Derrick Garwood, Freelance Medical Writer and Editor

In 2005, when there have been no outbreaks of poliomyelitis in the Western hemisphere for over a decade, it is difficult to appreciate the stark terror that accompanied epidemics in the 1950s. Of the known cases, one third developed paralysis and around 5% died. These recollections of a patient from the pre-vaccine era (just click on Listen) convey the helplessness of both victims and doctors, and the human cost is illustrated in this photograph of a child in an iron lung. For younger readers, the iron lung (or negative pressure ventilator) was essentially a metal cylinder whose internal pressure was varied so that a patient whose respiratory muscles were paralysed could breathe, and some polio victims remained in them for many years. More information on them is available here and the ward photograph gives an idea of the large numbers that were required.

This is an electron micrograph of the causative agent, an RNA virus of the Picornaviridae family. The University of Texas Microbook covers the whole family in some detail and makes some useful points about the poliovirus, which attaches itself to a specific receptor on epithelial cells lining the alimentary tract and cells of the central nervous system. In geographical areas with poor hygiene and sanitation, non-immunised individuals are frequently infected, but many cases do not give rise to symptoms and only about 1% progress to the paralytic form of the disease. In these areas, most infants acquire active immunity while still protected by maternal antibodies. However, when living conditions improve children may escape early contact and are therefore susceptible if an outbreak occurs.

The three types of poliovirus are Type 1 (Brunhilde), Type 2 (Lansing) and Type 3 (Leon). Type 1 is the most common form and the one most closely associated with severe symptoms and paralysis. Unfortunately, immunity to one type of virus does not confer immunity to the other two types.

For a basic overall account of the disease, try this page from the University of Utah, or go to emedicine for a clinical perspective. Transmission, by the faecal-oral route or by ingestion of contaminated water, is followed by an incubation period of 5 – 35 days. Viral particles initially replicate in the nasopharynx and gastrointestinal tract, and then invade lymphoid tissue, with subsequent spread via the bloodstream. The infection can manifest in 4 different forms: inapparent infection, abortive disease, nonparalytic poliomyelitis, and paralytic disease. This last form is characterised by death of motor neurones and asymmetric loss of muscle function, as well as by muscular atrophy (as in this photograph) which becomes apparent several weeks after the onset of symptoms. The degenerative changes which occur in the anterior horn of the spinal cord are shown in this photomicrograph.

Two vaccines are available to protect against all three types of poliovirus. The Salk vaccine, which was licensed in 1955, consists of a mixture of the three serotypes grown in monkey kidney cell cultures and inactivated by 37% formaldehyde. It is administered by subcutaneous injection and almost 100% of recipients are immune after 3 doses. An oral vaccine was developed by Dr Albert Sabin in 1961 which contains attenuated (weakened) live virus. The advantage of the Salk vaccine is that it carries no risk of vaccine-associated polio paralysis, but it confers little immunity in the gut, so the virus can still multiply in the intestines and be shed with the faeces. Thus it protects the individual but cannot help to contain an outbreak. It is also much more expensive than the oral vaccine and trained healthcare workers are needed to administer it. The Sabin vaccine can be administered by volunteers, and in areas of poor sanitation the shedding of live attenuated virus by recently vaccinated children can 'passively immunise' others in the community. However, in rare circumstances (approximately 1 in 3 million doses) the oral vaccine can cause paralysis in the recipient or a close contact. This extremely low risk is generally accepted because without this vaccine thousands of children would become paralysed each year.

As the only host for the poliovirus is man (there is no animal reservoir) it was a prime candidate for a global eradication programme, which has become a remarkable public health success story. Between the beginning of the World Health Organisation initiative in 1988 and 2003, the number of cases world-wide dropped by over 99%, from more than 350,000 to 1,919. You can read the latest monthly situation reports covering all aspects of the initiative here; for example, Nigeria is currently posing the greatest risk to eradication. A global case count for 2005 up to the current week, and the location of the cases, is also published.

To date, 5 million people who would otherwise have been paralysed are fully mobile because they were immunised as part of this programme. One tactic has been to hold National Immunisation Days (NIDs); these have been responsible for immunising more than 2 billion children. To discover just what they involve, look at the staggering statistics and video/audio diary covering an NID in India......truly remarkable.

Derrick Garwood, a Freelance Medical Writer and Editor submitted this tour.

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