The More the Merrier

Viral zoonotic risk is homogenous among taxonomic orders of mammalian and avian reservoir hosts (2020) Mollentze & Streicker, PNAS. https://doi.org/10.1073/pnas.1919176117

Image Credit: Tom Spinker, CC BY-NC-ND 2.0

The Crux

Diseases that jump from other animals to humans, or zoonotic diseases (see Did You Know?) have become something that all of us are now very familiar with. COVID-19 is one such disease, and the impact it has had on the world as a whole is all the evidence that anyone could ever need for understanding why it is important to know where these diseases come from. Classically, specific groups of animals have been thought to act as reservoirs for the viruses that cause these diseases. Take rabies, for example. This is the disease that results in rabid animals, but you may not know that bats act as a reservoir for rabies, meaning that the rabies virus survives within bat populations and can be spread by them.

This is known as the “special reservoir hypothesis”, and it posits that there are certain traits associated with these reservoir species and/or their ecology that make them more likely to act as reservoirs for these viruses. In contrast, it could be that all animal species are equally likely to act as a reservoir for zoonotic viruses, and the risk of virus transmission is instead due to how many host species are within a given group of animal hosts. All this means is that you expect to find more diverse groups of animals hosting a more diverse group of viruses. This is known as the “reservoir richness hypothesis”.

In order to better manage zoonotic disease emergence and even predict where it is likely to occur in the future, it is important to understand if there are indeed special reservoirs among animal hosts, or if disease emergence is instead a consequence of host species richness. Today’s authors utilized data on zoonotic viruses and host species to understand this relationship.

Did You Know: Zoonotic Diseases

A zoonotic disease is one that has “jumped” from a given animal species to humans. The most famous and recent example that most everyone knows is COVID-19, but swine flu, bird flu, Lyme disease, and SARs are all zoonotic diseases. The reason they are such a concern is that humans don’t have a history with the virus because it previously didn’t infect us, but when it does we do not have an established defense against it. That is why zoonotic diseases tend to have such strong effects on humans.

What They Did

The authors searched the scientific literature for records of viruses and their associated reservoir host species. After this they then ran statistical models to test for relationships between given host groups and zoonotic status of the viruses. These models allowed them to understand if there are indeed specific host groups that are more likely to act as reservoirs for zoonotic viruses. Importantly, the models the authors used allowed them to control for sampling effort, meaning that including a host-virus association that was more heavily studied than another wouldn’t bias their results.

What They Found

Despite including data on 429 host reservoir-virus associations, there was no evidence to support the special reservoir hypothesis. This means that there are not certain groups of animals that are more likely than others to host zoonotic viruses. Instead, the number of zoonotic viruses was associated with the total number of virus species (see image below), which was in turn associated with more species-rich groups of hosts. Therefore, there are not certain groups of animals that act as special reservoirs, instead the number of zoonotic viruses is a product of virus species diversity, which is itself a product of host species diversity.

The risk of zoonotic disease increases with host species richness. Shown is Fig. 3 from today’s study, and I include it because it is one of the most striking plots I’ve ever seen in a paper. The R2 in the plot represents how strongly the number of virus species correlates with the number of zoonotic species. Because 1 is the strongest correlation you can have, it’s clear to see that there is strong relationship between viral species richness and zoonotic species richness (Image Credit: Mollentze & Streicker 2020 PNAS, Image Cropped)

Problems?

This study involved using data from previous studies, and as such it was limited by what previous studies have investigated. Namely, certain host and virus species were much less represented than others in the database. While this is not a fault of the authors, it instead highlights how much more work needs to be done to understand how these results relate to other reservoir-virus associations.

So What?

Zoonotic diseases are an enormous concern for humans, and a considerable amount of money and effort has been put towards monitoring and responding to disease outbreaks. The results from today’s study imply that these efforts should not be focused on special host groups such as bats or rodents. Instead, future work should focus on more species-rich groups of reservoir hosts. Doing so will not only result in more efficient surveillance, but it could also result in more effective responses to these deadly diseases.

Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions and has very recently (and successfully defended) his PhD. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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