Health Wanted Show Notes: Mosquitoes—The Bane of Summer and Public Health
About Mosquitoes
Mosquitoes are the deadliest animal in the world, killing more than one million people a year via the infectious diseases they transmit.
Only females take blood meals. They must, to get enough protein to help the growth of their eggs.
Males, who feed on sugars, couldn’t even bite you if they wanted to (which they don’t). They lack the proper mouth piece, or “stylet,” necessary to break through our skin, and even when scientists have tried to literally serve the blood up on a platter, they are uninterested.
There are 3,500 species of mosquito, but fewer than 3% of species are known to carry diseases that can infect humans. Today, we’re just going to focus on the Big Three: Culex, Anopheles, and Aedes.
The Big Three
Culex mosquitoes are known as the “common house mosquito” but transmit West Nile virus, which is the number one cause of mosquito-borne illness in the U.S. They also transmit St. Louis encephalitis and Japanese encephalitis, named so for their potential to cause brain swelling.
Anopheles mosquitoes are responsible for transmitting malaria.
You might be surprised to learn that malaria isn’t a virus, but a parasite. In 2022, malaria sickened an estimated 249 million people and caused over 600,000 deaths, with 80% of those deaths happening in children under 5 years old.
Anopheles transmission of malaria is extremely localized. 94% of cases and 95% of deaths happen in Africa.
The Aedes family of mosquitoes is responsible for transmitting such diseases as Zika, dengue, yellow fever, and chikungunya.
Zika is probably a virus you remember hearing about in the mid-2010s, because, even though we’ve known about the virus since 1947, in 2015-2016 there was an outbreak in Brazil that established a connection between Zika infections in pregnancy and neurological birth defects in babies.
Chikungunya and dengue are similar viruses in that they are both terrible, causing fever and horrific joint pain. The nickname for dengue is “break-bone fever”, because that’s what it feels like.
Unlike other viruses that build your immunity after you’ve been exposed to them, subsequent dengue infections can have worse symptoms each time.
In the United States, we have all three of these types of mosquitoes. So, if you’re a budding entomologist or just want to know what’s biting you, here’s how to identify all three:
Culex mosquitoes are dark brown or black, will hold their bodies parallel to you when landing on you, and feed at dawn and dusk.
Anopheles are also brown and will feed from sundown to sunrise, but they have a unique feeding position. Feel free to borrow this little ditty I made up in grad school: face down, bum up, that’s the way Anopheles likes to …feed.
Aedes are much more distinct, often with black and white striped bodies (one species of Aedes is called the tiger mosquito). They are aggressive biters and will feed all day from dawn to dusk. They also prefer to live in cities.
While we have all three kinds of mosquitoes in the U.S., they can only transmit diseases that are available to them, either in humans or in what we call an “animal reservoir”: wild animals that have the pathogen in their bodies.
Mosquito Territory
You might not know it, but the CDC was founded explicitly with the purpose of stopping malaria in the United States.
The U.S. South used to be the heart of the malaria zone before the 1950s. In 1947, the land that the CDC headquarters currently occupy was bought for $10 from Emory University, which is of course the home of the Rollins School of Public Health, which is why our campuses are right next to each other.
The initial staff of fewer than 400 people got to work spraying insecticides, digging irrigation ditches to clear wetlands, installing window screens, and using preventative treatments. Malaria was eliminated from the U.S. in 1951, never to be seen again….or was it?
It’s true that locally transmitted malaria has stayed off the radar since that time. But every year, the United States sees about 2,000 imported cases of malaria, meaning malaria in people who have recently returned from travel in countries where malaria is much more prevalent and likely got infected there before coming home.
However, in 2023 there were at least nine cases of locally acquired malaria; that is to say, malaria in people who had no recent history of travel and could only have gotten infected by a local mosquito that already had the malaria parasite
Nine cases are not enough to sound the alarms, and the average person’s risk of getting malaria in the U.S. is still extremely low, but it does raise some concerns about what the impact of travel, along with climate change, will be on mosquito-borne diseases.
Certain species of mosquito, such as Aedes aegypti, also love the increased temperatures. Research has found that higher temperatures make them grow faster, lay more eggs, and bite more people.
We are seeing the effects especially when it comes to dengue virus, which has absolutely exploded this year. In the first six months of 2024, there have been nearly ten million cases of dengue in the Americas—twice as many as there were in all of 2023. Given that dengue is asymptomatic in over half of all cases, ten million is likely a vast underestimate.
So, what do people do? How do they protect themselves? We have a few different choices. Let’s begin with the American favorite of personal responsibility.
If your body is a temple, how do you build permanent mosquito nets around it?
One option is literal nets. Bed nets in areas with a high burden of mosquito-borne diseases are a staple of public health, but they have their limitations.
Most nets these days are treated with insecticides to prevent and deter mosquitoes, but, like most persistent pests, they’ve begun to grow resistant.
However, a recent study was released on the second generation of insecticides in bed nets and found they reduced the risk of infection by up to 55% compared to the old insecticides.
But bed nets have other limitations. They have to be used properly, and they only work for evening or night-biting mosquitoes.
Another option is to use EPA-registered repellents that contain active ingredients such as DEET, picaridin, or oil of lemon eucalyptus.
But we also have internal options!
If you’ve ever visited a malaria endemic area, your doctor has likely prescribed an antimalarial such as doxycycline or Malarone. These can be highly effective at preventing illness by killing the parasites before they can fully develop in your liver or blood cells.
Depending on the pill, they need to be taken daily or weekly and continued for a time after leaving the malaria area. They can also have some unpleasant side effects, so they aren’t ideal for people who live in endemic areas.
Vaccines for Moquito-Borne Illnesses
There’s a chikungunya vaccine that became available in the U.S. at the end of 2023 that was highly effective in phase three clinical trials, but it is currently only used for travelers.
Yellow fever does have an affordable and effective single dose vaccine that gives lifetime immunity. But vaccines are only as good as vaccinations, meaning you have to get shots in arms, and that can be hard to do in low income or remote areas.
Malaria finally got two options for pediatric vaccines within the last three years. Each option recommends four doses starting at age 5 months, and both were shown to reduce malaria by half in the first year of life, which is when kids are most susceptible.
But four doses is a lot and can be hard to get to everyone in need.
There was a 13% reduction in death attributed to the vaccination.
For dengue, the United States has nothing at the moment. The issue of antibody-dependent enhancement, which I mentioned earlier, has made vaccine development really hard because just like an infection, vaccinations also produce antibodies.
There was a dengue vaccine available to kids aged 9-16 who live in areas with high prevalence, but it was only available to kids with a laboratory-confirmed prior infection with dengue. A study in the Philippines found that if you gave the vaccine to kids who had never had dengue, and then they got exposed to dengue, it made it worse.
I say “was” because in April of this year, Sanofi-Pasteur, which manufactured the vaccine, announced it was going to stop production due to low demand resulting from the laboratory testing requirement, which is not great for the kids in Puerto Rico currently experiencing a dengue outbreak.
There was another pediatric dengue vaccine approved by the WHO this year, but the producer could not provide some of the clinical trial data that the FDA wanted and pulled their application from the U.S. market.
Mosquito Control in the Environment
Better than having to control our own bodies would be if we could control the bodies of those tiny little menaces.
A great option that everyone should be practicing is reducing the places where mosquitoes can lay eggs. Three out of four stages of their life cycle are either in or near water, so making sure there’s no standing water around your property can do a lot to reduce mosquito numbers.
Make sure empty flowerpots, tires, or garbage cans that are left outside are turned over to avoid collecting water.
There are, of course, bodies of water much too big to be treated with dunks. In those cases, we turn to science to try to control the ability of mosquitos to harbor and pass on disease.
One novel method that’s showing incredible promise uses harmless bacteria to infect mosquito lineages for generations and is proving to reduce the incidents of both yellow fever and dengue in the areas where it’s been in use.
