Polio Factsheet

Introduction
Poliomyelitis is a viral illness caused by infection with Poliovirus. Poliovirus is a non-enveloped enterovirus, which lives and replicates within the gut of an infected host and is usually transmitted via the faecal-oral route or, more unusually, by contaminated food or water. Worldwide incidence of Poliomyelitis is low, due to high rates of polio vaccination. The last case of polio recorded in Ireland was in 1984. Infection with poliovirus is generally asymptomatic but can cause mild viral prodromal symptoms such as fever and myalgia. In rare circumstances, infection can lead to permanent paralysis following viral invasion of the spinal cord. Polio is highly infectious and therefore early recognition of polio cases is extremely important to prevent spread of infection. Polio is a notifiable disease and all suspected or confirmed cases must be urgently reported to the Medical Officer of Health (MOH).

Epidemiology
Traditionally, three serotypes of wild polio virus (WPV) circulated globally: WPV1, WPV2 and WPV3. WPV2 was eradicated in 2015 and WPV3 was eradicated in 2019. Therefore, only WPV1 continues to circulate in endemic settings. Four countries reported a total of 11 incident cases of WPV1 in 2022: Pakistan, Afghanistan, Mozambique and Malawi. However, the majority of polio cases worldwide are not caused by infection with WPV, but instead by infection with vaccine-derived polio virus (VDPV). VDPV is a virulent form of poliovirus derived from the live-attenuated oral polio vaccine (OPV). Rarely, over generations of replication and various RNA mutations, the OPV virus can revert to a virulent form of poliovirus capable of infection and disease in unvaccinated hosts. Vaccination is thus the most important tool for preventing the spread of VDPV.

The worldwide incidence of VDPV is much higher than that of WPV, with 40 countries reporting circulation of VDPV between 2016 and 2021 amounting to 1,818 incident cases. VDPV has recently been detected in wastewater samples in the USA, the UK and Israel with one case of VDPV-associated poliomyelitis with paralysis in the USA.

Poliomyelitis
Poliovirus infection occurs usually by faecal-oral transmission. The virus replicates within the gut and is shed in faeces. The incubation period of infection is usually ~7 – 14 days, but can range from 3 – 35 days, and infected hosts are usually considered infectious for one week before, and for two weeks after the onset of symptoms. In most cases, poliovirus infection causes a self-limiting viral illness of fever, myalgia and sore throat. In about 1/200 cases, infection can lead to progressive acute flaccid myelitis (AFM) and paralysis. Acute flaccid paralysis (AFP) usually occurs progressively over a period of days and is a purely motor deficit, without sensory or cognitive involvement. AFM occurs when poliovirus invades the anterior horn cells of the central nervous system leading to inflammation (myelitis) and can affect any spinal level, although asymmetric paralysis of the lower limbs is most common. Bulbar involvement is particularly dangerous as it can lead to paralysis of the muscles for swallowing and breathing. Treatment of poliomyelitis is generally supportive, and early involvement of specialist neurology services is important. Vaccination is therefore the mainstay of management against polio. Early identification of poliovirus as a cause of AFP is important to ensure that all close contacts (including healthcare staff caring for the patient) are vaccinated.

Vaccination
Polio vaccination is part of the national childhood vaccination schedule in Ireland given at 2, 4 and 6 months as part of the 6 in 1 vaccine. Booster vaccination is then given at 4 or 5 years old as part of school vaccination programme. Protection is lifelong following complete vaccination. There are two polio vaccines currently licenced: the live-attenuated OPV and an inactivated polio vaccine (IPV). Both vaccines provide excellent immunity against infection with both WPV and VDPV. Since live-attenuated OPV allows viral replication within the gut, this vaccine provides a higher level of mucosal immunity within the gut and is therefore effective at preventing carriage and transmission of poliovirus as well as disease. OPV is therefore recommended in high incidence settings as it is more effective at reducing virus transmission. IPV is given as a parenteral injection and provides excellent immunity to poliomyelitis, however, does not provide mucosal immunity and therefore is not effective at preventing gut carriage and transmission. Given the inactivated state of the IPV vaccine there is no risk of reversion to virulent virus. IPV is recommended in low incidence settings. Ireland has administered only IPV since 2001.

It is important to obtain a complete vaccination history in anyone with suspected poliomyelitis. Anyone deemed to be at risk of infection (close contacts of patient or healthcare staff involved in patient care) must be vaccinated and catch-up vaccination is available as needed. For more information on national immunisation guidance on polio vaccination please see NIAC guidance

Surveillance
A condition of any country maintaining a “Polio Free” status is a robust system of ongoing poliovirus surveillance. In low incidence settings, this system is usually through AFP surveillance with some nations opting for additional wastewater surveillance.

• All cases of AFP in those <15 years old must be reported, and in adults any case of AFP where poliovirus is suspected, or the cause of AFP is not otherwise explained must also be notified to the MOH.
• For case definition, investigation, management, and notification/reporting details see the relevant sections in the “Guidance for Health Professionals” section.
• To notify the local public health department of a suspected polio-related AFP check here for contact information.
• Surveillance forms can be downloaded here, but should not delay immediate notification of any cases meeting the case definition, which should be done by telephone.

Last updated: 28 February 2023