What happens in polio, exactly?

What is it?

Poliomyelitis, commonly known as polio, is an infectious disease caused by the poliovirus. As many as 70% of infections show no symptoms. 25% of the infected have fever and a sore throat, while 5% have a headache, neck stiffness, and pain in limbs. In 0.5% of cases polio leads to muscle weakness severe enough to cause temporary inability to move. Of those experiencing muscle weakness, 2-5% of children and 15-30% of adults die. 25-80% of those surviving these severe symptoms develop years later a post-polio syndrome, which is characterized by a similar muscle weakness and fatigue.

During the first half of the 20th century polio was a severe problem. In 1952 the US suffered from 58 000 cases. Shortly thereafter, however, an effective vaccination began to dramatically inhibit the spread of the disease. The efforts of scientists have made possible the decrease of cases from 1988 to today by 99.97%. It is hoped that polio can be wholly eradicated by 2018, and the success story of the polio vaccination remains one of the most inspiring in modern medicine.

What causes it?

Polio is caused by the infection of the poliovirus. Viruses are not living things: they are protein cells, sometimes surrounded by an outer lipid layer, that hold genetic information within them. They don’t have metabolism, and they need living cells to reproduce. They invade the cell and turn it into a factory that produces new viruses, which eventually causes the cell to die.

Viruses often have a preferred cell type, and the poliovirus is no exception: it reproduces in the cells of the gastrointestinal tract. Because of this, infection primarily occurs when ingesting something that is contaminated by the feces of someone affected, or sometimes through oral-oral contact.

What happens during it?

When the virus has entered the body, it spends the first week reproducing in the gastrointestinal tract. At this point the affected rarely shows symptoms but is infectious. From there, the virus moves on to the lymphatic nodes and further to the blood circulation, which allows it to spread effectively all around the body. As the body’s immune system realizes there is an invader and as the virus does its damage to cells, the influenza-like symptoms occur.

Motor neurons are nerve cells that are responsible for producing both the voluntary and involuntary movements of muscles. The brain sends an electrical signal along them, and as the signal reaches a muscle cell, it provokes protein filaments to slide between one another which makes the muscle contract. In around 1% of polio cases, the virus spreads to the nervous system, preferring to invade and consequentially kill these motor neurons. It is not known why the virus does this, but it is believed to come down to chance. The most common type of paralytic polio involves the destruction of motor neurons in the spinal cord, which is the pathway of nerves between the brain and the body. This damage stops any signals the brain is trying to send to muscles and, because the muscles don’t receive enough stimulation, they become weak and eventually paralyzed. When the symptoms progress this far it indicates that over 90% of the neurons in the corresponding nerves have died.

How can it be treated?

The body’s immune system is very sophisticated. When we come into contact with a virus or bacteria, it takes our body some time to figure out how to kill it; but once we have done it, our body never forgets what it needs to do, and the immune reaction is much more quick and effective if the same pathogen tries to invade our bodies again. Vaccinations use this mechanism by safely introducing the pathogen to our body, so that our immune system can learn how to fight it while it isn’t under immediate attack. Vaccines are important in protecting the personal health of an individual, but they are absolutely vital in preventing the spread of a disease among the population. Use of the oral polio vaccine plays a big role in the prevention of the disease.

When infected, there is no effective cure for polio. Using antibiotics prevent bacterial infections in weakened muscles, and pain medications, light exercise, and a nutritious diet help to minimize discomfort and maximize the speed of recovery. Treatment of severe cases that have lead to deformations is a long-term process, including physical therapy and  sometimes orthopedic surgery. Ventilators are also sometimes necessary to support breathing.

 

Sources: http://emedicine.medscape.com/article/306440-overview#a5, https://en.wikipedia.org/wiki/Poliomyelitis, https://en.wikipedia.org/wiki/Post-polio_syndrome, http://tribune.com.pk/story/794408/a-short-history-of-polio/, 

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