UC Davis research finds strong links between emerging infectious diseases, threatened animal species

UC Davis research finds strong links between emerging infectious diseases, threatened animal species

Photo Credits: TESSA KOGA / AGGIE

Primates, bats among groups with higher viral transmission rates

There are newly discovered links between emerging infectious diseases and endangered or threatened species of animals, according to research from the One Health Institute within the UC Davis School of Veterinary Medicine.

Researchers are studying the rates of viral transmission from animals to humans and are particularly interested in zoonotic diseases that come from wildlife, as opposed to domesticated species. 

“We began this study about five years ago because we were really interested in which wild animal species in particular were most likely to be a source of zoonotic disease for humans,” said Christine Johnson, a professor of epidemiology and ecosystem health within the One Health Institute at UC Davis. 

In order to isolate particular species for the study, researchers examined data showing major zoonotic viruses that had spilled over from animals to people over the years. They then looked at which species had the highest rates of disease transmission for these viruses and combined their data with other data from the International Union for Conservation of Nature.

“We found that domesticated species had, of course, shared a lot of viruses with people,” Johnson said. “We knew that they’re really globally abundant. We were very interested in what wildlife species were sharing viruses with people. We found overall that there are about 58 species that are described as being very common [disease transmitters]. These are highly adaptable species that moved in with people and have essentially set up house [in humans].”

In addition to identifying common species with higher rates of disease spillover to humans, the research pinpointed major reasons for increased viral spillover in many species. 

“The other major finding we had was that species that were declining because of habitat loss in particular, or species that are living in marginal habitats, had more zoonotic viruses and were sharing [these] viruses with people,” Johnson said. 

The study also identified a few main processes that facilitate disease spillover for wildlife declining in abundance.  

“The exploitation of wildlife through hunting, capture and the wildlife trade typically involves very close contact between wild animals and humans, which facilitates disease transmission,” said Pranav S. Pandit, a postdoctoral scholar at the EpiCenter for Disease Dynamics, via email. “Viruses jump species when there is close enough contact to enable transmission between an infected animal and a susceptible person.” 

Animals in close contact with humans can share viruses through respiratory droplets, as well as through contact with animal feces, urine or blood. Marketplace environments provide optimal conditions for spreading diseases between diverse species due to their high density of animal and human populations. 

“In order for a zoonotic pathogen to spill over from a wild animal to a human, it has to have two things,” said Tierra Smiley Evans, a faculty member at the UC Davis One Health Institute. “It has to have the biological innate ability to infect a new host, and it also has to have the opportunity to come in contact with a human.”

Beyond these marketplace environments, research highlighted certain species of animal vectors that play a critical role in disease transmission from animals to humans. Among all the wildlife species living in close proximity with humans, data showed that primates are one such example of a high risk species. 

“Primates have a lot of genetic similarity to us,” Johnson said. “Being that closely related, it’s easy for us to get pathogens [from them].”

In addition to highlighting the importance of primates in viral transmission, data has revealed that rodents and bats were common disease vectors in the past and played a large role in the emergence of COVID-19. 

“Some species of wildlife, especially rodents, have also increased in abundance and have adapted very well to human-modified habitats,” Pandit said. “The widespread presence of these species, especially those that have adapted to sheltering with people or depending on people for food, means a higher likelihood of interaction with humans which facilitates disease transfer.”

Bats play a critical role in earth’s ecosystem and make up a large portion of mammals, one in five, according to Johnson. Because they are flying mammals, they have developed unique adaptations over time. These animals willingly take up residence in marginal habitats close to humans, where they are forced to redistribute because of habitat degradation and destruction.

Both the SARS viral spillover in 2002 and the MERS virus were linked to bats, and scientists suspect that Ebola viruses also originated in bats. 

“The exact origins of SARS-CoV-2 is yet to be determined, but some evidence suggests that it came from bat species, and the spillover might have occurred around the live animal market in Wuhan,” Pandit said. “Our research has already identified these activities as high risk for spillover of diseases from animals to humans, but to identify the exact source of SARS-CoV-2 there needs to be large scale surveillance of suspected bat reservoir hosts and suspected mammalian intermediate amplifying hosts.”

Although we know that these mammals are widespread disease vectors, they also have extremely beneficial effects on the environments in which they live. 

“There’s a lot of really dangerous pathogens that have come from bats, but bats are [also] a really important part of the ecosystem,” Johnson said. “They’re really important for pollination [and] they eat insects. I think we need to understand better ways to keep ourselves safe and watch out for bats.”

Written by: Dina Gallacher — science@theaggie.org