Why So Many Viruses Originate in the Far East

by Franky Elsmore

What is it about the Far East that makes it such a perfect breeding ground for viruses? New strains of flu virus originate from the Far East more frequently than could reasonably happen by chance alone. Why? I think it is to do with the collection and density of different animals found there and here’s why.

In southern China there is a higher density of waterfowl (mostly ducks), pigs, and humans living in particularly close proximity than anywhere else in the world. Birds are the main natural reservoir of flu viruses because these viruses act as harmless infections in the waterfowl. The viruses reproduce at a slow but continuous rate in the background of their immune system which prevents most symptomatic disease. These viruses contain all 15 HA and 9 NA types in almost any combinations. HA stands for haemagglutinin and are proteins acting as spikes on the virus surface.  Similarly NA stands for neuraminidase which is another form of surface protein which functions as a kind of ‘hook’ that can attach to and penetrate host cells. It is antibodies against these proteins that blocks the virus from binding to the cells and thus preventing infection and causing immunity. Antigenic drift is the slow and minor alterations in the structure of these proteins, but due to the previously ensured partial immunity a disastrous pandemic is rare. Pandemics occur when a major shift happens in a virus, creating an entirely new strain which the population hasn’t met before. This is a characteristic of influenza A virus, occurring roughly every 10-40 years and is called antigenic shift.

The genetic material of flu viruses is segmented into eight separate genes, so if two different strains of flu infect the same cell, a new virus may be produced containing a mixture of the parent genetics. For the most part viruses created by this ‘gene reassortment’ will be successful in finding a suitable host to infect and many will not be different enough from the original viruses to overcome host immunity. However, if the reassortment alters the HA and NA genes then it may be different enough to side step the pre-existing immune defences and have the potential to reproduce rapidly and cause a pandemic. 

For genetically shifted viruses to infect human populations it has to have been transmitted via an animal with an immune system somewhat similar to ours. This means that ideally they should be placental mammals that give birth to live young with a genome >80% similar to our own. Animals that frequently act as this reservoir and training ground for viruses with a potential to cause problems for humans as a species are pigs and bats. In order for bird viruses to do this, a bird-pig-human triangle is generally required. Once the virus has been created/altered and then reproduced harmlessly in the waterfowl, the virus strains are excreted out in their droppings which are then taken in by the intermediate host, the pig, which is very susceptible to both bird and human viruses. Once in the pig, the bird virus will come into contact with human viruses, creating a hybrid if they both infect the same cell at the same time. This resulting virus now poses a threat to humans as we as a species have had no prior immunity to it. These genetically reassorted viruses are the new strains of flu that occur a few times every century, ultimately capable of causing a pandemic. These conditions are extremely common in southern China especially, with this area having the highest density of waterfowl and pigs living in close proximity with humans. It is therefore unsurprising that so many new strains of flu originate from Asia, such as the Asian (1957), and Hong Kong (1968) pandemics as well as the fact that the Beijing, Shandong, Wuhan and Singapore strains all originated from there during the twentieth century. 

Knowing all of this, is it not reasonable to expect some kind of action in trying to alter the wet markets that are all too common in China as they provide the perfect conditions for new viruses to be transmitted from species to species as the animals are all living in ridiculously unhygienic condition amongst each others feaces and other excretions? As long as these conditions are continued, so will the threat of more potential pandemic causing viruses originating from those areas.

However it's not just waterfowl and pig density that is responsible for all of these concerning viruses that have an impact on the human race. It is also a fact that viruses come from bats incredibly frequently. Why bats? Bats function as a perfect viral vector because of their key role within many ecosystems, their genetic proximity to humans and their spectacular immune system. Bats in general are about as genetically close to us as dogs, at around 84%, which means that our cell surface proteins are pretty similar. This makes it a lot easier for viruses to cross the species barrier, especially if the virus has an effective viral entry protein (as the current coronavirus does). Viruses use their entry protein to bind to the target receptor protein to enter cells, so the more similar the target protein is between species, the easier it is for the virus to jump over that species barrier like it’s nothing.

Not only do any viruses that bats carry have a fighting chance at being potentially harmful to humans, but they also carry a lot of them. Bats have perfected the art of allowing the background replication of viruses to prevent any symptomatic disease. Plus, as the bats' immune system is so good at preventing viral harm to their own cells, it acts as a kind of training camp for the viruses where they can adapt to mammalian immune response mechanisms. This means that if we as a species do have the misfortune to come into contact with bat viruses, they are equipped to deal with immune systems which are far superior to ours, making it devastating for our comparably pitiful immune system’s capabilities. 

Bats come into contact with numerous viruses because they tend to drink the blood of multiple different animals as well as commonly eating mosquitoes and subsequently also ingesting all of the viruses from the animals they have fed on as well (mosquitos may also feed off the bats, causing viral transmission in this way instead). They also tend to excrete all over other animals' food sources, causing them to be exposed to their viruses. They are also prey for animals such as hawks, weasels and controversially some humans. Furthering their impact is the easily overlooked aspect of flight. This allows the transport of their viruses over hundreds of miles and bats have a surprisingly long lifespan, which often exceeds 20 years in the wild. They also tend to live in huge colonies in incredibly high density when roosting (for example the mexican free-tailed bat roosts with approximately 300 bats per ft2) which allows for large populations for the viruses to survive in. Bats are crucial to ecosystems so we cannot simply limit their numbers, which is hardly feasible anyway considering they account for somewhere between 20-25% of all mammals. Instead humans need to stop hunting bats for food, and try to limit the contact that the bats can have with our more viable food sources. Due to the huge bat population and the nature of human interaction with the bats in the East it is hardly surprising how frequently these viruses cross the species barrier and cause problems for humans.  

Clearly it would be overly simplistic and incorrect to say that the Far East is responsible for all of the viruses. This is evidently not the case as seen in the arrival of viruses like Ebola which came from the Congo basin and Spanish Influenza, which is mis-named as it most likely originated in the United States. But, it must be recognised that a disproportionately high number of diseases originate from Asian countries due to human activity such as the unhygienic and dense collections of livestock as seen in wet markets, and the culture of eating or coming into frequent contact with bats and other wild animals that carry a serious risk of possessing these viruses. Hence I feel it is entirely necessary that interventions take place in order to alter these standard conditions or we will undoubtedly face pandemics like the current COVID-19 and worse with increased frequency and severity as our global population combined with the ease of travel continues to rise.