Inhibition of host-seeking response and olfactory responsiveness in Anopheles gambiae following blood feeding

The effect of a single blood meal on the host-seeking response of Anopheles gambiae was investigated in the laboratory using a behavioural bioassay. w hereas possible changes at the chemosensory level were monitored using elec troantennogram recording (EAG). To avoid the possible confounding effect of body size, mosquitoes of a large size class only were used. Five-day old f emale mosquitoes were given a blood meal on a human arm and exposed to the emanations of a human hand in an olfactometer at 3, 24, 40. 48 and 72 h fol lowing the meal and their behaviour and EAG response to host stimuli were c ompared with that of unfed mosquitoes (controls) of corresponding age. Duri ng egg development, mosquitoes had access to glucose and an oviposition tra y. The ovarian development of blood-fed mosquitoes that responded to host o dours was compared with that of blood-fed mosquitoes that had not been expo sed to host odours. The EAG response of blood-fed and control mosquitoes to host odour was examined upon stimulation with air led over incubated human sweat, hexanoic acid, indole and geranyl acetone. EAGs were recorded at ti mes after a blood meal corresponding with those used in the behavioural exp eriment. There was no host-seeking response at 3 and 24 h post blood meal ( pbm). Seven percent of the mosquitoes responded to human emanations 40-h pb m, 27% at 48 h and 68% at 72 h following a blood meal. The average response of controls to host stimuli varied from 35 (at t-40 h) to 67%. There was n o ovarian development in the unfed group of mosquitoes. Of the mosquitoes t hat responded to host odour 48 h pbm, 12.5% (n=5) had ovaries in Christophe rs stage IV and the remainder in stage V. Of the mosquitoes that responded 72 h pbm, 66.7% (n=94) had ovaries in stage V and 31.2% (n=44) had recently oviposited. Maximum EAG amplitudes recorded from blood-fed and control mos quitoes were similar for mosquitoes in Christophers stages I-III. whereas i n stage IV EAG amplitudes recorded from the blood-fed group were significan tly lower than those of the corresponding control group in response to head space of incubated human sweat and to indole. The results show that there w as a strong inhibition of host seeking in An. gambiae for a period of at le ast 40 h following a blood meal. Host-seeking returned to pre-blood meal le vels 72-h post feeding and was associated with egg maturation. The inhibiti on of host-seeking behaviour was accompanied by an inhibition of olfactory sensitivity to headspace of incubated sweat and indole just before the resu mption of the host-seeking response. The implications of these findings for mosquito surveillance with host odours are discussed. (C) 2001 Elsevier Sc ience Ltd. All rights reserved.