The polio virus which causes poliomyelitis in humans is an enterovirus which belongs to the picornavirus (small, RNA) family. Polio virus is rapid, acid-resistant, stable, highly tissue specific and consists of a single-stranded, positive RNA. Polio virus is able to reside in the throat or intestinal tract of humans. Poliomyelitis is a highly contagious infectious disease which has three strains, poliovirus 1 (PV1), PV2 and PV3. Polio virus, although rare in developed countries, can be found in many under-developed countries due to the uncommonness of vaccinations there. Polio is known as a disease of development. The oldest known record of polio is in an Egyptian stone engraving of a young priest from 1350 B.C. with a withered leg, characteristic of a polio survivor. Loeffler and Frosch were the first individuals to see polio in 1898. The largest US epidemic was in 1916 in New York City.
Encounter and Entry
The polio virus affects humans by the fecal-oral route. A given individual ingests water or food contaminated with polio virus, the virus infects the individual, the individual passes the virus in their feces, the virus is in the sewage which enters a watershed where another individual ingests the water and consequently the virus. Polio virus can also spread by person-to-person contact, especially in young children. After entering the host, polio virus travels down the digestive tract to the small intestine where it replicates itself in the B-cells and T-cells of the gut mucosa lining the intestine.
Spread and Replication
Poliovirus binds to a specific cell surface protein, polio virus receptor (PVR). This protein is an immunoglobin which contains three loops, Ig domains. Polio binds at loop one. After binding, a conformational change, or alteration, in the virus capsid occurs; this is thought to prepare the virus for uncoating (extracellularly). Receptor-mediated endocytosis is thought to take the receptor into the cell. Polio virus is tissue tropic, meaning it replicates only in specific tissue types, generally lymphoid tissue in the pharynx and intestine. After uncoating, polio virus, which is an RNA virus, takes a single RNA molecule in its protective capsid. This RNA can be converted directly to a protein in the cytoplasm. The virus must then replicate its RNA using viral RNA-directed RNA polymerase. After replication of its own RNA, the virus must package the new RNA into capsids in order to infect more cells.
After replication in the mouth and intestine, polio virus spreads through the body via the blood. Polio virus is contained in the Peyer’s patches of the small intestine. Transmission to the central nervous system and...