Poliomyelitis, more commonly known as polio, is caused by, you guessed it, the poliovirus.
Polio can spread from person to person and is known to infect the spinal cord, which
causes paralysis.
The first major documented polio outbreak in the United States happened in June of 1894,
when 18 people died, along with 132 cases of permanent paralysis.
Back then, we didn’t know that polio could be spread from person to person.
It wasn’t until 1905 that Swedish scientist Ivar Wickman published two important findings
about polio: first, that the disease is infectious, and second, that some people who have polio
don’t get a severe form of the disease.
Finally, in 1908, two scientists in Vienna -- Karl Landsteiner and Erwin Popper -- conducted
experiments to find that an infectious particle smaller than a bacterium caused the disease.
They concluded that polio was caused by a virus, though we didn’t have the technology
to actually see the virus until the 1950s, when the electron microscope became available.
Even though scientists knew the disease was caused by a virus and spread between people,
we didn’t yet have an effective treatment or prevention method.
In 1916, a polio epidemic cropped up in New York City that killed over 2,000 people within
the city, about 6,000 people across the country, and left thousands more paralyzed.
A large problem is that not only can the poliovirus cause paralysis of an arm or leg, but it can
also paralyze the diaphragm and muscles required for breathing.
That’s why by 1929, the development of an artificial respirator for patients suffering
from paralytic polio was welcome news.
The artificial respirator device later became known as an iron lung, which is basically
a sealed tube-like structure that a patient’s entire body is enclosed in, except for their head.
The machine uses pressure to force air into the lungs, and then force air back out, effectively
breathing for the patient.
Jumping forward a bit, we now know that poliomyelitis is caused by three serotypes of polioviruses
-- poliovirus 1, or PV1, poliovirus 2, or PV2, and poliovirus 3, or PV3.
These viruses are non-enveloped, single-stranded RNA viruses that are classified as members
of the genus Enterovirus and the family Picornaviridae.
The three serotypes are just different enough that immunity to one serotype typically doesn’t
provide immunity for other serotypes.
As far as transmission goes, it turns out that polioviruses can only infect humans.
Polioviruses are quite stable under natural conditions and have been found in places like
swimming pools.
They typically enter the body through the mouth via the fecal-oral route, infect the
throat and intestinal tract, and then move on to invade the bloodstream.
Today we know that over 70% of people infected with poliovirus will not have a single visible symptom.
About a quarter of people might have flu-like symptoms that last only a few days, but for
the most part, the immune system can conquer the infection and achieve complete recovery.
It’s therefore interesting to think about the large outbreaks of the past, considering
that only around 1% of cases involve weakness or paralysis.
In those rare cases, the virus binds specific receptors on motor neurons, which are the
neurons within the spinal cord and brain that we learned about in the anatomy & physiology series.
It then replicates and destroys the infected cells.
This means that if a single case of poliomyelitis is observed, it’s likely that the disease
is already rampant in that population.
To be clear, the term “poliomyelitis” is reserved for cases specifically involving
paralysis.
For those people that do show serious symptoms, they might experience paresthesia, which is
a feeling of pins and needles in the legs, meningitis, which is an inflammation of the
membranes surrounding the brain and spinal cord, or paralysis, which is the most severe symptom.
In some cases, muscle pain, weakness, or paralysis can come back anywhere from 15 to 40 years
after infection, which is called post-polio syndrome.
Zooming out a bit, polioviruses are endemic in areas with crowding and poor sanitation.
In these nations, newborns typically have antibody protection from their mothers for
the first few months of life, and are likely to be exposed to the poliovirus at a young age.
They might develop a mild illness, but because of their mother’s antibodies, the virus
is blocked from spreading to the motor neurons, and they develop lifelong immunity.
In countries with good sanitation, however, poliovirus immunity is less common.
So, a poliovirus outbreak can have devastating effects.
Without widespread immunity, it is more likely that there are more cases of paralysis.
That is precisely what happened in the U.S. in the 1950s, when cases of respiratory paralysis spiked.
For poliovirus infections, the main types of treatment involve supportive care, which
includes rest, fluids, pain medications, and in some cases, an iron lung.
There are two types of vaccines that can prevent polio.
The first is an inactivated poliovirus vaccine, or IPV, which is given as an injection in
the leg or arm, and the second is an oral poliovirus vaccine, or OPV.
Thanks to a successful vaccination program, the United States has been polio-free since
1979, meaning that year-round transmission no longer occurs.
The virus has been brought into the country by travelers since, but the last case of such
an event was in 1993.
Globally, poliovirus 2 was eradicated in 1999, and there hasn’t been a case of poliovirus
3 since 2012.
According to the World Health Organization, poliovirus 1 affects only two countries as
of 2020, Pakistan and Afghanistan, so with any luck, this will soon be eradicated as well.