What are the causes?
The poliomyelitis virus is part of the Picornaviridae family and belongs to the Enterovirus genus. There are three poliovirus serotypes (1, 2 and 3), all of which are capable of causing the disease.
The virus enters the body through the mouth and travels through the mucous membrane of the throat or intestine to multiply in the cervical and mesenteric (small intestine) lymph nodes. In 1 to 2% of infected subjects, the virus attacks motor neurons in the central nervous system. The virus replicates in these neurons, permanently destroying them. This results in muscle paralysis, since the motor neurons innervate the muscle fibers. The extent of such paralysis depends on the number of neurons destroyed. All the skeletal muscles can potentially be affected.
How does the disease spread?
The poliomyelitis virus multiplies in the pharyngeal mucosa and small intestine and can be found in the throat and stools. It is spread exclusively through person-to-person contact, mainly by the fecal-oral route and particularly through contaminated water and food contaminated by feces. Infected individuals can spread the infection as long as the virus remains in their throats (a week) and feces (3 to 6 weeks or more).
The strategy for stopping the spread of the wild poliovirus is based, firstly, on the introduction of routine vaccination coverage for all infants (see "How can the disease be prevented?" below). Moreover, in countries where there is the greatest risk of the disease, additional doses of the OPV are administered to all children aged under 5 during National Immunization Days.
What are the symptoms?
The disease initially manifests in flu-like symptoms (fever, tiredness, headaches) and sometimes vomiting, neck stiffness and pain in the limbs. Irreversible paralysis (most commonly affecting the legs) occurs in approximately one in 200 infected individuals.
Unless palliative measures are taken, between 5 and 10% of paralyzed patients die of asphyxiation due to paralysis of the muscles used for breathing. Residual paralysis can be observed in patients who survive and it causes varying degrees of disability. This can range from minor paralysis with complete independence to extremely debilitating paralysis potentially requiring lifelong respiratory support.
Several decades after acute poliomyelitis, some former patients develop "post-polio" syndrome, which is characterized by new, slowly progressive muscle weakness. The exact causes of the development of this syndrome have not yet been clearly identified. It may be due to the persistence of the virus in some patients.
How is the disease diagnosed?
Poliomyelitis is diagnosed based on symptoms, in particular sudden-onset asymmetric paralysis. Diagnosis should be confirmed by testing a stool or a nasopharyngeal sample for poliovirus.
What treatments are available?
There is no curative treatment.
Steps are therefore mainly taken to improve symptoms and prevent complications. Common treatments include pain-relief medications, drugs to manage muscle spasms, and physiotherapy to help patients regain their muscle movement.
How can the disease be prevented?
As polioviruses are mainly spread by the fecal-oral route, key preventive measures focus on improved hygiene. Since there is no treatment for the disease, vaccination is the only possible preventive medical action. The disease has been preventable since the 1960s thanks to effective vaccines.
Two preventive vaccines are available:
- The inactivated poliovirus (IPV) vaccine by injection, developed by Jonas Salk in the 1950s. It contains inactivated versions of all three virus serotypes and provides protection through good overall immunity. As this vaccine requires several injections and regular boosters, it must be used in controlled sterile conditions. It is completely safe, but for a long time its cost and the logistics associated with its use limited distribution to certain developed countries like France. However, its global coverage is constantly expanding.
- The oral polio vaccine (OPV), developed by Albert Sabin, also in the 1950s. The OPV vaccine contains live polio viruses attenuated by mutations: the strains contained in the vaccine (one strain of each of the three serotypes) infect vaccinated individuals but are much less likely to infect the central nervous system than non-attenuated strains. This vaccine has many advantages, which explains why, until now, it has been the key tool in the eradication program – it is easy to administer as it does not require injection and it quickly confers good overall immunity and local immune response in the intestine, thus reducing the spread of the wild poliovirus (more effectively than the IPV); furthermore, the OPV is inexpensive. Its principal disadvantages are those generally associated with live attenuated vaccines: the potential to induce the disease in some vaccinated individuals and the introduction of live strains into the environment which, although attenuated, may establish transmission chains in settings where vaccination coverage is low after they are shed by vaccinated individuals.
A new version of the OPV has recently been developed. It is based on new attenuated strains that entail a much lower risk of reversion than historical strains. This new version of the OPV has been used since 2021, and is gradually replacing the historical version.
Can poliomyelitis be eradicated?
In the pre-vaccine era, poliomyelitis was one of the worst childhood diseases. In the 1980s, poliomyelitis still affected hundreds of thousands of children throughout the world every year.
To overcome the disease, WHO launched the Global Polio Eradication Initiative in 1988. This consists of two key components:
- Surveillance to detect the circulation of poliovirus as soon as possible
- Vaccination of all children throughout the world (see above).
Surveillance of poliomyelitis is carried out by the WHO-led Global Polio Laboratory Network of around 150 laboratories that test for polioviruses in all children aged under 15 who present with acute flaccid paralysis, a classic symptom of the disease. This network is also responsible for detecting polio viruses in wastewater, an indicator that these viruses are actively circulating.
Issues related to insufficient vaccination coverage tend to considerably delay the final eradication program deadlines. Some countries fail to maintain a sufficient level of vaccination coverage, which sometimes allows poliomyelitis to return due to wild viruses imported from the two countries in which they remain endemic.
As well as allowing the circulation of wild polioviruses, low vaccination coverage can sometimes lead to a new type of poliomyelitis outbreak reported in several countries since 2000, where vaccine strains contained in the OPV are naturally transmitted to unvaccinated children. If left unchecked for several months, this can encourage the genetic drift of vaccine strains and cause reversion to a pathogenic state. Such vaccine strains that revert to pathogenicity then cause the same type of paralysis as wild strains. A new OPV used since 2021, that offers the same benefits as the original OPV but contains much more genetically stable strains, should limit the risk of vaccine strains reverting to virulence.
The eradication program is faced with the further challenge that the vast majority of poliovirus infections are asymptomatic (unlike smallpox infections which always exhibited visible symptoms). Polioviruses are therefore able to spread silently, delaying the implementation of countermeasures to contain them.
The effectiveness of the surveillance system therefore needs to be increased as the number of poliomyelitis cases falls.
It should be noted that political instability in certain regions, conflicts, population displacement, anti-vaccination sentiment on religious grounds or based on unfounded rumors, and failings on the part of certain states sometimes make it difficult to implement routine vaccination campaigns and an effective surveillance system.
How many people are affected?
In the pre-vaccine era (1960s), poliomyelitis affected over 600,000 children every year throughout the world. In 1988, the estimated global incidence of poliomyelitis was over 350,000 cases per year. In 1994, thanks to vaccination, the Region of the Americas (36 countries) was certified as free of wild polioviruses (natural strains as opposed to attenuated OPV strains), followed by the Western Pacific Region (37 countries and areas including China) in 2000, the European Region (51 countries) in 2002, the South-East Asian Region (11 countries including India) in 2014, and the African Region (47 countries) in 2020.
Wild serotype 2 strains have not circulated since 1999 and no cases associated with type 3 strains have been reported since November 2012. These two serotypes have therefore already been declared eradicated. So only wild type 1 strains are still circulating today.
Thanks to the WHO initiative, poliomyelitis incidence has been reduced by over 99% (only a few hundred cases are currently reported per year), two of the three serotypes have been eradicated, and the final serotype has been contained, persisting only in two countries, Afghanistan and Pakistan.
October 2024