What is Poliomyelitis?
Poliomyelitis is an infectious disease caused by a virus called the poliovirus. The virus has the potential to cause a serious neurological illness with paralysis that can be fatal. Cases of poliomyelitis of sudden onset (acute infection) are now almost unknown in the UK and other developed countries, due to highly successful vaccination programmes and other public health measures.
The World Health Organisation (WHO) has been working since 1988 on a worldwide programme to eradicate poliomyelitis. In 1988 it was estimated that there were more than 350,000 cases worldwide. The number of reported cases had reduced to1951 by 2005. Only four countries were considered to be at risk of outbreaks of polio by 2006; these are India, Nigeria, Afghanistan and Pakistan.
A minority of cases of acute poliomyelitis are contracted as a result of vaccination (see below).
40 cases of paralytic polio were reported in the UK between 1985 and 2002
The last major epidemics of poliomyelitis in the UK occurred in the 1940s and 1950s before the widespread introduction of effective vaccines. People who survived the illness were left with a variety of disabling effects. After a period of stability lasting many years some of these people have developed additional functional restrictions, which are described as late functional deterioration of polio and the post polio syndrome. The term late effects of polio are also used to describe the clinical presentations of this group.
Acute poliomyelitis and the late effects of polio will be discussed under separate headings in this guidance.
What is Acute poliomyelitis
The poliovirus is an enterovirus, that is, a virus that predominantly causes infection of the gastrointestinal tract. The virus enters through the mouth and is excreted in the faeces. It causes an illness of sudden onset (acute infection) after a short incubation period and is highly infectious. The disease spreads through oro - faecal contamination within households, and more widely in the community via contaminated water supplies if public hygiene and sanitation are inadequate.
In developing countries acute poliomyelitis usually affects children and young adults who have not been vaccinated. Acute poliomyelitis is usually a mild illness with full recovery in 95% of cases. However a minority of people experience a serious neurological illness with profound muscle weakness and paralysis – paralytic polio. The polio virus attacks the motor nerve cells (the nerves that control movement) in the brain and spinal cord. The disease can be fatal if the muscles of respiration become paralysed. Adults are more likely than children to develop paralytic polio.
Acute poliomyelitis and vaccination
Virtual eradication of poliomyelitis worldwide has been achieved by routine vaccination programmes of the whole population initiated in childhood. Two types of vaccines are used to protect against polio – the Salk vaccine and the Sabin vaccine. The Salk vaccine consists of inactivated (dead) polio viruses injected into the body causing the individual to develop immunity to any subsequent infection by the polio virus. The Sabin vaccine is given by mouth and consists of attenuated live polio viruses; these are wild polio viruses that have been modified in the laboratory and cause a mild internal infection that gives immunity against future wild infection. In some circumstances the viruses in the Sabin vaccine can revert to the wild type leading to a more serious infection including paralytic polio. This may occur in the person who has received the oral vaccine, or in contacts within their household. Household contacts, for example, another child or adult, are at risk if they are not immune, usually because they themselves have not been vaccinated as part of the routine programme. Since 2004 the oral vaccine is no longer used in the routine vaccination programme in the UK, and hence it is unlikely to be a source of potential new cases in the future.
What are Late Effects of Polio and the Post Polio Syndrome
The last polio epidemics in the UK occurred in the 1940s and 1950s affecting predominantly children and young adults. It is estimated that about 30,000 people survived these outbreaks. These people are now (2007) in their fifties or older. Between 20 – 60 % of people with residual disabilities develop new functional restrictions some years after recovery from the attack of acute poliomyelitis.
Typically the functional deterioration develops 15 –30 years after the original illness and is not directly related to the patient’s age. The cause of the deterioration has provoked debate amongst medical experts specialising in the care of people with polio, medical researchers, people with previous polio and disability groups. The debate centres on whether deterioration is mainly due to complications arising from existing neurological and orthopaedic impairments, or is due to new changes in affected muscles. People with residual disabilities are prone to develop, for example, degenerative arthritis in the joints of paralysed limbs, and also in joints of normal limbs due to overuse. However it is argued by some that the main cause of the deterioration is new wasting and weakness of affected muscles, and also in normal muscles not originally affected by the acute infection. There is some scientific evidence that motor nerve cells (motor neurones) degenerate further. The term post polio syndrome is used to describe the latter scenario, but it is also used in a more general way to describe any type of late deterioration.
Whatever the causation of the late effects of polio/post polio syndrome, it is clear that some people develop new functional restrictions that affect activities of daily living, and may compromise their capacity to manage independently. In reality the causes are likely to be multifactorial. The late effects/post polio syndrome appear to occur more often in people who contracted polio as adolescents or adults rather than in children hood, or in those who had the most severe acute illness with paralysis and respiratory difficulties.
Amended June 2008
