The Enigmatic Role Of Biodiversity In Zoonotic Disease Emergence And Transmission
Covid-19 aside, we’ve been fighting zoonoses — diseases transmitted between non-human vertebrates and people — for a long time. Various transient microbes have caused population dips throughout our short history, ranging from the plague and smallpox to tuberculosis. More recently, we’ve been struggling to overcome HIV, Ebola, severe acute respiratory syndrome (SARS), and Middle East respiratory syndrome (MERS). It seems that zoonotic diseases are emerging more frequently these days.
Until recently, habitats of high biodiversity were thought to be hotspots for the emergence of new zoonotic pathogens, presenting a hazard to humans. Conversely, some researchers have been suggesting that under some conditions, high biological diversity can actually decrease the transmission of zoonotic diseases. Such conflicting findings have complicated the clarity with which scientists can provide useful data and inform public policy managers about real diversity–disease relationships. In this study, researchers wanted to set the record straight. They carried out a review looking at how biodiversity affects both the emergence of new zoonotic diseases and the transmission of established ones, and how data is gathered and evaluated.
Do we know what determines whether a given pathogen will spill over from one animal to another? Apparently, cross-species transmission involves a complex interplay between the characteristics of the pathogen, including:
- the original host’s infection, behavior, and ecology;
- how the pathogen enters and survives in the environment;
- how humans become exposed to the pathogen;
- and how susceptible those humans are to infection.
Previous models of linking biodiversity and zoonotic potential focused on the total host diversity in a given ecosystem, assuming that all taxa are equally likely to be sources of zoonotic pathogens. However, we now know that certain groups of animals such as bats, rodents, or farmed animals might be significantly more likely to spread such pathogens. A second model based on this knowledge can be used to predict the emergence of zoonoses, where the diversity of these specific hosts is the most important factor when determining the probability of a new outbreak. Using a database of 800 zoonotic pathogens, a group of researchers had previously found that ungulates (a group of animals that includes many hooved mammals) and carnivores could be the sources of the greatest numbers of zoonotic pathogens.
Meanwhile, other researchers highlighted the importance of bats. Others still proposed that rodents could be the most likely source. Domestication status was often stressed as a strong predictor of shared pathogens, while wild animals, the source of many spillover events, suggest that the former may act as secondary sources. Despite the intense and ongoing bickering within the scientific community, in practice, the primary source of a zoonotic pathogen is rarely identified definitively. We still don’t know what animals were the primary source of SARS-CoV-2, for instance.
Human impacts such as land-use change might play a significant role in this, too. Contributing anthropogenic activities have been linked to emerging infectious diseases in many studies. A group of researchers found that wild zoonotic host species are more abundant and more diverse in human-impacted habitats compared to less disturbed habitats, suggesting that wild biodiversity might indeed help dilute a disease’s spread. The dilution effect occurs when the recorded transmission of a pathogen increases as diversity declines, a phenomenon observed in both zoonoses and other types of diseases. The third, so-called “zoonotic host diversity and abundance”, model is suggested to be the most realistic so far as it takes into account both the relevant species and the numbers of potential individual hosts in a given area. The researchers argue that future studies should focus on collecting and analyzing data on the diversity, abundance, and capacity to transmit disease, focusing on the taxa that actually share zoonotic pathogens with humans.
It was observed that host species with short lifespans appear to be more likely to transmit pathogens, thus zoonotic emergence and transmission could be highest in areas where such animals are most abundant. The researchers highlight that when biodiversity is lost, the species most likely to disappear are large-bodied animals with slower life histories, while the smaller-bodied animals tend to increase in abundance. It is therefore critical that we should study both the effects of natural biodiversity levels and how this diversity is changing over time, for example, via human impacts.
Animal advocates will be interested to learn that we are slowly uncovering more secrets that surround the mystery of zoonotic diseases and that biodiversity in the wild might help thwart future outbreaks after all. The study calls for more research to better understand how our behavior and its effects on natural biodiversity levels impact this biological battle of microbes vs. immune systems. But the emphasis should not be placed solely on better understanding our impacts — it should also include a look at how we could mitigate such impacts in the affected ecosystems. Research into the restoration of biodiversity could be an important frontier in the improved management and policy surrounding zoonotic disease risk.