Zoonotic Diseases and the Spread of Epidemics

Bats in a cage
Zoonotic Diseases and the Spread of Epidemics
How the wildlife trade and disruption of ecosystems have increased the spread of epidemics

In response to the Covid-19 outbreak, Internews’ Earth Journalism Network is sponsoring a series of webinars on how to cover the pandemic and related environmental issues, including zoonotic diseases and wildlife trafficking. One of our first webinars explores the threats posed by human-wildlife interactions and how environmental disruption is heightening our risk of zoonotic pandemics like this new coronavirus.

Our guest speaker Maarten Hoek is a senior public health manager and Non-Executive Director of Madaktari Africa, a non-profit organization working to educate and train health care workers in Sub-Saharan Africa. A doctor in veterinary medicine, epidemiologist, risk analyst, and medical consultant in public health, Hoek has worked on health strengthening projects in Africa for eight years and is currently carrying out research on coronaviruses. 

You can watch a recording of the entire webinar here that includes a Q&A at the end. Note that the answers from Hoek have been edited for length and clarity, and they reflect his own views, not that of any institution.


Most people think zoonotic diseases – illnesses transmitted from animals to humans – are rare or happen only occasionally, and when they do occur we see outbreaks, like the one we’re seeing today with Covid-19. But in fact, Hoek explained, they are a major part of human history and have shaped our evolution. They are also everywhere – and most people have likely suffered from one.

The key message, said Hoek: “Everything is very tightly linked.”

Zoonosis: What is it?

A disease passed from animals to humans. There is also reverse zoonosis, which is passed from humans to animals. Common zoonotic diseases include SARS, MERS and Covid-19. According to the World Health Organization (WHO), 61% of all current human diseases are of zoonotic origin, while 75% of newly discovered diseases are zoonotic.

Why do zoonotic diseases pose risks?

  1. Global climate change: The Earth is warming up, which leads to changes in vegetation, animal and human behavior. And all those changes may cause diseases to spread to new geographic areas. They may also allow diseases to infect new animal species that weren’t previously in that area, and they may allow diseases to spill over into other animal populations.
  2. Health care: Overuse of antimicrobials in medicine, low doctor/patient ratios, lack of early detection.
  3. Farming and animal husbandry: Putting huge numbers of animals in a confined space – as is the case with industrial farming – is very risky. Shipping them around the world can also facilitate the spread of disease.
  4. Human factors: Population increases lead to more interactions between humans and animals, increasing the ability for pathogens to spread. In addition, aging populations and those with chronic illness create a pool of people who are more vulnerable and more likely to suffer from the ill effects from infections.

Many people have heard of zoonotic diseases like SARS, MERS and Swine Flu. But what is Disease X?

Listed among the priority diseases identified by the WHO for research and development in public health emergencies, it’s not a specific disease but a label for the disease that is yet to come. In 2019 Disease X was Corona, and then Corona was named Covid-19 and became a known entity. “But there are many more diseases that we are just not aware of yet, and therefore we have to find ways to prepare for them, to identify them and to act on them quickly – possibly quicker than we did now with Covid-19,” Hoek said. 

What’s being done or needs to be done to address the problem?

  1. OneHealth: A collaboration between institutes that work on human, animal and environmental health.
  2. Increased education: On hygiene and practical know-how.
  3. More economic development: Poverty reduction gives people the means to protect themselves.
  4. Ecosystem restoration: Biodiversity is very important. The more homogenous species are, the easier it is for a virus to adapt and spread between them.
  5. Changes in dietary practices: Vegetarian diets are safer and better for the environment.

What role does the media play in the spread of diseases?

A powerful one! “You may have all the knowledge on zoonotic diseases … but the way you bring the message is super important,” Hoek said. Sensationalist reporting and fake or misleading news is hugely detrimental to human health because it causes fear, creates confusion and can be counterproductive.


Here are some questions we address in our webinar.

Q: How do we know Covid-19 is a zoonotic disease and what other criteria distinguish between zoonotic and human-transmitted diseases?

A: We know it’s zoonotic in origin because we looked at its genetic makeup. There are a couple of coronaviruses in the human population and a lot of them in animal populations. Scientists have been monitoring them since the SARS and MERS outbreaks because they know these diseases can jump species [what’s known as “spillover”].

Researchers monitoring the genetic makeup of the coronavirus have found similar viruses in bats and pangolins, but it’s not a perfect match. They’re still looking for the exact animal source and may never find it. But there are strong indications that it came from bats via a pangolin-like animal. SARS, for example, was found to come from bats via a palm civet cat, since researchers were able to perfectly match the genetic makeup of a virus found in both civets and humans. 

Q: Can infected pets transmit Covid-19 to humans and vice versa?

A: Researchers are looking into it. There have been cases where pets have been infected, which researchers believe to have come from their owners, so an example of reverse zoonosis. But while the role pets play in the transmission of Corona is still being investigated, it’s safe to say it is almost exclusively human to human, so no need to be concerned about infected pets for your health. If you’ve been diagnosed with Covid-19, you may want to keep an eye on your pet. [Public Service Announcement: Folks, don’t abandon your pets.]

Q: Is it possible to quantify how much greater the risk of zoonotic diseases are due to human activities, such as deforestation, and climate change?

A: It’s very difficult to make accurate calculations or bold statements. I can only say that from the changes we see now and from what we have experienced, it will become an increasing problem due to our increased population and, therefore, our increased human-to-human interactions but also due to loss of biodiversity and ecological damage. I can’t say how much it will be, but let me say that we do expect to see more in the not-too-distant future.

Q: What happens to the virus’s DNA as it mutates? Will that prevent a problem for developing a vaccination?

A: Yes, each time a virus replicates in a human cell, it’s genetic material changes. In most cases this results in the virus not being viable, not being infectious, and in some cases, it results in the virus being more infectious. And also as we see viral infections evolving with us, we see that the mortality rate of the virus actually goes down, while the virulence – the ability to infect us – goes up, so it learns to live with us. That is a natural selection process.

At this stage, the genetic variety is not so large that it will cause us trouble [in developing a vaccine]. What we do expect at this stage is that the coronavirus will start to behave similarly to what we see with influenza, in that it will keep on changing, and we’ll do our best to keep an eye on which strains are circulating either in the global South or global North so we can start to make vaccines for that. But we still have a lot of work to do on vaccines. They’re still far away, and each vaccine works slightly differently, so it also depends on what type of vaccine we can find.

Q: Does the virus transmit through waterways or food?

A: No. It is mostly transmitted now through pretty close contact, as well as through the droplets produced when you sneeze [or cough], and by [a lack of] hand hygiene, [meaning you can become infected by touching your unwashed hands to your face].  

Q: Do animals shed more viruses when stressed?

A: When animals and humans are stressed their immune system is suppressed, and therefore the virus is less controlled and is able to infect more cells within their bodies. A good example of that is with the 1918 Spanish influenza outbreak [after World War I]. We saw that a lot of people were not just mentally but physically stressed – there was malnutrition and very poor health care – and all these stress factors result in sub-optimal functioning of your immune system. Therefore, a virus can do its thing for longer before it’s controlled, if at all, by the body, and [the virus] therefore spreads more quickly, as well.

Q: In that case, do zoological gardens (zoos and animal parks) pose a threat?

A: It depends on the condition of the zoo. Luckily more zoos now mimic the natural habitat of the animal, so these animals are less stressed. The international trade in animals is of course a risk for the spread of disease and introducing new disease, and zoos have a role to play in that as well. There are zoos that are very aware of this and deal with it well, and there are places where zoos are a part of the problem. But if you compare the trade for zoo animals to the trade in bushmeat, then bushmeat stands out for risk. 

Q: Can we only identify these zoonotic diseases when they’ve jumped the species barrier into humans? Or are there scientists going into the wild to try and identify potential threatening microbes?

A: That’s absolutely correct. Bats, particularly, are subject to study, and bats are interesting to study because they live in huge colonies and they cover large geographical areas so they have a lot of exposure to all kinds of pathogens. Because they live in large populations, pathogens have a lot of opportunity to infect those bats and then learn to live with them, so to speak [that’s because if a pathogen kills off its host quickly, it’s not very successful.] And bats have been around for millions of years, so bats carry a lot of diseases that are of interest to us. They are also an important reservoir for zoonotic diseases to people. What we then look at is the type of diseases they carry – the family of viruses – and then we try to assess how much change a virus or pathogen needs to undergo before we know or think it may be a problem for us.

There has been a lot of work done on engineering viruses and studying what makes it risky for us and what type of changes need to happen so we can make a rough assessment of how likely it is to jump the species barrier to us. As we progress with our science and capacity, that will become much more accurate.

Q: You talked about how industrial farming poses a risk also, the fact that when animals or livestock are given antimicrobials it can create resistance within a pathogen. What kind of practices should factory farms use to avoid the spread of these zoonotic diseases?

A: The reason we resort to anti-microbial use is because we put too many animals in a space that is not suitable to house those animals, therefore they’re stressed and therefore they need artificial interventions to stay healthy. So if we do animal farming, we need to do it in a way where animals are not stressed but are actually in an environment that’s not endangering their health, and therefore endangering our health.

Q: Are there other ways food systems should be made more sustainable or re-planned to prevent the risk of zoonotic diseases?

A: There are a lot! It basically has to do with the fact that if you put a lot of animals together into a confined space, you create trouble. The rest is really up for the next webinar.

Q: Why are digital surveillance of citizens and forceful quarantine being used as counter measures now, but not for Zika, Ebola, SARS and past outbreaks that had a higher mortality rate?

A: Well, there are technological developments that have only been feasible recently, partly. Partly there is now a lot of money available because it’s a proper pandemic, affecting also richer countries and therefore more resources. The huge Ebola outbreak in Western Africa affected mostly developing countries and therefore resources were much slower to pour in, and IT development is very expensive.

We also see that there is a shifting attitude and acceptance of using digital information in controlling disease outbreaks. For example, in China there is already a mandatory app you have to download on your phone and it shows a QR code and it is green as long as you haven’t been in close contact with a confirmed or suspected case, and your phone tracks all your whereabouts and if somehow your phone tells you you’ve been in close proximity with an [infected] case, your phone beeps and shows a red QR code and then you’re supposed to quickly self-isolate until you’re tested.

Such technology, on the one hand, is very exciting. On the other hand, it’s a bit scary because it very much infringes on our freedom, [privacy] and independence. But slowly, as people start to realize these lockdowns are also very drastic measures, and in fact using technology, such as apps on phones, may allow us to start society back up, more and more people will be more accepting of technological advances that do infringe on our private life.

Q: Many of these diseases seem to be generated from Africa and Asia, is there any reason for this? Is that even correct?

A: That is correct. The reasons are slightly different. In Asia, we see a huge population of people, we see a huge trade in live animals, live animal markets, and therefore there’s a lot of human-animal interaction and there is a lot of opportunity for pathogens to spill over into the human population and start causing outbreaks.

In Sub-Saharan Africa, we see that there is generally a lack of means for people to create safe spaces for themselves in terms of bed nets for mosquitos or in terms of safe and hygienic cooking. So there it’s mostly an economic issue that needs to be addressed.

Q: As you know, China has at least temporarily banned the wildlife trade and closed down wildlife markets. Would you recommend that ban be permanent or what other regulations do you think need to be put in place?

A: There’s a lot of pressure globally on China to permanently ban live animal markets, and I hope this will have an effect because the impacts on both local and the global populations are very far-reaching, and each country has a responsibility to mitigate these risks, and China has a clear risk factor in the live markets and needs to address it.

Q: Any sense of how long this pandemic will this last?

A: What we expect now is that this coronavirus will start to act as the influenza virus is doing. We will likely see a seasonal affect. At the stage we are in now, globally this will last another couple of months if not much longer before everything goes back to reasonably normal. This is really an unprecedented event that will have a profound impact on how we interact with the environment, with the world and just generally. It will hopefully be an event that will make us think about our ecological footprint and the way we treat and live with animals, but also each other in society. There’s a lot of need for reflection on how we deal with the planet and with each other.

Banner image: Bats in a cage / Credit: Steven Pahel on Unsplash

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