Picornaviridae
Picornaviruses are named for their small (“pico” + “RNA” = picorna) size. Nevertheless, they sure do have a massive and diverse array of viruses – over 100 serotypes! These viruses can even be traced all the way back to Ancient Egyptian records of polio epidemics, but are still around and cause a menagerie of diseases today.
Unique features, Morphology and Genome
Ø Picornaviruses contain positive sense, single-stranded RNA that is approximately 7-8 kilobases long.
Ø The genome is monopartite and polyadenylated at the 3’ end, but has a VPg protein at the 5’ end in place of a cap.
Ø The viral RNA is infectious and replication takes place in the cytoplasm.
Ø The virus has an IRES (Internal Ribosomal Entry Site) which distinguishes it from many other RNA viruses.
Ø The virus is naked with an icosahedral capsid.
Ø The triangulation number is 3, while the capsid has four unique proteins: VP1, 2, 3, and 4.
Ø The capsid is one of the smallest of all viruses with a diameter of only 27-30nm.
Ø Translation and cleavage of viral polypeptides produces eleven distinct proteins.
One example of a Picornaviridae virus is Rhinovirus.
With as many as more than 100 serotypes, rhinoviruses are transmitted through the respiratory route and replicate in the nose (“rhino”). The many serotypes are then divided into “major” and “minor” groups and all cause a similar syndrome – the “common cold.” The large number of serotypes allows many rhinovirus infections to occur in one person over time, since immunity only develops for one serotype and each newly acquired rhinovirus causes a new “cold.” About half of all colds can be attributed to rhinoviruses, particularly those that occur in the winter.
Pathogenesis includes an Upper Respiratory Tract infection with a short incubation period of 2 to 3 days. Production of the endogenous IFN (Interferon) will help battle the virus to protect the body. IgA (a type of immunoglobulin present in blood and body secretions which may aid in fighting infections) is then locally synthesized, though it will degrade with time. However, our serum IgG (memory cell of immune system) would confer lifetime immunity!
And how would you know that you’ve been infected by rhinovirus?
A lab diagnosis of this virus will include culturing the virus, nasal washings, EIA and PCR.
This epidemic occurs practically throughout the year, albeit there are a few serotypes circulating simultaneously! This is most probably due to the fact that the virus is abundant in nasal discharge.
How does one control this “common cold” then?
Well, you could wash your hands regularly, avoid touching your eyes or nose. If you happen to sneeze into a tissue, be sure to discard it right after! You might also want to avoid people who have a cold! Or, if you are the one with the cold, do stay at home! This will ensure that other people would not be affected!
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