It’s Professor Dave, let’s talk about the common cold.
Most of us get a cold at least once a year.
Why does this happen exactly?
While there are many pathogens that can cause the common cold, rhinovirus is considered
to be the main culprit, responsible for more than one-half of cold-like illnesses.
In fact, somewhere in the ballpark of one hundred different strains, or serotypes, of
rhinoviruses have been associated with cold-like symptoms in humans.
Rhinovirus, which is sometimes referred to as RV, was isolated back in 1956 by Dr. Winston
Price at Johns Hopkins University.
The virus itself is what’s referred to as a positive-sense, single-stranded non-enveloped
RNA virus belonging to the picornavirus family.
Let’s break that down a bit using some of the basic knowledge we went over in the previous tutorial.
Let’s recall that viruses are simply genetic material, either RNA or DNA, surrounded by
a protective coat.
In the case of single-stranded RNA viruses, there are two types, either positive-sense
or negative-sense RNA.
With a positive strand, the RNA can act as mRNA and be directly translated in the ribosomes
of the host cell.
With a negative strand, this will act as the complement of what would be the sense strand,
so the virus must also contain an enzyme called RNA-dependent RNA polymerase that can use
the negative strand to generate the positive strand through complementary base pairing.
Then the newly made positive strand can be translated to generate the viral proteins.
In the case of rhinoviruses, the positive RNA genome serves as mRNA, which codes for
proteins that are involved in replication, virus assembly, and the structural aspects,
namely the capsid.
We also know that rhinoviruses don’t have an envelope surrounding their RNA.
Now let’s dig into what it means to be a part of the picornavirus family, which is
one of the largest families of viruses.
Let’s start with the name.
These viruses are small, hence the prefix pico, and contain ribonucleic acid, hence the RNA.
They also have a naked capsid structure.
The picornavirus family has over 230 members, which are divided into five genera: Enterorvirus,
Rhinovirus, Heparnavirus, Cardiovirus, and Aphthovirus.
Compared to the other genera, rhinoviruses are known to be sensitive to acidic pH and
replicate poorly at temperatures above 33 degrees Celsius, which typically limits them
to causing upper respiratory tract infections.
Now let’s talk about structure.
The positive-strand RNA of picornaviruses is surrounded by an icosahedral capsid, which
is that soccer ball looking shape we talked about earlier.
The icosahedral capsid has 12 pentameric vertices, which are each composed of sets of proteins.
For all picornaviruses, the capsid is stable in the presence of detergents or heat, but
in the case of rhinoviruses, not in the presence of acid, as we mentioned.
Now let’s dig into how these viruses do their damage.
Picornavirus particles attach to host cells by binding to specific host cell receptors.
And remember, these host cell receptors have normal functions in host cells, it just so
happens that these viruses have adapted to bind to them.
To be more specific, upwards of 80% of rhinoviruses bind to specific molecules called ICAM-1,
which stands for intercellular adhesion molecule-1, on the surface of epithelial cells, fibroblasts,
and endothelial cells.
The process of the virus binding to the appropriate receptor on the appropriate type of cell is
what we refer to as attachment.
Attachment then triggers conformational changes in surface proteins of the capsid, which causes
penetration and uncoating.
The rhinovirus RNA is injected directly into the cytoplasm of the host cell.
Once inside, the genetic material doesn’t have to undergo transcription, of course,
because it’s already mRNA.
The host cell ribosome then translates the viral genetic material, triggering a very
intricate and tightly controlled process of protein chopping and virus assembly, all orchestrated
by the directions encoded in the viral genome.
At the same time, the host cell generates tons of viral mRNA.
Ultimately, many, many rhinovirus particles are made, as many as 100,000 virions per cell,
which are then released via cell lysis, where the cell sort of explodes.
Thinking in more detail about rhinovirus infection, it turns out as little as ONE infectious viral
particle can initiate an infection.
The virus typically enters through the eyes, mouth, or nose, either by airborne droplets
generated by a sneeze or cough, or by passage on fomites, and then initiates an infection
in the upper respiratory tract.
Most of the viral replication going on there happens in the nose.
The severity of symptoms really depends on how much, and when, the virus is shedding.
At the peak of infection, the nasal secretions of an infected person, commonly referred to
as snot, can contain concentrations of 500 to 1000 infectious virions per milliliter.
The infected cells release chemicals like histamine, which cause the characteristic
runny nose and other irritating symptoms associated with the common cold.
Since there are so many serotypes of rhinoviruses, it’s almost impossible to build up a lasting
immunity to them, which is what results in rhinovirus-induced colds being so “common.”
There are other viruses and bacteria that cause similar symptoms too, like enteroviruses,
coronaviruses, and adenoviruses.
For the most part, it’s difficult to identify which pathogen made you sick.
And furthermore, any cold is likely to pass pretty quickly.
Thinking about rhinoviruses out in the wild, there are two main mechanisms by which they
are spread, as we’ve discussed, either by breathing in aerosol droplets, or by fomites,
which means on hands or contaminated inanimate objects.
Some experts think that hands are the major vector for viruses, and direct person-to-person
contact is the predominant mode of spreading.
Plus, these non-enveloped viruses are extremely stable and can survive on things like doorknobs
and toothbrush handles for hours.
At the same time, we know that rhinoviruses cause clinical illness in only half of the
people infected, though asymptomatic people can also spread the virus, even if they make
much less of it.
For the most part, infants and children have the highest rates of rhinovirus infection,
though they can spread these infections to their family members pretty easily.
And especially during the colder months, it’s not surprising for rhinoviruses to be passed
around in school or other social settings.
When it comes to treatment, there are tons of over-the-counter remedies that help treat
the common cold, but rhinoviruses aren’t a solid candidate for vaccination, since there
are so many serotypes.
Handwashing and disinfecting commonly handled items is the best way to prevent spreading
of the virus.
So now with the most common virus covered, let’s move forward and talk about other
viruses that cause common diseases.