Tests of Olyset nets by bioassay and in experimental huts.

BACKGROUND: Olyset nets are a type of long-lasting insecticidal net made of polyethylene fibre with permethrin incorporated into it and with a 4 mm mesh size. BIOASSAYS: Olyset netting was wrapped around a wire frame and the mosquitoes were released inside for bioassays. There was significantly faster knockdown and higher percent mortality than bioassays with the netting attached to a WHO cone with a piece of cardboard on the other side of the net to prevent escapes through the large mesh. It is suggested that with the latter method some mosquitoes place their tarsae through the mesh on to the cardboard, thus avoiding insecticide contact. TRIALS IN EXPERIMENTAL HUTS: Four mm mesh nets were compared with conventional 1.5 mm mesh nets treated with permethrin. In further trials in huts Olyset nets which were either unwashed or five times washed, with or without subsequent heating, and a Olyset net which had been in domestic use for four years or a new Olyset net were compared with a net treated with bifenthrin. RESULTS & CONCLUSION: In all cases Anopheles biting on sleepers under the nets was reduced and Anopheles mortality was increased by the use of the insecticidal nets. No significant impact of washing or heating was detected and an Olyset net was as good as new after four years use, but did not cause as much mosquito mortality as bifenthrin treated nets.

Seasonal and spatial variation in natural densovirus infection in Anopheles minimus S.L. in Thailand.

We report the first detection of a mosquito densovirus in anophelines, An. minimus species A and species C, and describe temporal and spatial variation in natural densovirus infection. A total of 814 (136 species A; 678 species C) adult mosquitos, obtained over a one year period from human biting catches at three locations in a village in western Thailand, were PCR tested for densovirus infection. Overall infection prevalence did not differ between species (15.4% species A; 14.5% species C). Infection prevalence showed significant seasonal variation. Some spatial heterogeneity in infection was also noted, with timing of peak infection prevalence varying between sites. PCR-screening of An. minimus s.l. larvae found an overall infection prevalence of 18.8%. Larval infection showed a significant positive association with rainfall recorded two months previous to larval collection. Infection in adult mosquitos showed a moderate relationship to environmental variables, but a significant negative correlation with larval infection in the previous month.