Viral diseases have shaped human history. A sudden emergence has the power to cause huge social changes, like ones we’re currently experiencing. But how do they do this? What causes a virus to jump, or ‘spill over’ into a new species?

What is an emerging virus and how do they emerge?

An emerging virus is a virus which has entered a new population where it previously didn’t exist or is expanding its geographical range. Global disease emergence is increasing for many reasons and we’ll discuss why this is occurring and why most emerging viruses that normally infect animals are now infecting humans.

What’s important to remember is there are no singular reasons for viral disease emergence, and we might never know what occurred to cause a disease to break into new populations. But there’s many ways this can happen, ranging from the virus’s genetics all the way through to human-environment interactions. Here we’re going to focus on RNA viruses as these are the viruses which most commonly cross species barriers1. Let’s start with the genetics first.


Genetic

Enzyme Errors

RNA viruses contain an enzyme called RdRp. This enzyme replicates RNA. RNA is a form of genetic code like DNA, so it encodes genes. However, RdRp is prone to making mistakes, it might miss a base, it might use the wrong one and its common for this to happen. This means a virus can be produced with different properties, this can range from being able to target a new cell type to replicating faster. One case where this occurred was during the 1918 Spanish Flu, where a mutation allowed the virus to replicate in tissues outside the respiratory tract2.

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Figure 1: Replication of RNA with mutation highlighted in yellow.


Reassortment of segmented genomes

Reassortment is the process where genetic material might get ‘mixed up’. When a cell is infected with two different but closely related viruses there’s a chance this might occur. Think of it like mixing your favourite drinks. It could work, and you might have a new flavour, or it might not! It’s easier for segmented genomes, so the genetic code is in multiple segments and is common in viruses like influenza. H1N1 is an influenza virus which is made up of bird, pig and human influenza strains3.  

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Figure 2: Viral co-infection of two viruses (A and B). Inside cell shows 2 different genomes, with reassorted virus containing genomes of both virus A and B having egressed.



Recombination of RNA genomes

Recombination is a random event which occurs when RdRp, the enzyme which makes new RNA, falls off the genome its copying onto a different one. It ultimately will produce an RNA genome which is a combination of two different viruses. This is another common event and many virus families have evidence of this occurring, including Herpesviruses, HIV and even Coronaviruses4,5

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Figure 3: Recombination of RNA genome in 5′ to 3′ directionality.


Environmental

Changes in weather

Climate change isn’t only impacting our weather, it’s also changing disease distributions through temperature but also causing changes in host territory. Ultimately this changes how we interact with hosts of viruses as well as their biology. For example, Japanese Encephalitis Virus (JEV). This virus is carried by mosquitoes, so a higher temperature alters host territory as well as allowing for mosquito development to occur where it didn’t previously. This is because mosquitos have a minimum temperature where development will occur, and for the mosquito which carries JEV its 22-23 °C. However, viral diseases can have a minimum transmission temperature, and JEV has one of 25-26 °C. If more countries have temperatures above this range, then the virus can be transmitted in new populations. What this all means is as global temperatures rise it’s very likely countries will experience diseases they haven’t previously6


Bush meat and live animal markets

Consumption of bush meat and live animal markets remove natural barriers in place, meaning that close contact between animals and humans now occurs. Outbreaks may occur due to consumption of an animal which died of a disease, and not of more natural causes. This is how Ebola outbreaks have started before. However, it is important to consider the socio-economic conditions found within regions where consumption of bush meat occurs. Protein sources in these regions can be expensive and the local population may not have the choices we do7. Reducing disease emergence from live animal markets can be done safely by reducing inter-species interactions, essentially handling the animals less and making the markets less crowded. However, it could also be done through limiting the days of operation8.  

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Figure 4: Wet market in South Korea. Source: Stocksnap, Pixabay.



Changing land use and farming practices

Deforestation of land for farming and urban development is forcing disease hosts to come into closer contact with humans, one example where this is occurring is Australia. Here, horse farms are traditionally where fruit bats reside however urbanisation has resulted in loss of the natural habitat, forcing greater interactions with the human population9.  

So, there we have it. Several mechanisms on how viruses can emerge into human populations. But what about SARS-CoV-2? Well, the jury’s still out. Though early cases were linked to a seafood market many weren’t, indicating the source of the virus likely wasn’t here. In the meantime, scientists will be hard at work hoping to solve many puzzles, including this!  If you have any questions about what was discussed drop us a message below or on Facebook, Twitter or Instagram and we’ll get back to you.


For more information on the origins of SARS-CoV-2 read our blogpost breaking down a Nature paper by Dr Jordan Clark here

For more information on viral disease emergence check out ‘Spillover’ by David Quammen. 


References:

1.J Woolhouse, M. E., Adair, K. & Brierley, L. RNA Viruses: A Case Study of the Biology of Emerging Infectious Diseases. Microbiol. Spectr. 1, 10.1128/microbiolspec.OH-0001–2012 (2013). 

2.Taubenberger, J. K. The origin and virulence of the 1918 ‘Spanish’ influenza virus. Proc. Am. Philos. Soc. 150, 86–112 (2006). 

3.Vijaykrishna, D. et al. Reassortment of pandemic H1N1/2009 influenza A virus in swine. Science 328, 1529 (2010). 

4.Fleischmann, W. J. Medical Microbiology. (University of Texas Medical Branch, 1996). 

5.Su, S. et al. Epidemiology , Genetic Recombination , and Pathogenesis of Coronaviruses. Trends Microbiol. 24, 490–502 (2016). 

6.Wu, X., Lu, Y., Zhou, S., Chen, L. & Xu, B. Impact of climate change on human infectious diseases : Empirical evidence and human adaptation. Environ. Int. 86, 14–23 (2016). 

7.Kurpiers, L. A., Schulte-Herbrüggen, B., Ejotre, I. & Reeder, D. M. Bushmeat and Emerging Infectious Diseases: Lessons from Africa BT  – Problematic Wildlife: A Cross-Disciplinary Approach. in (ed. Angelici, F. M.) 507–551 (Springer International Publishing, 2016). doi:10.1007/978-3-319-22246-2_24 

8.Karesh, W. B., Cook, R. A., Bennett, E. L. & Newcomb, J. Wildlife trade and global disease emergence. Emerg. Infect. Dis. 11, 1000–1002 (2005). 

9.Plowright, R. K. et al. Urban habituation, ecological connectivity and epidemic dampening: the emergence of Hendra virus from flying foxes (Pteropus spp.). Proceedings. Biol. Sci. 278, 3703–3712 (2011). 


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Written by Charlotte Rigby.