All language subtitles for D D S02E03 Mosquitoes 1080p AMZN WEB-DL DD2 0 H 264-playWEB_track3_[eng]

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.