The Wuhan novel coronavirus is dominating the news, with reports of thousands infected and new cases reported worldwide every day. Are we at the verge of a pandemic? That’s not a question I’m equipped to answer, and I encourage our readers to follow all advice from the CDC and WHO regarding the spread of the disease. Does this emergent disease have anything to do with urbanization? That’s a question I might be able to chime in on. Let’s take a closer look at the relationship between zoonotic diseases and urbanization.
What is a zoonotic disease?
Zoonotic diseases occur when pathogens — including bacteria, fungi, parasites, and viruses —jump from wildlife to humans. Zoonotic diseases are not uncommon; we have all heard of zoonotic diseases, even if you don’t know them by that name, such as the ones in the infographic below. In fact, most new emerging diseases are zoonotic. These pathogens make the jump from wildlife to humans through direct and indirect routes of infection. Direct interactions with wildlife can lead to scratches and bites, or can otherwise lead to contact with bodily fluids such as blood, mucus, saliva, or feces. Close interaction with wildlife can also increase the exposure to disease vectors such as mosquitos and ticks carrying the pathogen, and to indirect routes of exposure via contaminated surfaces or food. Once the jump from wildlife to human occurs, the pathogen may transmit directly between humans or via alternate hosts such as livestock or pets which humans more closely interact with on a daily basis.
The Wuhan coronavirus — what do we know about it’s jump to humans?
Now that you have a little background knowledge of zoonoses, let’s talk about Wuhan and the novel coronavirus. Wuhan is a big city: it has 11 million residents and is the 7th largest city in China. It has a humid subtropical climate and it is surrounded by rural lands. And, as you have likely heard in the news by now, “wet markets”, where live wildlife for human consumption is sold, can be found within the city.
Scientists have been quick to try to figure out where the coronavirus came from and how it made it to human hosts. The wet markets, in particular the Huanan Seafood Wholesales Market, have been zeroed in on as a possible source. The Chinese CDC found samples of seafood from the market tested positive for the virus, and that the positive samples were clustered at the portion of the market where live wildlife is sold.
Given this potential source, China has taken steps to curb the sale of wildlife in the region temporarily, a move that scientists with the Wildlife Conservation Society say should be permanent to prevent future zoonotic outbreaks. And while the market does seem to be a source for infections, some scientists are also raising doubts and suggesting that the virus jumped to humans before it made it to the market; in other words, the market was just the perfect combination of live animals and humans to incubate and spread the emerging disease.
There has also been some debate about the original animal host of the disease. Early reports by Chinese scientists suggested that the virus originated in snakes. This conclusion is controversial, and scientists have since found that the virus shares 96% similar DNA with coronaviruses originating in bats — suggesting bats, not snakes, are the original host.
What does urbanization have to do with zoonotic diseases?
So what does this have to do with urbanization? With expanding human populations comes an intensification and expansion of urban areas. We are now at a point where most humans live in urban areas globally and most of the world’s terrestrial surfaces are impacted by human activities in some way. As wild areas are increasingly encroached upon, human-wildlife interactions are becoming more common. Urban expansion, wildlife health, and human health are intricately interwoven.
Urbanization may increase the likelihood of a novel emergent zoonotic disease through a number of avenues. A 2019 publication reviewed the many ways in which zoonotic diseases are impacted by urbanization and explored what that means as the human population continues to grow. Their findings touched on three major themes:
Patterns of urbanization and socioeconomic conditions can promote disease transmission — Concentration of humans and inequality in the distribution of health services, clean food and water, and sanitation lead to hotspots of disease within cities.
Urban areas promote increased movement of people, animals, and wildlife-products between rural and urban locations — Migrants may introduce zoonotic diseases to cities if they travel with wildlife, or may also arrive in cities already infected with pathogens. Industrial-scale animal production and movement of animals en-masse can rapidly spread diseases, with consequences most severe for food-insecure urban poor.
Urban habitat modification alters the environment and wildlife interactions — Increased impervious surfaces and buildings alter the hydrology of the urban environment, which may increase the prevalence of water-borne diseases and aquatic vectors such as mosquitos. As human settlements increasingly encroach on wild areas, human-wildlife interactions increase. Humans and their domesticated animals (including livestock) are increasingly exposed to diseases and disease vectors that previously only impacted wild animals.
The authors also highlight the complex nature of urban drivers of zoonotic disease. The factors highlighted above interact to make cities the perfect place for zoonotic diseases to emerge and rapidly spread, especially in poor and marginalized communities. Importantly, the authors discuss the trend that with zoonotic outbreaks, informal food markets such as the wet-markets in Wuhan often get blamed. This blame may be misplaced, and is also an oversimplification of the synergistic environmental, ecological, and socioeconomic factors promoting zoonoses in cities.
Cities as zoonotic hotspots
Although the factors contributing to zoonotic emerging diseases are complex and synergistic, evidence does seem to suggest that cities may be more prone to such outbreaks than rural areas. As long as we continue to expand our urban areas, we will likely continue to see emerging zoonotic diseases. What does this mean for the humans and the wildlife in cities? Here’s my takeaway thoughts:
With emerging zoonoses, we should be cautious not to demonize the host animals. Wild animals are not to blame for zoonotic diseases, humans are. We need to examine our patterns of interaction with wildlife and the socioeconomic conditions that propagate disease outbreaks. Eradication of wildlife hosts is not a viable long-term solution; addressing the root causes of disease emergence is.
Cities lead to increased human-wildlife interactions. These interactions take many forms: increased wild-land–settlement interfaces, rural-urban migration, the movement of animals, and urban tolerant animals occupying cities, to name a few. With increased wildlife interaction comes increased disease risk. Animals that thrive in cities may be sources or reservoirs of zoonotic disease. Only by studying the ecology and evolution of these animals in cities can we fully understand the disease dynamics to appropriately manage the risk while maintaining functional urban wildlife communities.
Might urban wildlife adapt? On this blog we often talk about evolutionary change in response to urban pressures. We know that many urban animals have divergent immune and stress responses compared to their non-urban counterparts. What role might this play in disease susceptibility and transmission? For example, perhaps selection for improved immune function might make urban wildlife more resistant to zoonotic diseases? Alternatively, perhaps impaired immune function and elevated stress responses might make them more susceptible. Urbanization may impact the zoonotic vectors as well. For example, artificial light at night appears to extend the infection window, and thus the epidemic potential, of West Nile Virus transmission by mosquitos to house sparrows. How might this and other evolutionary changes in urban vector species impact urban zoonotic disease transmission?
The intersection of human health and urban ecology and evolution is seemingly understudied. Does your research on urban wildlife have implications for zoonotic diseases or human health? Let us know in the comments if so, we would love to hear more about it!
Want to know more?
To read more about urbanization and zoonotic disease, check out:
Ahmed, S., Dávila, J.D., Allen, A., Haklay, M., Tacoli, C. and Fèvre, E.M., 2019. Does urbanization make emergence of zoonosis more likely? Evidence, myths and gaps. Environment and urbanization, 31(2), pp.443-460.
To learn more about the transmission and spread of the Wuhan novel coronavirus, I encourage you to follow the advice of the United States Center for Disease Control and Prevention (CDC) and the World Health Organization.
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