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The change from larval to adult mosquito control in the Gezira area of Sudan resulted in a decrease in the prevalence of malaria in this region. House spraying with malathion began in 1975 and resistance to this compound was first detected in 1978. Laboratory tests showed that adult Anopheles arabiensis Patton were resistant to malathion and phenthoate but susceptible to all other organophosphates tested. The larvae of this strain were susceptible to malathion. The malathion resistance in the adults was synergized by triphenyl phosphate, but not by piperonyl butoxide. This suggests that a carboxylesterase enzyme may be the basis of malathion resistance in this strain. Analysis of general esterase levels to alpha- and beta-naphthyl acetate showed that there was no quantitative change in the amount of carboxylesterase enzyme present in the resistant strain as compared to the susceptible. The absence of larval resistance suggests that house spraying rather than agricultural spraying is the major source of selection pressure. The presence of a high level of adult malathion resistance in A. arabiensis may decrease the efficacy of malathion for malaria control in Sudan. The lack of cross-resistance to organophosphates which do not contain a carboxylester bond means that insecticides such as fenitrothion are still practical alternatives.
Janet Hemingway (Sat,) studied this question.