History & Culture

Wet Markets & Coronavirus | Ask the Expert

SARS, MERS, Ebola, and now the coronavirus (COVID-19) reported in Wuhan have all in some way been linked to animal food sources or wet markets as the origin of outbreaks. Virologist Dr Mark O’Dea gives an insight into how these diseases evolve, spread, and ultimately, what role wet markets play in facilitating their transmission from animals to humans.

We interviewed Murdoch University’s Virologist Dr Mark O’Dea to get a breakdown of what these diseases really are, and whether food safety really plays a key role in outbreaks.

Oliver: Probably the most obvious place to start, what exactly are zoonotic viruses? ​

Dr O’dea: Zoonotic viruses can be transmitted from animals to humans. Same for zoonotic bacteria. They don’t have to be in direct contact, and can of course be on fomites (inanimate objects or foodstuffs). In some cases, we have reverse zoonoses whereby pathogens from human hosts infect animals. A good example is human flu infecting pigs or poorly treated human wastewater being used in paddocks which stock then feed on.

Oliver:  Where do these viruses and other animal-borne diseases usually come from? 

Dr O’dea: How long is a piece of string... Usually, with the emerging ones, it is due to humans encroaching on an animal’s environment or changing the management of animals. It is very rare that they just appear from nowhere. Sometimes it is due to a breakdown of manufacturing processes – i.e. Hepatitis A in fruits and vegetables when wash water is contaminated. However, in that example, the contamination is of human origin rather than animal.

Oliver: Is this why wet markets are often blamed as a source of zoonotic diseases like the Wuhan coronavirus? 

Dr O’dea: Generally, because you have wild animals bought in which would not usually come into contact with each other or humans. A wet market is the perfect place to allow transmission from any animal carrying a potentially zoonotic virus to humans – basically it greatly increases the odds of an animal with a potentially zoonotic virus coming into contact with a human (or coming into contact with another animal species which can then act as an intermediate). It is the fault of the humans – not the animals.

Oliver:  I’ve heard a lot about viruses being formed in farming processes, and wet markets simply act as a medium for virus transmission. Is there any truth to this? 

Dr O’dea: Not necessarily, they evolve. They exist in their hosts in the wild – but as you said, the wet markets act as a transmission site. They also combine animals which wouldn’t normally come into contact with each other – and this can lead to viruses adapting to try to infect or transmit to another animal species. These animals and viruses would coexist in their ecological niche in the wild; human intervention is a large part of the problem.

Oliver: How are the diseases or viruses transmitted between animals and humans? 

Dr O’dea: It varies greatly depending on the virus. Obviously, the current virus is predominantly through aerosols and inhalation, however some can be through ingestion - such as Hepatitis A or Hepatitis E.

Oliver: Could more stringent controls on farming conditions theoretically reduce the risk of zoonotic viruses? 

Dr O’dea: No, it doesn’t have much, if anything at all, to do with farming conditions. I feel that it’s easy to get mixed up between the wet markets and farming, which are very different systems. I would say that given the number of uncharacterised viruses likely to be circulating in wild animals, we have had very, very few spillover events into humans. Our livestock production systems (at least in developed countries), have a very good understanding of animal health and biosecurity to minimise risks.

Learn more about wet markets.

Oliver: How do viruses stack up against bacteria in food-borne illnesses/diseases? Are bacteria in food sources predominantly to blame for these kinds of viruses?

Dr O’dea: Bacteria do not affect viruses in this scenario. There are bacteriophages, which are viruses infecting bacteria, but these don’t infect multi-celled organisms (like us). While you can definitely have cases of viral and bacterial foodborne illness occurring at the same time, the two are exclusive, and a change in the level of one somewhere in the food production process doesn’t have a direct effect on the other.

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