字幕表 動画を再生する 英語字幕をプリント 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.