In this post I will try to answer three questions that are related to each other. How do new viruses evolve, how do viruses spread from animals to humans, and can viruses change when they start to spread.
In my last post, I talked about how viruses contain a strand of DNA or RNA which holds all the information needed to make new copies of the virus. When a virus reproduces, part of the process is making a new copy of either the RNA or the DNA.
If you’ve ever had to copy something out, you’ll know that from time-to-time, you make a mistake. The same thing can happen when new copies of the virus’ DNA or RNA are made. These mistakes are called replication errors. The altered DNA or RNA can still be used to make new viruses, but they will be different – this is called a mutation.
Replication errors do not produce completely new viruses. Instead, they produce new versions of the original virus – these are called strains. Viruses which store their information as RNA have more replication errors than those with DNA. There are two reasons for this. RNA is designed to store information for shorter periods of time, so it is less stable than DNA. Also, remember, the virus is using the organelles of a cell to copy itself. The organelles in a cell are designed to copy DNA, not RNA. They have all sorts of safety mechanisms to prevent replication errors when copying DNA, but these won’t work for RNA. The common cold is caused by RNA viruses, this is why there are so many different strains of it.
Flu is also caused by an RNA virus. As well as replication errors, flu viruses can do something called re-assortment. Basically, if two different strains of flu infect the same person or animal, they can mix their RNA together and then recombining it, giving a new strain which is very different to the old ones. The most recent example of this was swine flu in 2009.
So, what about coronavirus COVID-19? It’s an RNA virus, so whenever it reproduces there may be replication errors. Most replication errors actually cause changes that make viruses weaker, and these strains die out very quickly. Very occasionally a replication error will give a new strain that survives.
So, is COVID-19 a completely new virus? The answer is no, and this relates to the second question, which is about how viruses spread from animals into humans. If you’ve read my post about how viruses work, you’ll know that they invade cells and take them over in order to make new viruses. The first step in this is to stick onto the surface of a cell.
The envelope of a virus is covered in proteins, called spikes, which it uses to attach itself to a cell. These proteins will attach to proteins on the cell surface called receptors. For this to happen, the shape of the spike has to match up with the receptor – like a key fitting into a lock, or two jigsaw puzzle pieces fitting together. If the key, or the puzzle piece, is even slightly the wrong shape, it won’t fit.
There are a lot of viruses that affect animals but which humans can’t catch – this is because the spikes are the wrong shape to attach to human cells. Every so often though, a mutation will occur which alters the shape of the spikes, so that they can attach to human cells. Humans can now catch the new strain of the virus.
So, how did this happen with COVID-19? Viruses are more likely to cross from animals into humans if there is a lot of contact between them. Wuhan, in China, where the virus was first seen, has a large market where live animals are sold. The city is also very overcrowded, so it’s easy for viruses to spread. Scientists think that the virus spread to humans from animals being sold in the market. They aren’t sure which animal, but coronaviruses are common in bats, and in an animal called a pangolin – both of those were on sale in Wuhan.
The final question is, do viruses change when they start to spread? The answer is, yes. The more a virus spreads, the more it reproduces, so the more opportunity for replication errors. With a virus like flu, there is also more chance of the same person or animal being infected by two different strains, leading to re-assortment. So, how will COVID-19 change as it spreads? The answer is that any changes are likely to be very small, because they will be due to replication errors – coronaviruses can’t undergo re-assortment like flu viruses do. Changes are likely to be very minor and will not alter how the virus affects humans. Scientists working on a vaccine will also be making sure that it will still work if there are changes to the virus.
I hope you have found that useful! In my next post, I will be looking at how scientists study new viruses and find out how they work.