Disease factsheet about poliomyelitis

factsheet facts

Poliomyelitis, also known as polio or infantile paralysis, is a vaccine-preventable systemic viral infection affecting the motor neurons of the central nervous system (CNS). Historically, it has been a major cause of mortality, acute paralysis and lifelong disabilities but large scale immunisation programmes have eliminated polio from most areas of the world. The disease is now confined to a few endemic areas and global eradication of the wild polio virus (WPV) is being attempted. The last WPV infection in Europe was in 1998.

The pathogen

  • Polioviruses are small single-stranded RNA viruses that belong to the Enterovirus subgroup of the family Picornaviridae. Humans are the only reservoir of polio virus. 
  • There are three distinct serotypes of WPV, types 1, 2 and 3, and infection or immunisation with one serotype does not induce immunity against the other two serotypes. 
  • Poliovirus type 1 has historically been the predominant cause of poliomyelitis worldwide and continues to be transmitted in endemic areas. Transmission of WPV 2 virus has been successfully interrupted globally and was last reported in 1999.
  • Oral polio vaccine (OPV) is produced from live attenuated WPV which on very rare occasions can transform into pathogenic strains called vaccine derived poliomyelitis virus (VDPV).

Clinical features and sequelae

  • Poliovirus infections can lead to a spectrum of clinical presentations ranging from subclinical infection to paralysis and death. Ninety to ninety-five per cent of all poliovirus infections are asymptomatic.
  • Paralytic poliomyelitis occurs in less than 1% of all infections. The disease is traditionally classified into spinal, bulbar and bulbospinal types, depending on the site of the affected motor neurons. 
  • Spinal poliomyelitis starts with symptoms of meningitis followed by severe myalgia and localised sensory (hyperaesthesia, paraesthesia) and motor (spasms, fasciculations) symptoms. After 1–2 days, weakness and paralysis sets in.
  • The weakness is classically an asymmetrical, flaccid paralysis that peaks at 48 hours after onset. This is classified as an acute flaccid paralysis (AFP). Proximal muscle groups are affected more than distal groups. Any combination of limbs may be paralysed although lower limbs are predominantly affected.
  • Bulbar poliomyelitis is a serious form of the disease resulting from paralysis of the muscles innervated by the cranial nerves, leading to dysphagia, nasal speech, pooling of secretions and dyspnoea. Rarely polio can present as encephalitis, clinically indistinguishable from other causes of viral encephalitis.
  • The mortality rate for acute paralytic polio ranges from 5–15%. 
  • The paralysis can progress for up to one week. Permanent weakness is observed in two-thirds of patients with paralytic poliomyelitis. By 30 days, most of the reversible damage will have disappeared, although some return of function can be expected up until nine months.
  • Post-polio syndrome is a poorly understood condition characterised by the onset of fatigue, muscle weakness and wasting in patients who have recovered from paralytic polio, starting several years after the acute disease. It is not an infectious disease and further discussion on this condition is beyond the scope of this factsheet. Current European consensus guidelines on diagnosis and management of post-polio syndrome are available from the European Federation of Neurologic Societies (EFNS).

Epidemiology

  • In the pre-vaccine era, virtually all children were infected with polio virus early in life.
  • Immunisation with OPV started towards the end of the 1950s and has significantly reduced the incidence of poliomyelitis. 
  • The Global Polio Eradication campaign launched in 1988 interrupted transmission in most populations and the number of cases fell from an estimated 350 000 in 1988 to 22 WPV and 96 cVDPV cases in 2017.
  • The World Health Organization (WHO) European region, which encompasses the European Union Member States, was declared polio-free in 2002.  As of 2018, the Region has retained its polio-free status, as assessed by the European Regional Certification Commission (RCC) for Poliomyelitis Eradication at its 32nd annual meeting, held in May 2018 in Copenhagen, Denmark. Three countries, however, (Bosnia and Herzegovina, Romania and Ukraine) are of special concern to the RCC due to suboptimal immunisation coverage, weakness in surveillance, supply shortages and other vulnerabilities.
  • Three countries - Nigeria, Pakistan and Afghanistan – have WPV transmission, and within these countries the disease is confined to certain pockets.
  • The following factors have been identified as contributing to continued polio transmission: high population density; poor health service infrastructure; poor sanitation; high incidence of diarrhoeal diseases; and low oral polio vaccine coverage.
  • Data on poliomyelitis surveillance and disease incidence are reported in ECDC’s Annual Epidemiological Report on Communicable Diseases in Europe available here.
  • The EU case definition of paralytic poliomyelitis (2002/253/EC) for the purpose of reporting communicable diseases to the community network can be found here

Transmission

  • Humans are the only known reservoir of polio virus.
  • The virus is transmitted via droplets or aerosols from the throat and by faecal contamination of hands, utensils, food and water. The majority of transmissions occur through person-to-person contact via the faeco-oral route, although the oro-oral route is also possible. 
  • The incubation period is approximately 7—10 days (range 4–35 days) and about 24% of infected individuals develop clinical symptoms including fever, headache and sore throat.
  • Infected persons are most infectious from 7 to 10 days before and after the onset of symptoms. However, poliovirus is excreted in the stools for up to six weeks. 
  • Immunodeficient patients can, in rare cases, become asymptomatic chronic carriers of WPV and VDPV.
  • Poliovirus can survive at room temperature for a few weeks. Soil, sewage and infected water have been shown to harbour the virus.
  • All unimmunised persons are susceptible to the infection. Infants in the first six months may have some protection from passively transferred maternal immunity. Children younger than five years are at highest risk.
  • Poliovirus is highly infectious, with sero-conversion rates of 90–100% among household contacts.

Prevention

  • Provision of clean water, improved hygienic practices and sanitation are important for reducing the risk of transmission in endemic countries.
  • Immunisation is the cornerstone of polio eradication. Two types of vaccine are available: an inactivated poliovirus vaccine (IPV) and a live attenuated OPV. 
    • Oral polio vaccine has been the vaccine used predominantly in the past in global campaigns and is still used in endemic areas. It has the advantages of inducing both humoral and intestinal immunity and of being cheap and easy to administer. The disadvantage is the small risk of vaccine associated paralytic poliomyelitis (VAPP), which occurs in about 4 out of every 1 000 000 vaccinated children and unvaccinated contacts.
    • Inactivated poliovirus vaccine is injected intramuscularly and does not carry any risk of VAPP. The disadvantage is that it does not confer intestinal immunity and is not effective for outbreak control. It is more expensive and requires better trained staff for deliverance.
  • European countries have gradually shifted from OPV to IPV over the last decades and today all EU Member States use IPV in their childhood immunisation programmes. National immunisation schedules and policies are available here (For more recent updates, please refer to the ECDC Vaccine Scheduler and to national vaccination websites.)
  • Sensitive surveillance for acute flaccid paralysis (AFP) cases or, alternatively, testing for virus in sewage water (which is routinely done in several EU countries) along with rapid case investigations and containment of outbreaks are essential for maintaining polio-free status in Europe.
  • For surveillance purposes, the WHO and the European Centre for Disease Prevention and Control (ECDC) have approved clinical, laboratory and epidemiological criteria for defining poliomyelitis.
  • Mandatory surveillance of all cases of AFP (even non-polio) is a part of the WHO strategy to monitor the effectiveness of the reporting system. In Europe, 43 of the 53 WHO member countries are involved in active AFP surveillance and reporting. 
  • Testing for WPV and VDPV in sewage water is a sensitive surveillance method for poliomyelitis risk.

Management and treatment

  • All cases of AFP should be investigated for poliomyelitis. 
  • There is no specific treatment available for acute poliomyelitis and cases are managed supportively and symptomatically.
  • Even a single case of poliomyelitis is considered an epidemic and requires urgent action. The standard operating procedures required are provided by the Polio Global Eradication Initiative “Responding to a poliovirus outbreak”. 
  • In the instance of a confirmed polio case, an outbreak response plan will be made by experts, which includes the administration of OPV to household contacts of the index case and persons in the neighbourhood.

Note: The information contained in this fact sheet is intended for the purpose of general information and should not be used as a substitute for the individual expertise and judgement of healthcare professionals.

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