The life of a virus

 
 
Tags: | | |
 
 
POSTED: August 16, 2020
 
 
 
 
 

The final episode of the current series of The Infinite Monkey Cage examines the question “What is Life”. The geneticist Professor Aoife Mclysaght described viruses as “on the very edge of life”. Whether you thought of them as alive or not depended, apparently, on precisely how you chose to define life.

This set me thinking about how little I actually knew about the life and times of a virus. I then had a conversation in which the lifespan of the virus came up. I realised that I had no idea how long a virus lived for, how it reproduced, and how it died (all of which it must do if it wished to meet any standard definition of “being alive”):

On the Science Focus website Maggie Staple stated that “Viruses exist at the boundary of what counts as life, and most scientists now agree they don’t make the cut”. She went on to say that

Strictly speaking, viruses can’t die, for the simple reason that they aren’t alive in the first place. Although they contain genetic instructions in the form of DNA (or the related molecule, RNA), viruses can’t thrive independently. Instead, they must invade a host organism and hijack its genetic instructions.

That said, it makes sense to talk of how long viruses can remain viable and capable of infection. Some – including the influenza virus, and HIV, the virus responsible for AIDS – can’t survive for more than a few hours outside a host organism unless kept under carefully controlled conditions.

But others, notably the deadly smallpox virus, can easily remain infectious for years.

I found more detailed information in the North Carolina State University news pages. There Matt Koci, a virologist and immunologist whose work focuses on host-microbe interactions in birds, answered the question “How long would viruses in a dead body pose a biological hazard?” He said that

Probably the biggest factor is what kind of virus it is. What is its virion made of? Does it have a capsid or an envelope?

He expanded upon this:

Virion is the $5 word for virus particle. In order for a virus to move its genome from victim cell to victim cell it needs protection from the environment. There are two main ways viruses do that. One way is for a virus to surround its genome with a lipid bilayer membrane (a two-layer shell made out of fat that it steals from the cell it’s hijacked). These viruses are called enveloped viruses, because they’re in those membrane “envelopes.” The second way a virus can protect itself from the environment is by building a little geodesic shell of proteins around its genome, forming what is essentially a little protein BB or golf ball. These shells are called capsids, so viruses that form these protective layers are called capsid viruses.

Because the virus particles of enveloped viruses are largely made up of lipids (fats) they tend to be less able to survive in the environment as compared to viruses with capsids. Enveloped viruses are more prone to drying out, and they are sensitive to a lot more disinfectants than capsid-based viruses. That’s why handwashing with simple soap is effective against viruses like influenza virus and coronaviruses (which are enveloped viruses), but soap won’t work for viruses like norovirus (which has a capsid shell).

Soap works because it has lipids in it too, and the lipids in the soap insert themselves into the virus envelope in a way that causes the envelope to fall apart – inactivating the virus. Soaps don’t work on capsid-based viruses because there’s no lipid in the virus particle for the soap to interact with. That’s why we need other types of cleaners, like bleach, to tackle non-enveloped viruses.

Finally I found an article in the Smithsonian magazine that discussed how viruses mutate, evolve, and jump from one host species to another. This argues that

making the jump from one species to another isn’t easy, because successful viruses have to be tightly adapted to their hosts. To get into a host cell, a molecule on the virus’s surface has to match a receptor on the outside of the cell, like a key fitting into a lock. Once inside the cell, the virus has to evade the cell’s immune defenses and then commandeer the appropriate parts of the host’s biochemistry to churn out new viruses. Any or all of these factors are likely to differ from one host species to another, so viruses will need to change genetically — that is, evolve — in order to set up shop in a new animal.

Successful viruses make an “evolutionary two-step — first spillover, then adaptation to the new host”. This

is probably characteristic of most viruses as they shift hosts, says Daniel Streicker, a viral ecologist at the University of Glasgow. If so, emerging viruses probably pass through a “silent period” immediately after a host shift, in which the virus barely scrapes by, teetering on the brink of extinction until it acquires the mutations needed for an epidemic to bloom.

The articles contain much more detail, all of which you should read. The above, however, outlines some of the main features in the life of a virus determined to spread.