Chapter 11 - Family Picornaviridae

Abstract

Members of the family Picornaviridae are small (pico) RNA viruses. Virions are unenveloped with T=3 icosahedral symmetry (20–30nm diameter). The family Picornaviridae is one of five named families in the order Picornavirales, a large group of unenveloped positive-strand RNA viruses of plants, insects, and animals. Picornaviral genomes are a single molecule of positive-sense, RNA ranging in size from 7500 to 9000 nt. The 5′ termini of picornaviral genomes are covalently linked to a small protein called VPg. Picornaviral genomes have relatively long (600–1200 nt) 5′ noncoding regions. These contain highly base-paired stem-loop structures that function as internal ribosome entry sites (IRESs) by serving as a scaffold for the assembly of ribosomes. The presence of an IRES to direct ribosome assembly means that picornaviral mRNAs can be translated even though they lack a 5′-methyl-G cap. Picornaviral genomes have a single open reading frame encoding a polyprotein of ~2300 amino acids. The protein expression strategy of the picornaviruses is to generate all protein products from a single polyprotein precursor, by stepwise proteolytic cleavages. To date there are 26 names Genera in the family Picornaviridae. Of importance to human health are several members of the genus Enterovirus. Primary sites of replication of members of the genus Enterovirus are epithelial cells and lymphoid tissues of upper respiratory and gastrointestinal tracts. In the upper respiratory tract enterovirus infections can cause the common cold. In the gastrointestinal (GI) tract enterovirus infections are very often asymptomatic. However some enteroviruses can escape the GI tract to infect other organs. For example, paralytic polio is caused by one of three serotypes of poliovirus, an enterovirus that can occasionally cross the blood–brain barrier. Hepatitis A virus (genus Hepatovirus) is another human picornavirus transmitted via the fecal-oral route. It replicates in the liver, causing acute hepatitis. Foot and mouth disease virus (FMDV, genus Apthovirus) infects hoof-stock such as cattle, sheep, pigs, and horses. FMDV is very stable in the environment and is easily transmitted over long distances. In FMDV-free regions of the world occasional incursions and outbreaks cause major disruptions of agriculture and huge economic losses.