We know from Covid-19 that the closer and longer time we spend in contact with infectious people, the more likely we are to become ill.
People do not just share diseases with each other; we have several diseases in common with animals. These are called zoonoses.
Some of the highest impact zoonoses include Ebola, Covid-19, mpox and HIV, all of which likely originated in wild animal populations and were passed to humans before mutating and evolving to become better adapted to infecting human populations.
Since the 1990s, more than 30 new human pathogens – which can be bacteria, viruses, fungi or other parasites – have been detected. Three-quarters of these new diseases have come from animals.
READ MORE
Contact with wild animals can occur due to people living in proximity to wild animal populations, such as bats or rats; through hunting and consumption of wild animal meat (bush meat) or through trade in wild animals for meat, fur, the pet trade or medicinal products.
Covid-19 most likely arose from human contact with wild animal populations that were traded at the Huanan Seafood Market in Wuhan, China, where a cluster of the earliest known human cases was discovered.
Very similar viruses to those that cause Covid-19 in humans were found in horseshoe bat populations in southeast Asia and from surface sampling of stalls in the market that held susceptible populations of animals such as raccoon dogs and palm civets.
When animals are traded, disease transmission can happen at multiple stages, including harvesting of the animal from the wild, captive breeding, transportation, stockpiling, warehousing, retail, consumption and maintenance as a pet
Trade in wild animals is widespread, with a quarter of all mammal species involved as either living animals for the pet trade and biomedical research or as products from dead animals, such as meat, fur or traditional medicine.
A recent study by Swiss ecologist Dr Jerome Gippet and colleagues showed that of 2,000 mammal species that are traded, 800 of them share at least one pathogen with humans. Traded mammals are also much more likely to share pathogens with humans than non-traded mammals.
When animals are traded, disease transmission can happen at multiple stages, including harvesting of the animal from the wild, captive breeding, transportation, stockpiling, warehousing, retail, consumption and maintenance as a pet.
Hunting and consumption of wild meat have been linked to HIV and Ebola outbreaks, but more unusual outbreaks include anthrax infections from animal skins used for drums and the mpox outbreak in North America linked to the pet trade in prairie dogs.
When live animals are traded, the risk of sharing pathogens with humans is increased, compared to when only animal products are traded.
By examining 40 years of trade data used to monitor endangered species, Gippet and colleagues showed that the longer a species was traded, the more likely it was to share a pathogen with humans. For every 10 years that a species was traded, it picked up an extra shared pathogen. Illegally traded species were also more likely to share pathogens with humans.
The concept of “One Health” links environmental, animal and human health together. Increased contact between humans and wild animals can arise as space for nature gets squeezed due to urban or agricultural development.
Domestic animals and animals that are commonly found in human-dominated environments are reservoirs for zoonotic diseases and can act as intermediate hosts or stepping stones for disease transmission between wild animal populations and humans.
For example, bird flu can pass from wild bird populations to domesticated chickens and turkeys and then to humans.
Chances are that the next global pandemic will come from a zoonosis, and likely a species that has already been in the wild animal trade for some years, that is traded live and which is from an illegal source.
This highlights the possibility of monitoring the pathogens of traded species, as it may enable early warning of potential new human diseases. Controls on the animal trade, including better facilities and sanitary practices, could also help to reduce the risk of disease spread.
Our own health depends on the health of our animal companions, animal products and those wild animals we encounter.
Investment in animal welfare, environmental protection and restoration is therefore part of the public health agenda.
- Yvonne Buckley is Professor of Zoology at Trinity College Dublin and co-director of the Co-Centre for Climate + Biodiversity + Water
