The coronavirus SARS-CoV-2 that has dominated the news since early 2020 has something in common with other diseases that have hit the headlines in recent years. SARS-CoV-2, just like Ebola, HIV and MERS before it, originated in wildlife before ‘spilling over’ into humans. SARSCoV-2 currently appears to have originated in horseshoe bats and was potentially transferred to humans via an unknown species, possibly pangolins. But other so-called zoonotic diseases (illnesses that spread from animals to humans, and vice versa) originated in the likes of chimps, camels and mice.
While the existence of zoonotic diseases has been known for decades, the coronavirus pandemic has brought into sharp focus how closely our health is connected to the health of the animal species with which we come into contact. “This pandemic is just a tragic wake-up call,” says epidemiologist Dr Jonna Mazet at the University of California, Davis.
Mazet was principal investigator on the PREDICT project, a $207m (£150m approx) global effort run from the US to build a clearer picture of the viruses lurking in wild animals that could spill over into humans and wreak havoc. From 2009 to 2019, the project’s scientists collected samples of animal blood, saliva and dung from fields and forests in 30 countries. They found 940 virus species that hadn’t been previously identified, including 160 coronaviruses and one new Ebola virus that were previously unknown. But this may just be the tip of the iceberg. “We estimate there are probably about 500,000 viruses that could infect people that have not been characterised or detected by science,” says Mazet. Not all of these would cause disease, but it shows the scale of the problem.
Combine the number of viruses capable of spilling over into humans with deforestation, bushmeat hunting, and farming activities encroaching into wildlife-rich areas, and it’s a dangerous cocktail. It’s no wonder then that the US Centers for Disease Control and Prevention (CDC) estimates that 75 per cent of the new or emerging infectious diseases we are contending with originated in animals.
TOO CLOSE FOR COMFORT
A clear demonstration of the close connection between human and animal health came in June 2020, during the first months of the pandemic. Danish authorities reported that SARS-CoV-2 had leapt from humans to mink and spread extensively on mink farms. Not only that, but the virus also jumped back to humans, causing a spike in the country’s coronavirus cases. What was particularly worrying was that some of those who caught coronavirus from mink had a new virus variant. Thankfully, tests showed that the variant’s mutations did not make it more contagious or deadly. SARS-CoV-2 has also been detected in a raft of other species that humans come into contact with, including gorillas at San Diego Zoo Safari Park and a snow leopard at Louisville Zoo.
Dr Kaitlin Sawatzki, a postdoctoral researcher at Tufts University in Massachusetts, has been testing pets brought into a veterinary clinic to see whether any of them picked up SARS-CoV-2 from their owners. While Sawatzki is reticent to say how many of the dogs and cats tested positive for coronavirus antibodies before the research is published, she says that what she found was “comparable” with an Italian study of pets from 2020 in which 3 per cent of dogs and nearly 6 per cent of cats tested positive. The good news is that dogs don’t appear to get ill with SARS-CoV-2, and for cats it may only cause a mild illness.
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