Effectiveness of a lambda-cyhalothrin bednet impregnation against forest/border malaria in northwest Thailand

A cohort study was carried out in Mae Sariang district close to Thai-Myanmar border. This was aimed at measuring the pattern and size of movements as well as their relevance to malaria illness. Overall 494 people in 95 households were randomly selected from 5 small villages. They were fortnightly visited and interviewed by interpreters on the details of activities causing them to spend nights outside the village. A malaria illness was detected by the case detection systems already available. The results showed that short term movements during the transmission season were common. About 74% of villagers moved at least once. An average of 11% of their nights were spent outside the villages. Adult males predominated. The commonest and second most common reasons for movements were agricultural (41%) and forest (17%) activities respectively. Movements carried a 7.8 times higher risk than staying in the village (p < 0.01). Forest work (including illegal) had about 6 times higher risk than the other activities (p < 0.05). A randomise placebo control trial of a lambda-cyhalothrin bednet impregnation, on a community basis, was also carried out in the same area. This was aimed at measuring the effectiveness of the insecticide treated bednet programme and involved 12 pairs of small villages, called bans in Thai. The outcome measure was a malaria incidence, detected by the existing case detection system, and a prevalence of parasitaemia detected by mass blood surveys,once a year during the transmission period. The outcome measures were collected 12 months before and 16 months after the impregnation. A bednet fund programme was introduced in the first year and also evaluated using the baseline incidence rate. The programme significantly increased the availability and use of bednets. These resulted in about 28% effectiveness against malaria incidence and the density of persons/bednet was directly related with malaria incidence (r = 0.48, p<0.05). In the second year, bans in each pair were randomly allocated either a lambda-cyhalothrin or placebo bednet impregnation. The post-impregnation data showed a small and slow trend of 26% effectiveness. This was not statistically significant. However, the significant effect of 84% reduction was detected amongst adult males after the second impregnation. There was no significant effect on the prevalence of parasitaemia. The sleeping patterns, prevalence of bednet use, dosage and coverage of the impregnation, side-effects and washing rates were also studied. The possible mechanisms of the effect are discussed.