Polio (poliomyelitis) is a highly contagious viral disease. There are three types of poliovirus, types 1, 2 and 3. Infection can result in irreversible paralysis, usually of the leg muscles. In some cases the muscles that control breathing are paralysed and this can lead to death.
The Global Polio Eradication Initiative aims to eradicate the disease by providing polio vaccines to every child globally ensuring a polio-free world for future generations. Since 1988, polio case numbers have reduced by over 99.9%.
The only countries with endemic wild-type poliovirus in 2016 were Nigeria, Afghanistan and Pakistan. Wild type-2 polio has been eradicated globally since 1999 and no cases of type-3 have been reported worldwide since 2012.
In countries affected by conflict and disease-outbreaks, disruption to immunisation programmes increases the risk of re-emergence of polio.
Prior to the development of polio vaccines nearly every person became infected, with the highest disease rate being in infants and young children. New Zealand began immunising with the oral polio vaccine (OPV) in 1961. The oral vaccine contains attenuated (weakened) live poliovirus, but is associated with a rare risk of vaccine-derived poliovirus infection. In 2002, New Zealand introduced an inactivated polio vaccine (IPV) to stop the risk of polio caused by vaccine-derived poliovirus.
Although the oral polio vaccine continues to be in widespread use internationally a global discontinuation of the type-2 virus component of OPV has occurred in 2016 and IPV is being introduced worldwide.
Until poliovirus is completely eradicated worldwide, there remains a risk of polio returning to any polio-free country, including New Zealand. Since 1962, seven cases of polio have been reported in New Zealand, the most recent was in 1998. Four cases were confirmed as vaccine-associated and two, classified as probably vaccine related, occurred before New Zealand changed from OPV to the IPV in 2002. One case of wild-type polio (not vaccine related) was acquired in Tonga and imported into New Zealand.
Humans are the only host for polioviruses. People infected with a poliovirus excrete the virus in their saliva and faeces, whether they have symptoms of the disease or not. Polioviruses are passed from person-to-person through the faecal-oral or oral-oral route. The viruses may be passed on through contaminated water, milk or food.
The majority of cases (approximately 95%) have no symptoms. However, while infected these people shed the poliovirus in their faeces and oral secretions and can spread the disease.
Approximately 4 to 8 in 100 cases have symptoms of minor non-specific illness, which can include a low-grade fever, fatigue, headache, vomiting, neck stiffness, sore throat or muscle pain.
In about 1 to 5 in 100 cases, the person develops aseptic (sterile) meningitis a few days after they have recovered from the non-specific symptoms. They usually recover fully.
Up to 2 in 100 cases see a rapid onset of acute flaccid (floppy) paralysis. Paralysis is either of a single limb or the respiratory system. The incubation period between exposure and development of paralytic disease is usually 7-21 days, but can vary between 3 days to 35 days.
Poliomyelitis is incurable.
During the acute illness, only supportive care can be provided to reduce complications associated with paralysis. After the person has recovered, rehabilitation may prevent or reduce post-polio deformities and increase independence.
The complications from polio depend on which part or parts of the body are affected by the poliovirus. People may experience permanent muscle weakness or paralysis, difficulty passing urine and urinary tract infections, uneven limb growth (the affected leg does not grow and the unaffected leg continues to grow normally), bone deformities, heart problems, breathing problems, cranial nerve damage, brain damage and death.
People who have had polio may develop post-polio syndrome, which is characterised by significant muscle pain and weakness 15-40 years after recovering from polio. It is thought to be related to the aging of muscles and nerves that are compensating for the original damage.
Polio can only be prevented by immunisation. Travellers are recommended to be up-to-date with polio vaccination prior to visiting countries with circulating poliovirus and be vigilant about hygiene and avoiding any risk of contaminated water or food.
Household contacts of a person with polio are usually infected before the polio has been diagnosed in the first case, so isolation of the person with polio in the home environment after diagnosis is not useful.