5. Human poliovirus causes Poliomyelitis (Polio)
Poliovirus is an Enterovirus, separable into three serotypes, that is the causative agent of
poliomyelitis. Poliomyelitis is an acute and highly infectious disease involving invasion of the
gastro-intestinal tract by one of the three serotypes. In a minority of patients it invades the
nervous system, and can cause total paralysis in a matter of hours. The incubation period is from
3-21 days. Signs and symptoms - include fever, fatigue, headache, vomiting, stiffness in the neck
6. and pain in the limbs. One in 200 infections leads to irreversible paralysis (usually in the legs).
Amongst those paralysed, 5%-10% die when their breathing muscles become immobilised.
A polio virion is composed of a single-stranded positive-sense RNA genome of approximately
7,450 nucleotides enclosed in an icosahedral particle 27nm in diameter. The RNA is the
infectious agent for poliomyelitis, while the viral capsid serves as a protective coat for the RNA.
Polioviruses occur as three distinct serotypes, which differ structurally primarily in the loop
regions of their viral coat proteins.
The only natural host of Poliovirus is humans. Poliovirus can survive in the body, and in raw
sewage or freshwater systems; polio is frequently found in areas where raw sewage directly
enters a water source without treatment. Transmission of the virus occurs either by direct person
to person contact, or by indirect contact with infectious saliva or faeces, or with contaminated
sewage or water.
Poliomyelitis, was first described in 1789, but not fully recognized until 1840. Reports on
clinical and epidemiological aspects of poliomyelitis came in the late 19th and early 20th
centuries, but the basic study of Polioviruses only began in 1908 when the disease was
successfully transmitted to monkeys.
The incidence of poliomyelitis declined radically in the United States when a mass immunisation
program with the Salk vaccine was begun in 1955. By 1961 the Sabin vaccine, a preparation
made from living organisms and taken orally, was released for use. Since then the disease has
been virtually eliminated in the Americas, Europe, and Australasia, but vaccination programs
continue because of polio's existence in other parts of the world (mainly South Asia, Egypt, and
West and central Africa) and the ease of travel.
In 1988 the World Health Organization began a global vaccination campaign to eradicate the
disease—which continued to paralyse hundreds of thousands of children each year—by 2000.
Although the date of eradication was later pushed back to 2005, by 2003 there were less than a
thousand new cases of polio worldwide.
Researchers are working on turning the Poliovirus into a possible treatment for brain tumours.
Poliovirus together with a harmless genetic coding element from the common cold has resulted
in a modified virus with a remarkably strong anti-cancer agent that rapidly killed cancer cells in
laboratory cell cultures and in animals, research is continuing.
