Poliomyelitis (polio) is a highly contagious disease caused by the human enterovirus of the Picornaviridae family. There are three serotypes (types 1,2 or 3) of Polioviruses responsible for this paralytic disease.
In the prevaccination era almost every child used to be infected by this nasty virus. It used to be a matter of concern for every parent in the world that their child might become paralytic. Progression of Polio infection to disease typically occurred in one out of 200 children. Thanks to the Pulse Polio Immunization Program (1995), along with the Universal Immunization Program ran in India on the guidelines of WHO Polio Eradication Initiative (1988), India has been declared polio free country on 27 march 2014 with no case of Polio since last 3 years.
Symptoms of Poliomyelitis
Poliovirus infection is usually without symptoms, with viral replication limited to the alimentary tract. In some ~24% cases, symptoms of the disease appear after an incubation period of 7-10 days (or 4-35 days). A person can have mild sickness with fever, fatigue, nausea, headache, flu like symptoms, stiffness in the neck or back and pain in limbs which resolve completely on its own. Paralytic Poliomyelitis develops in less than 1 % infections when the virus invades the central nervous system and replicates in the motor neurons of the spinal cord. Temporary or permanent paralysis of the affected muscles may occur depending on the extent of degradation of the motor neurons. Rarely, viral destruction of respiratory muscles can cause respiratory paralysis or even death due to complete respiratory arrest, especially in adolescents and adults. The lower limbs are mainly affected by polio and the paralysis is often permanent.
Innate or natural immunity
Infected individuals with a strong immune system develop humoral (due to circulating antibody) and mucosal (secretory immunoglobulin) immunity. A person is considered immune when neutralizing antibodies against Polioviruses are present in blood. Natural immunity gained against 1 serotype does not provide protection against the other 2 serotypes. Mucosal immunity of intestines decreases the replication and excretion (shedding) of the virus, and thus acts as a potential barrier to viral transmission. Persons with B-cell related immune defects are at increased risk of suffering from paralysis caused by Polioviruses.
How does the Polio virus spread?
Poliovirus spread mainly by faecal-to-oral route in places where sanitation facilities are poor. It can also be transmitted via oral-to-oral mode in the developed countries equipped with satisfactory sanitation facilities.