Original subtitles

Every year, natural disasters such as earthquakes,

wildfires...

and tsunamis...

kill around 90,000 people worldwide.

These disasters dominate our news

and fill our screens with their devastation and suffering.

Yet the havoc they wreak,

the deaths they cause,

are dwarfed by humankind’s deadliest killer:

the mosquito.

Mosquitoes are the biggest killers of humans.

Weighing a barely measurable two-and-a-half milligrams,

the mosquito carries diseases

that are mind-bogglingly deadly.

They have killed half of the people who have ever lived--

A staggering 54 billion.

Per year, mosquitoes transmitting malaria

kill almost half a million people

and infect another 200 million or more.

Including other mosquito-borne diseases,

such as yellow fever, West Nile virus,

dengue and Zika,

mosquitoes kill over 700,000 people each year.

Many more are left with severe side effects

for the rest of their lives.

Mosquito-borne disease kills more people

than any other individual animal.

In this episode of Deadly Disasters,

we’ll hear fascinating expert insights

and firsthand accounts from victims,

activists and researchers.

We will be looking at two different types of mosquitoes

that transmit and spread multiple diseases

across the globe.

We will present the struggle

to reduce the impact of these diseases

and the difficulties they cause in different countries.

They are one of our most ancient foes.

Mosquitoes preserved in amber date back millions of years,

and some were carrying malaria, even then.

Mosquitoes and the diseases they carry

have changed human history.

Over centuries, entire armies have been reduced or contained

by the fatal bite of a minuscule infected insect.

The most dramatic devastation brought by mosquitoes

came when old diseases

journeyed to different continents

and affected many native populations.

When Columbus arrived in the New World,

the mosquitoes there were irritating,

but they carried no diseases.

However, the blood of the new arrivals,

and the mosquitoes that crossed with their ships,

changed everything.

Only a few decades after Columbus

first stepped onto the Caribbean island of Hispaniola,

the local Taíno people had all but been wiped out.

It is estimated that,

from a pre-contact population of about a hundred million,

diseases such as smallpox, influenza and malaria,

along with brutality and murder,

killed 95 million of America’s indigenous inhabitants.

Mosquitoes have even changed human DNA.

Normally evolution is driven by a selection pressure,

that is something that will kill individuals

that either do or don’t have a particular mutation.

This has been seen with malaria,

particularly in sickle cell mutations,

where people with sickle cell anemia

are much more resistant

to the malaria parasite spread by the anopheles mosquito.

Consequently, where you have people dying from malaria,

or malaria reducing their ability to reproduce,

you have a selection pressure, which will favor a mutation

that’s resistant to that selection pressure,

changing DNA.

Around 8000 years ago,

when yam farmers of the Bantu people

started expanding into West-Central Africa,

they encountered a new species of mosquito.

It was host to the falciparum variant of plasmodium,

the organism that causes malaria.

The falciparum variant of this disease is brutal.

It kills between a quarter and a half of those it infects.

The human evolutionary response was extraordinary.

Within 700 years, a sickle cell mutation appeared

that granted some protection.

Plasmodium was unable to latch onto

sickle cell’s crescent-shaped red blood cells.

Before modern medicine,

if you inherited sickle cell from one parent,

you had 90 percent immunity to falciparum.

But the cost was high.

Your life expectancy was dramatically reduced

by the sickle cell condition,

and you would die as a young adult of sickle cell anemia.

If both parents transmitted the sickle cell condition,

you would most likely die in infancy.

Sickle cells were not

humanity’s only genetic response.

Some 10 percent of the world’s population

has some imperfect genetic protection against malaria.

The protections themself,

such as the blood disorder thalassemia,

often cause other serious or even fatal health issues.

Mosquitoes spread diseases on different,

sort of, spatial levels

so that if you’ve got disease in a village,

the mosquitoes will come and go to the village

and transmit that disease within that community.

But then you also have mosquitoes

that travel over longer distances,

moving it to a nearby village.

And then you also have mosquitoes

that will hitch a ride on a truck

and end up going to a community that’s further away.

So they’ll be resting on the load of the truck somewhere,

the lorry will drive away,

and then when the lorry stops, the mosquito can fly out.

And this is also the same reason

why they spray insecticide in aeroplanes

when you’re there sometimes,

to stop mosquitoes going intercontinentally

with the diseases that they may be carrying.

Only female mosquitoes bite.

They need the protein and amino acids in a blood meal

to fertilize their eggs.

The mosquito uses various clues

to zero in on human beings and other animals.

She can detect carbon dioxide

exhaled by her hosts many feet away.

As she gets closer to her target,

she looks for the movement of dark objects.

Once she finds someone, she lands,

inserts her proboscis

and probes for blood vessels beneath the skin.

When she finds one,

she injects saliva into the wound.

The saliva contains an anticoagulant,

ensuring a constant flow of blood.

But the mosquito’s saliva may also contain pathogens,

such as the malaria parasite or the encephalitis virus.

Her spit may be fatal.

I don’t think the average person does realize

how dangerous mosquitoes can be.

And I think they’re very seriously underestimated

by the population in general.

The diseases mosquitoes can inflict on humans

can be devastating.

Even when it doesn’t kill,

malaria takes a huge toll on human health.

Social and economic costs in developing countries,

particularly in sub-Saharan Africa and South Asia,

are stark.

Around 219 million people suffered from malaria in 2017.

Families have to buy drugs to treat malaria at home.

They have to pay to travel to,

and for treatment at clinics.

They lose days of work,

and time at school.

They have to pay for preventive measures,

and for burials.

Governments have to maintain,

supply and staff health facilities.

They have to buy drugs and supplies.

They need to fund public health interventions,

such as insecticide spray

or distributing insecticide-treated bed nets.

Opportunities are lost

for joint economic ventures and tourism.

Direct costs, such as illness,

treatment and early death,

are estimated to be as high

as $12 billion U.S. dollars a year.

The costs in lost economic growth are far greater.

There are many species of mosquitoes,

and some of them transmit no diseases at all.

Some of them are just an irritant.

And then others, like the group that transmit malaria,

which are anopheline mosquitoes,

they create an awful lot of trouble

by transmitting malaria all over the world.

Different mosquitoes

carry different viruses.

Some are not well-known,

but no less damaging:

dengue fever,

chikungunya,

yellow fever

and most recently, Zika.

But malaria is the most infamous and the deadliest.

Malaria is the most serious of the parasitic diseases

that affect human populations.

The main effects are on children and on pregnant women,

though everybody can become infected with malaria.

And it causes hundreds of thousands of deaths every year.

Malaria is transmitted to humans

by females of the Anopheles genus,

which includes approximately 430 different species.

However, only about 30-40 of these

are actually able to transmit the plasmodiumparasites

that cause malaria in humans.

Generally speaking, malaria mosquitoes, they like,

sort of, open water or sort of, like, muddy pools,

areas where livestock are reproducing.

Some of them will even reproduce in cattle’s footprints

when they’re full of water.

Others are reproducing when there’s been irrigation.

So you get wet ponds

that have, like, muddy areas around the fields,

and they’ll be reproducing in those.

Anopheles gambiae has an overwhelming role

in transmitting our most dangerous malaria parasite:

Plasmodium falciparum.

Plasmodium is the organism

that can reproduce in the stomachs of mosquitoes,

where it forms bumps or cysts.

When these break, the plasmodium cells

make for the insect’s salivary glands.

When the mosquito bites you to feed on blood,

they also inject saliva,

transferring the plasmodium parasite.

The parasite first infects the liver,

but also, eventually, the red blood cells.

If another mosquito comes along and bites the same person,

it picks up the plasmodium and becomes infected as well.

As soon as they are injected into the bloodstream,

the parasite spores head straight to the liver.

In less than 30 minutes,

the spores invade the liver cells,

where they develop into merozoites.

Then they multiply rapidly.

They usually stay in the liver cells for 10 days,

although some types of the parasite

can lie dormant in the liver for months or years

before they reactivate.

Erupting from the liver,

the merozoites invade red blood cells,

multiplying further.

Within 48 hours,

their multiplication causes red blood cells to burst.

This releases more merozoites into the bloodstream.

These, in turn, infect more red blood cells.

Over 10 days, some merozoites develop

into the sexual form of the parasite,

known as gametocytes.

When another mosquito sucks up blood from an infected human,

they take the gametocytes into them,

where they mature, reproduce

and form a thousand new sporozoites.

The reason why Anopheles gambiae is such a good vector

is that it bites almost exclusively human populations,

it bites at specific times,

at dusk and dawn, when people are moving about.

And so, it’s very well designed to be able to spread malaria.

Malaria is endemic, and like road accidents,

kills so frequently,

we become immune to the huge number of deaths,

if not the disease itself.

It flares up regularly, particularly in Africa.

For example, in 2019,

malaria erupted in Burundi.

The outbreak in this Great Lakes country

infected about 70 percent of the population

and killed over 3000 people.

Malaria is not a new problem in the country.

According to World Vision International,

Burundi had over 19.7 million cases of malaria infection

in the years 2015 through 2017.

With an estimated population of 11.53 million people,

that’s an infection rate of about 170 percent,

or the equivalent of almost every citizen of Burundi,

contracting malaria twice in that time span.

There are several reasons why the outbreak was so deadly.

They illustrate vividly the problems faced

by so many communities around the world,

blighted by the disease.

The U.N. reported a number of factors

that helped make the outbreak so calamitous.

Preventative measures weren’t used enough.

The population was vulnerable,

having low levels of resistance.

There was an increase in drug-resistant strains

of the disease.

One of the reasons

why the diseases that the mosquitoes carry

are so deadly to human populations

is because we don’t have vaccines against them.

The real challenge the vaccine has

is in achieving long-term protection,

or at least protection for long enough that we can actually

significantly lower transmission rates.

The climate crisis contributed.

Mosquitoes spread higher up the mountainous country.

Their behavior changed,

including more aggressive feeding.

And Burundi’s agricultural policies

encouraged an increase in rice production,

leading to farmers working in mosquito-infested areas.

Malaria devastates people with little or no immunity.

This includes young children,

pregnant women, and travelers or migrants.

Malaria infection during pregnancy

is a significant public health problem.

It brings substantial risks for the pregnant woman,

her fetus and the newborn child.

N’DOUR [IN FRENCH]:

Malaria’s ability to evolve to resist drugs

is crucial to its lethal success.

In terms of fighting malaria,

it’s a constant battle against evolution,

and that’s taking place on more than one level.

The mosquitoes are evolving resistance

to the insecticides that we use against them.

And in some cases,

they’re changing their behavior,

so that they’re biting in an area

where they wouldn’t have bitten before.

They’re changing which time of day they’re active.

Or they’re biting outside of huts,

whereas they used to bite inside of huts.

At the same time, the malaria itself can’t always be treated

by the common drugs anymore.

It has evolved resistance to those.

Malaria parasites, resistant to the key drugs,

have spread rapidly in Southeast Asia.

They have moved from Cambodia to Laos,

Thailand and Vietnam,

where half of patients are not being cured

by first-choice drugs.

It raises the terrifying prospect

that drug resistance could spread to Africa.

Should it spread to Africa,

as it has not once but twice before,

the mortality could be substantial.

We could have millions of people dying as a result.

Malaria is commonly treated

with a combination of two drugs:

artemisinin and piperaquine.

The drug combination was introduced in Cambodia in 2008.

But in only five years, by 2013,

the first cases of the parasite mutating

and developing resistance to both drugs were detected.

The parasite’s DNA showed that resistance

had spread across Cambodia to Laos,

Thailand and Vietnam.

It also picked up further mutations,

making it even more challenging to treat.

In some regions,

80 percent of malaria parasites were drug resistant.

But this didn’t mean the disease was untreatable.

Although half of patients

were not being cured with standard therapy,

alternative drugs were used instead,

including different drugs alongside artemisinin,

or combining three drugs, to overcome resistance.

This highly successful, resistant parasite strain

is capable of invading new territories.

It can acquire new genetic properties.

Most worryingly, it could spread to Africa,

where most malaria cases occur.

And in the same way malaria developed resistance

to chloroquine in the 1980s,

this new strain could, once again, kill millions.

Malaria’s evolutionary resistance

is matched only by the insect that spreads it.

Insecticides are the main way

to kill mosquitoes that bite indoors.

However, repeated exposure to an insecticide

over several generations

can lead mosquitoes to develop resistance.

As mosquitoes can have many generations a year,

it can develop high levels of resistance relatively fast.

Some mosquitoes have become resistant to insecticides

only a few years after the insecticides were introduced.

Aedes aegypti,

opportunistic and tenacious,

aedes transmits dengue, chikungunya,

yellow fever and Zika.

More than half of the world’s population

lives where this species is present.

Experts describe Aedes aegypti as "opportunistic."

It is remarkably able to adapt to changing environments,

especially those created by humanity.

Increases in international travel and trade,

and rapid unplanned urbanization.

Aedes aegypti has exploited these opportunities

with striking efficiency.

Most ominously, while they used to breed in water,

collected in tree holes and plant leaves,

they have adapted to breed in urban areas.

They flourish in impoverished, crowded places

with no piped water and poorly collected garbage.

These adaptations make aedes a container breeder.

They can breed wherever rain collects

or water is stored.

Larvae are found in all sorts of man-made objects,

such as plastic cups,

bottle caps, plates under plants

and pets’ water bowls.

The mosquitoes can also breed in septic tanks,

toilet tanks and showers.

Construction sites, used tires

and clogged rain gutters are all fertile spaces.

Once laid, the eggs can survive

for very long periods of time if they’re dry,

often over a year.

The eggs are sticky,

virtually gluing themselves to the insides of containers.

Submerged in water, they hatch immediately.

In cool temperatures,

mosquitoes can remain in the larval stage for months.

Aedes aegypti bite aggressively and during the day.

They bite most intensely at dawn and dusk.

In well-lit homes, they will even bite at night.

Females often sneak attack.

They approach their victims from behind

and bite their ankles and elbows,

protecting them from being noticed and hit.

Aedes aegypti are sip feeders.

Instead of drawing enough blood for a meal in one bite,

they take lots of little sips during many bites.

This increases how many people

a single mosquito with the virus can infect.

The females produce an average of 100 to 200 eggs,

depending on the size of the blood meal.

Unlike most other mosquito species,

a female Aedes aegypti

can produce up to five batches of eggs during her life.

To help survival,

she lays her eggs at several different sites.

All these features

make Aedes aegypti extremely hard to control,

and the diseases they spread make a much greater threat.

One of the deadliest diseases

the aedes transmits is dengue fever.

Astonishingly, dengue has risen 30-fold in the past 30 years.

And the disease is spreading to more countries.

Infection is usually mild, but can be very serious

and potentially life-threatening in rare cases.

There is no specific treatment

or widely available vaccine for dengue.

Symptoms include fever, severe headache,

muscle and joint pain and a red rash.

Each year, an estimated 390 million dengue infections

occur around the world.

Of these, 500,000 cases develop into dengue hemorrhagic fever,

a more severe form of the disease.

This results in up to 25,000 deaths annually worldwide.

If temperatures rise by above 2 degrees Celsius

for the year 2080,

some experts affirmed that 2.25 billion people

could be at risk of getting infected.

Aedes also spreads the chikungunya virus.

The name comes from a word in Makonde,

meaning "that which bends up."

This is because of the crippling pain

in the joints it causes.

Symptoms include fever and joint pain.

Luckily, most patients make a full recovery.

Chikungunya virus emerged in the western hemisphere

in late 2013 and has continued to spread

to neighboring countries.

As of 2016,

about 1.8 million cases were reported

from about 45 countries and territories.

The aedes also transmits yellow fever.

Symptoms include high temperature,

headache, vomiting, muscle pain

and light sensitivity.

Most people recover after three to four days,

but serious symptoms can develop:

yellowing of the skin and eyes [jaundice],

bleeding from the mouth, vomiting blood.

Half of those who get these symptoms die.

The yellow fever virus

is estimated to cause 200,000 cases

and 30,000 deaths each year,

with 90 percent occurring in Africa.

It is endemic in 34 countries in Africa

and 13 countries in Central and South America,

all in the tropical and subtropical regions,

the natural geographical range of its vector.

But dengue fever, chikungunya and yellow fever

are not the only dangerous afflictions

that the aedes mosquito has given humanity.

It is spreading a disease

that could rival the malaria of its anopheles cousin.

In 2016, something happened in South America

that made headlines worldwide.

On the 1st of February,

the World Health Organization made an announcement.

A link between the Zika infection

and clusters of microcephaly

and other neurological disorders,

constituted a public health emergency

of international concern.

The announcement seemed to have come out of nowhere.

What was Zika?

Why hadn’t we heard of it before?

Why was it so devastating?

Zika virus can be dangerous to adults

because it can cause, um,

an interaction between the immune system,

which your body uses to fight off pathogens,

and nerves in the periphery of your arms and legs.

But more specifically, it can be very dangerous

to unborn children in the womb,

because it can cause their brains to develop

in a very, very slow way, which is called microcephaly.

Before 1947,

we knew nothing about Zika.

Scientists conducting routine surveillance

for yellow fever in the Zika forest of Uganda

found it by accident.

They isolated the virus in samples taken from a captive,

sentinel rhesus monkey. The next year,

the virus was recovered from the mosquito Aedes africanus,

caught on a tree platform in the Zika forest.

The first human cases were detected in Uganda

and the United Republic of Tanzania in 1952.

Between 1969 and 1983,

the virus spread to equatorial Asia,

including India, Indonesia, Malaysia and Pakistan.

However, the disease in humans continued to be seen as rare,

with mild symptoms.

But in 2007, this changed.

Zika spread from Africa and Asia

to cause the first large outbreak in humans

on the Pacific island of Yap, in Micronesia.

Before this, there had been no outbreaks,

and only 14 cases of human Zika virus disease

had been documented worldwide.

The outbreak was surprising, but ultimately reassuring.

Although around 5000 people were infected,

roughly three-quarters of the island’s tiny population,

no one was hospitalized, and no one died.

And the outbreak was brief, lasting just three months.

But Zika’s next outbreak was more worrying.

Erupting six years later in 2013 in French Polynesia,

it caused an estimated 30,000 infections.

The virus spread to another seven island nations.

Again, there were no deaths,

but the disease had established a strong foothold

in the Pacific.

It no longer looked so harmless.

These outbreaks were dwarfed by what happened next.

Zika arrived in Brazil.

The outbreak of Zika in South America was a big surprise.

Nobody was expecting that to happen.

And what we think occurred

was that some people who were infected,

probably in the western Pacific, traveled to Brazil.

And there, they happened to be bitten by mosquitoes

that were able to take up and develop

and transmit this virus

to other people who were living there.

The mosquitoes that transmit Zika in Brazil

are actually ancestrally African mosquitoes

that were brought by humans many years ago

into South America,

almost certainly through the slave trade.

When researchers compared viruses,

their results suggested

that the virus from French Polynesia

made its way via Easter Island

before entering the Americas.

Zika spread explosively throughout Brazil

and then throughout Latin America

and the Caribbean.

In under a year, the virus had been detected

in nearly every country or territory

infested with Aedes aegypti.

Two factors helped its dramatic spread:

the population’s lack of immunity

and the mosquito’s behavior.

Aedes aegypti’s ability to adapt to urban environments

made it Zika’s perfect partner.

In Brazil, particularly in the early days,

there was a lot of anxiety and concern among pregnant women

as to whether their babies might be affected or not.

Of course, in places like that,

people are being bitten by mosquitoes on a daily basis,

and it’s impossible to tell,

unless you have quite serious investigations,

whether you might have been affected by Zika

and whether it might have affected your baby.

During the outbreak in French Polynesia,

doctors began seeing

unusually rare neurological complications.

These included Guillain-Barré syndrome,

a severe neurological disorder.

Three months after they were discharged from hospital,

around half the patients needed help to walk.

If Zika was responsible,

the burden on health services would be enormous.

In July 2015,

Brazil told the World Health Organization

that they were finding

increasing neurological disorders.

These included Guillain-Barré syndrome,

mainly in the northeastern state of Bahia.

Colombia, the Dominican Republic,

El Salvador, Honduras, Suriname

and Venezuela all followed.

A link with Zika was strongly suspected.

A second report from French Polynesia

told the World Health Organization

that more research had now linked Zika

to the surge in cases of Guillain-Barré syndrome.

But Guillain-Barré was not the worst that Zika had in store.

The most dramatic news came at the end of October in 2015.

Brazil told the World Health Organization

that 54 cases of microcephaly among newborns

had been detected since August.

The chance that a mosquito bite in pregnancy

could cause severe brain abnormalities in newborns

frightened the public and stunned scientists.

Zika virus has been deadly.

It can cause morbidity and mortality

through a number of mechanisms.

One is encephalitis.

There have been a number of deaths by encephalitis

directly attributable to Zika virus.

Another is an interaction with the immune system,

which can cause your peripheral nerves

to not communicate very well

with the muscles that they innovate.

This can slow down respiration and can result also in death,

because effectively, you have respiratory paralysis.

However, the most notable trouble that Zika causes

is with unborn children and pregnant women

and that can cause something called microcephaly,

which is where the developing child’s brain

doesn’t develop properly.

There was little evidence,

but pediatric neurologists in Brazil were convinced

that the brain abnormalities were linked to Zika.

They were to be proven right.

Some of the fetuses studied

had an extreme form of microcephaly,

in which the fetal brain stops growing,

the skull partially collapses,

and excess scalp skin shows distinctive folds.

The idea of a "congenital Zika syndrome" emerged.

It included severe microcephaly,

brain calcifications

and sometimes impaired eyesight and hearing loss.

We’re yet to discover the full range of effects

on our nervous system.

Many experts believe that a virus

which can cause such severe abnormalities

could cause other neurological problems as children develop.

The evidence was compelling.

Zika’s emergence in the Americas

stunned a world ill-prepared to cope,

especially with the heartbreaking neurological

abnormalities in newborns.

Microcephaly, once a child is born,

leads to all sorts of neurological defects,

which can manifest as a lack of coordination,

intellectual impairment, difficulty seeing,

difficulty hearing.

It’s an absolutely horrendous condition.

There are no vaccines.

Doctors can offer women little protection

beyond advising them to avoid mosquito bites,

delay their pregnancy

or not to travel to areas with ongoing transmission.

Because there are no reliable diagnostic tests,

pregnant women, possibly exposed to Zika,

are left to worry about the consequences

for their unborn babies.

Ultrasounds can detect brain abnormalities

only in the third trimester of pregnancy.

The tragedy is greatest for the poorest.

In rich countries, like the United States,

the costs of caring for a single child with microcephaly

have been estimated to be as high as $10 million.

In Brazil, many women

whose babies are born with microcephaly

are young and poor.

In poorer families,

the burden of care will largely fall on mothers,

who may have to give up work

or struggle to find the time and transport

to get help from health and social services.

And in tropical cities

throughout lower income countries,

the poor cannot always afford air-conditioning,

window screens or even insect repellents.

With no piped water, and poor sanitation,

they can be forced to store water in containers.

This provides ideal conditions

for the proliferation of mosquitoes.

Will Zika virus ever be eradicated?

One would like to hope so,

but I’m not sure, realistically, that it will.

Why do I think this?

Because it’s so difficult to detect.

You may have the virus and not know you’ve got it,

at which point you’ve got a reservoir

that surveillance won’t be able to pick up.

Some experts speculate about what might happen

if these recent epidemic strains from Asia

begin to spread throughout

Africa’s and Asia’s densely populated cities.

The latest developments for the Zika virus

in the Americas are not encouraging,

in terms of virus persistence

and the potential for further spread.

In April 2016,

Zika was found in monkeys in Ecuador

and the northeastern part of Brazil.

It suggested a new transmission cycle

that was allowing the virus to persist.

Equally alarming, in the same month,

researchers in Mexico detected the Zika virus

in Aedes albopictus mosquitoes.

Known as the Asian tiger mosquito,

it is an invasive species

that has expanded well beyond Asia

and adapted to flourish in a variety of human habitats.

As it can survive the winter in temperate climates,

it could expand the areas at risk

of Zika virus transmission significantly.

Public health officials are calling Zika

a global emergency.

There is no vaccine and no known cure.

There are millions of mosquitoes around the world,

and to combat these diseases,

it is sometimes best to cut them off at the source,

the vectors themselves.

There are measures that you can take

that reduce the population of mosquitoes,

and what those are

depends very much on which species of mosquito and where.

There are larvicides that you can use to control.

So it kills the larvae

as they’re feeding and growing in the water.

And you can spray chemicals on the inside of houses

so that when mosquitoes rest...

So killing some mosquitoes is always within our reach,

but very often, there are millions of them,

and they’re spread out everywhere.

One way of tackling mosquitoes is by larviciding.

Larviciding typically involves applying pesticides to water

where mosquito larvae develop.

As they feed, the bacillus are ingested.

The bacterial toxin perforates the mosquito’s guts

and kills it.

Some larvicides cover the surface of the water

with thin films of liquid,

which stop the larvae from obtaining oxygen.

Control of adult mosquitoes

usually involves releasing fine droplets of pesticides.

This kind of "fogging"

can significantly reduce local mosquito populations.

But while larviciding

selectively kills mosquito larvae,

fogging can kill beneficial insects,

as well as mosquitoes,

and is also much more costly.

One of the best ways to control mosquitoes

is to deny them a place to develop.

This is source reduction:

removing water from places where mosquitoes lay eggs.

This can be achieved by draining ponds,

ditches, backwaters and lagoons,

and by keeping water out

of natural and artificial containers.

Preventative steps

have helped halve malaria in Africa since 2000

and saved 6 million lives.

Insecticide-treated bed nets last for two to three years.

They can be used to protect the most vulnerable,

such as children and pregnant women.

Bed nets are routinely distributed

in sub-Saharan Africa,

but some still sleep unprotected.

Indoor spraying works for at least four months

and kills mosquitoes that land on sprayed surfaces.

In 2005, U.S. President George W. Bush

launched a malaria initiative.

President Barack Obama then doubled the funding

when he came to office,

investing $5.8 billion.

Millions of homes were sprayed,

insecticide-treated bed nets were provided,

together with anti-malaria treatments

and rapid diagnostic tests.

But these measures are not a solution.

Between 2010 and 2015,

60 of the 78 countries reported mosquito resistance

to at least one of the insecticides

used in indoor spraying and the nets.

Given the ability of mosquitoes and the diseases they carry

to adapt and resist our efforts,

gene editing could be the answer.

The gene-modification method of mosquito control

might change a mosquito’s DNA,

so that it’s not as good at transmitting a virus

but still able to produce viable offspring.

Alternatively, stopping the reproduction of mosquitoes

could eventually eliminate a species altogether.

In 2018, some 10,000 sterile male mosquitoes

were released in Burkina Faso,

a country in the front line of the war against malaria.

In the year before,

there had been 9.8 million infection cases,

resulting in almost 4000 deaths.

This was the first step in a program

to dramatically reduce the mosquito population

in the country, and hopefully beyond.

Releasing the 10,000 mosquitoes

would help researchers understand

how long they lasted, where they traveled

and their interactions with the natural insect population.

It’s not just modifying the mosquito’s genome

where genetics may be able to help us

to fight back against our most deadly enemy.

A genetically modified fungus

that can kill mosquitoes carrying malaria offers hope.

Trials in Burkina Faso have found that a fungus,

modified to produce spider toxin,

quickly killed large numbers of mosquitoes carrying malaria.

Within 45 days,

mosquito populations were drastically reduced

by more than 90 percent.

These experiments, however,

are not without their controversy.

To remove one species of mosquito

could affect the ecosystem

and cut off the food supply of other species

that feed on these insects.

Mosquitoes form an integral part of the ecosystem.

They form food for a number of animals, like bats, for example,

which also help regulate other pathogens.

So if you starve the bats,

you may very well have other insects responsible

for other diseases which become much more prevalent

as you’ve removed their natural predator.

Human ingenuity is even recruiting bacteria

in the war against the Aedes aegypti.

Wolbachia are bacteria present

in almost 60 percent of insect species,

including some mosquitoes.

But wolbachia is not commonly found in the Aedes aegypti.

Wolbachia is a type of bacteria.

It affects some mosquito species naturally,

but it is possible for scientists also to make sure

that it can infect vectors of diseases

so that it’s a potentially effective way

of controlling diseases, such as malaria or dengue.

The World Mosquito Program’s research

has shown that when introduced into the Aedes aegypti,

wolbachia can help to reduce the transmission

of its viruses to people.

Diseases such as dengue were shown to not develop well

in mosquitoes that also had wolbachia present.

And wolbachia is safe for humans,

animals and the environment.

Climate change isn’t necessarily doing anything to the virus,

but climate change may very well influence

the areas the mosquito can inhabit.

That means, rather than being located

in a very-- Fairly narrow equatorial band,

that band is going to widen,

which will encompass Europe

and possibly quite a lot of Continental America.

In towns and cities in China,

the United States and Europe,

mosquitoes could become year-round threats

to people who haven’t had time to build up immunity.

Climate change could also influence

the spread of mosquitoes.

Aedes aegypti used not to be able to reach

more temperate regions.

But a warming climate

could increase its northern or southern reach.

And its diseases will spread too,

striking people with no immunity.

In 1909, Dr. Rubert Boyce,

founder of the Liverpool School of Tropical Medicine,

declared human civilization’s fate

boiled down to a simple choice: "mosquito or man?"

Over a century later, that choice remains.

The mosquito has profoundly affected humanity.

It has changed our DNA and our history,

and it continues to devastate lives today.

Despite the massive advances

in technology and medical science,

it remains our deadliest killer.

With the effects of globalization

and climate change increasingly uncertain,

we remain extraordinarily vulnerable

to this tiny assassin:

one of the deadliest disasters known to humans.