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Story Publication logo January 19, 2021

The New Mosquito Bringing Disease to North America

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Coronavirus is the canary in the coal mine. Meet the diseases to come, the teams of scientists working to prevent them, and learn how our intrusions into the natural world will be the cause of it all.
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Today, zoonotic diseases cause an incredible 2.4 billion cases of human illness each year and 2.2...

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The Guantanamo province in Cuba. Image by Libor Píška / Shutterstock. Cuba, undated.
The Guantanamo province in Cuba. Image by Libor Píška/Shutterstock. Cuba, undated.

During the night of 18 June 2019, on the US naval base in Guantanamo Bay, Cuba, an intruder was caught in a trap.

Soldiers are accustomed to prisoners wishing to break out of Guantanamo. The base is best known as the place where the US indefinitely confines suspects in its “war on terror”, without due process or trial. For an intruder to make her way in was unusual. Even stranger, no one had ever seen anything like her on this side of the world.

The first witnesses to get a close look described the interloper this way:

"Proboscis dark with median spattering of pale yellowish scales."

"Wing: Scales mainly dark and narrow on all veins."

And most striking of all: "Abdomen … with large median white spot.”

The intruder was an Aedes vittatus mosquito. One of 3,500 mosquito species found across the globe, it is a new addition to the dozen or so species in North America that carry parasites or pathogens harmful to humans. Other mosquito species, like Aedes albopictus and Aedes aegypti, can transmit diseases like dengue, yellow fever and chikungunya. But unlike those others, Aedes vittatus is capable of carrying nearly all of the most dangerous mosquito-borne diseases, except for malaria.

“Being in close contact with these mosquitoes is not good news. They’re breeding in your bird bath and they’re feeding off your kids,” says Yvonne-Marie Linton, research director of the Walter Reed Biosystematics Unit and curator of nearly 2 million specimens in the Smithsonian Institution’s US National Mosquito Collection.

The Aedes vittatus is endemic to the Indian subcontinent and has never been seen in the western hemisphere until now. The mosquito is “a proven vector of chikungunya, Zika, dengue, yellow fever viruses and many other diseases,” according to the team of scientists who discovered it.

Most likely, the first specimens traveled here as eggs on a shipping container or possibly aircraft. The species’ likely proliferation across the Caribbean and the southern US will be equally manmade: climate change is shortening North American winters, allowing mosquitoes to breed many more times in a single season and thus spread viruses further afield.

Mosquitoes garner far less attention than, say, the swarms of so-called “murder hornets” first noticed in North America in 2020. Originally thought to be from Japan, they spread across America’s Pacific Northwest, killing off honeybee colonies.

“There is a parallel between the murder hornet and Aedes vittatus in that they came from somewhere else – they are in an area now that they didn’t exist in before,” says Ben Pagac, an entomologist for the US Army’s Public Health Command who conducts biosurveillance for the military across the Caribbean. He says this is a lesson for the public about the threats human travel and global trade carry by dispersing zoonotic diseases far and wide.

Mosquito-borne diseases kill more than 1 million people and infect nearly 700 million each year – almost one out of every 10 people on Earth.

But they have had a devastating effect for millennia. Historian Timothy C. Winegard, author of the 2019 book The Mosquito, believes they even made up an early example of biological warfare: in the Peloponnesian war of 415 to 413 BC, the Spartans lured the Athenians into mosquito-infested swamps. “Malaria killed or incapacitated over 70% of the [Athenian] force,” writes Winegard. Some of the most infamous warriors in history were laid to rest by these biting bugs. Genghis Khan was killed by a mosquito. So, by one theory, was Alexander the Great.

Meanwhile, the Aedes vittatus isn’t the first mosquito to wreak havoc on North America. Just a century ago, malaria – carried by the Anopheles mosquito – was endemic, sickening thousands each year.

“Huge sections of the American South were malarial swamps before the disease was eliminated from the continental United States,” wrote the late journalist Matthew Power. Iconic images from the 1940s and 1950s show men, women and children being sprayed down by the pesticide DDT, now known to be toxic to humans. Today, mosquitoes are kept at bay by more environmentally friendly, but no less stringent, measures. “Wetlands are drained, forests are felled, farmers migrate to cities, houses are built with windows,” all to protect us against mosquito-borne diseases, wrote Power.

But as climate change makes North American winters shorter and less cold, Linton and her colleagues warn in their study that mosquitoes may soon cause “disease outbreaks (that) pose serious threats to public health”.

A different war

When it comes to the mosquito, the US military has long been preparing. Having battled mosquitoes in the Pacific during World War Two, the US Military began mosquito research in earnest in the 1950s to 1970s, says Linton.

“More soldiers were dying in the Vietnam War from mosquito-borne disease than from bullets or combat,” says Linton. Even today, she says, “20 of the top 50 diseases that affect the military are vector-borne”. Many of the nearly 200,000 US service members currently stationed abroad are deployed to tropical areas where they’ve never been before – meaning they have no immunity to the area’s pathogens.

Five hundred years ago, the situation was the reverse. It was Christopher Columbus and his European followers who introduced mosquitoes to the so-called New World, unleashing new pathogens on the continent’s inhabitants. "Scientists agree that the Americas were malaria-free for thousands of years before Europeans arrived," writes Sonia Shah in her book The Fever. "When colonists from England arrived in the early 1600s, they had parasites roosting in their veins, and they encountered a land teeming with mosquitoes and wetlands, much more so than today. Swamp, bog, wetland and marsh covered more than 220 million acres of the region that would become the United States."

Soon, those parasites spread from the colonisers to the mosquitoes – and back to people. Ships sailing from the Caribbean carried mosquitoes that spread yellow fever and malaria up the Atlantic coast. The diseases devastated Native American communities. They impacted British colonists as well.

“Prior to the American Revolution, there were at least 30 major yellow fever epidemics in the British North American colonies, striking every major urban centre and port on the 1,000-mile stretch of seaboard from Nova Scotia to Georgia,” writes Winegard. He believes that the impact of yellow fever on British forces may have “ultimately decided the fate of the revolution”.

Thanks to widespread DDT spraying and other measures, such as the draining of swamps, the 20th Century saw enormous reductions in mosquito-borne disease. But since 1999, outbreaks of new diseases never before seen in North America have once again brought mosquitoes into the spotlight.

First, there was the West Nile virus, which jumped from infected birds to mosquitoes to humans, killing hundreds of Americans between 1999 and 2003Hundreds of cases per year still occur today. Then it was dengue fever and Zika virus, the latter of which can cause microcephaly among infants. In 2013, an outbreak of chikungunya, which causes debilitating fever and joint pain, spread across Latin America, sickening nearly 800,000 people across 31 countries, including at least 1,600 travelers to the United States.

Events like the 2013-14 chikungunya outbreak in the Caribbean and the 2015-16 Zika outbreak in Brazil are expected to become more and more frequent.

“Fifty years ago, there were no vectors of those big diseases” in the western hemisphere, says Linton. “The Caribbean got hit by these invasive viruses because it had established populations of these dangerous, invasive mosquitoes.”

By the time a zoonotic outbreak escalates into an epidemic, it’s often already too late to control. “It’s unexpected. It happens like Covid-19 happened. It takes everybody by surprise, and therefore there’s no mitigation in place,” says Linton. And when governments react by trying to purchase and distribute necessary medicines or medical equipment, they often find everyone else is scrambling for the same products, she says.

That’s why we need to be prepared, says Linton, by doing what’s known as biosurveillance: “actively looking for those vectors which might be problematic”. “Unless you know your enemy, you’re never going to be able to tackle it,” she says.

This is her job. Linton is tasked with identifying, classifying and assessing the risks posed by mosquitoes to US soldiers both at home and abroad.

“That’s how we ended up finding what we found in Cuba,” she says.

Enter the Aedes vittatus

Since 2016, preventive medicine specialists have collected mosquito samples around the base at Guantanamo Bay. Each week, a technician puts down a mosquito trap, usually next to soldiers’ and civilians’ sleeping quarters. The trap attracts mosquitoes with light; when they hover nearby, a fan sucks them in. Sometimes, the haul is as many as 3,000 of the insects.

Once roughly sorted, technicians put them into tubes and send them to a US army lab at Fort Meade, Maryland, where researchers continue to sort the specimens by hand. 

“People get very good at recognising the mosquitoes that they see every day, like the typical Asian tiger mosquito [Aedes albopictus] or Aedes aegypti, which have distinct black and white markings that stand out in the pots,” says Linton.

Mosquitoes that can’t be identified visually are sent for DNA testing. That’s what happened in June 2019.

“Looking through the microscope, there was one that looked a little different. So I zoomed in on it,” said Pagac. “It had a pattern on the thorax, the back of it, that was completely different than anything I had seen before” – those eye-catching white spots.

When Pagac and a colleague looked the species up, they realised it was Aedes vittatus. “That made our eyes go wider – not only is this an oddity but it could have some pretty significant health implications,” Pagac says. Immediately, Pagac phoned Linton.

The next day the two were at her lab outside Washington DC, examining the mosquito under a microscope.

To try to determine where these particular specimens came from, they compared the specimens’ DNA barcode signature to other Aedes vittatus populations from all over the world – Kenya, Ghana, India. The most likely result was India.

“I knew that was not a good thing,” says Linton. “I knew it was invasive, knew it had not been found in America before [and that] it’s a very effective vector of dengue, chikungunya, Zika and yellow fever.”

The mosquito’s discovery raised an important question – could this Guantanamo Bay intruder be the culprit of the recent outbreaks of Zika, dengue, or other mosquito-borne diseases appearing in the Caribbean?

World on the move

To find out, Linton and her team first needed to figure out how it arrived in Guantanamo in the first place.

Natural events such as hurricanes have been known to move mosquitoes from one Caribbean island to the next. But humans, with their trucks, ships and airplanes, can unwittingly transport small disease vectors like mosquitoes farther and faster than any storm.

Linton and Pagac also knew that Aedes vittatus was what entomologists call a “container breeder”.

“Their eggs can tolerate desiccation, they can be moved around – and as soon as they hit water, they emerge,” says Linton. “And if it’s a warm, wet climate, they will survive.”

This trait reminded Linton of another mosquito species that spread in the same manner over the last 40 years: Aedes albopictus, the Asian tiger mosquito. In Southeast Asia, it had been a vector for dengue, yellow fever, and chikungunya. Then, in 1979, some of its eggs were inadvertently transported to Albania on a shipment of used tires, which often sit around in junkyards or ports collecting water – the perfect habitat for mosquito eggs to hatch. “Since then, the mosquito has traveled and established itself in almost every country in the world,” Linton says.

All of a sudden, entire continents were plagued with a new vector of existing zoonotic diseases. Africa already had Aedes aegypti, which also can transmit dengue, yellow fever, chikungunya and Zika, says Linton. “So now you have two massive vectors in the same place. The chances of maintaining an outbreak become higher and higher.”

Linton and her colleagues believe the same may now be happening with Aedes vittatus. Cuba, after all, is an island – and shipping containers are the perfect way to transport not only goods, but, inadvertently, mosquito eggs.

Stopping the vittatus

Immediately upon identifying the mosquito, US Navy Preventative Medicine Unit personnel at Guantanamo Bay began weekly sprayings of two residential areas near the spot where the first specimens were found. They also began collecting more samples in more locations, employing special traps that use dry ice to emit carbon dioxide – which is what attracts mosquitoes to the human body – to entrap more mosquitoes at a time.

Still, the mosquitoes kept coming. In December 2019, Aedes vittatus larvae were found in a bird bath less than 50m from the original test site. When they were metamorphosed by scientists in a lab, they produced 10 male specimens and seven females. On 24 February 2020, another female vittatus was found in a trap, followed by four more mosquitoes on 2 March, one a kilometre from where the original specimens were found.

On 18 April, scientists’ fears that the mosquito may be proliferating even beyond Cuba came true when two specimens were discovered in the Dominican Republic, 260 kilometres east of Cuba. Even more alarming, these specimens didn’t share the same molecular makeup of the ones from Guantanamo. Rather, these intruders seem to have been independently transported from Southeast Asia.

“There seemed to be a global trade route that was bringing these mosquitoes into the Caribbean,” says Linton. She and her colleagues created a “cheat sheet” with diagrams of the Aedes vittatus and passed it along to entomologists across the Caribbean to enlist their help in identifying whether the species had spread. “The species could well be hiding in plain sight across other islands,” the scientists wrote in their study.

“If it’s in the Dominican Republic, it’s definitely in Haiti,” says Linton. “We’re assuming Jamaica, Puerto Rico, and it could be already in Florida”, as well as Texas, Alabama, Louisiana and Mississippi.

Unless public health officials and state departments of natural resources act quickly, it may be only a matter of time before these mosquitoes spread across North America. This will require measures such as destroying the habitats where mosquitoes breed, such as by spraying chemicals or bacteria into slow water, emptying stagnant water sources or putting up devices similar to light traps that attract mosquitoes and kill them.

Part of what makes mosquitoes so tricky to control is that “they can adapt to habitat and human objects”, says Linton. “We put bird baths in our garden, we have small children’s pools, and this is where this mosquito is found.”

Making matters worse, Aedes vittatus mosquitoes bite during the day. That means that some personal protection methods most people would think of – such as closing doors, using mosquito nets on beds at night, or covering up come dusk – are ineffective.

But Pagac says spraying chemicals alone won’t solve the underlying problem, either. Rather, Americans may need to get used to incorporating preventative measures into their daily lives, such as regularly emptying kiddie pools, flowerpot basins and plastic toys.

“That’s the way you really address mosquitoes,” Pagac says.

Climate change

Military movements or industrial shipping may have brought Aedes vittatus to the western hemisphere. But human-accelerated climate change is helping the species – which loves warm, wet weather, exactly the kind that many regions are seeing more of – continue to thrive.

A mosquito will lay eggs about 36 hours after biting its host and, if the host was infected, can pass along the virus. It will produce 100-120 eggs that will hatch already carrying the disease.

Normally, there may be six new mosquito generations in a given year. But that’s changing.

“These mild winters, the longer seasons that we’re having – that means the mosquitoes have a chance to do 10 generations instead of six,” says Linton. “That means they have more time to acquire viruses” before winter freezes them off.

Humans, however, are not helpless. Biosurveillance processes like that in Guantanamo can help us predict, and then head off, where diseases are spread. But unless robust, coordinated biosurveillance programs are funded around the world, by the time a new disease vector is discovered, it may be too far along to stop, says Linton.

Focusing on travel hubs can help. In recent years, England has seen cases of so-called “airport malaria” – malaria brought in from a mosquito via a flight. Now, airports around the world are equipped with light traps. “Today, eight times more malaria patients arrive at clinics and hospitals across Europe than did in the 1970s,” writes Shah.

Aedes vittatus might be one of the few examples where we’re ahead of the curve,” says Pagac.

By funding research across the Caribbean and North America, experts like Pagac say, governments could enable scientists to monitor the species’s spread and project where the mosquitoes are headed next.

Only then can national and local governments enact campaigns using known methods like spraying, emptying stagnant water and encouraging people in at-risk areas to protect themselves by wearing long sleeves, pants and bug spray.

In a world reeling from Covid-19, that kind of focus may seem like a difficult challenge. But, he and Linton say, we must not lose this opportunity to “stop the next one”.

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