Picornaviruses are small (20-30nm) and non-enveloped and contain a single-stranded RNA genome (MW 2-3 X 10^6). The nucleocapsid has cubic symmetry and contains 32 spherical subunits (capsomeres). Virus maturation takes place in the cytoplasm. Enteroviruses and rhinoviruses commonly infect humans.
Enteroviruses exist in many animals, including humans, cattle, pigs, and mice. Enteroviruses of human origin include the following:
- Polioviruses, types 1-3
- Coxsackie viruses of group A, types 1-24
- Coxsackie viruses of group B, types 1-6
- Enteroviruses types 68-71.
Enteroviruses are transient inhabitants of the human alimentary tract and may be isolated from the throat or lower intestine. Rhinoviruses, on the other hand are isolated chiefly from the nose and throat. Among the enteroviruses that are cytopathogenic (Polioviruses, echoviruses, and some Coxsackie viruses), growth can be readily obtained at 36 to 37 degrees Celsius in primary cultures of human and monkey kidney cells and certain cell lines (such as HeLa); in contrasts, most rhinovirus strains can only be recovered in cells of human origin (embryonic human kidney or lung, human diploid cell strains) at 33 degrees Celsius.
The enterovirus Capsid is thought to be composed of 32 morphologic subunits, possible in the form of a rhombic triacontahedron rather than a regular icosahedron. Rhinovirus capsid architecture appears to be similar. Infective nucleic acid has been extracted from several enteroviruses and rhinoviruses. Enteroviruses and rhinoviruses differ in buoyant density. Enteroviruses have a buoyant density in CsCl of about 1.34g/mL; human rhinoviruses, about 1.40g/mL. The first three listed enteroviruses are the commonest, but the last is not so common even though it has its effect to human health. We can stop by and take a look at what it is about.
Enterovirus Types 68-71
Four enteroviruses (types 68-71) grow in monkey kidney cultures, and 3 of them cause human diseases. Enteroviruse 68 was isolated from the respiratory tracts of children with bronchitis or pneumonia. Enterovirus 70 is the chief cause of acute hemorrahic conjunctivitis. Acute hemorrhagic conjunctivitis has a sudden onset of sub-conjunctival hemorrhage ranging from small petechiae to large blotches covering the bulbar conjunctiva. There may also be epithelial keratitis and occasionally lumbar radiculomyelopathy. The disease is commonest in adults, with an incubation period of a day and a duration of 8-10 days. Complete recovery is the rule. The virus is highly communicable and spreads rapidly under crowded or unhygienic conditions. There is no effective treatment.
Enterovirus 71 was isolated from patients with meningitis, encephalitis, and paralysis resembling poliomyelitis. It continues to be one of the main causes of central nervous system disease, sometimes fatal, around the world. In some areas, particularly in Japan and Sweden, the virus has caused outbreaks of hand, foot and mouth diseases.
Human rhinoviruses include more than 100 antigenic types. Rhinoviruses of other host species include those of horses and cattle.
Other picornaviruses are foot-and -mouth disease of cattle (Aphthovirus) and encephalomyocarditis of rodents (cardiovirus). The host range of the picornaviruses varies greatly from one type to the next and even among strains of the same type. They may readily be induced, by laboratory manipulation, to yield variants that have host ranges and tissue tropisms different from those of certain wild strains; this has led to the development of attenuated poliovirus strains now used as vaccines.