Picornaviruses represent a very large virus family with respect to the number of members but one of the smallest in terms of virion size and genetic complexity. They include two major groups of human pathogens: enteroviruses and rhinoviruses. Enteroviruses are transient inhabitants of the human alimentary tract and may be isolated from the throat or lower intestine. Rhinoviruses are associated with the respiratory tract and isolated chiefly from the nose and throat. Less common picornaviruses associated with human illness include hepatitis A virus, parechovirus, cardiovirus, and Aichi virus. Several genera of picornaviruses are also associated with animal, plant, and insect disease.
Many picornaviruses cause diseases in humans ranging from severe paralysis to aseptic meningitis, pleurodynia, myocarditis, vesicular and exanthematous skin lesions, mucocutaneous lesions, respiratory illnesses, undifferentiated febrile illness, conjunctivitis, and severe generalized disease of infants. However, subclinical infection is far more common than clinically manifest disease. Etiology is difficult to establish because different viruses may produce the same syndrome, the same picornavirus may cause more than a single syndrome, and some clinical symptoms cannot be distinguished from those caused by other types of viruses. The most serious disease caused by any enterovirus is poliomyelitis.
A worldwide effort is making progress toward the goal of total eradication of poliomyelitis.
PROPERTIES OF PICORNAVIRUSES
Important properties of picornaviruses are shown in Table 36-1.
TABLE 36-1Important Properties of Picornaviruses ||Download (.pdf) TABLE 36-1 Important Properties of Picornaviruses
|Virion: Icosahedral, 28–30 nm in diameter, contains 60 subunits |
|Composition: RNA (30%), protein (70%) |
|Genome: Single-stranded RNA, linear, positive sense, 7.2–8.4 kb in size, molecular weight 2.5 million, infectious, contains genome-linked protein (VPg) |
|Proteins: Four major polypeptides cleaved from a large precursor polyprotein. Surface capsid proteins VP1 and VP3 are major antibody-binding sites. VP4 is an internal protein. |
|Envelope: None |
|Replication: Cytoplasm |
|Outstanding characteristic: Family is made up of many enterovirus and rhinovirus types that infect humans and lower animals, causing various illnesses ranging from poliomyelitis to aseptic meningitis to the common cold. |
Structure and Composition
The virion of enteroviruses and rhinoviruses consists of a capsid shell of 60 subunits, each of four proteins (VP1–VP4) arranged with icosahedral symmetry around a genome made up of a single strand of positive-sense RNA (Figure 36-1). Parechoviruses are similar except that their capsids contain only three proteins because VP0 does not get cleaved into VP2 and VP4.
Structure of a typical picornavirus. A: Exploded diagram showing internal location of the RNA genome surrounded by capsid composed of pentamers of proteins VP1, VP2, VP3, and VP4. Note the “canyon” depression surrounding the vertex of the pentamer. B: Binding of cellular receptor to the floor of the canyon. The major rhinovirus receptor (intercellular adhesion molecule-1 [ICAM-1]) has a diameter roughly half that ...