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Polio is an infectious disease caused by a virus. It can strike at any age, but affects mainly children under the age of three (over 50% of all cases). The disease causes paralysis, which is almost always irreversible. In the most severe cases, polio paralysis can lead to death by asphyxiation. It was a very common disease in Ireland in the 1950s.
Europe was declared polio-free in 2002 because of extensive vaccination programmes over many years. However, polio is still endemic in India, Pakistan, Nigeria and Afghanistan.
Polio follows infection with any one of three related enteroviruses - poliovirus types 1, 2, or 3. The virus enters through the mouth and then multiplies inside the throat and intestines.
The incubation period is four to 35 days and the initial symptoms include fever, fatigue, headaches, vomiting, constipation (or less commonly diarrhoea), stiffness in the neck, and pain in the limbs.
Poliovirus is spread through person-to-person or faecal-oral contact.
Where hygiene and sanitation are poor, young children are especially at risk. Young children who are not yet toilet-trained are a ready source of transmission, regardless of their environment.
Polio can be spread when food or drink that is contaminated by faeces. There is also evidence that flies can passively transfer poliovirus from faeces to food.
Once established in the intestines, poliovirus can enter the blood stream and invade the central nervous system - spreading along nerve fibres. As it multiplies, the virus destroys nerve cells (motor neurons), which activate muscles. These nerve cells cannot be regenerated and the affected muscles no longer function. The muscles of the legs are affected more often than the arm muscles.
The development of effective vaccines to prevent paralytic polio was one of the major medical breakthroughs of the 20th century.
Poliovirus infection provides lifelong immunity against the disease, but this protection is limited to the particular type of poliovirus involved. This means that people infected with one type of poliovirus could suffer further acute episodes of polio from either of the other two types of poliovirus. This is the reason why polio vaccines contain the three strains of poliovirus.
In the past, polio vaccines were administered orally on a sugar lump. However this route of administration has changed with the advent of the six-in-one vaccine used in the Childhood Immunisation Programme.
This vaccine is administered at two, four and six months of age and is given by injection.
Although polio paralysis is the most visible sign of polio infection, fewer than 1% of polio infections ever result in paralysis. Most cases (90%) produce very mild or no symptoms and usually go unrecognised. No one knows why only a small percentage of infections lead to paralysis.
Several key risk factors have been identified as increasing the likelihood of paralysis in a person infected with polio. These include:
Because no drug developed so far has proven effective, treatment is entirely symptomatic. Moist heat is coupled with physical therapy to stimulate the muscles and antispasmodic drugs are given to produce muscular relaxation.
Large polio epidemics crippled thousands of children and caused panic every summer during the 1940s and 1950s in industrialised countries. The last major epidemic in Ireland occurred in Cork in 1956. At that time, people with polio affecting the respiratory muscles were immobilised inside ‘iron lungs' - huge metal cylinders that operated like a pair of bellows to regulate their breathing and keep them alive.
Soon after the introduction of effective vaccines in the late 1950s and early 1960s, polio was brought under control and practically eliminated as a public health problem in industrialised countries.
Last Reviewed: 30th June 2008