[Menydh]

Materials and Methods

Screening for malaria was carried out with thick blood smears, stained with 10% Giemsa and examined microscopically with (100x) oil-immersion lens for the presence of sexual and asexual parasites. As we did not use molecular techniques, infections with low densities of parasites may have been missed. Thus our results compare, strictly speaking, individuals with no parasites or a low level of parasitaemia with individuals with more intense infections, rather than comparing uninfected with infected individuals. In addition, microscopy and Giemsa staining detect only those gametocytes circulating in the blood, so that we missed the sequestered gametocytes. However, as mosquitoes do not pick up sequestered gametocytes, missing these parasites in an assay is indeed desired.

Attractiveness of the children in each group to mosquitoes was measured with a three-way olfactometer consisting of a central chamber attached to three tents¹² (see Figure 1). At sunset, the volunteer children entered the tents to rest or sleep. The infection status of the children was changed randomly among tents among the 12 replicates, so that any residual odour in the olfactometer could not bias the results (but rather, at most, increase random variation). The tents were connected with PVC tubes to a central cage, where a fan was placed to draw air from the tents. About 100 mosquitoes were released into the central cage, and were given 30 min to follow their preferred odour into the entry traps placed between the PVC tubes and tents. The mosquitoes were uninfected females from a colony of Anopheles gambiae s.s. that had been established from specimens collected in Njage village, Kilobero District, Tanzania in 1996 and is maintained at the ICIPE laboratory. We used females 5 d after emergence that had had no prior access to blood, had been maintained on glucose solution (6%) provided with a cotton wick, and had been starved (i.e., provided with water only) for 6 h before the experiment.