Humans as the main source of infectious diseases in animals

We tend to think of animals as a source of disease, but what if we were responsible for most viral transmissions? A new study sheds light on spillovers

It’s common for us to hesitate before letting our dogs lick us, fearing the possible transmission of bacteria. Similarly, interacting with wildlife raises concerns about disease spread. But what if the tables were turned? What if humans were the primary source of infectious diseases, passing them on to our four-legged friends and wildlife in various ways?

A recent scientific study published in the journal Nature Ecology & Evolution suggests this might be the case. The research team delved into publicly available viral genomic data to understand the mechanisms behind pathogen spillovers – the jumping of diseases from one species to another.

The analysis of nearly 12 million viral genomes aimed to identify cross-species transmission in 32 virus families and their mutations. Most of the sequences were associated with SARS-CoV-2.

The findings revealed that “viral sequences associated with vertebrates represent 93% of this dataset, with 93% of those associated with humans,” the researchers noted. Surprisingly, the study found a much higher frequency of reverse zoonosis – from humans to animals – compared to traditional zoonotic transfers within the viral families studied.

Out of 599 recent host jumps identified, 64% were related to humans. This suggests that host jumps from humans to other animals occur roughly twice as often as the reverse.

These jumps also result in host-to-host mutations, indicating the virus’s continuous evolution.

The data are particularly fascinating because they highlight how much more attention has been paid to zoonoses, which are responsible for epidemics and pandemics, than to jumps in the opposite direction.

“We should consider humans as a node in a vast network of hosts that constantly exchange pathogens, rather than as a reservoir of zoonotic bugs,” commented Francois Balloux, co-author of the study.

This aspect has been underestimated, and current scientific knowledge on it is limited. Monitoring virus transmission modes, interactions, and impacts is crucial.

The research underscores the need to explore this field further to be better prepared for future events with implications for human and animal health.

“Understanding how and why viruses evolve to pass among different hosts across the broader tree of life can help us comprehend how new viral diseases emerge in humans and animals,” explained lead author Cedric Tan.

Source: Nature Ecology & Evolution