Poliomyelitis Polio

Organism Poliovirus (Enterovirus) types 1, 2 and 3.

Clinical features Infection commences with fever, general malaise and headache, the majority of cases resolving after these mild symptoms, but approximately 1% proceed to paralytic disease. The virus has a predilection for nerve cells, especially those with a motor function (the anterior horn cells of the spinal cord and the motor nuclei of the cranial nerves). These cells are destroyed and a flaccid paralysis results.

As a generalization, paralysis is more common in the lower part of the body, becoming less common the higher up it affects. Unilateral lameness is more common than bilateral lameness. The severe form of bulbar poliomyelitis is generally fatal in poor countries where respirators and intensive nursing care are not available. Site of paralysis is associated with injections or operations and such procedures should be avoided if there is any suggestion of poliomyelitis.

Diagnosis of the disabled case is made on clinical grounds, differentiating from the spastic paralysis of birth injury with which it is commonly confused. In polio, there will be a history of normal birth with commencement of walking, followed by a feverish illness and the development of flaccid paralysis. The paralysis is limited to well-demarcated muscle groups and there is no sensory loss. A similar history may be given for meningitis, but the damage will be central with accompanying mental deficiency.

Transmission is generally via the faecal-oral route, although the virus initially multiplies in the oro-pharynx; hence airborne transmission can also occur. The virus then invades the gastro-intestinal tract, where it is excreted for several weeks.

A disease of low hygiene, young children (4-5 months) meet the virus with only a small proportion showing overt disease. Of these, 80-90% have an inapparent subclinical disease, 5-10% suffer from fever, headache and minor clinical signs, with only 1% going on to paralysis. Paralysis is more common with older age. Therefore, a non-immune person going into an endemic environment is at far greater danger of developing paralytic poliomyelitis. Raising standards of hygiene will also have the same effect because it spares people from meeting the virus as young children and allows a pool of susceptibles to develop. In the course of time, the number of nonimmunes will be sufficient for an epidemic to take place. There will also be a higher proportion of paralysed cases (peak age 5-9 years) and many deaths. Sadly, the raising of living standards will change polio from an endemic disease with a few paralysed cases to an epidemic disease of increased severity. In epidemic poliomyelitis where sanitation is good, pharyngeal spread becomes a more important method of transmission.

Poliovirus strains vary in their neurovirulence with the more virulent strains having a greater tendency to spread. This could be due to a lower infective dose of the virulent virus being required to produce disease.

Incubation period is from 5 to 30 days with a mean of 10 days.

Period of communicability From 2 days after exposure up until 6 weeks.

Occurrence and distribution Poliomyelitis formerly occurred throughout the world, endemic in the poorer regions and epidemic in those with good sanitation, but this has changed considerably with the WHO programme of eradicating polio from the world. The Americas, Europe and Western Pacific are now free of infection, while there are very few cases remaining in the rest of the world. The end of 2005 has been set as the target date for global eradication of polio.

Control and prevention The main method of prevention and control is with polio vaccine. Two types of vaccine are available, the killed (Salk) and the attenuated living (Sabin). The Salk vaccine is given by intramuscular injection, inducing a high level of immunity not antagonized by inhibitory factors in the gut, but is expensive to produce because it contains many organisms. The Sabin vaccine is administered orally making it easier and cheaper as well as producing intestinal immunity, which can block infection with wild strains of poliovirus. Multiplication of the virus in the intestine makes it very useful in preventing epidemics and allows it to spread to non-vaccinated persons in conditions of poor hygiene, thereby protecting them as well. Unfortunately, the inhibiting action of antibodies in breast milk and colonization of the gut by other entero viruses can reduce its effectiveness. Increasing the dosage and telling mothers not to breast-feed for at least an hour after administration can help.

Because there are three strains of the poliovirus, the vaccine should be given on three separate occasions, separated by periods of at least 1 month to ensure that immunity develops to each of the strains. Polio vaccine is conveniently administered at the same time as DTP. A preliminary dose can be given soon after birth in areas where wild poliovirus is circulating.

There is a slight risk of a live attenuated virus becoming more virulent, so it is preferable to vaccinate the majority of the population all at one time. Also, in a situation of raising sanitary standards, epidemic poliomyelitis will only be prevented if there are sufficient people immunized to produce 'herd immunity'. For these reasons, mass campaigns can be effective. These should always be followed up by static clinics vaccinating newborns and missed persons. The WHO, in its bid to eradicate polio from the world, recommends National Immunization Days (NID) on which all children under the age of 5 years are vaccinated, irrespective of previous immunization status. Two doses are given at a month's interval followed by mop-up operations in areas of low coverage or where continuing transmission has been identified.

School children and adults, who have received a full course of childhood vaccinations, should have booster doses every 10 years. Maintenance of vaccination coverage should continue even in countries now free of infection and is essential for travel to parts of the world where polio has not yet been eradicated.

The long-term aim of prevention should be to raise standards of hygiene with the provision of water supplies and sanitation, but as mentioned above, this must proceed at the same time as an adequate vaccination programme.

Treatment There is no specific treatment for the acute stage, but rest and the avoidance of physical activity are beneficial. Specific supportive measures can be given to those with disabilities.

Surveillance developed for poliomyelitis eradication looks for cases of acute flaccid paralysis (AFP) in children under 15 years of age. These are investigated by stool examination, inquiry and search for other cases in the area. Remedial measures are carried out around the case, vaccinating all contacts.

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