The pangolin is a peculiar yet unassuming mammal. Found primarily in the tropical forests of Southeast Asia, pangolin species remained largely unknown to those who did not partake in local conservation efforts until just four years ago, dubbed “the most trafficked mammal you’ve never heard of” by CNN in 2014, due to extreme levels of international pangolin trafficking for the purposes of consumption (as pangolin meat is considered a luxurious delicacy throughout Southeast Asia) and healthcare (with pangolin scales comprising a key ingredient of a number of folk medical treatments). Between December 2019 and March 2020, the popularity of the search term “pangolin” on Google increased by over twenty five times, with the most popular search incorporating this term being “coronavirus pangolin”. At the time, early studies had revealed key spike glycoprotein similarities between SARS-CoV-2 and Chinese pangolin-CoVs, inciting suspicion that the humble pangolin had served as the intermediate host, or ‘bridge species’, through which zoonotic transmission had occurred. It is worth noting that, as of September 2024, reports remain inconclusive as to whether this hypothesis is actually veridical - but the associated surge in public awareness of the dangers of pathogen spillover has been invaluable in stimulating the development of novel pandemic prevention strategies.
When investigations began, the initial outbreak of coronavirus was linked back to the Huanan Seafood Wholesale Market, a large wet market in Wuhan wherein the sale of live animals, including both live and dead bamboo rats (considered a delicacy when grilled) and, reportedly, pangolins, had become commonplace. Whilst the absence of biosecurity regulation governing the sale of fresh meat in wet markets throughout the world has been repeatedly linked to bacterial and parasitic infections in customers, with every TripAdvisor-registered wet market in South America and Oceania reporting at least one case of food poisoning, the risk presented by the sale of live animals is far more dire. For one thing, the stress to which live animals are subject whilst displayed in a wet market is associated with increased viral shedding, i.e. an increase in the number of viral particles released from the infected animal into their local environment.
The experience of stress can trigger an inflammatory response via the activation of an infected animal’s sympathetic nervous system which, in turn, optimises the animal’s internal environment for increased viral replication. A stressed animal is also likely to groom themselves more frequently, contaminating their body and surroundings with saliva, and may even urinate or defecate in response to provocation, further increasing the volume of bodily fluids to which customers may be exposed. In Australia, the stress experienced by fruit bats as a consequence of urbanisation and subsequent habitat loss led to the re-activation of Hendra virus, which typically remains latent state within a fruit bat’s cells, causing no symptomatic manifestation. The virus was therein shed in large quantities via the bats’ saliva, urine and feces, with viral particles entering the food and water sources available to horses, which act as a ‘bridge species’ in the zoonotic transmission of Hendra virus to humans. Since the mid-1990s, over sixty outbreaks of Hendra virus have occurred in Australia, many of which can be linked back to the stress experienced by local fruit bat populations - as evidenced by the prevalence of outbreaks in the winter and early spring, wherein the flowers, fruit and leaves upon which the bats subsist grow more scarce, stimulating increased competitive stress.
Furthermore, stress is far from the only factor which can increase viral shedding and, subsequently, zoonotic transmission. Rapid postmodern urbanisation, paired with an upsurge in large-scale deforestation efforts, has brought the borders of human civilisation ever-closer to those of the natural world, increasing the contact shared between the average human and local wildlife. Rodent species, in particular, comprise a significant proportion of the animal community of tropical forest ecosystems, and are also considered the planet’s “most important hosts of zoonotic disease” - hosts which, due to their small body size, can easily enter a human dwelling, potentially contaminating food stores with their bodily fluids and, therein, associated viral particles. Alongside the innate risks associated with close human-to-animal contact, certain agricultural practices further increase the risk of zoonotic transmission. For example, rice farmers in China are at a high risk of contracting hemorrhagic fever, as the rice paddies in which they work provide ideal environments for striped field mice, a species serving as a key reservoir for the Hantaan virus causing this condition. Throughout the 1980s and 90s, improper handling of rinderpest (a highly virulent Morbillivirus referred to as the ‘cattle plague’) vaccines (e.g. lapses in maintenance of a “cold chain”) contributed to persistent outbreaks in cattle across the African continent. Whilst rinderpest is not considered zoonotic, this decade-long failure to contain a prevalent mammalian pathogen highlights the risk associated with viral spillover. These are just two examples of agricultural mismanagement resulting in viral outbreaks, with many more occurring at any given time. Additionally, malnourishment experienced by animal populations as a consequence of habitat destruction and human resource overuse can increase the amount of time required by their immune systems to ‘fight off’ infection, allowing for longer periods during which viral shedding and zoonotic transmission may occur.
❓ A ‘wet market’ is a market wherein fresh and perishable goods, such as meat, fruit and vegetables, are placed on display in the open air. These are described as ‘wet’ because goods are often submerged in cold water or ice to prevent degradation. Whilst it is important to distinguish between wet markets (the vast majority of which do not offer live wild animals for consumption) and wildlife markets, certain wet markets, such as the Huanan Seafood Wholesale Market, have incorporated the sale of wild (and sometimes trafficked) animals into their stock.
Mandatory Resources 🦠
A Long-Form Analysis of Different Interventions Designed to Reduce Pathogen Spillover
A Set of Two Posters Summarising Land-Induced Spillover
A Short Article Outlining the Spillover Potential of Viruses Afflicting Fur-Farmed Animals
A BMJ Article: Ecosystem Conservation & Pandemic Prevention
“Zoonotic Diseases: How The Next Pandemic Could Happen”: A Swiss Learning Exchange Video
Optional Resources 🔍
🌟 A Presentation by The Institute for Science and Policy Examining the Risk of Zoonotic Spillover