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VOL. 71 (2), 321-339, 2005 HISTORY AND FUTURE OF POLIOVACCINATION...

the affected muscles no longer function. The muscles of the legs are
affected more than the arm muscles. The limb becomes floppy and
lifeless – a condition known as acute flaccid paralysis. More extensive
paralysis, involving muscles of the thorax and abdomen, can result
in quadriplegia. In the most severe cases, poliovirus attacks the
motor neurons of the brain stem – reducing breathing capacity and
causing difficulty in swallowing and speaking. Without respiratory
support, polio can result in death (1, 2).

POLIOVIRUS

Poliovirus causing poliomyelitis is the type species of the
Picornaviridae, a close-knit group of viruses of man and other
vertebrates, with small (30nm), unenveloped, spherical virions. The
genome consists of one molecule of single-stranded RNA. The RNA
is of a positive polarity, meaning that is ready to act as a messenger
RNA. The hallmarks of the picornaviruses are the single translation
unit, which occupies up to 90% of their RNA and the resulting
synthesis of a single, giant polypeptide which has to undergo a
cascade of postsynthetic cleavages to functional proteins.

FIGURE 1. The poliovirus RNA, translation and postsynthetic cleavage (from
Rueckert, 1985, with permission).

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