|Genome: RNA Virus, Positive ssRNA Virus||Structure: Nonenveloped Virus, Icosahedral Virus|
- Inoculation of Polio Virus is by ingestion and the virus is shed in salivary secretions and fecally following infection. Therefore, transmission is likely through oral exposure to salivary secretions or fecal-oral transmission]]. The declining incidence of the disease in the US has been due to vaccination efforts as well as improved sanitation.
- Following inoculation, polio virus initially replicates in the GI mucosa and then continues replicating in the associated MALT and Peyer's Patches. The virus then disseminates hematogenously and probably infects the CNS through viremia. Replication within presynaptic as well as postsynaptic motor neurons destroys these cells and is responsible for paralytic disease.
- In the vast majority of patients, infection with polio virus is either subclinical or results in a mild flu-like syndrome primarily manifesting with constitutional symptoms. A small minority of individuals develop aseptic meningitis described below and only a fraction develop paralytic disease. The probability of developing paralytic disease increases with age which may explain the gradually increased incidence of paralytic disease as sanitation improved and the age of primary infection rose.
- Aseptic Meningitis
- This syndrome, also termed "Non-paralytic Poliomyelitis" occurs in a fraction in infected patients. It is characterized by the traditional syndrome of a viral meningitis, including meningismus as well a lymphoctyic CSF pleocytosis. Recovery is usually complete within 1-2weeks, except in a small fraction of patients which progress to paralytic disease.
- Paralytic Poliomyelitis
- Following the aseptic meningitis syndrome some patients develop a muscle weakness and then a flaccid paralysis. In some cases, derangement of motor neuron coordination may result in fasciculations and muscle spasms. Some patients slowly recover motor control over months but recovery may never be complete.
- Both attenuated and formalin-inactivated vaccines to polio virus have been used effetively. Polio has been eliminated from the western hemisphere but cases continue to be reported in Asia despite efforts by the World Health Organization to eradicate the disease.