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COVID-19. H1N1. Rabies. Ebola.
Like many others, all of these infectious diseases originated in animals before humans. According to the Centers for Disease Control and Prevention, the US national health protection agency, more than 75% of emerging infectious diseases in humans are zoonotic, and their origins have deep linkages to our rapidly changing environment. “They are a major part of human history and have shaped our evolution,” says Maarten Hoek, a senior public health manager and Non-Executive Director of Madaktari Africa during a 2020 EJN webinar on Zoonotic Diseases and the Spread of Epidemics. “[Zoonotic diseases] are everywhere – and most people have likely suffered from one.”
The key message, Hoek says, is that “everything is tightly linked.”
Through scientific underpinnings, story examples and ideas, case studies and further reading, this tipsheet aims to provide journalists with the resources and knowledge to report on the growing global issue of zoonotic disease threats in their region.
🔊Listen to this quote from Hongying Li, Senior Program Coordinator & Research Scientist at the EcoHealth Alliance
Starting with the basics: What does zoonotic mean?
Zoonotic is the term that describes a disease that can jump between animals, other invertebrates and humans. Zoonotic diseases, like COVID-19, present major challenges to human health and the economy. As of February 20, 2022, COVID-19 has led to the death of almost six million individuals globally, while economic response costs to the pandemic are estimated at $11 trillion with an estimated future loss of $10 trillion in earnings. As urbanization and deforestation increase, we are witnessing the loss of biodiversity, damage to local ecosystems, climate change, rising air pollution, desertification, growing antimicrobial resistance in the animal health sector and serious inequity and poverty among the human population.
As human-wildlife contact increases—whether through habitat encroachment or poaching—new zoonotic pathogens are bound to emerge.
As human-wildlife contact increases and humans and wildlife live in closer proximity—whether through habitat encroachment or poaching—new zoonotic pathogens are bound to emerge. Over the last several decades, the risk of spillover of these pathogens into the human population has rapidly increased, driven by land-use change such as deforestation, the wildlife trade and wild meat consumption, climate change and the intensification of industrial animal agriculture.
How can a disease spread from an animal to a human?
When a disease crosses from one species to another, it’s called spillover. Just like coffee can spill from your mug, a disease, like COVID-19, can ‘spill’ from one species to another. Zoonotic diseases tend to spill over in places where humans and animals are in close contact, like wet markets, intensive agricultural settings, or areas where wild animals have encroached on human civilization due to loss of habitat.
For example, scientists believe that Ebola was likely transmitted from primates to humans through the hunting, trading and eating of bushmeat. The SARS outbreak in 2002 was traced to the consumption of wild animals in the southern city of Guangzhou, in China.
“Everything is tightly linked.”
Maarten Hoek, Senior Public Health Manager and Non-Executive Director of Madaktari Africa
While it may seem like disease spillover is immediate, (one day it’s in a bat, the next day it’s in a human), it is actually a complex transmission process that requires multiple epidemiological and environmental factors to align.
Usually, the virus starts in a reservoir, which is an area where the virus is prevalent. Some examples of reservoirs include humans, animals (e.g, bats, pangolins) and the environment (e.g water, soil). Then, the virus replicates in a primary host. In some cases, this infection may not be harmful to them, meaning the primary host may be a carrier of the virus without displaying any signs of infection. But in other cases, the infection may be harmful to the primary host.
When a species comes in contact with the reservoir, which is called exposure, they may contract the virus. Contact with the virus can lead to exposure but does not always guarantee that the recipient host will become infected. The recipient host's probability of becoming infected depends on a few different factors, including their innate immune response and their unique molecular compatibility with the pathogen.
“Each time a virus replicates in a human cell, its genetic material changes,” says Hoek, in the 2020 EJN webinar on zoonotic diseases and the spread of epidemics. “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.”
In the stage called breakthrough, the virus mutates and overcomes the response of the new host, and transmission occurs. Spillover can only occur when the genetic material in the virus changes enough to enable the virus to survive and replicate in another species, like humans. Once a new mutated version of the virus exists, the disease can then spread throughout the new species, much like it did during the COVID-19 pandemic.
For scientists, understanding how and why spillover events happen can improve our ability to predict or prevent future outbreaks, or pandemics. But for journalists, communicating these scientific findings is about identifying who needs access to science-based and fact-checked information, and how to best communicate it. “Scientific research, new technologies, paradigm shifts, challenges to accepted scientific 'truths' aren't just science stories. They play a major role in key political, economic, cultural and social policy discussions, as well as in public dialogue,” says the Science Literacy Project. Applying a socio-cultural and regional lens to understanding the intersections of human, environmental and animal health that drive zoonotic diseases can help drive funding toward further study or policy action.
This graph illustrates how different types of viruses, such as SARS and MERS, can replicate between hosts and spread among humans.
A closer look at the science
Journalists reporting on zoonotic diseases would do well to gain a functional understanding of the science at play:
In the first phase, the amount of pathogen available to the human host at a given moment, known as the pathogen pressure, is determined by the human’s interactions with the reservoir host. This also depends on the prevalence of the pathogen in the area, and how well the pathogen can survive, develop and disseminate among a new species.
- Illustrative example: A woman moves a dead bat to the side of the road, unaware that the bat (the reservoir host), is infected with a novel virus that has been allowed to replicate and mutate enough to survive in a human host. She does not wash her hands well after handling the bat, and eats a meal two hours later, ingesting the pathogen.
Second, human behavior determines pathogen exposure; specifically, the likelihood, route and dose of exposure. Human actions that change our natural environment, such as deforestation, bushmeat hunting and consumption, illegal wildlife trade and climate change increase the likelihood of our exposure to pathogens. When our exposure is increased, the event of a spillover event rises.
- Illustrative example: This woman has been exposed to the pathogen because illegal logging in the forest surrounding her village has destroyed bat habitat and caused this species of bat to move closer to human settlements. Thus, human behavior has increased both pathogen pressure and pathogen exposure.
Third, genetic, physiological and immunological attributes of the recipient human host, together with the dose and route of exposure, affect how likely an individual is to become infected with the pathogen, and how severe the disease will be. This can vary from person to person.
Which animals are of particular concern?
Scientists have determined that bats, primates, rodents and other wild mammalian species are commonly involved in spillover events. Threatened and endangered wildlife species can also share more viruses with humans due to loss of their native habitat.
Intensive agricultural practices that keep livestock animals in tightly packed environments allow diseases to spread, as well. The animal products that we eat including eggs, poultry, dairy and meat have the potential to spread disease. In 2009, the H1N1 virus, a common infection in pigs, caused a pandemic. In February 2022, the pathogenic avian influenza (HPAI) A(H5N1) viruses were detected in U.S. wild birds and commercial and domestic poultry, according to the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspective Service (APHIS).
🔊Listen to this quote from Jason Gale, Senior Editor and Biosecurity Correspondent, Bloomberg News Australia:
Countering fearmongering and misinformation
An environmental journalist reporting on zoonoses should ensure that wherever possible, viewpoints are attributable to a primary source, and all facts, figures and studies cited are carefully fact-checked. Stories will need to stand counter to sensationalist and inflammatory stories or attempts to spread misinformation.
In the context of COVID-19, the ‘lab leak theory’, which gained traction throughout 2020 and 2021, proposed that SARS- CoV-2, the virus that caused the COVID-19 pandemic, originated from and leaked from the Wuhan Institute of Virology in Wuhan, China. In early 2022, two landmark studies emerged, disproving those claims and pointing to the origins of COVID-19 in a wet market – which has long been the leading theory among the scientific community.
“Together, these analyses provide dispositive evidence for the emergence of SARS-CoV-2 via the wildlife trade,” the study said.
In the context of a novel virus like COVID-19, when there is no database of pre-existing research to draw on and the evidence is shifting in real time, accurate science and health journalism becomes imperative.
“More and better reporting can build an informed citizen, who would likely be less susceptible to disinformation during a zoonotic disease outbreak and avoid the situation we have seen with Covid-19 with lives lost due to misinformation.”
Kok Eng Amy Sim, Asia Pacific Project Manager, Earth Journalism Network
As we have seen through the “anti-vax”, anti-mask, and COVID-19 conspiracy movements, public confusion and rapidly changing research can be weaponized to spread misinformation. As the nexus between scientific research and the public, journalists have not only the power, but the responsibility to curb misinformation.
Remember, stories can do more than just report the latest evidence and public health recommendations; they can actively engage with and tackle the reasons why people believe fallacies driven by xenophobia, fear or misinformation. You may find it illuminating to explore how external social, political and behavioral shifts contribute to the spread of fearmongering and misinformation. Consider reaching out to social scientists, behavioral specialists and misinformation researchers for their insight.
Reporting on zoonotic disease outbreaks: Finding the local context in a global problem
Zoonotic spillovers and outbreaks are driven by social, behavioral and economic factors. How do these factors apply to your region and community? Consider some of the following questions to ground your story, and make it relevant to your audience and region:
- What is the relationship between conservation and human wildlife-conflict? How is this being addressed by government officials and policymakers in your region?
- What is the role of the bushmeat trade in the local economy and in industrial agriculture? Can you convey its threats while remaining sensitive to cultural practices and food traditions?
- Are there education initiatives and campaigns underway to educate people and communities on zoonotic disease outbreaks?
What is a vector-borne disease?
A vector-borne disease is a disease transmitted to humans by arthropods (insects) such as mosquitos, ticks and fleas. Many of these insects ingest pathogens during a blood meal from an infected host, transmitting it (often at a later point in the pathogen’s life cycle) to a human host. Some parasites, such as malaria, rely on a combination of arthropods and animal hosts to complete their life cycle. Vector-borne viruses are also known as arboviruses: viruses transmitted to humans by arthropod vectors.
Are vector-borne diseases zoonotic diseases?
There are two points to address in answering this question. If we use a loose definition of zoonotic diseases—that is, that zoonotic diseases are simply those spread from animals to humans–then vector-borne infections make the cut. This is why you may see reporters and organizations sometimes grouping diseases like malaria as zoonotic. However, if we use the technical definition of a zoonotic disease, then there is an important difference.
The CDC and WHO define zoonotic diseases (viruses, bacteria, parasites and other pathogens) as diseases that have jumped from vertebrate animals (animals with backbones) to humans. Many vector-borne diseases are found in animal populations but are primarily transmitted to humans by intermediary arthropod hosts. Others, such as most kinds of malaria, do not involve vertebrate animals in their life cycle at all.
Transmission of zoonotic disease may happen with or without the involvement of vectors such as ticks or mosquitos—as long as the original reservoir or host is a vertebrate animal.
Let’s consider the example of Lyme disease. Lyme disease is a zoonotic disease because although the Lyme bacterium Borrelia burgdorferi is transmitted from blacklegged ticks to humans, its first hosts are typically rodents, which are a vertebrate animal.
Should I refer to malaria as a zoonotic disease in my reporting?
There are five types of malaria parasites that infect humans: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, Plasmodium malariae and Plasmodium knowlesi. Only the last one, P. knowlesi, meets the U.S. Centers for Disease Control (CDC) and World Health Organization (WHO) definition of a zoonotic disease. Most cases of malaria are technically not zoonotic because the parasite has no initial vertebrate host. In most cases, it is transmitted directly to human hosts from Anopheles mosquito hosts. There is one exception, however: P. knowlesi malaria is a strain of malaria that is transmitted from monkeys such as wild macaques to mosquitoes to humans, thus implicating vertebrate animals in the chain of transmission.
Read EJN-supported stories on malaria:
- How the fight against malaria reached a fever pitch on Odisha
- Breaking the cycle of malaria and malnutrition in eastern India
Cases of P. knowlesi malaria have been recorded in Southeast Asia, notably Malaysia, Indonesia, Thailand, Myanmar, Vietnam, Cambodia, the Philippines, Brunei and Singapore. So, when considering a technical definition of zoonotic malaria, it is important to ascertain which type of malaria parasite is implicated.
Can genetically modified (GM) mosquitoes still spread vector-borne and zoonotic diseases?
Ae. aegypti mosquitoes can be genetically modified in a laboratory and used to control other Ae. aegypti mosquitoes in a community. They work by altering a mosquito’s gut genes to make them spread antimalarial genes to the next generation of their species. When released into the environment, mosquitoes would spread genes that either reduce mosquito populations or make the insects less likely to spread the malaria parasite.
GM mosquitoes have been successfully used in parts of Brazil, the Cayman Islands, Panama and India to control Ae. aegypti mosquitoes. Since 2019, over 1 billion mosquitoes have been released. Though studies on GM mosquitoes are promising to curb vector-borne diseases such as malaria, the release of GM mosquitoes is not intended to stop an ongoing disease outbreak. Instead, GM mosquitoes are meant to help prevent disease outbreaks. Reducing the numbers of mosquitoes that can spread germs can help reduce the chance of an outbreak starting, but the best way to prevent disease is to control mosquitoes before an outbreak happens.
Is dengue fever technically a zoonotic disease? What about chikungunya and Zika?
Chikungunya, dengue and Zika are all viral diseases transmitted to humans primarily by infected Aedes mosquitos. They do not need non-human vertebrate animals to complete their life cycle, but they have been found in non-human vertebrate animal populations such as primates. So, sometimes they are transmitted zoonotically and other times they aren’t. The important thing to remember is that these diseases only technically count as zoonoses when transmission between vertebrate animals and humans is proven in the chain of transmission — which is not an easy thing to prove for every case of dengue fever!
Read EJN-supported stories on dengue:
- From Sri-Lanka to China: What India can learn from the world about combatting dengue, malaria
- How climate change has altered the rules of Odisha’s battle against vector-borne diseases
Incidentally, why do we capitalize the first letter of Zika virus, but not malaria and leishmaniasis?
The first letter of Zika is capitalized because it is named after a place—the Zika forest in Uganda, where the disease was first identified. Another example is Ebola, named after a river in the Democratic Republic of Congo, or Rift Valley fever, which is named after a location in Kenya. In 2015, the WHO released new guidelines, calling for a halt to the practice of naming emerging diseases after places, to mitigate stigma. At the beginning of the COVID-19 pandemic, the virus was incidentally referred to as the ‘Wuhan Virus,’ or the ‘China Virus,’ given its emergence in China. In mid-2021, the delta variant was was incorrectly referred to as the ‘Indian Variant,’ given its emergence in India.
Read EJN-supported stories on malaria and leishmaniasis:
Where does this leave us? Should we refer to malaria, dengue and Zika as zoonotic diseases in our reporting?
Unless you have sourced knowledge that specifies that transmission has taken place from vertebrate animals to humans, best practice is to refer to these three infections as vector-borne diseases. If you need to use an umbrella term in your reporting to cover all spillover infections, it’s optimal to use the joint description of ‘vector-borne and zoonotic diseases’.
How can we prevent more zoonotic diseases from spilling over in the future?
“Zoonotic diseases are a stark reminder of how people and nature are interconnected. Human activities that encroach upon wild places increase our contact with wildlife and the risk of spillover events. To lower that risk, we must rebalance our relationship with nature,” according to World Wildlife Fund. This could mean reducing or stopping the sale of wild mammals and birds at food markets, reducing illegal wildlife trade, cutting down on intensive agriculture and leaving wild environments intact.
It is also critical that communities and individuals are educated about zoonotic diseases, and practice hygiene when handling food products and animals.
🔊Listen to this quote from Dr Salome Bukachi, Medical Anthropologist at the University of Nairobi:
- Wildlife Trafficking
- Bushmeat Trade and Consumption
- Zoonotic Diseases and Inequities
- Human-wildlife Conflict
- How to report a disease outbreak or pandemic
- Nieman Guide to Covering Pandemics
- Journalism in the COVID-19 era: Why Health Reporting Must Focus on Human, Animal and Environmental Issues to Help Combat Future Pandemics
- National Geographic, How do Infections like the Coronavirus Jump From Animals to People?
- The Kissing Bug: A True Story of a Family, an Insect and a Nation’s Neglect of a Deadly Disease by Daisy Hernández
- Inflamed: Deep Medicine and the Anatomy of Injustice by Rupa Marya and Raj Patel
- Infections and Inequalities: The Modern Plagues,by Paul Farmer
- Nature, Bats are Global Reservoir for Deadly Coronaviruses by Amy Maxmen
- No Time to Lose: A Life in Pursuit of Deadly Viruses by Peter Piot
- Pandemic: Tracking Contagions from Cholera to Ebola and Beyond by Sonia Shah
- Rats, Lice and History by Hans Zinsser
- Social media misinformation model, developed by Stanford University evaluates individuals’ ‘susceptibility’ to false information, and their likelihood to spread it to others.
- Pro Med Mail produces a weekly email digest of infectious disease threats, many of which are spillover events and can serve as ideas for stories.
- To read about tips for reporting on One Health, and incorporating the One Health approach in your reporting, click here.
This tipsheet was produced by Signi Livingstone-Peters, with input from Katherina Thomas, Ida Jooste, Amrita Gupta, Stella Paul and Kok Eng Amy Sim.
Banner image: Zoonotic diseases are everywhere – and most people have likely suffered from one / Credit: Viktor Forgacs / Unsplash.