Why are some people more attractive to mosquitos?

Buzz off

Also in this week's column:

• Does eating fish improve brain function?
• What is a Cro-Magnon man?
• How can objects in the same room be different temperatures?

Why are some people more attractive to mosquitos?

Asked by Sarah Charles of Salisbury, Wiltshire, UK

Science is still working on the definitive answer to this Odd Body Question. Actually, mosquitoes don't bite at all, they suck the blood out of their victim. Most mosquitoes are attracted to the odour of carbon dioxide (C02). All humans exhale C02 with every breath. Yet this fact hardly helps explain why some people are "bitten" more than others.

Three early theories involving gender were proposed and then discarded. First, a theory claimed that women were more likely to be "bitten" than men because mosquitos were supposedly repelled by the strong odour of human sweat. Since the stereotyped view was that men are more likely to wash less often and be sweatier than women, so women received greater mosquito attention. But this simply wasn't true. Some men are bitten more than some women (and vice versa).

Second, a variation of this gender-based theory was that mosquitoes prefer thin-skinned people. Women generally have thinner skin than men, so women are more likely to be targeted by mosquitoes. But this too proved not to be true.

Third, it was theorised that women had some secret hormonal attractant that brought them to the attention of mosquitoes more than men. Even menstruation and ovulation could be factors in this. But such an attractant was never found. Gender does not now seem to be the all-important factor in mosquito "bite" susceptibility.

One current theory of why mosquitoes "bite" some people more than others is that diet makes one more or less attractive. Dr Randolph Morgan, director of the Insectarium at the Cincinnati Zoo, claimed that regular intake of some materials (such as yeast), which ultimately are exuded through one's pores, changes our smell and has proven effective in deterring mosquito "bites". More recent thinking is that substances in perfumes, soap residues, facial make-up, deodorants, and other compounds on the skin resulted in someone becoming more or less attractive to mosquitoes.

According to the American Mosquito Control Association of Mount Laurel, New Jersey there are over 400 such "magnetic compounds". It appears that some compounds come from within the body and some from without.

Recently, researchers have found that "masking odors" are given off by the potential victim which prevents mosquitoes from finding them.

James Logan, a research student at the Rothamsted Research in Herfordshire and Professor Jenny Mordue of the University of Aberdeen, found that "unattractive" individuals give off different chemical signals compared with "attractive" individuals. They tested the behavioural reaction of yellow fever mosquitoes to the odour of volunteers.

According to the January 2005 BBSRC Business, in one experiment, the mosquitoes were placed into a y-shaped tube and given the choice of moving upwind down one of two branches. The air flowing down one branch was laced with odour from the volunteer's hands. The other was without this odour. Their results suggest that differential attractiveness is due to compounds in unattractive individuals that switch off attraction either by acting as repellents or by masking the attractant components of human odour.

This theory differs from that of other research groups who have suggested that unattractive individuals lack the attractive components. The researchers are now testing these theories further using foil sleeping bags to collect whole body odours from volunteers.

Interesting facts

• A mosquito can "smell" its human blood dinner from a distance of up to 50 kilometres (30 miles) away.
• A mosquito can transmit fatal diseases. Among these are Malaria, Dengue Fever, Ross River Fever, and more. At least one million people die of Malaria world wide each year.
• Mosquitoes cannot transmit HIV. The virus cannot survive in the mosquito.
• The female mosquito needs blood for protein to make her eggs. Since male mosquitoes do not make eggs, they need no blood and do not "bite".
• In 2001, the Queensland Institute of Medical Research in Brisbane, Australia, announced the results of a study of mosquito bites in identical and non-identical twins. The researchers concluded that 85 per cent of human mosquito "attractiveness" is genetic in origin.
• According to the New Jersey Mosquito Homepage maintained by Rutgers University, mosquitoes actually rely upon sugar as their main source of energy. Both male and female mosquitoes feed on plant nectar, fruit juices, and other plant liquids. Sugar is burned as a fuel for flight and must be replenished daily. Blood is needed only for egg production and is consumed less frequently.
• According to Dr Steven Schutz of the Mosquito Control Research Laboratory of the University of California at Davis, although it was once believed that blood types were an important factor in varying attraction rates, this theory has been discredited.
• Is it true that the warmer the temperature, the more likely mosquitoes will bite? Air temperature may be a factor in mosquitoes "biting", but sometimes mosquitoes prefer it cool! According to Dr Leslie Saul-Gershenz, entomology director of the San Francisco Zoological Society, the Aedes mosquito (one of the more than 3,000 mosquito species throughout the world) is attracted to humans only when the temperature is below 15°C (59°F).

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to [e-mail]