Diseases in the Jungle: How Habitat Destruction Could Lead To Future Pandemics

By Kunal Patel

Video Credit: Andrea Vale

As the number of vaccinated people steadily increases, it seems that each day brings us closer to a return to some sense of normality. However, as unprecedented as the current COVID-19 pandemic has been, it is by no means the first disease to wreak havoc upon humanity and, if we fail to curb environmental destruction, it certainly won’t be the last.

Emerging and re-emerging diseases are not a new phenomenon in human history and pathogens have been shaping our species as far back as the neolithic revolution. An examination of the human genome will quickly reveal the presence of intrusive viral DNA strands that have infiltrated our genetic code, illustrating the perpetual arms race between our cellular machinery and invasive microbial hijackers.

It seems that infectious diseases have only become more prevalent in the modern world, with the emergence of 335 pathogenic species between 1940 and 2004. In the grand chronology of human history, the advent of the COVID-19 virus is merely the most recent example of a novel pandemic, though its emergence and rampant global transmission mark our entry into a pandemic era.

A vast majority of the influential pathogens of the past century have been viruses which can be attributed to the genetic instability of the viral genome, which allows for rapid adaptation to changing conditions. This ability has been perfected by RNA viruses (like COVID-19), which lack error correction mechanisms, allowing them to rapidly mutate and spread in swarms of genetic variants. These clouds of “quasi-species” allow for increased rates of transmission as there is a higher probability that one species will be infectious to the host cell.

The emergence of diseases into humans may be a common occurrence though they have not historically achieved sustained rates of transmission. As such, the contemporary spate of emergences we’ve witnessed is a concerning trend. Of particular note is the overwhelming emergence of zoonotic diseases or those that cross over from animals to humans. Inter-species disease transmission naturally necessitates inter-species contact and it would seem that modern anthropogenic factors have intensified the animal-human interface, increasing interactions and the risk of disease spillover.

By this point, we’re all too familiar with the fresh food “wet markets” that are characteristic of communities in the Asia-Pacific region, and that likely provided an interface for the emergence of both the 2002 SARS outbreak and the current COVID-19 virus. In a similar, though perhaps more concerning, way the expansion of industrial agricultural land-use change appears to be generating a similar, albeit much larger, interface.

The conversion of wildland to farmland results in dramatic reductions in species populations, throwing trophic relationships into disequilibrium. In order to survive in these new, anthropized ecosystems species must adapt to new ecological niches, causing geographic shifts that facilitate contact between previously separated species. This disruption is limited not only to the visible environment but also includes the microscopic relationships between microorganisms and their hosts.

Thus, agricultural expansion results in ecosystems that are in continual states of disruption which favors adaptable species such as birds and bats that have a high tolerance for humans. The success of these organisms, in turn, allows for the growth of their viral populations and encourages the selection of new strains. All of this, coupled with the occurrence of livestock and human populations permits the redistribution of these viruses into human environments. To make matters worse, agroindustrial operations are also characterized by genetically uniform livestock populations, and the combination of this with over-enriched diets, close quarters, and high-stress levels create the perfect storm for infection success and the generation of new mutations.

Already we’ve seen the emergence of novel pathogens in response to anthropogenic ecosystem disruption. In Malaysia, the spread of Nipah virus was catalyzed by the burning of forests for agriculture that triggered bat relocation and interaction with pig livestock. This in turn allowed for the transmission of the virus from bats to pigs via droppings and then from pigs to humans. Deforestation and environmental degradation has also been linked to the emergence of Zika and Hendra viruses as well as the spread of Bolivian hemorrhagic fever and HIV. It’s evident that human activity is inextricably tied to the emergence of novel diseases but it’s equally clear that the environment and animal communities play key roles in interspecific pathogen transmission.

As much as humanity tries to set itself apart from the natural world, human systems are very much part of broader environmental networks. If we are to have any hope of controlling or preventing the spread of emerging diseases we must work to reframe public health within this context. Going forward we need to decentralize humanity in our consideration of disease control and employ strategies that recognize ecosystems as the ultimate source of pathogens.

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