an area of more than 3000 km(2).A skin-snip survey, in which 6748 persons were examined (of whom 5700 lived within the controlled area and 1048 in a comparison area) during 1966, indicates that there has been no obvious decrease in prevalence of onchocerciasis, particularly among children born since the commencement of the scheme, that could be attributed solely to the effects of the control measures, although there are indications that a drop in severity of infection has been achieved.Dissections of wild-caught S. damnosum suggest that, although the fly-density was greatly reduced by the control measures, the proportion of infective flies increased half-way through the period, probably owing to the greater proportion of older flies in the post-control populations; as a result, the estimated number of infective bites was only halved. By 1966, however, the infective bite rate was down to about 1/30th of its original level.It is concluded that the control scheme has demonstrated that the S. damnosum population in a small area within a larger endemic area can be substantially reduced by treating the rivers with DDT during the first 12 weeks of the wet season. It appears, however, that onchocerciasis may be transmitted by a very low density of S. damnosum.The author suggests that an ophthalmic survey would reveal whether there has been any change in the incidence of onchocercal blindness due to the control measures, and that an entomological investigation might show whether the continued transmission is due to residual, resident or immigrant fly populations or whether a second vector is involved.