When it comes to bugs, mosquitoes aren't anyone's favorite. From carrying diseases to leaving behind itchy bites on our skin, they're a flying menace responsible for much hardship around the world.
A team of researchers set out to Africa to study a subspecies of mosquito that is a disease-carrying vector and determine what exactly makes them so drawn to human blood in order to halt their spread.
Summer In The City
It's summertime in your city and you're looking forward to enjoying bonfires under the stars, sitting on the beach watching the sunset, and going for late evening walks, hoping to make the most of the extra daylight.
But there's another creature out there who enjoys the warm weather just as much as you do.
The Inevitable Itching
After you come inside from a bonfire with friends or a late evening walk through the park, your skin begins to itch—a lot.
You take a look at your skin and peppered throughout your arms and legs are small pink and very itchy bumps. In between the vigorous itching, you realize that you've been bitten by mosquitoes.
Meet The Mosquito
When the heat of summer rolls around, we expect to see lots of creepy crawlies emerge and make themselves known to nature. From ants to wasps and spiders, one of the most dreaded pests is the mosquito.
In fact, there are over 3,000 types of mosquitoes worldwide, but only a few species are responsible for biting people and spreading disease. Needless to say, however, for such a small bug, they're capable of causing big problems.
The Deadliest Animal In The World
When it comes to the deadliest animal in the world, you might think of something carnivorous and dangerous like a great white shark or alligator. But, as fearsome as these predators are, you'd be wrong.
The deadliest animal in the world is the mosquito, and they sicken approximately 100 million people each year.
Male Versus Female
Turns out male mosquitos are actually harmless and drink nectar. Because they don't bite humans, they're also incapable of spreading disease. Unfortunately, the same cannot be said of their female counterparts.
The biting mosquitoes we're familiar with are actually all female, and the reason they need blood is to produce their eggs and reproduce. When they bite, they pierce the skin and suck up blood, and in the process inject saliva. Our bodily reaction to it is what causes the pink bump and itching.
Anopheles Stephensi
Anopheles stephensi is a subspecies of mosquito and when it comes to researchers and health experts, it—and another subspecies of mosquito—is a grave cause for concern.
This single mosquito is the sole reason for the spread of the viral disease malaria through the African continent. As a subtropical species, it also thrives in parts of India, the Middle East, and South Asia.
Aedes Aegypti
Aedes aegypti is also known as the "yellow fever mosquito" and is characterized by the white markings on its body. This species of mosquito spreads viruses such as Zika and dengue fever and thrives in tropical, subtropical, and temperate climates, such as Africa, Asia, and even parts of the United States.
This mosquito was recently the star of a scientific study that sought to determine why mosquitoes target humans as opposed to other animals, and, most importantly, how to halt their spread.
Malaria And Dengue Fever
When it comes to mosquitoes, malaria and dengue fever are two closely associated viruses, and because these mosquitoes live near urban areas, they are more likely to spread disease.
Malaria is a disease caused by a parasite spread by mosquitoes and causes severe flu-like symptoms. Dengue fever also causes severe flu-like symptoms and in serious cases can impact the body's vascular system. Both illnesses can prove fatal, and in 2018 "there were 228 million cases of malaria worldwide and 405,000 deaths," according to the CDC.
The Study
Knowing the danger of these disease-carrying mosquitoes, a team of researchers decided to launch a study and investigate the "globally invasive subspecies" Aedes aegypti.
Published in the Current Biology journal, researchers sought to understand "why mosquitoes would strongly prefer humans over the domestic animals that almost always accompany us in domestic environments."
What They Knew So Far
Going into the study, researchers knew that "host-seeking females take up to 95% of their blood meals from humans in nature" and the subspecies Aedes aegypti is believed to have evolved in Africa approximately 5,000-10,000 years ago.
But despite this knowledge of the mosquito and the diseases it carries, researchers acknowledged that "no work to date has explicitly examined the ultimate drivers of human-biting in mosquitoes."
How The Study Was Conducted
Researchers examined a number of Aedes aegypti colonies in their native habitat throughout sub-Saharan Africa. They used ovitraps to collect mosquito eggs from a total of 27 outdoor sites.
The locations included non-urban areas such as trees of wild areas where humans and mosquitoes rarely meet to densely populated urban areas in big cities. The specimens were also collected in a variety of ecosystems, such as rainy forests to semi-arid habitats.
What Did The Results Show?
The results revealed that mosquitoes as a whole choose their host based on body odor, and in the case of Aedes aegypti, they showed a "robust preference for human odor."
While most of the studied mosquitoes preferred biting animals, their overall preference for biting humans varied widely across all test sites.
Human Populations
However, this range of human preference researchers said can be explained by two different factors, particularly "by local differences in human abundance."
Researchers found that "the number of humans living within a [12-31 mile] radius around collection localities was a strong predictor of preference...This effect helped explain why mosquitoes from cities in Burkina Faso, Ghana, and Gabon were all more responsive to human odor, on average, than those from less populated areas of the same countries."
The Importance Of Climate And Water
The second variable includes the lifeblood of the planet: water.
Aedes aegypti lay their eggs on a wet substrate just above the waterline in tree holes or even artificial containers. If the area is wet, the eggs can hatch immediately. However, if the eggs are laid at the end of the rainy season, they must survive the dry season and wait for the rains to return. "Human water storage helps Ae. aegypti in harsh environments by providing a year-round aquatic niche for larval development."
Genetic Testing
Researchers noted that a mosquito's preference for human odor has a strong genetic component, so they decided to study their DNA. Sequencing the genome of 375 mosquitoes, they determined that genetic changes in "a few key chromosomal regions" correlated to a human preference and attraction to human blood.
In fact, "Preference for humans has a single, shared genomic basis inside and outside Africa."
In Conclusion
By studying human population distribution, climate, and DNA sequencing, researchers determined that "long, hot dry seasons [are] the key selective factor driving Aedes aegypti specialization on human hosts, most likely as a by-product of dependence on human-stored water for breeding."
However, researchers issued a warning at the end of the study.
Looking Ahead To The Future
Researchers note that "both climate and human population density are changing rapidly in Africa" and that rapid urbanization will foster "dramatic increases" in human population density.
With this in mind, researchers also predict that a change in denser human populations, combined with hotter and drier environments brought on by climate change, might "trigger transitions to human-biting in many cities across the continent by 2050."
A Final Recommendation
With these predictions in mind and knowing the suffering that Aedes aegypti can inflict, researchers made one scientific recommendation.
"We urgently need to incorporate such environmentally structured variation into epidemiological models and other efforts to predict and manage the transmission of Ae. aegypti-borne disease in Africa."
Keep The Mosquitoes Away
There are a few things you can do in order to reduce the risk of being bitten.
The CDC recommends you use an insect repellent that contains an active ingredient, such as DEET. When outside during hours when mosquitoes are active, make sure to dress in long-sleeved shirts and pants. Finally, control mosquitoes by installing a screen over doors and windows, and prevent them from laying eggs by emptying outdoor objects that can contain stagnant water.
Mosquito Treatment Options
If you do get bitten by a mosquito, the CDC recommends several things to treat the bite.
First, wash the area with soap and water and apply an ice pack for 10 minutes to reduce swelling. People can also apply a baking soda and water mixture to the bitten area to reduce itching and swelling, or use an over-the-counter cream. Finally, do not scratch the bite to prevent infection.