We have only eradicated one virus from the world – smallpox. We were so close to eradicating poliovirus, but like the old saying: ”close only counts in horseshoes and hand grenades.”
Poliovirus causes asymptomatic infection in most people (~70 – 75%). There is wide-ranging potential illness in the 25 – 30% who become symptomatic. The most feared manifestation of poliovirus infection is poliomyelitis, first described in 1789 in England.
Poliovirus caused increasingly severe epidemics in the northern hemisphere each summer and fall in the first half of the 20th century. If you were a parent with small children in the early 1950’s, you no doubt remember the fear of poliomyelitis in children, the uncertainty as to how children were being infected, the pool closures for fear of contracting poliomyelitis, the public service announcements at the beginning of movie shows at theatres and the images of children in iron lungs as well as the recovered children and adults in leg braces to assist them in walking.
By 1952, there were more than 21,000 cases of poliomyelitis (sometimes called paralytic polio) reported in the U.S.
Parents expressed great relief when effective vaccines were introduced (inactivated poliovirus vaccine (IPV) in 1955 and oral poliovirus vaccine (OPV) in 1961). There were long lines wrapping around buildings where vaccines were being administered for those awaiting the opportunity to get vaccinated.
The vaccines worked. The rate of new poliovirus cases precipitously declined, and poliovirus infections from the wild-type virus were eliminated in the U.S. in 1979.
Poliovirus
Poliovirus is a picornavirus that belongs to the group of viruses referred to as enteroviruses. It is an RNA virus.
There are three serotypes (a serotype refers to an antigenically distinct form of the virus – think of this in a similar way that you would think about strains, although technically, they are not the same) of the poliovirus (type 1, type 2 and type 3). My reference to the serotypes being antigenically distinct means that immunity to one type does not confer significant cross-protection to the other types. (Recall that antigens are proteins on the surface of bacteria and viruses that the body recognizes as not being itself. In response, the body produces antibodies that bind to the antigens, hopefully preventing the virus from being able to enter a cell (we call these neutralizing antibodies), but even if neutralizing, the antibody can “tag” the virus so that certain immune cells (cytotoxic T-cells) recognize it as something to ingest and destroy with their intracellular enzymes and chemicals.)
Poliovirus enters the body through the mouth and begins to replicate in the mouth and throat, but continues its infection and replication in the gut. The virus is then passed in the infected person’s stool for several weeks (even if the infected person is without symptoms) and can infect others when the other person’s hand comes into contact with virus from the infected person’s stool and they in turn ingest it. Symptomatic polio disease (other than poliomyelitis) generally appears within 3 – 6 days of infection. (Paralytic poliomyelitis generally doesn’t appear until 7 – 21 days following infection.)
While the poliovirus is infecting cells lining the gut, it can get into the lymph tissue and nodes that are in close proximity, and in turn, enter into the blood stream, the route by which poliovirus can infect the brain and spinal cord, destroying the major cells responsible for movements of muscles. The resulting paralysis is often permanent, especially if there has not been significant improvement over the course of the first year following infection.
The majority of those who develop symptomatic infection have symptoms such as low-grade fever and sore throat. They typically recover in days to a week.
Anywhere from 1 – 5% of children infected with poliovirus will develop aseptic meningitis (this can manifest as headache, stiff neck, fever, vomiting and intense discomfort of the eyes when in bright light). However, these children do not experience paralysis (it may appear initially that they have muscle weakness, but generally that is because of stiffness and aching in the extremities and therefore they avoid movements or putting weight on their legs, which generally resolves within days).
The frequency of paralytic polio (poliomyelitis – literally, inflammation of the spinal cord due to poliovirus) varies with each different serotype, but generally is seen in less than 1% of cases.
The case fatality rate (CFR = number of deaths divided by the number of patients identified with the disease) for poliomyelitis is 2 – 5% in children. For adolescents and adults who develop poliomyelitis, the CFR can be as high as 15 – 30%. While most paralytic cases involve the legs, a small percentage of people develop what we call bulbar polio that unfortunately causes weakness of the facial muscles, the muscles involved in talking and swallowing, and potentially even the muscles associated with breathing (thus the pictures of patients in iron lungs in hospitals). The CFR for these patients can range from 25 – 75%.
Few people have an appreciation for the fact that infection with certain viruses earlier in their lives may cause problems later in life. For those who are somewhat aware, they likely will cite infection with chickenpox that can cause shingles decades later in life as one of the most common such examples. However, there are many examples, including viruses that can cause certain cancers (we refer to these as oncogenic viruses), and the most recent discovery that infection with the Epstein-Barr virus in children or young adults can cause multiple sclerosis later in life. I have written many blog posts exploring the long-term health consequences we are seeing in patients who had seemingly recovered from COVID-19.
With poliovirus infection, 25 – 40% of persons who developed poliomyelitis in childhood experience new onset of muscle pain and a worsening of their weakness or new weakness or paralysis 15 – 40 years after their initial infection, a condition referred to as post-polio syndrome.
We nearly were able to add poliovirus as the second virus to be eradicated, however, Pakistan and Afghanistan did not successfully vaccinate their populations sufficiently to eliminate the wild-type virus from their countries, and, as a consequence, polio is endemic in those two countries. Of concern, we have started to see wild-type virus cases pop up in certain African countries.
When outbreaks of polio occur, we vaccinate those around them. Ideally, we would use the inactivated vaccine (IPV) because that vaccine does not contain viable virus that can infect people. However, IPV is administered by injection, and this is much more difficult to mobilize to a large group of people in remote and underdeveloped areas of the world. For that reason, the oral vaccine (OPV) is used, however, this vaccine contains attenuated (weakened, but not inactivated) virus. While it protects the person being immunized, the attenuated virus can “revert” in that person’s gut so that when the vaccinated person passes the virus in their stool, it now may infect people who come into contact with that person and who are not protected. The most recent case of poliomyelitis in a resident of New York state was due to a reversion of a vaccine strain of virus – most likely, because of no recent international travel by this person, a result of another person from one of those countries that still uses OPV (we stopped using OPV in the U.S. in the 70’s) who was passing the vaccine-derived virus in their stool and traveled to NY, where the American, who was unvaccinated, came into contact with the virus.
Thus, there is concern with the risk for wild-type virus infection among travelers to countries where the wild-type virus remains endemic, but also to countries that are immunizing their populations only with OPV and have circulating vaccine-derived poliovirus.
Thus, if you plan on international travel with children in the future, it is essential that you get them fully vaccinated if they have not previously been. The polio vaccination schedule for children is 4 doses in total of vaccine given: (1) at 2 months of age; (2) at 4 months of age; (3) sometime between the ages of 6 and 18 months; and (4) on or after the 4th birthday with at least a 6-month interval since the third dose.
The CDC has issued a Level 2 travel advisory that calls for enhanced precautions if you are travelling to any of the following countries:
- Afghanistan
- Algeria
- Benin
- Botswana
- Burundi
- Cameroon
- Canada
- Central African Republic
- Chad
- Cote d’Ivoire
- Democratic Republic of the Congo
- Djibouti
- Egypt (healthcare facilities, refugee camps and humanitarian aid settings only)
- Ghana
- Indonesia
- Israel
- Madagascar
- Malawi
- Mali
- Mozambique
- Niger
- Nigeria
- Pakistan
- Republic of the Congo
- Somalia
- Sudan
- Togo
- United Kingdom
- Yemen
- Zambia
Before traveling to any of these countries, adults who have completed the full routine vaccine series should receive a single, lifetime booster dose of polio vaccine.
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