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This May Be the Deadliest Creature on Earth

Scientists aim to alter the genetics of mosquitoes to prevent them from being vehicles of deadly diseases.

Enlarged below

Labium

Fascicle

Capillary

The Feeding Tube

The mosquito’s proboscis consists of six tightly bound stylets called the fascicle, surrounded by a sheath known as the labium—which doesn’t penetrate the skin.

Multipronged Strike

The fascicle pierces the skin. Each of the stylets has a different role in extracting blood.

H

MAXILLA

Blades like saws pierce tissue.

MANDIBLE

Pointed spears advance the fascicle.

HYPOPHARYNX

A ribbonlike tube injects the saliva.

LABRUM

A flexible feeding tube draws blood.

Stealth Attack

Mosquito saliva acts as an anesthetic to numb a victim’s skin, making the pain of penetration less noticeable, and as an anticoagulant to keep the blood flowing.

Vicious Cycle

Viruses and parasites pass through the mosquito and into a new host in three general stages, usually over several days.

An infectious agent enters the mosquito in consumed blood, making its way to the gut.

It then moves through the mosquito’s body, accumulating in the salivary glands.

Once in the saliva, the agent infects a host when the mosquito sucks blood.

JASON TREAT AND RYAN WILLIAMS, NGM STAFF

ART: THOMAS POROSTOCKY

SOURCE: ALEXANDRA WESTRICH, FIELD MUSEUM

The Feeding Tube

The mosquito’s proboscis consists of six tightly bound stylets called the fascicle, surrounded by a sheath known as the labium—which doesn’t penetrate the skin.

Multipronged Strike

The fascicle pierces the skin. Each of the stylets has a different role in extracting blood.

Labium

MAXILLA

Blades like saws pierce tissue.

MANDIBLE

Pointed spears advance the fascicle.

Stealth Attack

HYPOPHARYNX

A ribbonlike tube injects the saliva.

Mosquito saliva acts as an anesthetic to numb a victim’s skin, making the pain of penetration less noticeable, and as an anticoagulant to keep the blood flowing.

LABRUM

A flexible feeding tube draws blood.

Vicious Cycle

Viruses and parasites pass through the mosquito and into a new host in three general stages, usually over several days.

Skin

Fascicle

An infectious agent enters the mosquito in

consumed blood, making its way to the gut.

Capillary

It then moves through the mosquito’s body,

accumulating in the salivary glands.

Once in the saliva, the agent infects a host

when the mosquito sucks blood.

JASON TREAT AND RYAN WILLIAMS, NGM STAFF

ART: THOMAS POROSTOCKY

SOURCE: ALEXANDRA WESTRICH, FIELD MUSEUM

This story appears in the August 2016 issue of National Geographic magazine.

We squash mosquitoes with our enormous hands. We poison-bomb them from spray trucks and airplanes. We irradiate them, drain their habitats, breed them experimentally in laboratories to confound their DNA. We’ve known for more than a century that a mosquito’s bite can pass on brutal disease: Zika is the virus receiving the most attention now, but malaria alone kills more than 400,000 people a year, and scores of thousands die from mosquito-borne yellow fever and dengue. To this day, insects smaller than a child’s thumbnail remain the most dangerous nonhuman animals on the planet.

And we are still trying to figure out how to vanquish them. There’s a line one hears frequently from entomologists and other mosquito experts, especially amid the Zika alarms: “We have no silver bullet.” What they really mean is no stake through the heart; silver bullets are for werewolves. Mosquitoes—some of them, anyway—are vampires. Of the 3,500 species that researchers have identified so far, only a few hundred feed on human blood, including the Zika-carrying Aedes aegypti and Aedes albopictus. Some, notably Ae. aegypti, turn out to be assailants of astonishing formidability.

Start with their physical equipment, especially in the mosquitoes that are the most anthropophagous, which is an elegant way of saying they prefer human blood. A mosquito homes in on you by sensing the proximity of blood from your sweat, your breath, your warmth. Her feeding apparatus, that elaborate proboscis, is a multipart marvel with a skin-piercing fascicle of tiny stylets that can suck your blood while injecting mosquito saliva laced with an anticoagulant. A mosquito can slip that fascicle into your skin so gently that you have no idea what’s happening until the blood meal is already under way. She can sip your blood until she’s more than twice her weight and has to lumber off someplace to rest, jettisoning the liquid and retaining the nutrients, before she can fly properly again.

Skeeter Scatter

Mosquitoes thrive in a tropical climate. Scientists expect this zone to widen toward the poles as the planet warms.

N.

AMER.

EUR.

ASIA

AFRICA

S.

AMER.

EQUATOR

AUSTRALIA

Projected change in suitable habitat by 2050

for

Aedes albopictus

+100%

No change

–100%

Up to 600 million more people may be exposed to invasive Zika-carrying

Aedes Albopictus

Some regions could become too hot for mosquitoes.

Jason Treat, Ryan Williams,

and Charles Preppernau, NGM Staff

Source: Yiannis Proestos, Cyprus institute

Skeeter Scatter

Mosquitoes thrive in a tropical climate. Scientists expect this

zone to widen toward the poles as the planet warms.

NORTH

AMERICA

EUROPE

ASIA

Up to 600 million

more people may be

Tropic of cancer

exposed to invasive

Zika-carrying

Aedes albopictus.

AFRICA

Projected change

in suitable habitat

by 2050 for

Aedes albopictus

EQUATOR

Some regions

+100%

could become

too hot for

SOUTH

AMERICA

mosquitoes.

No change

Tropic of cAPRICORN

AUSTRALIA

–100%

Jason Treat, Ryan Williams, and Charles Preppernau, NGM Staff

Source: Yiannis Proestos, Cyprus institute

Mosquito Maladies

Pathogens have adapted to thrive in ­different species of mosquitoes with ­characteristics that make them good hosts.

ZIKA VIRUS

DISEASE

This rapidly

spread

ing virus

can

cause

defects in the

unborn babies

of infected

Aedes aegypti

pregnant

Found in cities, this insect feeds almost exclusively on human blood.

women.

CHIKUNGUNYA

Named after a

Kimak

onde word

meaning “to be

contorted,” the

virus, discovered

in Tanzania,

causes severe

joint pain.

Aedes albopictus

YELLOW

The aggressive, adaptable species can easily colonize the habitats of other mosquitoes.

FEVER

Nearly 60,000

deaths

a year

are attributed

to this skin-

yellowing

disease, which

can be prevented

with a vaccine.

DENGUE

FEVER

Spreading since

Haemagogus*

the 1970s, the

These forest vectors of yellow fever can be identified by their metallic sheen.

potentially fatal

virus now

threatens

50 percent of

the world’s

population.

MALARIA

The parasitic

disease

killed

more than

400,000 people in

Anopheles*

2015. Most of the

The only genus that transmits malaria, it is known for its long front feelers.

fatalities were in

sub-Saharan

Africa.

LYMPHATIC

FILARIASIS

This tropical

disease alters the

lymphatic system

and causes

devastating

disfigurement and

enlargement of

Culex

body parts.

quinquefasciatus

WEST NILE

These night feeders, common around the world, prefer to lay eggs in dirty water

FEVER

In 1999, the

appear

ance of

this virus

in the

U.S. highlighted

the threat of

vector-borne

diseases outside

native ranges.

*

Many species within the genus can transmit the disease

Jason Treat, Ryan Williams and Charles Preppernau, NGM Staff

SourceS: World Health organization;

Centers for disease control and

prevention

Mosquito Maladies

Pathogens have adapted to thrive in ­different species of mosquitoes with ­characteristics that make them good hosts.

LYMPHATIC

WEST NILE

DENGUE

YELLOW

ZIKA VIRUS

FILARIASIS

MALARIA

FEVER

FEVER

FEVER

DISEASE

CHIKUNGUNYA

This tropical

The parasitic

In 1999, the

Spreading since

Nearly 60,000

This rapidly

Named after a

disease alters the

disease

killed

appear

ance of

the 1970s, the

deaths

a year

spread

ing virus

Kimak

onde word

lymphatic system

more than

this virus

in the

potentially fatal

are attributed

can

cause

meaning “to be

and causes

400,000 people in

U.S. highlighted

virus now

to this skin-

defects in the

contorted,” the

devastating

2015. Most of the

the threat of

threatens

yellowing

unborn babies

virus, discovered

fatalities were in

disfigurement and

vector-borne

50 percent of

disease, which

of infected

in Tanzania,

enlargement of

sub-Saharan

diseases outside

the world’s

can be prevented

pregnant

causes severe

body parts.

Africa.

native ranges.

population.

with a vaccine.

women.

joint pain.

Aedes aegypti

Aedes albopictus

Anopheles*

Haemagogus*

Culex quinquefasciatus

Found in cities,

The aggressive, adaptable

The only genus that

These night feeders,

These forest vectors of yellow fever can be identified by their metallic sheen.

this insect feeds

species can easily

transmits malaria,

common around the world,

almost exclusively

colonize the habitats of

it is known for its

prefer to lay eggs

on human blood.

other mosquitoes.

long front feelers.

in dirty water.

*

Many species within the genus can transmit the disease.

Jason Treat, Ryan Williams and Charles Preppernau, NGM Staff

SourceS: World Health organization;

Centers for disease control and prevention

Yes, your vampire is always a female. In the mosquito world, males live off plants. The female is the biter, the worker, the source of human peril; she lives off plants too, but all those blood nutrients are for her eggs, the nourishing and laying of which are the great project of her short, purposeful, and somewhat solitary life. A single mating may be all an Ae. aegypti needs; she stores sperm inside her body, fertilizing separate batches of eggs as needed, up to several hundred at a time. Five or six occasions of egg laying are common for an Ae. aegypti that escapes extermination by swat or insecticide and reaches her expected month-long life span. The multiplication possibilities are staggering.

Ask biologists what natural advantage different mosquito species might have gained by spreading disease—why Aedes became the primary carrier of the Zika virus, for example, and Anopheles the carrier of malaria parasites—and they’re likely to tell you that you’re thinking about the question backward. It’s the pathogens, those disease-causing organisms driven to multiply in mammalian bodies, that over millennia “learned,” evolutionarily speaking, what excellent transport and delivery services some mosquitoes happen to provide. It’s not an easy ride for the pathogens: They have to survive being sucked into a mosquito’s gut, exposed to digestive enzymes, and then pushed through membranes into a mosquito salivary gland before being injected into the next warm-blooded host. The injectors, on the other hand, are simply perpetuating their family line. “It’s such a rare confluence of evolution that has allowed this to happen,” says Karl Malamud-Roam, a mosquito research scientist who helps direct a pest management program based at Rutgers University. “It’s hard to be a successful germ or mosquito.”

A modicum of respect seems in order, then, for this remarkable confluence and the very resourcefulness of the flying vampires. Consider the reproductive strategies of Aedes aegypti, which because of Zika has been the subject of international symposia and plans of attack. An Ae. aegypti will lay her eggs in the random bodies of water that humans tend to create just by living day to day: A pet dish will do, or an upturned jar top, a discarded tire, a cistern with a cracked lid. She will spread each egg batch around, making it much harder for natural or man-made interventions to wipe out a whole brood at once. She can find egg-laying spots that aren’t wet yet but will be, when the weather changes; she’s that ingenious. She bites all day long; bed nets (which have helped reduce worldwide malaria deaths because the malaria-carrying Anopheles tends to bite at night) aren’t as effective against Zika and other Aedes-carried diseases.

And when you reach down to slap a biting Ae. aegypti, she’s likely to dart lightly away, escaping the descending palm of death, and then come back to bite you again. “So she makes sure you get a multiple dose,” says University of Kentucky entomologist Grayson Brown, who in March went to Brazil, where Zika has hit hard, to help lead an Aedes aegypti summit.

“Crisis in the Americas” was the summit’s billing, and Brown says the discussion included more crises than the potentially explosive spread of Zika. Yellow fever remains a terrible worry, as do dengue, chikungunya, and Mayaro, a mosquito-spread monkey virus infecting people in northwestern Brazil. Defensive strategies under consideration range from simple to scientifically ambitious: campaigns to clean out breeding spots, experimental trap designs, larva-killing acoustic signals, plans to prevent mosquitoes from reproducing successfully by infecting them with bacteria or altering their genetic makeup. One presentation described an “autocide” technique that takes lethal advantage of the way Ae. aegypti spread each brood to multiple sites: lace the first with larva poison that the mosquito takes in when she lands. Then at her next site, she poisons her own offspring.

No silver bullets, though. “There is not going to be a silver bullet,” Brown says. “It’s going to be hard work. But it has to be done, on a year-to-year basis—for forever.”



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