Permethrin impregnated bednets and DDT residual spraying : multicentre comparative trial in Solomon Islands
A malaria intervention comparative trial was carried out in Solomon Islands between 1987 and 1992, involving 7 pairs 14 communities, with 29,182 people. Seven communities were given bed nets impregnated with permethrin annually at 0.5g/m2, and the others sprayed biannually with DDT at dosage 2g/m2. Each pair was evaluated for about two years, by measuring entomological indices, prevalence of infection, incidence of infection, and levels of community compliance. An intensive differential cost analysis of both interventions was done in one pair of the communities and an analysis on cost and benefit of PCD mechanism was done in three pairs. Anopheles farauti, the main vector, (A. punctulatus, an inland vector became very rare with interventions), maintained the early evening biting peak and high outdoor biting. The highest transmission potential was indoor with high parity (54.1%) and sporozoite rate (1.42%). It avoided contact on DDT sprayed surface and was not killed even though it was still sensitive with a 75.1% mortality. There were neither changes in biting density, nor parous rates with DDT spraying. Permethrin impregnated bed nets reduced biting density by an estimated 53.69%. The parous rate indoor was reduced by 11.64% when compared with that in the comparison area, and those caught in the bed nets area did not have any sporozoites. Prevalence of infection, by quarterly prevalence surveys revealed a 21.2% difference between the intervention areas after two years. The most significant decline was in the under 10 year old group (p<0.01) in permethrin treated bed nets, including in infants (p<0.05). The decline was especially marked with Plasmodium falciparum. There were increases in the DDT area, including P. falciparum in the younger age groups. DDT spraying did not have any effect on the incidence of infection. Permethrin impregnated bed nets reduced malaria incidence, by an estimated 49%. This reduction was particularly significant however on children under ten years old (p<0.0001) and marked with P. falciparum. Compliance with DDT declined by 30% but with bednets it remained high above 85%. These results confirmed that permethrin impregnated bednets are more effective than DDT residual spraying in controlling malaria in Solomon Islands. The operational costs for DDT spraying was $8.53 and impregnated bed nets $3.85 per capita per year. The mean cost of processing and examining a PCD slide is $0.40. These cost indices took account of all materials, personnel and administration involved. It took a mean of 6.1 days from the time the smear was taken to the time examined (SE = 0.21,95%CI 5.71 to 6.53 days). It would take twice this time for a result to be received by the health workers managing patients. Only 20% of blood slides could contribute to patient management. Based on these findings, all that is necessary is to make blood smears of patients less than 10 years of age for epidemiological evaluation of vector control interventions in malaria programme. This will save scarce resources at primary health care level. Making blood slides of everyone would not further add significant information and benefit, at an extra cost. The only exceptions are, those critically ill with malaria, complicated malaria and a patient suspected to have drug resistant malaria. Permethrin impregnated bednets are a cost effective way to control malaria in primary health care and the most cost benefit way to evaluate vector control intervention is careful monitoring of PCD results, especially with P. falciparum malaria of children under 10 years old.