Bitten! Why some people are just more attractive… to mosquitoes


 

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Have you ever wondered why you seem to attract every mosquito within a mile radius while the person standing next to you appears completely immune? Or maybe you’re the lucky one that they tend to ignore in their quest for a tastier meal. Either way, there are a number of factors, from your skin microbiome to what you drank with dinner, that can affect your relative attractiveness to mosquitoes.

There are about 2,700 species of mosquitos in the world, but not all of them bite humans. Most prefer birds or other mammals to us. Mosquitoes get their daily nutrition from sipping flower nectar, but when it comes time for the female to reproduce, she needs the protein of a blood meal to develop her eggs. Male mosquitoes don’t drink blood at all.

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                                     A mosquito sips nectar from a flower

 

When a female mosquito lands on your skin, she inserts two tiny tubes. The first one injects an anti-clotting agent, while the second draws out the blood that pools as a result. Most of the time, the bitten human is unharmed, except for the itch caused by our body’s histamine response. But mosquitoes are also vectors for several human diseases, such as Yellow Fever, West Nile Virus, and Malaria. Because of this, we would expect that over time some pretty complex evolutionary interactions have built up as the human, the mosquito, and the parasite or virus all try to come out on top and survive.

And they have…

One major draw to a hungry, human-biting female mosquito is blood type. About 85% of all people are ‘secretors’. They secrete chemical signals through their skin that allow mosquitoes to identify their blood type, and Type O is preferred over all others. Having Type O blood and being a secretor makes you 83% more likely to be bitten. People with Type A are least likely to be bitten, but are also the most likely to suffer severe symptoms or die from untreated malaria. Those with Type O blood are likely to have less serious cases of the disease, due to physiological differences in their blood.

So why might mosquitoes flock to Type O? Humans, mosquitoes and malaria all co-evolved in Africa beginning around 200,000 years ago, so there is a long history between the three of us. If people with Type O blood were always better able to survive long enough to pass the malarial parasite (Plasmodium falciparum) on through another mosquito bite, then they were the ones the parasite would have favored. We already know that Plasmodium can affect the behavior of its mosquito host, because infected mosquitoes have been shown to be more strongly drawn to human odors.

And speaking of human odors…

Body odor is another strong draw for mosquitoes. Human sweat doesn’t smell until the bacteria on our skin begin to break it down, and different bacteria produce different odors, giving us each a unique scent. Smelly feet are especially attractive to mosquitoes, and they will land on them over any other body part. Oddly, they are also attracted to Limburger cheese. Or maybe not so oddly, since Limburger is made by one of the same bacteria that causes foot odor.

Even though we all have about the same total number of bacteria on our skin, some people have a wider variety of species, while others are dominated by just a few types. Studies have shown that people with only a few types of bacteria are more likely to be bitten, even when the total number of bacterial cells is the same. Having a wide variety of bacteria seems to protect us, and since you acquire you skin flora from your mother at birth, they tend to be passed down through families (or at least they were until the age of antibiotics and disinfectants).

Interestingly, people with ancestry from malarial areas of the world are less attractive to mosquitoes than those descended from populations that haven’t had exposure to malaria, despite their blood type. This suggests some level of co-evolution between humans and their skin flora to avoid a deadly disease.

And a few other attractive things…

-Mosquitoes are drawn to the carbon dioxide we exhale. If you are a larger person or have been exercising, you are more likely to be bitten.

-Exercise causes lactic acid to be secreted through your skin, also a draw for mosquitoes.

-Mosquitoes have also been shown to prefer the scent of a beer drinker’s skin.

-One study showed that mosquitoes are 500 times more likely to bite when the moon is full.

 

So… if you are a person with type O blood and low microbial diversity on your skin that has just gone for a run, taken off your shoes while sipping a beer and noticing that the moon is full… prepare to be bitten.

 

 

Smallegenge, R.C. et.al. 2013. Malaria infected mosquitoes express enhanced attraction to human odor. PLoS ONE 8(5)

Verhulst, N.O. et.al. 2009. Cultured skin microbiota attracts malaria mosquitoes. Malaria Journal 8:302

Verhulst, N.O. et.al. 2011. Composition of human skin microbiota affects attractiveness to malaria mosquitoes. PLoS ONE 6(12)

 

H. pylori: old friend, new enemy?


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Up until the 1980s, doctors and scientists believed that gastritis and stomach ulcers were caused by peptic acid eroding the lining of the stomach. Then, two scientists from Australia, Dr. Barry J. Marshall and Dr. J. Robin Warren, made a discovery that changed everything: most people who suffered from ulcers also had a bacterial infection that was causing inflammation of the stomach lining. The newly discovered bacterium was named Helicobacter pylori, and doctors soon found that treatment with antibiotics could effectively cure most ulcers.  Good news for ulcer patients.

In 2005, Marshall and Warren were awarded the Nobel Prize for Physiology or Medicine for discovering H. pylori and its role in gastric disease, which led to wide-spread news and media coverage. People everywhere were hearing about this ‘bad stomach bacteria that you didn’t want to have’, but a few years have passed, and we are beginning to understand that it’s just not that simple.  You might want to have it, or you might not… it depends…

We now know that humans (along with many species of animals and birds) have been colonized by H. pylori for at least 60,000 years, and probably longer. We know this because geneticists can calculate average mutation rates of the bacterium’s DNA as it travelled along in the first Homo sapien exodus from Africa.

We seem to have lived together quite peacefully with H. pylori until very recently in human history. Not long ago, just about every human on the planet was colonized with H. pylori. It was (and still is) part of our normal microbial flora, but today only about half of all people are still carrying it.

Ironically, infection with H. pylori has been linked to gastritis and peptic ulcers, both of which have been on the rise in the last hundred years or so, just when the incidence of H. pylori infection is rapidly declining.  And, peptic ulcers were very uncommon before the 20th century, when just about everyone was infected with H. pylori.

Also practically unheard of before modern times was gastroesophageal reflux disease (GERD), a disease that has been increasing steadily, and is most common in people who aren’t colonized by H. pylori at all. GERD is not common in people who have H. pylori. So, here we have a bacteria that we apparently lived peacefully with for much, if not all, of our time as Homo sapiens, not even knowing it was there, and now suddenly we can’t live with it, can’t live without it…

What went wrong?

As it turns out, being colonized with H. pylori causes your stomach to produce less acid, and since humans have been colonized with the bacterium for so long, the ‘less acid’ state has become normal for us. Now, remove the H. pylori from the equation, and what happens? That’s right, a lot more acid is produced, and the incidence of GERD goes up as well.

But then why the ulcers?

Most people who are colonized with H. Pylori never get ulcers, but those that do have an abnormally low Treg response in their gastric system. ‘Treg’ is short for ‘regulatory T cells’, and their job is to keep the immune system in check, for example turning off the inflammatory response after fighting off a disease, or preventing the immune system from attacking things that aren’t really dangerous to us (and causing allergic reactions).

We have learned that much of our Treg response depends on our immune system learning early on in childhood what is friend and what is foe, and exposure to a large variety of bacteria early in life is important for all that to be sorted out. Yet with the advent of antibiotics, disinfectants, and improved sanitation, we are exposed to far less bacteria, both ‘good’ and ‘bad’, now than ever before in our history. Studies have already shown that children raised on farms where they are exposed to a wide variety of bacteria have far less incidence of asthma and other autoimmune diseases.

When we are not exposed to H. pylori as a child, our immune system does not recognize it as part of our normal gut flora when we acquire the bacteria as an adult. This means the Treg response doesn’t effectively control inflammation in the lining of the stomach, and ulcers can result. The excessive use of antibiotics may also have selected more virulent strains, which outcompete the more harmless ones..

And cancer?

Yes, prolonged infection with H. pylori is linked to stomach cancer, or at least it has been in the recent past, and here’s where things get even more complex. It seems that you need pretty close contact in order to pass H. pylori from person to person, and therefore in the history of our co-evolution, families, tribes, and other close-knit groups tend to share the same strain of the bacteria. Over time, people and their local strain of H. pylori adapt to one another’s small differences.

But now people have started moving around faster and farther than ever before, and are constantly encountering new strains of H. pylori. If you’re already colonized by your own familiar strain, that’s not really a problem. But if you acquire H. pylori for the first time from a ‘stranger’, it’s more likely not to agree with you and cause inflammation and other changes that might eventually lead to cancer.

A fascinating study of two populations in Colombia illustrates how closely H. pylori has evolved with its human hosts. One of these populations lives in the mountains, and is of Amerindian descent. The other population, living near the coast, is of largely African descent. Both populations have the same rate of colonization with H. pylori, yet gastric cancer rates are much higher among the Amerindian population.

Why?

When scientists sequenced the genomes of the H. pylori, they found that the bacterial strains colonizing the coastal population were largely of African descent, as were the people. But the bacteria colonizing the Amerindian population were largely Southern European. This mismatch seems to have led to a greater incidence of cancer.

Unfortunately, the absence of H. pylori also leads to an increase in cancer, because of the association of GERD with esophageal changes that can eventually become malignant.

We still have lots to learn about our physiological relationship with microbes, but now that the microbiome has moved into the spotlight, we are on the verge of a new age of understanding, and perhaps a more targeted and refined approach to medical care.

 

References

Atherton, J.C. and Blaser, M.J. 2009. Coadaptation of Helicobacter pylori and humans: ancient history, modern implications. J Clin Invest. 119(9): 2475-2487

De Sablet, T. et.al. 2011. Phylogenetic origin of Helicobacter pylori is a determinant of gastric cancer risk. Gut 60(9): 1189-1195