Poliomyelitis is a viral illness. It leads to infection of the nerves and sometimes the spinal cord and the brain leading to partial or complete paralysis.

Poliomyelitis is a disease caused by infection with the poliovirus. This virus spreads via person to person contact, via contact with infected mucus from the nose or mouth of the infected child and on contact with infected faeces usually through contaminated water. (1-4)

How does the virus cause polio?

The infection occurs through the gastro-intestinal route. On contact the virus enters through the mouth or nose and goes on to multiply in the throat and intestines.

From there it is taken up by blood vessels and lymphatic channels.

The time from being infected with the virus to developing symptoms (termed incubation period) ranges from 5 - 35 days (average 7 - 14 days).

In about 95% cases the infection is mild and harmless. However in 1% or less number of individuals there may be paralysis of limbs or other muscles, brain affliction or meningitis and encephalitis.

This happens when the virus invades the nerves of the spinal cord (especially a part of the spinal cord called the anterior horn) and lower part of the brain called the brain stem.

The brain stem deals with controlling the respiration. Damage to this area may lead to paralysis of muscles of respiration and even death. Damage to the spinal cord may lead to permanent muscle weakness and disability. (1-4)

Who is at risk of polio?

Children who have never been vaccinated are at a higher risk of getting polio.

Additionally, children, pregnant women and elderly who have never been exposed to the infection and never been vaccinated are at a higher risk of getting the infection if they visit an area where there is a polio outbreak. The disease is more common in the summer and fall. (1-4)

Types of polio

Types of polio virus include (2) –

• Wild virus – This is the naturally present polio virus. This is the culprit virus that causes paralytic polio in countries where the infection is widespread. It has three sub varieties. These are termed P1, P2 and P3.
• Vaccine-induced paralytic polio (VAPP) – The polio vaccine is made by live attenuated viruses. In other words weakened but living polio viruses are used in the vaccine.

When administered the body learns to recognise the virus and prepares its immune defences against the real active virus when it comes in contact with the individual.

Rarely the vaccine virus in the oral polio vaccine (OPV) can cause paralysis - either in the vaccinated child, or in a close contact. This occurs in about 1 in 2.5 million doses of the vaccine.

• Vaccine-derived polioviruses (VDPVs) – Very rarely a strain of virus in the OPV may genetically change to become a more virulent form and cause paralysis. This is called vaccine-derived polioviruses (VDPVs).

The polio virus

The polio virus belongs to the enterovirus subgroup within family Picornaviridae. Enteroviruses are those that are transient inhabitants of the gastrointestinal tract. They can sustain at acid pH.

These are small viruses that have RNA as their genetic material. The polio virus may be destroyed by heat, chemicals like formaldehyde and chlorine and by ultraviolet light. (3)