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  • Most of us probably think of malaria as a disease confined to the tropics. It

  • thrives in Southeast Asia, the eastern Mediterranean, parts of Central and South

  • America, and Africa which carries most of the global malaria burden. And when we

  • think of malaria we probably think of humid jungles or tropical lakes in

  • equatorial regions. As of 2017, 87 countries had ongoing malaria

  • transmission, with places like the US and Europe appearing to be safely off the

  • list. It's easy to assume that's because the US and Europe are not tropical

  • places. But in fact, both places used to be riddled with malaria. In the U.S. in

  • particular, it debilitated towns, caused thousands of deaths, and even determined

  • the settlement patterns of the country. But now it has been so thoroughly erased

  • that many people don't even know it used to exist there at all. So how is it that

  • malaria once thrived in the US, and why did it disappear? And perhaps most

  • importantly, can the methods of its elimination in the US be copied

  • elsewhere? To understand the spread of malaria in the US it's important to

  • consider the life cycle and behavior of the Plasmodium parasite and it's vector,

  • the mosquito. Malarial parasites are carried by the Anopheles mosquito which

  • breeds in still water such as marshy ponds and swamps, features that exist in

  • great numbers in the eastern and southern United States. When a mosquito

  • bites an infected person a small amount of blood is taken in which contains

  • microscopic malaria parasites, which then mate in the gut of the mosquito and begin

  • a cycle of growth and multiplication. About one week later, a form of the

  • parasite called a sporozoite migrates to the mosquitos salivary

  • glands. When the mosquito takes its next blood meal these parasites mix with the

  • mosquitos saliva and are injected into the next person being bitten.

  • The sporozoites then rides in the bloodstream towards the human liver. Once

  • it arrives, it enters a liver cell. There it undergoes many rounds of division and

  • multiplication. A single infected liver cell can lead to the creation of

  • thousands of new parasites. These new parasites then migrate to infect red

  • blood cells where they can hide from the body's immune system. Here they consume

  • the contents of the red blood cell and divide to create even more parasites.

  • Eventually the red blood cell they are inhabiting ruptures and the new

  • parasites called merozoite are released. These continue the cycle by invading

  • other red blood cells which subsequently also rupture. The parasites living in the

  • bloodstream is what causes the symptoms of malaria, which can range from

  • headaches, to fever, to seizures, to death if the parasites block arteries in the

  • brain and kidneys. At this point the parasite within the human bloodstream

  • can then be ingested once again by a mosquito and transmitted to another

  • unsuspecting human and the cycle begins again. No one knows for sure when malaria

  • was first introduced into humans but it is thought to have been in prehistoric

  • times. But as for the US its infestation with the disease was not

  • brought about until the 17th century with the first arrival of slave ships

  • from Africa. Once the parasite was introduced to North American shores

  • places like the wet low-lying plains of Virginia and South Carolina became

  • overrun with the disease. The Carolinas were initially thought of as a land of

  • paradise by European settlers but with the introduction of malaria became known

  • as a ghastly place to live. An English proverb at the time said "those who want

  • to die quickly, go to Carolina". Incoming immigrants thus labeled certain

  • colonies as healthy and others as dangerous. The Caribbean was understood

  • to be the most dangerous, with Florida and the Carolinas being a close second.

  • The Chesapeake was a bit better but only in the northern colonies - New York, New

  • England, and Pennsylvania, did European settlers thrive. And because most African

  • slaves had some tolerance to malaria and white indentured laborers from Europe

  • did not, demand for slave labor in malarious areas also increased. Malaria,

  • along with other diseases like yellow fever, thus had a substantial role in

  • determining settlement and labor patterns in the colonies - patterns that

  • would eventually lead to the Civil War. Malaria wreaked havoc in America for

  • centuries, and by the time of the first world war,

  • malaria was a huge problem, especially for the military, where men training in

  • the south were picking up the disease in rapid numbers. 10,500 admissions for

  • malaria were reported from April 1917 through December 1919, involving a loss

  • of 130,000 training days. By the 1930s malaria had

  • become concentrated in 13 of the country's southeastern states and there

  • were well over a million cases during the Great Depression. Malaria had become

  • a major national problem. By 1933, malaria deaths in America reached a new peak.

  • Malaria thrives when poverty is high where people lack access to adequate

  • health care and nutrition, and America had just descended into the Great

  • Depression. But for the next decade where malaria should have boomed, it instead

  • retreated. But the exact reason why isn't as clear as you might think. To this day

  • scientists and historians fiercely argue about which of the many factors was the

  • key to its eradication. At the time there were two camps of opinion on the best

  • way to get rid of malaria. The first, to attack the parasite inside the human

  • body. The second, to eliminate the vector for the disease, the mosquito. If it were

  • possible to treat all members of a community at once and eradicate the

  • parasite within them, then malaria could be wiped out, even if the adult

  • mosquitoes carry on biting everyone. For hundreds of years

  • people were aware that quinine obtained from cinchona bark could be used to

  • alleviate the symptoms of and even prevent malaria. So in 1916 scientists

  • carried out a study on 500,000 people in Bolivar County Mississippi,

  • a place plagued with malaria, to see if quinine could be used as an effective

  • treatment. They gave out doses of quinine for free and indeed found that it

  • reduced malaria infections by 90%. However even though high doses of

  • quinine were good for quickly ending an episode of fever and chills they found

  • that people would not take enough quinine on a regular basis to prevent

  • infection long term because of its bad side effects.

  • Therefore, it was useful for interrupting and infection and relieving symptoms but

  • more often than not the infection would just come right back. Thus quinine

  • distributed in large numbers definitely helped but it would not be the complete

  • answer to getting rid of malaria in America. It would take more than

  • medicating the population to get rid of this persistent disease. Once it became

  • known that the mosquito was the cause of malaria many people believed its death

  • would be the way to eliminate the disease. But killing the adult stage of

  • the mosquito is challenging since they infest the world in 3d space and at the

  • time there wasn't any reliable methods for doing so. Therefore many believed it

  • was best to attack the mosquito at the most vulnerable point in its lifecycle -

  • the larval stage. Mosquitoes lay their eggs in marshy stagnant water which then

  • hatch into larvae which will eventually develop into full-grown malaria

  • spreading mosquitoes. And so one way to hit them where it hurts is to eliminate

  • their breeding grounds, the marshy stagnant water. And unluckily for the

  • mosquitoes the 1930s brought a wave of public works projects intended to boost

  • the economy, including malaria control and lots and lots of digging. The Works

  • Progress Administration put people to work digging 32,000 miles of ditches and

  • draining 623,000 watered acres. And when draining wasn't possible, coating the

  • surfaces of ponds with oil and spraying their habitats with a compound called

  • Paris green further smothered the larvae. These efforts coincided with a sharp

  • decline in malaria transmission in the 1930s, but by 1940 while less common than

  • it once was, malaria still persisted.

  • During this time the US army still trained its men in many areas of the

  • Southeast US where malaria still had its grip. Not wanting to repeat the hard

  • lessons from the First World War, large-scale anti malaria operations were

  • undertaken. 40,000 acres of surface water was eliminated 4.7 million gallons of

  • diesel oil larvacide was used, and 9.8 million dollars was spent on this

  • all-out war against malaria. Then in 1944 one of the most effective mosquito

  • killers of all time was invented - DDT. Thanks in part to DDT by 1945 malaria

  • transmission in the US had dropped significantly and the disease's days in

  • the US were numbered. Then in 1946, the CDC was born, with the

  • primary mission of finally getting rid of malaria in America once and for all.

  • During the CDC's first few years more than 6.5 million homes were sprayed with

  • DDT. This along with even more wetland drainage pushed the disease out of

  • existence. By 1951 malaria was considered eliminated altogether from the country.

  • The onslaught of DDT, drainage works, habitat oiling, and preventive medication

  • had finally worked. However there are dozens of other factors which also

  • contributed to malaria's death in America. Some historians firmly believe

  • that the key factor in its eradication was actually population movement away

  • from rural areas, while others think it was simply better education about the

  • disease that did the trick. Others think it was general economic improvement and

  • the installation of screens on houses, and yet others think it was actually a

  • massive drought that led to malaria's demise. This multivariable attack on the

  • disease and the uncertainty that that causes has left scientists unsure of how

  • to best translate these results to other parts of the world today. This, along with

  • the fact that many of the approaches taken in America cannot or should not be

  • taken in other parts of the world has made eliminating malaria globally a

  • massive, still unsolved problem. Most can agree that draining marshy areas in

  • America helped lead to malaria's decline in some amount. However, in Africa where

  • most of the world's cases of malaria occur today, such methods are not

  • feasible, because the mosquitoes there breed in small pools of water that

  • form from rainfall, spread across the landscape. It is difficult if not

  • impossible to predict when and where the breeding sites will form and to find and

  • treat them before the adult mosquitoes emerge.

  • DDT too was instrumental in eliminating malaria in America since it's so

  • effective at killing mosquitoes. The problem is we now know it's also great

  • at killing everything else. It was banned pretty much everywhere in the world in

  • the 1970s and 80s, and although it's still being used in some places

  • scientists urged it to be used as a last resort to combat malaria. Distributing

  • anti malaria medicine also would certainly help if everyone vulnerable to

  • the disease had plentiful access to it but the cost of this can be prohibitive

  • and while some progress can be made with this method the geography of places like

  • Africa make it so even if you make gains in one area, the disease will just pop

  • back up again as it returns from a surrounding area. These problems along

  • with new drug and insecticide resistance within the parasite and mosquito makes

  • it so new approaches must be taken in this global battle.

  • Luckily organizations like the Bill and Melinda Gates Foundation are working

  • towards just that putting their resources in large part towards better

  • data collection along with research and development for creating new medicines

  • and vaccines. And recently scientists have been experimenting with genetically

  • modified mosquitoes intended to drastically reduce mosquito populations

  • in the wild. It's a drastic new development in the battle against

  • mosquito-borne illness and is something we'll cover in depth in a future video.

  • No single strategy to combat malaria will ever be effective everywhere and

  • unfortunately there's no silver bullet for this centuries-old problem. But with

  • long term commitment and a flexible strategy along with much more funding it

  • may be possible to eliminate malaria everywhere in the world

  • one day soon. Malaria was in part wiped out in America

  • because of effective military strategy - a strategy that took thousands of

  • man-hours and millions of dollars and at the time seemed impossible. There have

  • been many unlikely achievements like this throughout history, from malaria and

  • yellow fever eradication in different parts of the world, to the building of

  • the Panama Canal, to the successful invasion of Normandy on D-Day, all things

  • which took unprecedented levels of coordination and strategy, and all things

  • that easily could have failed with the wrong plans set in motion. You can learn

  • more about the detailed logistics for operations like this in Real

  • Engineering's new Logistics of D-Day series available exclusively on Nebula,

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  • Wendover Productions, Medlife Crisis, BrainCraft and our other channel Real

  • Engineering. The great thing about Nebula is all of the original content, content

  • that we can create with total freedom without worrying about the YouTube

  • algorithm or demonetization. The next episode of Real Engineering's Logistics

  • of D-Day series is out today, and an episode of working titles will be coming

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B2 中高級 美國腔

美国是如何消灭疟疾的(How Malaria Was Eradicated In The U.S.)

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    joey joey 發佈於 2021 年 06 月 09 日
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