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  • Only one human disease in history has been fully stamped out; smallpox, with the World

  • Health Organization declaring it eradicated way back in 1980.

  • But, we might be close to eradicating a second disease; polio.

  • Yes, the disease that brings up imagery of wheelchairs and iron lungs may soon be on

  • its way out, but as of 2019, we're not there yet.

  • Polio, also known as poliomyelitis, is an infectious disease caused by the aptly-named

  • poliovirus, which comes in three strains, and is spread mostly by consuming contaminated water.

  • And by contaminated we mean, well

  • So, the kid poops it out, the virus gets in the water around there, usually in the sewage.

  • And then, sometime in the next day or two or three or a week or something, some other

  • kid will drink the sewage and it'll get in that kid and they'll cycle around like that.

  • Hi, I'm Jay Wenger, I'm the Director of Polio at the Bill & Melinda Gates Foundation.

  • I started working on polio eradication in 2002, when I went to India, and I joined the

  • Gates Foundation in 2011, and have been there ever since.

  • Polio can be passed through the air with oral fluids, like from a cough or a sneeze, but

  • the cycle today usually begins with fecal oral transmission, mostly in areas with poor sanitation.

  • Once swallowed the virus heads straight to your intestines, finding places to replicate along the way.

  • Now, most of the time your body will take care of the invader with an immune response,

  • giving you flu-like symptoms or no symptoms at all.

  • There are two things that happen with a natural infection.

  • Well, there are lots of things that happen, but the two main ones are, you develop humoral antibodies.

  • And those are antibodies that float around your bloodstream.

  • And basically will, if the virus gets into your bloodstream, they'll attack it and you'll end up ejecting it.

  • The second way is producing mucosal antibodies that are secreted in the gut to ensure it doesn't reproduce.

  • But if those antibodies in the blood don't stop the infection, it could result in paralysis,

  • which occurs in about one out of every 200 people who contract the poliovirus.

  • That's because if the virus survives into your bloodstream, they can find a way into

  • your nervous system and eventually get into the anterior horn cells of your spinal cord.

  • Basically, they reproduce then they go into the cell and they reproduce

  • there, and they result in death of the actual anterior horn cell, and

  • is that cell that the message from the brain transmits to the muscle.

  • And it causes an irreversible cell death.

  • So, if these anterior horn cells die, it disrupts the transmission between the brain and the muscles,

  • such as the muscles in the torso, limbs, and between our ribs.

  • And which part of the body becomes paralyzed will depend on which cells die.

  • But, for instance, if it attacks the nerves that ultimately go to the leg, say, the right

  • leg, it can kill those nerves, and then, messages from the brain to the leg don't get there anymore.

  • And so, the leg will be paralyzed, you won't be able to move it.

  • That usually happens pretty quickly, usually within a day.

  • But, if the virus gets to the nerves that influence your breathing muscles, that's especially

  • bad, because you then can't breathe and your lungs don't work.

  • Weakened lungs could result in death, or in some cases, bringing in a machine to help

  • the patient breathe, like the iron lung

  • All of these effects of polio; weakened lungs, paralysis and death, are why so many people

  • are still focused on eradicating the disease.

  • Polio was a huge deal in the United States. It was actually the most feared infectious

  • disease of like the 40s and 50s in the United States.

  • So, there was a huge push to develop a polio vaccine.

  • And the first polio vaccine that got developed was developed by Salk, and that was based

  • on a killed poliovirus preparation.

  • And so, that one was great, it was a breakthrough of its time.

  • And if you inject killed poliovirus, what you get are the humoral antibodies.

  • This boost of humoral antibodies in the blood helps lower the chance that the virus will

  • get to your spine, reducing the risk of polio-induced paralysis.

  • And so, the vaccine is good in that it prevents paralysis, and it has a little bit of an impact on the amount that you excrete.

  • It reduces it a little bit, but not a lot.

  • So that, especially in tropical countries with rotten sanitation, the IPV is not actually

  • a good tool to get rid of the virus from a population.

  • That's because the IPV isn't as effective at boosting the mucosal antibodies, so the

  • virus still reproduces in the gut, ensuring large numbers are still passed on through poop into water supplies.

  • But a couple years later, Sabin came out with an oral vaccine.

  • This Sabin vaccine was developed using live, weakened strains of poliovirus and helped

  • not only trigger humoral antibodies to protect from paralysis, but also mucosal antibodies,

  • which greatly reduces the amount of poliovirus passed on through poop.

  • So, it was a huge difference and people found that soon that after using that in populations,

  • giving every kid in the population the OPV, a couple times, you would actually drive the wild virus away.

  • This happened in Latin America and wild poliovirus was eventually eliminated from the

  • entire Americas, and then in Europe and Eastern Asia which essentially became wild poliovirus free.

  • But things sort of got slowed down a little bit in the 2000s, when we got down to like four countries.

  • And that was India, Pakistan, Afghanistan and Nigeria.

  • And we think that we are making progress toward finishing off polio in these last places.

  • India had their last case in 2011, we're hoping Nigeria had their last case in 2016.

  • And we're down to only two countries right now, Afghanistan and Pakistan.

  • If we get rid of the wild poliovirus there, it won't be anywhere else, there's nowhere else to be.

  • And so that will basically stop that as a cause of paralysis for kids everywhere.

Only one human disease in history has been fully stamped out; smallpox, with the World


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B2 中上級

どのようにしてポリオをほぼ根絶したのか? (How Have We Almost Eradicated Polio?)

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    Jerry Liu に公開 2021 年 01 月 14 日