The spillover of viral pathogens from animals to humans has become a hot topic since the outbreak of COVID-19. IMAGE/UGC.
By PATRICK MAYOYO
newshub@eyewitness.africa
Tola Barisa, a father of four from Banisa village in Tana River County, faced a dire situation after a six-month drought decimated his cattle, leaving his family on the brink of starvation.
Desperate, he ventured into the forest, capturing two squirrels to feed his wife and children. Unbeknownst to him, this act of survival contributed to the spread of zoonotic diseases—germs that jump from animals to humans.
As climate change intensifies droughts and food shortages, wildlife encroaches on human habitats, escalating risks of disease transmission.
Experts now warn that illegal wildlife trade and habitat destruction exacerbate this crisis, linking human activities to zoonotic disease outbreaks like COVID-19 and Mpox.
Dr. David Obura, a marine scientist at CORDIO East Africa, emphasizes that environmental changes foster conditions for pathogens to leap between species.
“Although I have no specific expertise in this area, it is a very well established mechanism of ‘zoonoses’, or the movement of diseases (especially viruses) from animal into humans when habitats are changed and the animals move, or people penetrate further into the habitats, or the environmental changes make the disease vectors more able to jump between species,” Dr Obura said.
Climate change impacts like drought that result in food shortages for both animals and people is now at the heart of the spread of zoonotic diseases.
While animals are forced to migrate to new areas in search of food and water due to adverse weather events, people living near wildlife parks and sanctuaries are on the other hand forced to hunt wildlife for food and income.
The spillover of viral pathogens from animals to humans has become a hot topic over the last five years following the outbreak of the COVID-19 pandemic that is caused by the coronavirus and now the ongoing spread of Mpox that is caused by the monkeypox virus (MPXV).
What Research Has Revealed
Mpox (previously known as monkeypox) is a viral disease caused by an orthopox virus called monkeypox virus. PHOTO/WHO.
The World Health Organisation (WHO) says there are three classes of zoonotic diseases namely: endemic zoonoses which are present in many places and affect many people and animals; epidemic zoonoses which are sporadic in temporal and spatial distribution; and emerging and re-emerging zoonoses which are newly appearing in a population or have existed previously but are rapidly increasing in incidence or geographical range.
Examples of the latter include Rift Valley fever, SARS, pandemic influenza H1N1 2009, Yellow fever, Avian Influenza (H5N1) and (H7N9), West Nile virus and the Middle East respiratory syndrome coronavirus (MERS-CoV) reported in the recent past.
Other zoonotic diseases includes brucellosis, bubonic plague, glanders, Lyme disease, rabies, dengue fever, Ebola, hantavirus syndromes, leptospirosis, salmonellosis, tuberculosis, anthrax, bacterial pneumonia, gastrointestinal parasites, measles, Marburg, Crimean Congo hemorrhagic fever and Rift Valley fever, toxoplasmosis, Q fever, cryptosporidiosis and campylobacter infection among others.
Research shows that the illegal trafficking of wild animals and plants is not only damaging biodiversity around the globe, but it is also being linked to spreading of diseases from wild animals to humans.
Scientists say there are many drivers of the spread of zoonotic diseases from animals to people that include shrinking wildlife habitats due to climate change and human activities that include deforestation, poaching and trafficking of wildlife products, urbanization, mining, infrastructure development and agriculture and land use among others.
A study titled Climate change increases cross-species viral transmission risk says changes in climate and land use will lead to opportunities for viral sharing among previously geographically isolated species of wildlife and in some cases, this will facilitate zoonotic spillover—a mechanistic link between global environmental change and disease emergence.
Another study, Climate Change and Zoonoses: A Review of Concepts, Definitions, and Bibliometrics says climate change can have a complex impact that also influences human and animal health adding that climate change alters the conditions for pathogens and vectors of zoonotic diseases.
“Signs of this are the increasing spread of the West Nile and Usutu viruses and the establishment of new vector species, such as specific mosquito and tick species, in Europe and other parts of the world,” the report notes.
Another research; Climate change is driving the expansion of zoonotic diseases says climate change is expanding the range of many infectious diseases in particular, vector-borne diseases, such as dengue or malaria, which are advancing.
How Zoonotic Diseases Spread
Launch of COVID-19 vaccination in Kenya. PHOTO/UNICEF.
Dr Jeanne M. Fair and her colleagues at Los Alamos National Laboratory, New Mexico, USA, who are pioneering a multidisciplinary approach to the problem says as planet heats up infectious diseases that were once confined to warmer latitudes are slowly expanding their range.
“Our climate is changing. As the planet heats up, infectious diseases that were once confined to warmer latitudes are slowly expanding their range. In particular, zoonotic diseases – diseases that spread from non-human animals to humans – are taking advantage of the greater range made available by climate change. One group of zoonoses, vector-borne pathogens, pose an increasing risk to human health,” their study notes.
According to, Dr Gladys Kalema-Zikusoka, the founder and CEO of Conservation Through Public Health (CTPH), research shows that the illegal trafficking of wild animals and plants is not only damaging biodiversity around the globe, but it is also being linked to spreading of diseases from wild animals to humans.
“This is true because the risk increases especially among poachers, local market sellers and consumers. For example, the lack of proper hygienic measures during commercialization of wild meat increases the risk of disease spread,” she said.
Dr Kalema-Zikusoka said the main drivers of the spread of zoonotic diseases includes, global population growth and population mobility, urbanization, food preferences, companion animals and global food systems and food safety.
“Once a zoonotic disease has emerged, its spread in the human population is likely to be facilitated by population movements. Migration, also called long-term population resettlement, is likely to spread diseases that have a long period of latency or duration of infectiousness, whereas short-term mobility for periods of days or weeks, typical of ‘travel’ patterns, may rapidly spread diseases with short resolution periods,” she noted.
Dr Gladys Kalema-Zikusoka, the founder and CEO of Conservation Through Public Health (CTPH). PHOTO/CTPH.
Dr Kalema-Zikusoka said while populations in urban areas are typically less exposed to animal contact than rural populations, depending on the market structures and production systems of live food animals, urbanites may also live in more crowded conditions conducive to disease transmission.
She said food preferences puts consumers in contact with live fowl and freshly killed wild animals (primates, reptiles, bats, etc.) as well as domesticated animals (e.g., dogs, civets, pigs) and their faeces, which may be infected with pathogens and contribute to the transmission of zoonotic diseases such as SARS and HPAI H5N1.
“Consumer preference for fresh products of wet markets is a complicating factor for health authorities that are trying to reduce health risks,” she emphasized.
Dr Kalema-Zikusoka observed that companion animals like dogs and cats commonly referred to as pets increases the chances of zoonotic infection from several different types of diseases (e.g., salmonellosis and Giardia, Cryptosporidium and toxoplasmosis, rabies).
“The transnational trade in exotic animals from birds to non-traditional companion animals (e.g., prairie dogs that carry monkeypox in the United States) is growing and creating new challenges for both human and animal health professionals and demands their closer collaboration,” she added.
The Role Livestock Production System and Human-Wildlife Conflict Play
Zoonotic diseases are responsible for an estimated 2.5 billion cases of illness and 2.7 million deaths worldwide, each year. GRAPHIC/PROVEG INTERNATIONAL.
She elaborated that the livestock production system, farm and market structure, and farm geography are major variables that define the emergence and consecutive spread of a zoonotic disease.
Dr Kalema-Zikusoka said legal and illegal trade- direct contact between the humans and wild animals during poaching, handling and consumption of wild meat accelerate the spread of a zoonotic disease.
“Environmental factors that are anthropogenic such as deforestation, agriculture, habitat fragmentation that lead to climate changes that can cause global warming leading to occurrence of heat related diseases,” she noted.
Dr Kalema-Zikusoka said human wildlife conflicts increase contact between the wildlife and humans as it involves wildlife invading human settlements which exposes them to unhygienic conditions such as human fecal matter from open defecation, contaminated human clothing’s, hence acquiring zoonotic parasites.
Climate change is a phenomenon that is already happening and in some areas of Africa it is taking place at an alarming rate.
According to a study titled Impact of climate change on human-wildlife conflicts in East Africa climate change is expected to cause an increase in weather-related disasters and extreme weather events, such as droughts, heat waves, floods, desertification, and vector-borne infestations.
The study by GE Otiang’a-Owiti, S Nyamasyo, E EMalel and R Onyuro says long-term changes in climate exacerbate environmental degradation leading to loss of wildlife habitat in many vulnerable places.
“Furthermore, climate change will alter the location and nature of the geographical environment, and wildlife will be forced to migrate to new areas as a way of adapting or face extinction,” the study notes.
It adds that as there are limited natural places left for wildlife to move to, this will likely bring wildlife into more densely populated human areas, and create situations of human wildlife conflict.
How Climate Change Intensifies Natural Disasters and Alters Terrestrial Ecosystems
Assistant Secretary-General of the United Nations (UN) and Deputy Executive Director of the United Nations Environment Programme (UNEP), Ms Elizabeth Maruma Mrema. PHOTO/UN.
The review looked at how climate change intensifies natural disasters, alters the functions and structure of terrestrial ecosystems thus making human wildlife conflicts inevitable in some regions with East African countries.
It’s thought that if global temperatures continue to rise by an average of 4.5°C since pre-industrial times, which is likely to happen if we do nothing to reduce our carbon emissions, half of the world’s wildlife could be lost from Earth’s most bio diverse places.
As ocean temperatures melt ice sheets — the hunting grounds of polar bears — these large carnivores have to search new areas for food, which is why 52 polar bears “invaded” a Russian town , looking for their next meal.
Poor African villagers who have had their entire yearly crop destroyed by a herd of hungry elephants are unfortunately not going to get a solution soon unless the global temperature rise is going to be limited to below 2°C in line with the Paris Agreement as climate change is only going to make these negative interactions between humans and wildlife more common.
According to World Wide Fund for Nature (WWF) conflict between people and animals is one of the main threats to the continued survival of many species in different parts of the world, and is also a significant threat to local human populations. If solutions to conflicts are not adequate, local support for conservation also declines.
According to the Assistant Secretary-General of the United Nations (UN) and Deputy Executive Director of the United Nations Environment Programme (UNEP), Ms Elizabeth Maruma Mrema, said biodiversity loss, ecosystem degradation and disease share many common drivers.
“Human disturbance of ecosystems and biodiversity loss are increasingly linked to the spread of zoonotic and vector-borne diseases. In many instances, climate change also increases the risks. Such human disturbances to natural ecosystems affect patterns of infectious diseases,” she said.
Ms Mrema responded to questions about the spread of the zoonotic diseases through email when she was still the Executive Secretary of the Secretariat Convention on Biological Diversity.
How Human Disturbances to Natural Ecosystems Affect Patterns of Infectious Diseases
An Mpox patient receiving treatment. PHOTO/WHO.
Ms Mrema said these developments reduce the abundance of some organisms, cause or reduce population growth in others, and it can disrupt some interactions among organisms, altering interactions between organisms and interactions between biotic (e.g., animal, plants, fungi, bacteria) and abiotic (e.g., temperature, water, air, sunlight) components of the environment.
“Of course, this is reason for extreme concern and a call for urgent action. The continuing loss of biodiversity on a global scale represents both direct and indirect threats to our health and wellbeing,” she noted.
Zoonotic diseases represent a significant societal challenge in terms of their health and economic impacts. One Health approaches to managing zoonotic diseases are becoming more prevalent, but require novel thinking, tools and cross-disciplinary collaboration.
Emerging infectious diseases represent a significant public health concern as they become more prevalent worldwide. It is estimated that about 60 percent of emerging infectious diseases are zoonotic, 72 percent of which have been estimated to originate from wildlife.
In 2019, thirteen different zoonoses had confirmed cases in humans within the European Union. This has been promoted by human technological advances that have caused an exponential growth in global population size and mobility, leading to increased probability of human-wildlife interactions and, therefore, exposure to zoonotic diseases.
Zoonotic disease exposure is particularly worrying when combined with the current human population trend and the expected increase in contacts among humans, livestock and other captive animals, as well as wildlife species.
The Role One Health Approach
A multidisciplinary approach and coordinated collaborations between the public health sector, veterinarians, ecologists and wildlife managers have become key to managing existing and to prevent emerging zoonotic diseases.
Ms Mrema said the One Health approach, which recognizes the intrinsic connection between human health, animal health and the health and resilience of nature, can help guide the world towards an effective post-2020 Global Biodiversity Framework and help achieve the UN Sustainable Development Goals (SDGs).
The One Health approach is a collaborative, interdisciplinary strategy that recognizes the interconnectedness of human, animal, and environmental health. It aims to address health issues at the interface of these domains, emphasizing that changes in one area can significantly impact the others.
On how wildlife trafficking contribute to the spread of zoonotic diseases, Ms Mrema said there is significant evidence that the wildlife trade is involved in the emergence of a range of diseases, particularly where the trade is poorly regulated.
“According to the World Health Organization, animals, particularly wild animals, are the source of more than 70% of all emerging infectious diseases in humans, many of which are caused by novel viruses. Traditional food markets, where live animals are held, slaughtered and dressed, pose a particular risk to workers and customers alike,” she said.
GRAPHIC/ONE HEALTH TRUST.
Dr Tedros Adhanom Ghebreyesus, WHO Director-General says human health does not exist in a vacuum, and nor experts efforts to protect and promote it noting that close links between human, animal and environmental health demand close collaboration, communication and coordination between the relevant sectors.
“The High-Level Expert Panel is a much-needed initiative to transform One Health from a concept to concrete policies that safeguard the health of the world’s people,” Dr Ghebreyesus said.
The importance of interdisciplinary approaches is highlighted by the interlinked nature of human, animal and ecosystem health, which led to the new concept of “One World One Health”.
Due to the complexity of zoonotic disease dynamics, the One Health approach offers the most effective means of studying zoonosis thanks to the collaboration of experts in a variety of research fields.
Despite such multidisciplinary efforts, the effect of stressors – i.e., direct and/or indirect disturbances such as hunting, habitat loss, and more broadly habitat and climate change – on animal ecology within human dominated landscapes and the potential emergence of zoonotic disease is still understudied. Overall, there is a lack of empirical evidence on how human perturbations can alter disease transmission and the emergence of zoonotic diseases.
Dr Monique Éloit, Director General of the World Organisation for Animal Health says the COVID-19 pandemic is a stark reminder that collaboration across sectors is absolutely critical for global health.
“The newly established One Health High-Level Expert Panel will contribute to bringing together diverse scientific expertise. United, we will better anticipate global health threats and work to control risks at the animal source,” he said.
Speaking during the launch of a new One Health High-Level Expert Panel to improve understanding of how diseases with the potential to trigger pandemics, emerge and spread, Dr Éloit said his organisation was proud to provide high-level expertise, along with its partners, to develop science-based ‘One Health’ strategies and programmes.
Studies show that transmission dynamics of zoonoses involve multiple agents including humans and a diverse range of wild and domestic animals. In order to understand the processes behind their transmission, it is essential to clearly disentangle the role of each agent involved.
Dr Tedros Adhanom Ghebreyesus, WHO Director-General. PHOTO/WHO.
Due to the complexity of disease transmission and the maintenance of infection within multiple wildlife hosts, the individual components of the transmission chain are often studied separately, which can limit our understanding of the subtle underlying effects explaining disease emergence and transmission.
Therefore, a holistic approach is essential to develop a complete picture of the transmission dynamics of zoonotic diseases, with research on rabies being an outstanding example of how empirical data can be used to elucidate epidemiological dynamics.
In reducing zoonotic risk, knowing the distribution and abundance of wildlife vectors is also essential. To that aim, Species Distribution Models (SDMs) can be used to produce models of the distribution and abundance of species based on occurrence data.
Wildlife Poaching and Trafficking Link to Zoonotic Diseases
Kenya Wildlife Service (KWS) rangers watch as residents of Mfangano Island attempt to slaughter a hippo that had died in Lake Victoria. PHOTO/UGC.
The UNEP Deputy Executive Director said captured wild mammals carry the highest risk of infections going undetected and affecting humans who are exposed to them through close contact.
“The unfortunate reality is that hunting, trading, butchering and preparation of wildlife for consumption has led to a significant proportion of known zoonoses (e.g. Ebola virus disease, HIV/AIDS, Monkeypox, SARS, COVID19),” she observed.
Ms Mrema said to reduce the public health risks associated with the sale of live wild animals for food in traditional food markets, the World Health Organisation (WHO), the World Organisation for Animal Health (OIE) and the United Nations Environment Programme (UNEP) have issued guidance on actions that national governments should consider adopting urgently, with the aim of making traditional markets safer and recognizing their central role in providing food and livelihoods for large populations.
“The guidance recommends, as an emergency measure, the temporary suspension of trade in potentially high-risk live mammal species for food in those markets where risk assessments have not been carried out and where appropriate regulations are not yet in place,” Ms Mrema said.
A study by, Dr Jake Young titled; How Should Wet Market Practices Be Regulated to Curb Zoonotic Disease Transmission says consumption and trade of wild animals presents major zoonotic disease transmission risks.
The study says policies that aim to limit these practices must balance environmental health against the fact that trade and consumption of wild animals are important sources of livelihood and food security for many people.
The study says in the wake of the COVID-19 outbreak, for example, there were calls for outright bans of wet markets, but such extreme measures are generally regarded by experts as misguided noting bans are difficult to enforce and tend to drive the sale of wild animal products underground where they are harder to monitor and regulate.
Instead, the research notes there is general agreement that policies to intensify regulation, monitoring, and enforcement work better to reduce health risks associated with demand for and consumption of wild animals.
Implementing Measures to Curb Spread of Disease Outbreaks
The UN Assistant Secretary-General added governments must carefully consider the potential impacts to ensure suspensions do not unfairly disadvantage certain communities.
Ms Mrema said factors such as global travel, climate change, and overuse and misuse of antimicrobial agents can influence pathogen movement, host ranges, and persistence and virulence.
“Beyond direct infection risks for human and animals, there are also implications around food security, nutrition, medicine and other health related issues,” she added.
Ms Mrema said there is increasing evidence that the underlying drivers of almost all recent emerging infectious diseases (EIDs) are anthropogenic environmental changes, and socioeconomic changes that alter contact rates among natural reservoir hosts, livestock and people, or otherwise cause changes in transmission rates.
“As highlighted in the IPBES workshop report on biodiversity and pandemics, the risk of a pandemic is driven by exponentially increasing anthropogenic changes; the emergence of new zoonoses correlates with wildlife (mammalian) diversity, human population density and anthropogenic environmental change,” she opined.
Another study by the World Health Organisation (WHO) on reducing public health risks associated with the sale of live wild animals of mammalian species in traditional food markets says significant problems can arise when these markets allow the sale and slaughter of live animals, especially wild animals, which cannot be properly assessed for potential risks – in areas open to the public.
“When wild animals are kept in cages or pens, slaughtered and dressed in open market areas, these areas become contaminated with body fluids, faeces and other waste, increasing the risk of transmission of pathogens to workers and customers and potentially resulting in spillover of pathogens to other animals in the market,” the study says.
The study adds such environments provide the opportunity for animal viruses, including coronaviruses, to amplify themselves and transmit to new hosts, including humans.
Human-Wildlife Conflict and Solutions to Curb the Spread of Zoonotic Diseases
Climate change intensifies droughts and food shortages, escalating risks of disease transmission. PHOTO/LOU GOLD.
Ms Mrema added that the problem of human-wildlife conflict contribute to the spread of zoonotic diseases noting that the underlying causes of pandemics are the same global environmental changes that drive biodiversity loss and climate change.
“These include land-use change, agricultural expansion and intensification, and wildlife trade and consumption,” she said.
Ms Mrema reported that apart from COVID-19 pandemic that is spread by the animal origin coronaviruses, zoonotic disease has driven the Ebola virus disease, HIV/AIDS, Monkeypox, and SARS, all of which have hugely impacted our world.
“Some diseases, such as HIV, begin as a zoonosis but later mutate into human-only strains. Other zoonoses can cause recurring disease outbreaks, such as Ebola virus disease and salmonellosis. Still others, such as the novel coronavirus that causes COVID-19, have the potential to cause global pandemics,” she emphasized.
Another study on interventions to reduce risk for pathogen spillover and early disease spread to prevent outbreaks, epidemics, and pandemics says the pathogens that cause most emerging infectious diseases in humans originate in animals, particularly wildlife, and then spill over into humans.
The research notes the accelerating frequency with which humans and domestic animals encounter wildlife because of activities such as land-use change, animal husbandry, and markets and trade in live wildlife has created growing opportunities for pathogen spillover.
The scientists say multiple approaches can be used to address spillover caused by tropical and subtropical forest clearing and degradation among them integrated policies with increased enforcement aimed at removing incentives for deforestation while respecting rights of indigenous peoples and local communities (IPLCs).
They add regulatory and market-based measures can establish standards that affect international supply and demand for products and lead to decreased deforestation.
“Payment for ecosystem services gives landholders incentives to maintain or increase forest cover on their lands, thus enhancing levels of ecosystem services provided by those forests,” the study observes.
The study shows community-designed interventions provide local services, such as healthcare, to IPLCs living within and near forests to minimize reliance on deforestation to generate income.
The research says in many emerging infectious disease hotspots, little intact forest remains, and spillover events occur because dense human and domestic animal populations live closer to wildlife and commercial activities might involve intentional for example wildlife trade or unintentional contact with wildlife.
“Such areas make up only four percent of global area 10 percent of tropical area, but account for 60 percent of global spillover risk,” the study concludes.
Deputy Executive Director of the United Nations Environment Programme (UNEP), Ms Elizabeth Maruma Mrema. PHOTO/UN.
Ms Mrema noted that the major impact on public health of COVID-19, HIV/AIDS, Ebola, Zika, influenza, SARS and of many other emerging diseases underlines the critical need for policies that will promote pandemic prevention, based on this growing knowledge.
The interconnectedness of human health, animal health, and ecosystems underscores the urgent need for a One Health approach to mitigate these risks. As Tola’s story illustrates, the fight for survival can have unforeseen consequences in a world increasingly shaped by climate change.
