How the Poliovirus Works
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 within hours, makes thousands of copies of itself. The virus kills the cell and then spreads to infect other cells.
- 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.
- For every 200 or so virus particles that encounter a susceptible cell, only one will successfully enter and replicate.
- In tissue culture, poliovirus enters cells and replicates in six to eight hours, yielding 10,000 to 100,000 virus particles per cell.
- 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.
- 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.
- A person who gets polio is immune to future infection from the virus type that caused the polio.
Poliovirus Capsid Model and Scientific Art
Scientists use many types of models to visualize concepts about the real world. 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 that given time.
A bronze model of the poliovirus was made by for the "Whatever Happened to Polio" 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.