ABSTRACT
The separation of people from their feces is of major importance in the prevention of many infectious diseases. For the forseeable future a large proportion of the population of the world cannot be provided with water-borne sewerage because this is expensive and requires in-house piped water. Various forms of pit latrines are a more feasible alternative,1 but these tend to provide breeding sites for
1. Culex quinquefasciatus, vector of Bancroftian filariasis in many areas and the major urban nuisance mosquito2
2. Blowflies, especially Chrysomya putoria and C. megacephala, which may carry fecal pathogens to food,3 and when present in large numbers are generally considered a repulsive feature of pit latrines
One solution has been regular programs of insecticidal spraying. Oil has been used4 but is now considered less cost effective than organophosphate insecticides.5 A program6 using the organophosphate, chlorpyrifos (= Dursban), was carried out by the City Council in Dar es Salaam, Tanzania, for more than 10 years. It was shown6 that because of the persistence of chlorpyrifos, 6 cycles of spraying per year were sufficient to give permanent control when a dose of about 1 mg/1 was applied. In 1980 $135,000 was spent on the import of insecticide to carry on this program in over 100,000 pit latrines and about 60,000 cess pits, septic tanks, etc. More recently, because of foreign-exchange shortage there has been insufficient insecticide to spray all the pits. Furthermore, by 1980, some 10 years after the program began, a high level of organophosphate resistance was detectable by standard tests on the Dar es Salaam population of Culex quinquefasciatus.1 Studies on the impact of this on control in the field8 showed that the resistance did not prevent the larvae being killed when the insecticide was freshly applied, but the resistant larvae could survive in the insecticide residue left in a typical pit 2 weeks after spraying, whereas susceptible larvae would continue to be killed for at least 9 weeks after spraying. The stepping up of the spraying frequency to once every 2 weeks would have been prohibitively expensive and could not have been managed with the existing spraymen, each of whom could visit 40 houses per day and required 37 working days to complete a round of spraying.
