Preferential assault
it is an experience most of us would have had many times. How come mosquitoes and other winged creatures are partial in their biting habits? They seem to have distinct favourites, whose blood is selectively devoured. While scientists always suspected that it is something to do with one's skin or, more specifically, its secretions emitting odour, supportive evidence has been forthcoming only since the past few years.
The first conclusive evidence that people may vary in attractiveness to mosquitoes has been reported from Tanzania by Burt G J Knols and colleagues from the Department of Entomology, Wageningen Agricultural University, Wageningen, Netherlands. Field trials have shown that men are distinct favourites particularly those with large body-size; mercifully, children are targeted less frequently as compared to adults ( The Lancet , Vol 347).
In the study of Knols, three volunteers slept in separate tents outfitted with a mosquito entrance and two exit traps. The volunteers slept in the same tent, in the same bed and under the same unimpregnated bed net for some days. Then over the next nine nights they moved between these three different sites. Analysis of mosquitoes collected by the trap showed that one of the volunteers is significantly less attractive to two anopheles and one culex species than were the other two. Knols et al conclude that three of the mosquito species that they collected during the study period must have selected their hosts on their specific body odour.
The effect of expired carbon dioxide, a known mosquito attractant, was effectively discounted since the three volunteers were of approximately the same weight, size and age. The mosquitoes that were studied included Anopheles gambae, Anfunestes giles, Culex quinquefasciatus and Mansonia spp in north east Tanzania. While the anopheles species causes malaria, the culex and mansonia cause Japanese encephalitis.
Other recent experiments which have used wind tunnel bioassay, suggest that the bacteria and other microflora present in the human skin may be responsible for attracting the mosquitoes. The human body odour secreted by the two million odd eccrine sweat glands on the skin that release odourless sweat and a sparser number of apocrine glands which secrete substances which vary from human to human in respect of smell or stink, as the case may be. In fact, these scent glands produce a milky secretion that reacts with bacteria on the skin.
Some studies on blackflies, considered the national insect of the Canadian wilderness, have suggested that sweat, especially secreted by body parts below waist, particularly feet and ankles, was found to be especially attractive to these flies. Since trousers were found to be more attractive to flies than shirts, it has been suggested that the apocrine glands of the groin region might be involved in producing the attractant compounds. Scientists are trying to zero in on the composition of the secretions and have found L-lactic acid and as yet unidentified compound(s), isolated from acetone washings of human skin. The differential attractiveness of humans to Aedes aegypti (which causes dengue and dengue haemorrhagic fever) was found to be related to the amount of L-lactic acid skin washings, although lactic acid alone was not attractive without a carbon dioxide component in the air stream.
In fact, Knols and de Jong are using this feature of attraction of odour to mosquitoes to develop odour-baited traps ( Parasitology Today , Vol 12). They have chosen a particular typical and distinct odour of human feet caused by the resident skin microflora and the environment frequently described as