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The Virus and Vaccine, How the Polio Virus Works

Quote. To initiate infection, the virus must first attach to its cell-surface receptor and then deliver its RNA to the appropriate cellular compartment. End Quote. -David Belnap, biochemist, 2000

Poliomyelitis is a viral disease. There are three types of poliovirus and many strains of each type. The virus enters through the mouth and multiplies in the throat and gastrointestinal tract, then moves into the bloodstream and is carried to the central nervous system where it replicates and destroys the motor neuron cells. Motor neurons control the muscles for swallowing, circulation, respiration, and the trunk, arms, and legs.

Human nerve cells have a protruding protein structure on their surface whose precise function is unknown. When poliovirus encounters the nerve cells, the protruding receptors attach to the virus particle, and infection begins. Once inside the cell, the virus hijacks the cell’s assembly process, and makes thousands of copies of itself in hours. The virus kills the cell and then spreads to infect other cells.

Factoids
bullet Many types of human cells have receptors that fit the poliovirus; no one knows why the virus favors motor neurons over other cells for replication.
bullet For every 200 or so virus particles that encounter a susceptible cell, only one will successfully enter and replicate.
bullet In tissue culture, poliovirus enters cells and replicates in six to eight hours, yielding 10,000 to 100,000 virus particles per cell.
bullet One way the human immune system protects itself is by producing antibodies that engage the protein covering of the poliovirus, preventing the virus from interacting with another cell.
bullet There are three types of poliovirus: 1, 2, and 3. Type 1 is the most virulent and common. Both the Salk and Sabin vaccines are “trivalent” that is, active against all three virus types. Type 2 poliovirus has not been detected anywhere in the world since 1999.
bullet A person who gets polio is immune to future infection from the virus type that caused the polio.
Illustration of the poliovirus attached to neuron receptors
Poliovirus bound to a neuron receptor Illustration courtesy of Link Studio
Photo of poliovirus bronze models next to microscopic images
Scientifically accurate bronze models (without patina) of the poliovirus created for the Smithsonian by Edgar Meyer, 2005.

These models are an adaptation of James Hogle’s image of the poliovirus and were specially cast in bronze for the exhibit. They are the first three dimensioanl representations of the poliovirus

Poliovirus Capsid Model and Scientific Art
Scientists use many types of models to visualize concepts about the real world. Environmentalists and climatologists make computer graphics models of the entire earth. In 1953, James Watson and Francis Crick used a brass model of DNA's structure—the so-called “double helix”—as a physical analog of genes. All models are partly right and partly wrong because they represent only a level of knowledge at a given time. This bronze model of the poliovirus was made by for the exhibition by biochemist/artist Edgar Meyer, based on the first three-dimensional images of poliovirus that virologist James Hogle at Harvard obtained from X-ray crystallography in 2000. While the model represents the surface relief at a very high resolution, the shell (capsid) in nature is more complex than artwork or X-ray crystallography can show.

A Vaccine to Prevent Polio
Scientists could make vaccines even before they completely understood how they functioned. Eventually researchers learned that vaccines work by fooling the body’s immune system into producing antibodies even though there is no disease. Jonas Salk and Albert Sabin used this knowledge to create two different kinds of polio vaccines. 

Life Cycle of the Poliovirus Animation (popup window)
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Illustration of the life cyle of the poliovirus
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Life cycle of the poliovirus
Illustration courtesy NMAH

Photo of James Hogle
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James Hogle in his Harvard Medical School lab, 2000

Smithsonian National Museum of American History main site
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