While animals like mice are often seen as carriers of diseases, the reality is that when it comes to disease transmission, animals have more reason to fear humans. An analysis of virus genomes suggests that in cases where viruses spread between humans and other animals, 64% of the time it is humans infecting the animals, rather than the other way around. This research was recently published in Nature Ecology & Evolution.
Some zoo animals have contracted SARS-CoV-2 from humans.
Image Source: Sergei Supinsky/AFP/Getty
"The virus we pass to animals is more than what they pass to us," says Cedric Tan from University College London. For instance, after severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spreads from bats or other species to humans, humans then transmit it to many other species.
Tan and colleagues have been using a global virus sequence database to study the spread of viruses between species. The database contains nearly 12 million sequences, but many are incomplete or lack data on collection time and host species.
To address this, researchers narrowed down the 12 million high-quality sequences to about 60,000, along with complete relevant data. They then created "pedigrees" for the related viruses.
They identified nearly 13,000 viral lineages and 3,000 transmissions between species. Out of 599 transmissions involving humans, most were from humans to other animals rather than from other animals to humans.
Tan says the team didn't anticipate this, but in hindsight, it makes sense. "We have a large population size and are spread globally, basically everywhere."
In other words, viruses spreading in humans have many opportunities to spread to many other species worldwide, while the chances of viruses spreading among non-human species confined to a single region are much lower.
The study found that SARS-CoV-2, MERS-CoV, and influenza viruses are the most commonly transmitted from humans to other animals. This aligns with other research findings, such as SARS-CoV-2 spreading from humans to pets, zoo animals, farmed animals like minks, and wild animals like white-tailed deer.
However, even excluding SARS-CoV-2, MERS-CoV, and influenza viruses from the analysis, the research group found that 54% of transmissions were still from humans to other animals.
Tan says that virus transmission from humans to other species poses a threat to many endangered animals. For example, several wild chimpanzees have died in Uganda due to outbreaks of human respiratory viruses and human metapneumovirus.
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