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Spill the Z: The Tea on Zoonotic Spillovers

Updated: Jun 14

Habitat destruction and human encroachment on natural areas increase the risk of more pandemics.

zoonotic spillover between primate and human
Viral exchange by Tanishk Katalkar

The COVID-19 pandemic shook the world and brought a significant amount of attention towards the problem of zoonotic diseases and their intersection with climate change and mass epidemics. A spillover event is caused when a disease-causing pathogen is transferred from a non-human animal species to the human population, and resultantly, gives rise to a zoonotic disease.

About 75% of all recorded human infectious diseases have a zoonotic origin. For instance, the SARS-COV and the MERS-COV viruses have spilt over to humans from bats, the HIV virus has come over from non-human primates, etc.

(Also read: COVID and Climate: A Tale of Two Crises)

There are several ways through which such zoonotic pathogens find their way into the human population pool. It is important to understand that spillover transmission involves animals, humans, and the environments where they interact. The transmission can occur through direct contact (either between reservoir and host or between reservoir and vector, and then from vector to human), through slaughter and butchery, or through the animal excreta.

Though these are the primary modes of spillover transmission, it’s a much more complex process that demands several environmental, behavioural and epidemiological factors to align. For example, the probability of spillover would be dependent on the intensity of the infection in the reservoir population first, and how prevalent it is in that gene pool. Further, it is also dependent on the survival probability of the disease-causing pathogen when it leaves the reservoir and enters the environment and is exposed to human contracture.

If it’s through excreta, it will be determined by the rate at which the reservoir is shedding/excreting, the pathogen’s survival and the human activities that lead to the contact between the two. Similarly, the transmission is influenced by the butchering and transport of meat for consumption purposes, and how the pathogen survives that process. If the carrier is a secondary vector, the spillover hangs on the survival and behaviour of the vector and the biting rate between the vector and the human population. However, even if the pathogen and human host make contact, the spillover can be swayed by the inherent immune response of the host and the internal pathogen multiplication process.

Pathway of pathogen spillover transmission

From this, we can infer that essentially, the spillover transmission of a pathogen from non-human animals to humans hinges on the intensity of the infection in the reservoir, the survival rate of the pathogen, and the behavioural factors and immunity of the host.

Zoonotic diseases have become a pressing public health and medical research concern, primarily because the intensity of their occurrence has seen a dramatic increase, due to factors accelerated by the climate crisis. According to a study released by Nature, the climate crisis could lead to around 15,000 more infections over the next few decades.

(Also read: A Picnic: Doughnut, Jalebi, Economics, the Planet and Humanity)

For starters, the survival, development and dissemination of the pathogen can be facilitated by climate change, easing out the spillover process. The animal-human contact is also influenced by rising temperatures and deforestation and loss of habitat as more and more animals are looking for new places to live in. In addition to this, the alarming shrinking of diversity in species and the disruption of ecosystems is also expected to result in closer contact between the reservoirs, vectors, and hosts. Changes in land use patterns and conversion of forest covers into agricultural land and pastures increase the proliferation of small rodent species that may carry pathogens, and the practice also contributes to climate change which affects the inter-species dynamics as alluded to above.

Further challenges include armouring the current, insufficient medical infrastructure to combat this rise in zoonoses. The COVID-19 pandemic is the most contemporary, frightening and realistic example of the threat zoonotic spillovers pose to the global economy and health system. Within 4 months of its first identified case, the novel coronavirus had engulfed the world and ubiquitously overwhelmed the medical infrastructure. Strict and urgent measures need to be put in place to contain these spillovers to the best of our ability, and combating climate change through institutional and systemic reformations is the strongest measure that can be taken. Putting a stop to factory farming, prohibiting urbanization into forest areas, increasing investments in medical research, funding vaccine research and campaigns, strengthening public health infrastructure and making it more accessible, and controlling hunting and poaching of animals, among others, are only some steps that can help contain the spillovers to an extent.

(Also Read: This Is Not the Apocalypse - Flipping the Narrative)


Ellwanger, J. H., & Chies, J. (2021). Zoonotic spillover: Understanding basic aspects for better prevention. Genetics and molecular biology, 44(1 Suppl 1), e20200355.

Plowright, R., Parrish, C., McCallum, H. et al. Pathways to zoonotic spillover. Nat Rev Microbiol 15, 502–510 (2017).

Rodó, X., San-José, A., Kirchgatter, K. et al. Changing climate and the COVID-19 pandemic: more than just heads or tails. Nat Med 27, 576–579 (2021).

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