Facts about polio

The virus invades the nervous system, and the onset of paralysis can occur in a matter of hours. While polio can strike a person at any age, over fifty percent of the cases occur in children between the ages of three and five. The incubation period of polio, from the time of first exposure to first symptoms, ranges from three to thirty five days, thus Polio can spread widely before a polio outbreak is apparent. Most people infected with the poliovirus have no symptoms or outward signs of the illness and are thus never aware they have been infected. After initial infection with poliovirus, virus particles are excreted in the faeces for several weeks and are highly transmissible to others in a community. In all forms of polio, the early symptoms of infection are fatigue, fever, vomiting, headache and pain in the neck and extremities. Around 1 percent of unimmunized people develop paralytic complications, in some cases bulbar paralysis. Initial infection is of cells in the lining of the small intestine. Immunization interrupts the process here. After multiplication there, viraemia occurs, with distribution of virus to other parts of the body. Flu-like symptoms are typical of viraemia from any cause. The virus has an affinity for the cell bodies of motor neurones, which carry commands to the muscles. Non-paralytic polio may result in fever, vomiting, abdominal pain, lethargy, and irritability, and some muscles tender to the touch. In some cases there may be no significant symptoms whatsoever. The virus affects the anterior horn cells in the spinal column which control movement of the trunk and limb muscles including the intercostal muscles. An affected limb becomes floppy and poorly controlled – the condition of acute flaccid paralysis (AFP). This presentation can lead to permanent paralysis of the body yet it only occurs in around 1 percent of cases. The degree of paralysis is proportional to the extent of infection of the motor nuclei, which is likely to be proportional to the degree of viraemia, and inversely proportional to the degree of immunity. Extensive paralysis of the trunk and muscles of the thorax and abdomen (quadriplegia) may occur. If it affects the upper part of the cervical spinal cord then diaphragm paralysis requires ventilator support. Without respiratory support, polio affecting respiration is likely to result in death from failure of breathing, or aspiration of secretions and resulting pneumonia. Young children who contract polio may sometimes suffer only mild symptoms, and as a result they may become permanently immune to the disease. Hence inhabitants of areas with better sanitation may actually be more susceptible to polio because fewer people have the disease as young children. People who have survived polio sometimes develop additional symptoms, notably muscle weakness and extreme fatigue, decades later; these symptoms are called post-polio syndrome. Since it’s possible to have a polio infection without having significant paralysis, many people who are unaware they ever had polio may now be suffering from post-polio syndrome. The first effective polio vaccine was developed by Jonas Salk at the University of Pittsburgh, although it was the oral vaccine developed by Albert Sabin eight years later that was used for modern mass immunization. The Salk vaccine is based on formalin-inactivated poliovirus. The Sabin vaccine is a live-attenuated vaccine, produced by the passage of the virus through non-human cells at a sub-physiological temperature.

June 2006
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