SARS-CoV-2: A New Viral Pest

by

Gregory Beekman

 

SARS-CoV-2: A New Viral Pest


When a tornado hits, all you can do is batten down the hatches and hide until it passes. This is because some natural events are too powerful for us to fight. It’s always been this way (think of our ancestors hiding up trees from sabre-toothed cats with nothing but wooden spears to shake at them) and probably always will be this way.

Take our current pandemic. No matter how big and powerful you may think you are, you are but a weakling against one of Earth’s tiniest pests: SARS-CoV-2, the novel coronavirus first identified in Wuhan, China that causes the disease Covid-19 (characterised by fever, dry cough, and breathlessness, amongst other symptoms).

Although viruses evolved at least 1.5 billion years ago, and are thus widespread across the planet, our knowledge of them is barely a century old. It is difficult during this pandemic to imagine a time when we knew nothing of viruses and the other microscopic organisms that can infect us. Although bacteria were discovered in 1676 by Antony van Leeuwenhoek, it wasn’t until the 19th century that even smaller organisms were found.

Louis Pasteur, the famous French inventor of pasteurization, needed water that was free of bacteria (then the smallest organisms known) for his experiments. In 1884, one of his scientist-assistants, Charles Edouard Chamberland, developed a ceramic filter with pores smaller than the size of bacteria. This was a huge breakthrough and these filters became popular with the public for making their drinking water safe.

In 1892, Russian botanist Dmitri Ivanovsky used one of these filters to remove bacteria from the sap of a diseased tobacco plant. But, surprisingly, the filtered sap was still able to infect other tobacco plants. The sap thus contained something so small that it could easily pass through the tiny pores of the ceramic filter. Ivanovsky couldn’t see what in the sap was causing further infection because optical microscopes at the time just weren’t powerful enough. But he had discovered the first virus – the tobacco mosaic virus.

However, it wasn’t until 1938 that scientists were able to finally see this virus – the first virus to be imaged. This is thanks to the work by the German scientists Max Knoll and Ernst Ruska who developed the first electron microscope in 1931. By using a beam of electrons instead of a beam of light to create images, an electron microscope can see far smaller objects than an optical microscope can.

This meant that scientists could finally see viruses and study their structure in detail. You cannot underestimate just how important a step this was in understanding viruses. Over the decades, scientists have taken electron micrographs (as images from an electron microscope are called) of a variety of viruses, greatly adding to our understanding of them. 

So just what is a virus?

Viruses are small particles that infect larger organisms. There are many families of viruses, each with a different range of hosts that they can infect. It is not only humans that can be infected by viruses – just about every other living thing can too. Plants, fungi, algae, insects, fish, molluscs, crustaceans, reptiles, birds, and mammals can all be infected. Even other pathogens, including bacteria, yeast and larger viruses, can be infected. They are a menace to just about all life on Earth.

An individual virus is called a virion, or virus particle. They are tiny. A full stop on a page is about 0.3 mm (millimetres) in diameter, which is 300,000 nm (nanometres). The human eye can see objects as small as 100,000 nm. No virus is as large as this. The smallest virus discovered is the Porcine Circovirus at just 17 nm in length; in recent years, “mega viruses” have been discovered, with sizes of around 500 nm. But even these larger viruses are still far too small for the human eye to see.

Viruses are so small that they don’t even have cells. In animals and humans, we have cells that are like little factories. Inside these cells are all types of biological machinery, including the mitochondria that generate the energy of life. But viruses don’t have cells and thus don’t have any mitochondria. Therefore, some scientists think viruses are not really alive, but it is an on-going subject of debate; some scientists describe viruses as being at the edge of life.

One way to think of them is to think of them as a bit like a hen’s egg but with the shell made from protein and the egg inside replaced with a genetic instruction manual.  The virus “eggshell” is called the capsid and the genetic instruction manual is called the nucleic acid; some viruses have a further coating of fat around the capsid and this fat layer is called the envelope. Not all viruses are egg-shaped or spherical. For example, the Tobacco mosaic virus is rod-shaped – but the entire rod is the capsid and inside it is the nucleic acid.

Viruses come in two types – they either carry DNA or RNA inside their capsids. The Tobacco mosaic virus in an RNA virus – inside, it carries a single-strand of helical RNA. Cold sores are caused by a DNA virus, the herpes simplex virus – inside its capsid, there are two strands of genetic material, wound around each other in a double-helix.

The outside of the capsid is where the virus’s attack weaponry is located, which the virus uses to attack the cells of the host it is infecting. These attack spikes have been likened to keys; proteins on the surface of a cell are the locks. If the virus key fits, a pore opens, and the virus enters the cell. Once inside, the virus hijacks the machinery of the cell to manufacture copies of itself. A single cell can create between tens of thousands and millions of new virus particles in a few days, the number depending on what type of virus it is.

But what about coronaviruses?

Coronaviruses are a group of viruses that cause respiratory illnesses, including the common cold. The first human coronavirus wasn’t discovered until 1965 and the term coronavirus wasn’t coined until 1968. So, we have only known for half-a-century or so that these types of viruses exist. We now know that there are seven strains of coronavirus that infect humans; there are many others that infect animals. It has been estimated that 15% of colds in humans are caused by coronaviruses (the rest are mostly from rhinoviruses).

Electron micrographs show that SARS-CoV-2 is a spherical RNA virus around 100 nm in size and covered with attack spikes; its capsid has a fatty envelope, and this makes it susceptible to soap and alcohol. It was first reported to the World Health Organisation in December 2019 and the world has had barely six months to study it. Already, coronavirus has killed half a million people worldwide.

Some people have questioned the merit of going into Lockdown. But let’s look at the Spanish flu pandemic of 1918. Viruses are characterised by their R number: the higher the R number, the more contagious the virus. Estimates put the Spanish flu R number at around 3, meaning each infected person would infect three others on average; this is the same as the R number for SARS-CoV-2.

Spanish flu is estimated to have infected one-third of the world’s population and killed up to 50 million people. The cause of the disease was unknown – no one had yet seen a virus. Now we know it was caused by an influenza H1N1 virus, probably of avian origin; it is a spherical RNA virus whose capsid is covered in a fatty envelope and is about 100 nm in diameter.

Does this sound familiar?

Although both pandemic viruses are RNA viruses of similar size, they are not related to each other: SARS-CoV-2 originated in bats while H1N1 came from birds.

The Covid-19 pandemic has not caused the same death toll as the 1918 pandemic but that’s because scientific knowledge developed over the years has allowed the world to react to the threat. So far, over half a million people have died. It’s only not worse because of global efforts such as lockdowns (where many businesses are closed and citizens’ freedoms are severely restricted), medical interventions (ventilating patients and trialling existing drugs) and the unprecedented sharing of scientific knowledge around the world (including China publishing early the genome of the virus).

In short, the world has thrown everything it has at it – and still the deaths mount.

Our only hope is that scientists can develop a vaccine against this virus. Until then, all we can do is hide from it. Just as we do from tornadoes. And just as our ancestors did from sabre-toothed cats.


(C) Gregory Beekman 2020