Wellor more than two years, COVID-19 has been making its way among humanity. But humans are not the only victims of the virus. The disease that leading theories still show transferred from animals to humans at a wholesale seafood market in Wuhan, China, has now infected pets and animals from farms, laboratories and zoos. It has also found its way into the wild, infecting many non-domesticated species.
COVID-19 now appears to be widespread in the animal kingdom, according to recent study in the diary Scientific data which provides the first global case count of animal cases of COVID-19. But there’s good news: Another study found that the highly infectious variant of Omicron and its many sub-variants may affect animals less than they do us—transmitting more easily among them and causing milder disease.
“As far as I know, there has been no apparent increase in the reporting of SARS-CoV-2 in animals since the emergence of BA.5,” says Amelie Deswars-Larive, assistant professor at the University of Veterinary Medicine Vienna in Austria and co-author of Scientific data study. “Yet the kind of active surveillance and monitoring of animals that [has been] conducted is crucial. We must not think ‘human first’ but rather integrate knowledge about animals, humans and their shared environment and develop a holistic approach to surveillance and control of SARS-CoV-2.”
In the study, researchers collected reported incidents of COVID-19 by analyzing two animal health databases: Emerging Disease Monitoring Program, reporting system of the International Society of Infectious Diseases; and on World Animal Health Information System, on which veterinarians, conservationists, and other researchers report nonhuman COVID-19 diagnoses. From February 2020 to June 2022, there were 704 “animal events” of SARS-CoV-2 — defined as a single case or multiple related cases in a group, herd or other population of animals — in 26 different species. Outbreaks have occurred in 39 countries on five continents, with Australia and Antarctica reporting no cases. As for the total number of sick animals it represents? Only 2058.
But that small number has big implications. Most of the reports only state the number of animals that test positive, not the proportion they represent of the total number tested, so it is not possible to say what percentage of the animal population carries the virus.
“Obviously we’re only seeing the tip of the iceberg,” says Desvars-Larrive, as animals are tested for SARS-CoV-2 significantly less than humans. “It is impossible to answer how many animals are actually infected, but SARS-CoV-2 is a widespread coronavirus. Its capacity to adapt to new hosts is impressive.
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Of all the species studied, the American mink, with 787 cases reported, and white-tailed deer, with 467, leads the pack. To be fair, this is partly due to sampling bias, Desvars-Larive says. Minks are extensively tested because they are raised in densely populated farms. (In November 2020, the Danish government ordered the killing of 12 million minks on farms as the virus began to spread among the species.) Meanwhile, deer live close to humans and are hunted for their meat, making sampling them for COVID-19 something in our interest. Next on the list are domestic cats with 338 cases and domestic dogs, at 208. Further down are lions (68), tigers (62), and western lowland gorillas (23). The list is rounded out by various other animals, including the black-tailed marmoset, Canadian lynx, ring-tailed coatis, and giant anteater, with one case each.
Other animal species not listed have either not been tested or may have natural immunity – or at least resistance – to SARS-CoV-2. “Some animal species are more susceptible to coronaviruses,” says Desvars-Larrive. “This may be related to molecular mechanisms of virus entry or to some genetic mutations in the host.”
One question raised, but not answered, by the study is how animals are affected by Omicron and its subvariants, including BA.5which are so highly transmissible among humans.
However, several other studies have been or are currently being conducted to address this issue, and they show that the animals do well with the new strains. Before the Omicron variant and its many sub-variants appeared, researchers at Texas A&M University examined infection rates among dogs and cats living in homes where at least one person had tested positive for COVID-19. From a sample of a group of 600 animals, they found 100 infections – or 16% of all tested – possibly passed from human to pet. Some of the positive cases were symptomatic, with the animal coughing, sneezing, vomiting or acting lethargic; others were asymptomatic.
A second phase of the study is now underway, following the appearance of Omicron and BA.5, and although only 100 animals have been tested so far, the difference in results is striking. “Because Omicron and its subvariants are the dominant strains in humans, we have only had two positive infections in animals so far,” said veterinary epidemiologist Sarah Hamer, director of the study. “So it’s definitely a lower prevalence of infection now.”
Hamer stresses that the results are preliminary and the researchers have many more animals to test before the second phase of the study is complete — and she doesn’t have a definitive answer as to why infection rates in animals might be lower in the Omicron and BA .5 era. “Could there be something about this virus that just doesn’t infect animals as much?: she asks. “Is it possible that SARS-CoV-2 has been around for a while and these animals have developed an immune response? We don’t know yet, but we hope that the neutralizing antibody test we’re doing now will help fill in those gaps.”
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Similarly, other studies have shown that Omicron tends to cause less severe symptoms in animals than previous variants, and researchers have ventured some theories as to why. In one study published in Nature in January 2022, researchers found that the Omicron variant was less pathogenic in laboratory mice and hamsters than earlier strains of SARS-CoV-2, and the infected animals lost less weight and contained less virus in the upper and the lower respiratory tract. The researchers haven’t determined exactly what makes Omicron less virulent in rodents, but they offer some theories: With more than 30 mutations distinguishing the new variant from the original, the virus’ spike protein may engage less effectively with cell receptors in the animals. It’s also possible that changes in other proteins slow the virus’s replication in rodents, or even that the variant doesn’t replicate as efficiently at a rodent’s body temperature as it does at human temperature. Published study in Nature in May it gave similar results with the BA.2 variant. This time, the researchers also noticed a reduced inflammatory response in the animals’ lungs.
Another study published in April as a preprint in bioRxiv, analyzed 28 cats, 50 dogs and one rabbit living in households with humans infected with Omicron and found that just over 10% of the animals tested positive for the virus and none showed clinical symptoms. Lidia Sánchez-Morales, a veterinary scientist at the University of Madrid and lead author of the study, hypothesized what might protect the animals.
“Numerous studies have shown that animals are less susceptible than humans to infection with SARS-CoV-2, which may be due to a lower affinity between the cellular receptor and the binding viral receptor,” she wrote in an email. Specifically, she says, the ACE2 receptor in human cells, to which the virus attaches, is found to a lesser extent in animals, and Omicron may be less effective at overcoming this hurdle than the original virus. “We therefore conclude that the susceptibility of companion animals to this variant appears to be much lower than to other variants of concern known so far.”
But the danger remains. The seemingly endless variability of SARS-CoV-2 means that new variants are sure to emerge. Desvars-Larrive worries that the animals could serve as something of a laboratory for the virus to test out new variants before these new strains jump to humans.
“The introduction and further spread of SARS-CoV-2 into an animal population may result in the creation of an animal reservoir that can further support, spread and stimulate the emergence of new variants,” she says. “This is of particular concern for species that are abundant, live in social groups and have close interactions with humans.”
This fact, Desvars-Larrive argues, calls for much more aggressive testing of wild, captive and domestic animals. “Active monitoring and surveillance of animals is critical,” she says. “This is the only way to get more data and better understand the epidemiology of SARS-CoV-2 not only in animals but also at the human-animal interface.”
It is at this interface that our self-interest comes into play. What animals catch, we often do too. Taking care of them is one of the key steps in taking care of ourselves.
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