The Effects of permethrin-impregnated bed nets on mosquito behaviour and survival
The effects of permethrin-impregnated bed nets on blood feeding success, exiting behavior, temporal biting pattern and sporozoite rates in the malaria vectors Anopheles gambiae Giles and Anopheles funestus Giles were measured in western Kenya. Houses were paired with respect to size, construction, design and number of occupants. For each of 60 house pairs, occupants of one house were assigned one permethrin-i mpregnated bed net per sleeping place while the residents of the other house remained without nets. The mosquitoes exiting from one-half of the perimeter of each house were sampled using Colombian curtain traps while those that were resting inside the houses were sampled by the pyrethrum spray catch (PSC). The introduction of permethrin-impregnated bed nets in the houses resulted in a 151% (t=7.516, df= 21, p<O.OOI) and a 294% (t= 8.289, df= 26, p< 0.001) increase respectively in the proportions of An. gambiae and An. funestus " exiting from the houses. The house entering rates for An. gambiae and An. funestus dropped by 48% and 44% respectively. There was also a 92% (t=5.095, df=12, p<O.OOl) and an 84% (t= 3.095, dj=17, p<0.002) overall reduction respectively in the proportion of An. gambiae and An. funestus that were blood-fed. All-night hourly collections using human baited bed net traps were performed in two villages over a six-week period. In one village households had used permethrin-irnpregnated bed nets for one year while in the other village, perrnethrin-impregnated bed nets have never been used. The temporal biting patterns significantly differed between the intervention and .control villages for both An. gambiae (X2 =79.667,df 12, p=O.OOOI) and An. funestus (X2 =26.913, dj=11, p=0.0047) but these differences appear to be of little biological or operational significance. The temporal biting pattern of An. arabiensis did not differ from that of An. gamhiae. For An. gamhiae there was no difference in the sporozoite rates (X2= 0.0052 df =1, p=0.005) between the intervention and control villages. For An. junestus the sporozoite rate was almost nil in the two areas. Permethrin-impregnated bed nets have marked effects on the exiting behavior and the blood feeding success of the principal malaria vectors in Africa. Effects of wide-scale use of permethrin-impregnated bed nets on the time of biting in the malaria vectors were minor after one year of use.