MOSQUITOES feast on us for practical reasons. Only the females bite humans and they do it to get a ‘blood meal,’ deriving proteins from the blood to produce eggs. To help locate their prey, female mosquitoes use their antennae and palps, the organs between the antennae, to detect carbon dioxide and odour. This means that people who have a high metabolic rate and emit more carbon dioxide, including those who are pregnant, working out, or drinking alcohol, tend to be more attractive to mosquitoes.
Whether mosquitoes prefer a certain blood type is controversial. One theory suggests that blood type may also help to determine mosquito preference. If that is the case, what blood type do mosquitoes like? A 2019 study says that the major mosquito vector of dengue virus prefer people with type O blood to those with other blood types. However, separate research note that experimental and laboratory data evaluating whether blood type makes one person more, or less, attractive to mosquitoes has fuelled a lot of speculations, but the science is contradictory. Instead, researchers report that the likelihood of being a ‘mosquito magnet’ has more to do with skin odour and microbiota than the blood type.
A 2015 twins study published in the journal PLOS One says that DNA may account for nearly 67 percent of mosquito attraction similar to the levels at which height and intelligent quotients are considered genetically linked. A mosquito bite might not seem like a big deal, but the tiny insects can be a deadly menace. Mosquitoes are vectors that can transmit infectious pathogens, including mosquito-borne diseases such as dengue, zika, chikungunya and West Nile fever. The World Health Organisation estimates that such vector-borne diseases are linked to more than 700,000 death annually, making mosquitoes among the deadliest animals on the earth.
It might not be possible to find the genetic factors that make someone more attractive to mosquitoes, but steps can still be taken to reduce risks of mosquito bites. The Centres for Disease Control and Prevention recommends wearing shirts and long pants, especially fabrics treated with the insect repellent 0.5 per cent permethrin and applying insect repellents that contain ingredients such as diethyltoluamide or oil of lemon eucalyptus and reapplying as directed.
Steps can also be taken to control mosquitoes indoors and outdoors by fitting in window screens and keeping doors closed, using air conditioning during the warmer months and eliminating standing water in birdbaths, pools, buckets and flower pots. After all, mosquitoes bite humans to harvest proteins from the blood and Âresearch shows that they find certain blood types more appetizing than others. A study finds that in a controlled setting, mosquitoes land on people with type O blood nearly twice as often as those with type A. People with type B blood fall somewhere in the middle of this itchy spectrum. Additionally, based on other genes, about 85 per cent of people secrete a chemical signal through their skin that indicates which blood type they have while 15 per cent do not. And, mosquitoes are also more attracted to secretors than non-secretors regardless of which type they are.
One of the important ways that mosquitoes locate their targets is by smelling the carbon dioxide emitted in their breathÂ. They can detect carbon dioxide from as far as 164 feet away. As a result, a large number of people who simply exhale more of the gas over time generally attract more mosquitoes than others. This is one of the reasons children get bit less often than adult.
In addition to carbon dioxide, mosquitoes find victims at a closer range by smelling the lactic acid, uric acid, ammonia and other substances expelled via sweat and are also attracted to people with higher body temperatures. Because strenuous exercise increases the buildup of lactic acid and heat in the body, it makes some people stand out to the insects. Meanwhile, the influence of genetic factors, the amount of uric acid and other substances people emit make it easy for mosquitoes to find its victims.
Other research suggest that the particular types and volume of bacteria that naturally live on human skin affect the attractiveness to mosquitoes. In a 2011 study, scientists have found that having a large amount of a few types of bacteria made skin more appealing to mosquitoes. Having lots of bacteria but spread among a greater diversity of different species seem to make skin less attractive. This also might be why mosquitoes are especially prone to biting ankles and feet as they have more robust bacteria colonies.
In several studies, pregnant women are found to attract roughly twice as many mosquito bites as others. It could a result of an unfortunate confluence of two factors: they exhale about 21 per cent more carbon dioxide and are, on an average, about 1.26 degrees Fahrenheit warmer than others.
Mosquitoes also use vision, along with odour, to locate humans. So, wearing colours that stand out such as black, dark blue or red may make it easy for mosquitoes to the preys. As a whole, underlying genetic factors are estimated to account for 85 per cent of the variability between people in their attractiveness to mosquitoes Âregardless of whether it is expressed through blood type, metabolism or other factors.
Some researchers have started looking at the reasons a minority of people seem to rarely attract mosquitoes in the hopes of creating the next generation of insect repellents. Using chromatography to isolate the particular chemicals that these people emit, scientists at the UK’s Rothamsted Research lab have found that these natural repellers tend to excrete a handful of substances that mosquitoes do not seem to find appealing. Eventually, incorporating these molecules into advanced bug spray could make it possible for even type O, exercising, pregnant woman in a black shirt to ward off mosquitoes for good.
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Md Golam Sharower is a professor and head of the entomology at the National Institute of Preventive and Social Medicine, Dhaka.
