Ann Arbor. The world learned today that its hopes for finding an effective weapon against paralytic polio had been realized. —The New York Times, April 12, 1955
Enteroviruses constitute a major subgroup of small, naked capsid RNA viruses belonging to the family Picornaviridae (picornaviruses). Enteroviruses are transmitted by the fecal–oral route and readily infect the intestinal tract and further spread to cause paralytic disease, mild aseptic meningitis, exanthems, myocarditis, pericarditis, and nonspecific febrile illness. The enteroviruses of humans and animals are ubiquitous and have been found worldwide. Their name is derived from their ability to infect intestinal tract epithelial and lymphoid tissues and shed into the feces, but do not commonly cause gastrointestinal diseases. These viruses include the polioviruses, coxsackieviruses, echoviruses, parechoviruses, and other agents that are simply designated as enteroviruses. There is another member of the picornavirus family called rhinoviruses that are not enteroviruses, because they are transmitted through respiratory route and cause common colds (see Chapter 9). Over the years, renumbering and reclassification within the subgroups have occurred, primarily as a result of advanced sequencing analyses.
These viruses, which have many characteristics in common, are first considered as a group. Some of the special features of important serotypes are discussed in more detail later in this chapter.
ENTEROVIRUSES: GROUP CHARACTERISTICS
MORPHOLOGY AND BIOLOGIC FEATURES
As a group, the enteroviruses are picornaviruses that are extremely small (22-30 nm in diameter), naked capsid virions with icosahedral symmetry. They possess single-stranded, positive-sense RNA with a covalently bound small virus-encoded protein (VPg) and a capsid formed from 60 copies of four nonglycosylated proteins (VP1, VP2, VP3, and VP4). The basic building block of the capsid is the protomer containing one copy each of VP1, VP2, VP3, and VP4. Five protomers (pentamers) are placed at each of the 12 vertices of the icosahedron to form a capsomere of 60 protomers. The shell is composed of VP1, VP2, and VP3, whereas VP4 is attached on the inner surface. On the surface of the virus, there is a deep depression or canyon around each pentameric vertex. The receptor binding site is located at the floor of the canyon. The virion structure of a picornavirus member is shown in Chapter 13 (Figure 13–1). Replication and assembly occurs exclusively in the cellular cytoplasm; one infectious cycle can occur within 6 to 7 hours. This results in cessation of host cell protein synthesis and cell lysis with release of new infectious progeny. The replication cycle is shown in Figure 12–1. Picornaviruses enter the host cell via receptor-mediated endocytosis (viropexis) following interaction of a viral surface protein with a specific receptor on the host cell. Picornaviruses use a wide variety of host cell receptors, including PVR or CD155 (poliovirus), CD55 (coxsackieviruses, echoviruses, enterovirus 70), and ICM1 (some coxsackieviruses A). After the removal of capsid protein, ...