Effects of sublethal doses of pyrethroids on malaria vectors
Pyrethroids are currently the most widespread class of insecticide in public health use, although their future may be limited by resistance. The present study investigates the effects of these compounds on mosquitoes, other than mortality immediately after exposure. Longevity of both male and female An. gambiae was found to be significantly curtailed following sublethal exposure to the most commonly used pyrethroids, permethrin, deltamethrin and lambda-cyhalothrin. There was no direct effect observed on fecundity. Irritancy on direct contact with all 3 compounds was recorded, particularly with permethrin. Evening host-seeking flight activity was greatly reduced, and when females were treated with permethrin before release into a flight chamber, significantly fewer flew to a host to feed. Experiments investigating Plasmodium yoelii and P. falciparum development in the Asian vector An. stephensi revealed that sublethal exposure to pyrethroids inhibited development of oocysts in the midgut. Similar experiments using organochlorine, organophosphate and carbamate insecticides found no effect. Radio-labeled permethrin was traced inside the blood meal following exposure. However, none of the 3 pyrethroids were found to have a direct anti-malarial activity on cultured gametocytes. Likewise, permethrin did not affect exflagellation of P. yoelii. Time course experiments determined that for the reduction in oocysts there was a critical period of between 18 and 48 hours after the infective feed. Permethrin was not found to inhibit trypsin activity in assays, and the levels of trypsin in the midgut of blood fed treated and untreated females was not found to differ in a manner which could explain reduced infection rates. It is clear that sublethal exposure of mosquitoes can lead to a wide range of potentially important effects in terms of the impact of pyrethroids in malaria vector control.