Student presentation on
by Pamela Schell
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 neuronal cell destruction is seen in a small number of infected individuals.
Polio virus affects humans by a lytic cycle. PV1 which is usually
associated with epidemics causes paralysis and consequently the most deaths.
PV2 normally causes meningitis and a less severe paralysis. PV3 is usually associated
with sporadic cases of polio virus. The majority of polio cases include only
diarrhea symptoms or the individual is completely asymptomatic. Five percent
of polio cases show flu-like symptoms of fever, malaise, headache, nausea, sore
throat, upset stomach, and achy muscles. In one percent of individuals, where
polio virus is able to enter the gray matter of the central nervous system,
it causes paralysis. Incubation time is 3-5 days for minor symptoms and 1-2
weeks in the case of paralysis. From the time of ingestion, onset of symptoms
may be 3-35 days. An individual may still be secreting polio virus in their
feces weeks to months after symptoms have stopped.
The Life Cycle of Polio Virus
There are two types of polio vaccinations. The Salk Inactivated
Polio Vaccine (IPV) is an injection of killed virus which enables the immune
system to identify polio virus and eradicate it. The patient may still be a
carrier of polio. The disadvantage of this vaccination is its ability to mutate.
The Sabin Oral Polio Vaccine (OPV) is an oral form of all three types of live
polio virus. This causes the virus to replicate in the intestine and induces
life-long immunity but is not virulent enough to causes paralysis. The disadvantage
of this vaccination is its incapability to provide gut immunity. The CDC recommends
that children use a combination of the two vaccines. Other countries use only
OPV, but total eradication of polio is very realistic for the near future. Most
recently, the CDC has given information on the polio vaccinations contaminated
with SV40 and its link to cancer.
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Schaechter, M., Engleberg, N., Eisenstein, B., Medoff, G. (Eds). 1999. Mechanisms of
Microbial Diseases. Baltimore: Lippincott Williams & Wilkins.
© 2010, J.Graf. Site made by Pamela Schell, for comments please contact Joerg.Graf@uconn.edu