The authors describe the entomological, chemical and toxicological findings made by the WHO
Insecticide Testing Unit following the spraying of
malathion,
fenthion and
DDT in villages in Western Nigeria.Chemical observations of the deposits indicate that on the local type of mud
malathion is sorbed to a greater extent and to a deeper level than
fenthion, and
DDT less so. On surfaces of vegetable origin all three appeared to remain to about the same degree.The greatest effect on both anophelines and culicines as determined by the pyrethrumspray technique was shown by
fenthion on mud, followed by
malathion and then
fenthion on organic substrates, with
malathion and
DDT on mud as the least effective. As estimated by window-trap catches, the densities were most affected by
fenthion on organic surfaces, least by
fenthion on mud, and intermediately by
DDT on mud and
malathion on organic surfaces.Anopheline mortality was greatest with
fenthion on mud, followed by
DDT on mud and
malathion on organic surfaces, and least with
malathion with mud. With culicines,
fenthion was most active, and
DDT least so, on both types of surface.After spraying, the blood
cholinesterase levels in the inhabitants of the sprayed villages fell quite markedly with
fenthion and less so with
malathion. The falls were temporary, being reversed within six weeks, were not confined to any ascertainable population group and were not associated with detectable illness. The authors stress that until more is known of the significance of these changes and of possible effects on health, the introduction of organophosphorus
insecticides, especially
fenthion, for
malaria eradication should be carried out with caution.