On the basis of neutralization test of polioviruses, it can be divided into three serotypes.
Type 1: it is the common epidemic type
Type 2: it is usually associated with endemic infections.
Type 3: it is occasionally causes epidemics.
One of the main method of differentiation is antigenic property of polioviruses, as all three type have same chemical properties, physical properties, and resistance of the three types are all identical.
This virus have affinity for nervous tissue and narrow host range. Only man and some primates like cynomologous and rhesus monkeys are susceptible. These rhesus monkeys can be infected by oral route and develop paralysis.
Natural infection of polioviruses occur only in human. The virus is spread from man to man by faecal – oral route. The virus can also be spread by pharyngeal secretions (droplet infection) during first week of illness. No intermediate host is known.
On entering the body of a new host the virus multiplies in then tonsils and peyer’s patches of the ileum. Spread to regional lymph nodes (cervical and mesenteric) leads to a viraemia, enabling the virus to become disseminated throughout the body including cord and brain.
- Neural spread may occur in children with inapparent infection at the time of tonsillectomy. Poliovirus present in the oropharynx may enter nerve fibers exposed during surgery and spread to brain resulting in bulbar paralysis.
- A similar mechanism of viral spread via neural pathways may be responsible for paralysis of a limb recently injected with inflammatory injections such as diphtheria – pertussis – tetanus (DPT) vaccine.
- Pregnancy increases the incidence of paralysis.
- Muscular activity during the paralytic phase of the illness may lead to paralysis of the limbs used.
WHO work world wide for the eradication of poliomyletis. They set a target date of year 2000 to completely eradicate polio virus from human dwellings. Although overall the number of cases has fallen more than 95 %. 30 countries of south asia and west and central africa has recent reports of the cases. A new dead line has set to be 2005. The eradication has been attempted with annual national immunization days to ensure each child receives an adequate number of doses of oral poliovaccine
Two effective vaccines are available: (1) Inactivated polio vaccine (Salk) and, (2) Live attenuated oral polio vaccine (Sabin).
Reference: The Text Book Of Microbiology