By Nadia O’Hara

As soon as SARS-Cov-2 was identified in humans, scientists began searching for its origins, quickly determining that it was a virus of zoonotic origin, with bats being the source host. One only has to look back on the last 50 years to notice a disturbing trend: the frequency of zoonotic disease emergence has been drastically increasing – COVID-19, SARS, MERS, HIV, avian flu and many more zoonoses, diseases that are transmitted from non-human animals to humans, have caused epidemics in the last 20 years. There are, of course, a multitude of factors that contribute to the emergence of zoonotic disease. Most worryingly, anthropogenic activity that negatively impacts ecosystem health and biodiversity is among the leading causes of this uptick in zoonotic spillover events.

A comprehensive study published in the Lancelet reveals how several anthropogenic environmental changes, most notably land-use, livestock production and climate change help foster conditions which promote the emergence of zoonotic diseases. So, what exactly are these favourable “conditions?” According to Raina Plowright, an expert on infectious disease dynamics, zoonotic spillover events occur in three stages: pathogen pressure, exposure, and finally, infection.

In essence, pathogen pressure is the amount of a given pathogen that is “available” to humans at a certain point in time. This means that pathogen pressure is determined by the viral load circulating withing the reservoir host population and the method of pathogen release. For example, rabies is a disease that is passed on from the saliva of an infected animal. As such, the viral load in the saliva a particular animal as well as the “biting rate” of the infected animals control pathogen pressure. Exposure is simply that – the exposure of humans to the pathogen, opening them up to the possibility of infection. The infection stage is determined by the internal within-host mechanisms, such as the immune response, that could prevent the pathogen from replicating within the new host. We are primarily concerned with the first two stages, as the aforementioned “conditions” that help give rise to zoonotic diseases are ones that increase pathogen prevalence among the reservoir host and/or increase the chances of human exposure.

The demand for livestock production has been increasing for the past few decades, especially in the Global South, in response to population growth and urbanization. Livestock farms are often pushed into wildlife habitats through deforestation which increases the chances that wild hosts of zoonoses will encounter the livestock and transmit the pathogens. Consequently, the livestock can become a reservoir host for the disease and pass it onto the farmers. This is how a Nipah virus epidemic broke out in Malaysia – the expansion of pig farms led to the transmission of the virus from bats to the pigs.

Changes in land use also affect the composition of wildlife populations and animal behaviors. For example, if hunting or deforestation causes a decline in the predator populations of pathogen-host prey, then the hosts will suddenly become more abundant, increasing the chances of exposure. Clearly, loss of biodiversity is strongly linked to the likelihood of zoonotic spillover events. Species richness is a big natural barrier against zoonotic disease, so when richness declines, pathogen transmission increases. This effect is enhanced when the “disappearing” species is not a host for the pathogen, as reservoir hosts would become proportionally more abundant in that local ecosystem. 

Climate change can also drive the emergence of new zoonotic diseases. The shifts in climatic conditions change host and vector distributions and pathogen prevalence. Consequently, interaction between various hosts of the pathogen is also affected. Climate change especially influences vector-mediated diseases, since arthropod vectors are particularly vulnerable to changes in their environment. All these factors combined can drastically alter transmission dynamics, increasing the frequency of zoonotic spillover.

At this point, we are all intimately familiar with the impacts of the COVID-19 pandemic on our lives and the world. Unsurprisingly, the pandemic has also directly and indirectly negatively impacted ecosystems worldwide.

The pandemic strongly affected the global economy. Manufacturing firms struggled to keep up with demand as people were asked to stay home, the tourism and recreation sectors were all but locked down, and a huge number of people lost their jobs. Consequently, conservation projects lost their funding as governments prioritized the healthcare sector and keeping their economies afloat. Reduced funding to conservation efforts impedes their incredibly important work, such as achieving global biodiversity targets and blocking poaching and deforestation. The downturn of the tourism sector also includes a decrease in wildlife tourism. According to Conservation scientist Odette Lawler, the Tsavo National Park is one of the main sources of revenue for Kenya’s Wildlife Service, who do most of the work maintaining wildlife protection in Kenya. The immense reduction in tourists to the area could dramatically impact the Wildlife Service’s ability to complete their conservation goals and protect the park from poachers.

Indigenous Peoples manage over 38 million km² of land globally, which includes approximately 40% of all protected areas of wildlife. Lawler describes how Indigenous Peoples’ conservation work leads to better conservation outcomes, increased conservation innovation, strong support for climate change initiatives and realisation of Indigenous Peoples’s needs, which almost always coincide with what is best for the biodiversity in the region. Indigenous Peoples work hard and even risk their lives to protect their land from agriculture, mining and oil companies, who would raze the land to access coveted resources. Indigenous communities also have centuries worth of knowledge on how to sustainably live on their land but are disproportionately affected by biodiversity loss since they are reliant on the land and nature to survive. It is imperative that Indigenous Peoples around the world are included in conservation decision making. During the pandemic, Indigenous Peoples received severely inadequate support. This is not only extremely detrimental to the wellbeing of the Indigenous populations but also makes the protected ecosystems much more vulnerable to resource extraction.

We are trapped in a vicious cycle, where ecosystem disturbance leads to pandemics and pandemics exacerbate ecosystem disturbance in return. Unfortunately, most of the work done to prevent zoonotic spillover events are targeted at stopping individual outbreaks once they have already started, instead of working to minimize the chances of spillover to begin with. For example, just this January, millions of chickens were culled in Lancashire in response to an outbreak of bird flu. This likely stems from policy makers’ lack of awareness about the connections that exist between ecosystem and human health.

World governments must focus on sustainably recovering from the pandemic and investing further in conservation so that we have a chance to stop this feedback loop of disease in its tracks. Increasing job opportunities in the biodiversity support field, investing further in sustainable energy and offering better education to address these issues are big steps to prevent future zoonotic disease emergence.

Image: Relative map showing the origins of emerging zoonotic disease from 1940 – 2004Jones, K., Patel, N., Levy, M. et al. Global trends in emerging infectious diseases. Nature 451, 990–993 (2008). https://doi.org/10.1038/nature06536

The views expressed in this article are the author’s own and may not reflect the opinions of The St Andrews Economist.