Thursday April 23 2015
Mosquitoes are the world's deadliest animal
"Mosquitoes 'lured by body odour genes','' BBC News reports. Researchers tested a series of non-identical and identical twins, and found identical twins had similar levels of attractiveness to mosquitoes.
Researchers have long known that some people are more attractive to mosquitoes than others, and some think this is to do with body odour.
Body odour is, in part, inherited through our genes, so the researchers running this study wanted to find out whether twins with identical genes shared a similar level of attractiveness to mosquitoes.
They exposed the hands of sets of identical and non-identical twins to mosquitoes to see which twin the mosquitoes preferred.
The results showed identical twins were likely to have about the same level of attractiveness to mosquitoes, while non-identical twins' results differed more. This strongly suggests there is a genetic component, in the same way there is for height and IQ.
This could explain why one half of a couple is plagued by mosquitoes on holiday, while the other will be blissfully free of any bites. The research could eventually help scientists develop better insect repellents.
Where did the story come from?
The study was carried out by researchers from the London School of Hygiene and Tropical Medicine, the University of Florida, the University of Nottingham and Rothamsted Research. It was funded by the Sir Halley Stewart Trust.
The study was published in the peer-reviewed medical journal PLOS One, which is an open-access journal, meaning the study can be read for free online.
Generally, the media reported the study accurately, but did not question the reliability of results from the fairly small sample size (a total of 74 participants).
The Daily Telegraph suggested that using insect repellent made no difference to people with a genetic disposition to being bitten, but the study did not look at insect repellent, so we don't know if that is true.
What kind of research was this?
This was a laboratory-based twin study, which compared the relative attractiveness to mosquitoes of pairs of twins.
The researchers wanted to know whether identical twins, who share the same genes, were more likely to have the same level of attractiveness to mosquitoes as non-identical twins, whose genes are different.
Twin studies are useful ways to show how likely a particular trait is to be inherited. However, they can't tell us any more than that – for example, which gene is involved, or how genetics affects the trait.
What did the research involve?
Researchers took 18 pairs of identical twins and 19 pairs of non-identical twins. They tested them for attractiveness to mosquitoes by releasing the insects into a Y-shaped tube with two sections.
The twins put their hand into the top of a section, and the researchers counted the numbers of mosquitoes that flew up each side of the tube. They then looked at whether results were closer for identical twins than for non-identical twins.
The researchers did a series of experiments, testing the twins individually against clean air, and also pairing them against each other. They tried to avoid bias in the study by using randomisation to decide which side of the tube was used by which twin, and which twin was tested first.
All the twins were women and over the age of menopause. The twins had also been asked to avoid strong-smelling food such as garlic or chilli, to avoid alcohol, and to have washed their hands with odour-free soap before the experiment.
The researchers also checked the twins' temperatures to see whether body temperature had any effect on the results. The researchers used Aedes aegypti mosquitoes, which is the strain that carries dengue fever.
They analysed the data in two sets – firstly, which twin was more attractive to mosquitoes when tested against clean air, and then which was more attractive when tested against the other twin.
As well as seeing which tube the mosquitoes flew into (used to measure relative attraction), the researchers also counted how many flew at least 30 centimetres up the Y-shaped tube (used to measure flight activity).
The researchers used an average of 10 measurements for each twin to come up with estimates of the proportion of the attractiveness that was down to heritability.
What were the basic results?
The study found identical twins were much more likely to share the same level of attractiveness to mosquitoes than non-identical twins.
The study gives an estimate that 62% (standard error 12.4%) of relative attraction (the chances of the mosquitoes choosing that person's tube) was down to heritable factors, along with 67% (standard error 35.4%) of flight activity (the chance of the mosquitoes flying 30 centimetres up the tube).
The researchers say this would put attractiveness to mosquitoes at a level similar to height and IQ in terms of how much of it is inherited.
How did the researchers interpret the results?
The researchers say their results "demonstrate an underlying genetic component detectable by mosquitoes through olfaction". In other words, the study showed genetic differences account for at least some of the relative attractiveness of people to mosquitoes, and the difference is smelt by the insects.
They go on to suggest some people may have developed a body odour that is less attractive to mosquitoes, which could then have been handed down through natural selection of favourable genes, as it would protect against diseases such as dengue fever and malaria.
However, the researchers warn that the relatively small sample size and the nature of the experiment means they can't be precise about their conclusions. The standard error rates on their estimates of heritability are quite high, showing the level of uncertainty.
This research suggests the genes you inherit from your parents may determine your chances of being bitten by mosquitoes. However, the small size of the study limits how confident we can be in the results.
The researchers suggest differences in body odour determine how attractive a person is to mosquitoes. We know body odour is partly down to inherited genetic factors, so it would make sense that inherited body odour can make you more or less attractive to mosquitoes.
However, the study doesn't tell us whether the mosquitoes were attracted to people because of their body odour, or for some other reason that wasn't researched.
A lot more research needs to be done into which inherited components of body odour are linked to attractiveness to mosquitoes before scientists can use this information to produce better mosquito repellents.
At this stage, we don't know whether people who get bitten less often have less of a mosquito-attractive chemical in their body odour, or more of a mosquito-repellent chemical.
If you get bitten by mosquitoes more than other people, and one or both of your parents does too, this research suggests you might have inherited the susceptibility to being bitten.
Unfortunately, at this stage, there's not much you can do about it, except for wearing insect repellent. Wearing light, loose-fitting trousers rather than shorts, and wearing shirts with long sleeves may also help. This is particularly important during the early evening and at night, when mosquitoes prefer to feed.
If you are travelling to an area where mosquitoes are known to carry malaria, it's vital to get medical advice about which type of antimalarial medication you should take. You may need to start taking the medication before you leave the country, so it's important to plan ahead.
Read more about antimalarial medication.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.