Pandemics: Too many factors spoil the broth?

In the light of COVID-19 and possible future pandemics, research on zoonotic disease transmission has become more important than ever. Many possible factors which accelerate such transmissions have been proposed so far and are currently being investigated. However, it is not always clear which factors contribute to a specific disease outbreak and what the links in certain disease transmissions are. For this reason, the connection between ecological degradation and pandemics is still not quite in the minds of many people.

New viruses are on the rise. Globally, the number of pandemics has risen ten-fold since 1940 and Professor Serge Morand, who holds joint positions at the Centre National de la Recherche Scientifique (CNRS) and at Kasetsart University, suggests that the number and severity of pandemics will increase even more in the following decades. This is the result of several factors: Skyrocketing livestock breeding of just a few economically valuable animals, the acceleration of biodiversity loss, or the increasing contact between humans and wildlife. These stressors disrupt the fragile balance of ecosystems and, at last, we’re experiencing its effects in the form of new diseases.

International governing bodies have already recognized the problem. In the 14th edition of the annual global risk report, published by the World Economic Forum (WEF), climate change and deforestation were indicated as one of the most probable reasons for future infectious disease outbreaks. However, given the broad spectrum of potential factors, how reliable are such predictions really, especially in the light of the current COVID-19 pandemic?

According to Serge Morand, the mechanism which has made the virus of the COVID-19 pandemic able to infect humans is not investigated well enough yet. In addition to the general risks predicted by the WEF, the World Health Organization (WHO) states that wildlife trading or breeding may affect the risk of humans contracting further severe acute respiratory syndromes (SARS) through increasing contact between captive animals and people.

The lack of understanding of infection origin and spread mechanisms can be illustrated by two studies which investigated the involvement of pigs in the COVID-19 spread and came to opposite conclusions.

A joint study of the University of Nottingham and São Paulo hypothesized that the spread of COVID-19, which has most likely originated from bats, may have been intensified by pig farming. The authors believe that the receding natural habitat of bats has forced them to find shelter in pork factories. There, they were able to infect the pigs which, in turn, have transmitted the disease to humans. The scientists corroborated their hypothesis with the high number of COVID-19 cases in areas harboring pig farming factories.

On the contrary, another study indicates that there is no relationship between pig farms and the spread of COVD-19. Their reasoning was as follows: In 2018, an outbreak of African swine fever among pigs in China led to a sharp decrease in pork supply and a consequent increase in pork prices. To compensate for this, people sold a lot of wildlife meat which may have led to a higher exposure of humans to the SARS-CoV-2 virus (COVID-19 pathogenic agent) by an increased contact of wild animals with humans.

However, it seems plausible that livestock intensification, such as large pig farms, is very likely not responsible for most zoonotic infections, and the reasons are very simple: The hygiene standards in livestock farming are usually very high, and conventional livestock is not very receptible for these exotic pathogens. Therefore, we believe that the theory of wildlife as a disease-spreading factor is more accurate than livestock farms.

This is supported by the fact that more than 70% of newly emerging zoonotic diseases originate in wildlife. Prominent examples are Ebola, HIV, or the swine and avian flu. Also, current research seems to suggest that habitat destruction and, thus, exposing wild animals to human settlements is one of the major drivers of creating new zoonotic diseases.

Even though the exact mechanisms of many disease transmission are still not entirely clear, the link between various aspects of anthropogenic environmental changes and outbreaks of diseases is tighter than it meets the eye. Indeed, what does a malaria outbreak in Malaysian Borneo in 2002 has to do with deforestation? It might seem far-fetched, but the proportion of cleared land did, in fact, appear to be a strong predictor of a disease agent, the parasite P. knowlesi.

If things remain as they are now, the United Nations (UN) anticipate a constant stream of zoonotic viruses transmitting to people. Why aren’t people ringing all the bells to stop this steady self-destruction? Probably, because for the wide audience, disruption of ecological balance and disease upsurge are still not associated with each other. For most of us, pandemics are all about precautions, distancing, Zoom-seminars, such as “Rethinking Agriculture,” and, of course, those millions of people who paid with their lives for humanity’s imprudence. Maybe it is time to see things in a more holistic light.

Authors: Thomas Köhler, Maria Kunle