Hi friends! Today I thought I’d chat about zoonotic diseases. Lately, zoonotic diseases are frequently mentioned in the media, likely because COVID-19 is a zoonotic disease. But what are zoonotic diseases? And how do human interactions with the environment impact zoonotic disease transmission?
We will chat about what they are, and how things like animal agriculture and climate change influence disease transmission rates. Because yes, human interactions with the environment impact zoonotic disease risk and transmission rates in a major way.
First of all, what are zoonotic diseases?
Zoonotic diseases are diseases spread between animals, insects, ticks, and people.
There are many zoonotic diseases. Examples we hear about often include things like rabies, Lyme disease, SARS, Ebola, Avian (‘bird’) flu, salmonella, and of course, COVID-19.
What causes zoonotic diseases to spill over from animals to humans?
Scientists have identified three major types of changes that increase the likelihood that a pathogen will move from animals to humans: they include changes to the environment, host (the person or animal carrying the disease), or pathogen itself (meaning the pathogen may alter or mutate or develop resistance, etc).
But what factors increase the risk of these changes, and thus, increase the risk of transmission of zoonotic disease to humans?
What increases the risks of zoonotic disease transmission to humans?
Turns out, a lot of human activities are influencing the spread of these diseases, many of which have to do with how we interact with our environment.
In 2016, the UN Environment Program published a report detailing “Emerging Issues of Environmental Concern.” Zoonotic diseases were among the noted concerns.
The report noted that there has been a dramatic reduction in natural ecosystems and biodiversity, and that humans now keep more animals than ever before, which presents more opportunities for pathogens of animal diseases to spill over into humans.
The integrity of natural ecosystems is essential to maintaining a buffer zone that normally prevents spill over of certain pathogens from animals to people.
Unfortunately, human activities, including deforestation, mining, building, and use of land agriculture, have fragmented a lot of these buffering ecosystems, increasing the risk of pathogen spill over.
The more we disrupt the natural habitats of wildlife, the more likely they are to be enclosed into smaller spaces, and the more opportunities there are for pathogens to congregate in dense animal populations and carry into humans.
Increased and Intensified Agriculture, Particularly Increased Demand for Animal Products:
While many of these diseases start in wildlife, livestock often serve as the bridge between wildlife and human life.
Increased demand for milk and meat also increases likelihood of disease transmission, particularly under intense production conditions. In these conditions, large amounts of genetically-similar animals are clustered together.
Because they lack large genetic diversity, they become a group less resistant to infection and thus more likely to spread the disease. This is called the monoculture effect.
With the demand for animal products soaring (it’s higher than ever, despite the increasing popularity of plant-based products), the pressure to increase productivity of animal agricultural practices increase. This increases the risk of animal diseases spilling into humans.
As mentioned above, livestock often serve as the disease bridge between wild animals and humans. For example, it is believed that avian (‘bird’) flu was transmitted from wild animals to humans via livestock birds meant for consumption.
Furthermore, intensification of livestock also increases the use of fertilizer, and increased amounts of livestock animal waste, both of which can foster environmental conditions that allow some pathogens to thrive.
And then, of course, there’s the environmental impact of animal agriculture. Not only does animal agriculture encourage deforestation and ecosystem disruption, it also creates a significant amount of green gas emissions, which contribute to climate change.
Live animal markets where wild and livestock animals are closely packed together in suboptimal conditions, also known as ‘wet markets,’ have been identified as another source of risk, with many calling to ban these types of markets worldwide to prevent future outbreaks.
Climate change can serve as a major factor for disease emergence, because it influences conditions that help or hinder survival, reproduction and transmission of pathogens, vectors (animals or bugs that carry diseases) and hosts (animals or people who have the disease).
For example, climate change drives changes in things like temperature and humidity, and precipitation patterns. It also impacts seasonality; in some places, certain seasons come earlier, later, or last different amounts of time compared to how they have historically.
All of these conditions influence how certain pathogens reproduce and spread, and certain changes driven by climate change may cultivate optimal conditions or extend seasons for select pathogens to thrive.
Extreme weather events related to climate change, like flooding and wildfires, may exacerbate ecosystem fragmentation, and/or making environmental conditions even more amenable to disease outbreak.
And many of the other mentioned risk factors that encourage zoonotic disease transmission contribute to climate change, feeding a vicious cycle.
Overuse of antibiotics can increase the likelihood that pathogens are exposed to an antimicrobial so many times they build up resistance to the drugs.
The overuse of antimicrobials in animals encourages the emergence and spread of bacteria that are resistant to antibiotics. These antibiotic-resistant bacteria thus have a greater risk of being passed to humans.
Furthermore, antimicrobial resistance developed in livestock can then affect humans, meaning that when humans get sick and need antibiotics, they may have already acquired resistance to them due to resistance developed in livestock, and antibiotics will no longer work.
What can we do about this?
Many factors impact the rates of zoonotic disease transmission. While the differences between these factors exist, a theme clearly emerges among them: human activity has disrupted the ecosystem, and our detrimental impact on the earth has altered conditions that may foster infectious disease outbreaks.
As humans, we can take steps to change how we interact with our environment.
First, we should be striving to maintain natural ecosystems. This can start with proper monitoring of ecosystems to track, and preferably hault, substantial changes. The UN Food and Agriculture Organization has an initiate known as the UN Decade on Ecosystem Restoration to prevent future degradation of ecosystems and reverse existing changes
We must also work collectively to fight climate change. Furthermore, we should rethink intense animal agriculture practices and our overuse of antibiotics.
While these are large goals, they seem more feasible when you realize that many overlap. For example, decreasing animal agriculture will also decrease further ecosystem degradation, decrease antibiotic overuse, and decrease carbon and methane emissions that contribute to climate change.
If you feel helpless on a personal level, this post has some specific actionables for how to effectively combat climate change on an individual scale, and how to effectively advocate for sweeping changes to be made in the fight against climate change.
Zoonotic diseases are diseases spread between animals, insects, ticks, and people. Changes in the environment, pathogen, or host increase the risks of these diseases spreading. Ecosystem destruction, climate change, animal agriculture, and antibiotic overuse can increase the risk of zoonotic disease spread and outbreak.
Zoonotic diseases are not new and they are likely to not go away anytime soon. But there are things we can do now to slow ecosystem destruction, climate change, animal agriculture, and antibiotic overuse to help prevent these risks from increasing.
I’ll be posting a brownie recipe very soon! Super relevant, to this “How does human interaction with the environment impact zoonotic disease transmission?” post, I know. Ha!
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