A year ago humanity embarked on a project to vaccinate every person against Covid-19. But in recent months a shadow vaccination campaign has also been taking place. From giraffes to snow leopards, gorillas to sea lions, zoos around the world have been inoculating their animals with an experimental Covid vaccine as an insurance policy against what they fear could be a similarly fatal illness for certain mammals.
Meanwhile, veterinary scientists have been scrambling to understand the scale of Covid-19 infection in our furry household companions, and what the consequences could be for their health – and our own.
Last week two hippos at Antwerp zoo in Belgium became the latest in a coterie of creatures to contract Covid from humans. Fortunately, Imani and Hermien had no symptoms apart from runny noses, but other animals haven’t been so fortunate. In November three snow leopards died from Covid-related complications at a children’s zoo in Nebraska. Other zoos have reported infections in gorillas, lions, tigers and cougars.
Although Sars-CoV-2 is thought to have originated in an animal, most likely a bat, until recently most of the scientific focus has, understandably, been on human cases of disease. Yet ever since the early days of the pandemic, scientists have worried about the possibility of other animal infections.
“We’ve always recognised that coronaviruses have this tremendous capacity to jump species. So it was always predicted that there would be a variety of domestic animals, livestock and potentially wildlife that could be infected,” said Margaret Hosie, a professor of comparative virology at the University of Glasgow’s Centre for Virus Research.
If other animals can become infected and transmit the virus, this could put pressure on it to adapt and acquire new mutations, raising the prospect of new variants that could be transmitted back to people. “You could be concentrating on eradicating the virus in humans, but meanwhile the virus could be mutating away quietly in an animal species, and getting hotter and hotter,” Hosie said.
The first report of an animal infection came in February 2020 when a dog in Hong Kong tested positive, probably having contracted it from its infected owner. Since then there have been numerous reports of dogs and cats with Covid. Other pets generally appear less susceptible – no one has yet identified a Covid-positive goldfish – although ferrets and golden hamsters can catch Covid-19, and dwarf Roborovski hamsters can die from it.
Further research has suggested that cat and dog infections are relatively common. Scientists in the Netherlands found that in 20% of households they visited where pet owners had tested positive for Covid, the cats and dogs had antibodies for the virus.
Some infected animals develop symptoms – generally a runny nose, coughing, sneezing or conjunctivitis – and most make uneventful recoveries. However, some may experience more severe illness.
“The first cat that we identified as being infected from its owner was a young kitten that died of pneumonia,” said Hosie. “We didn’t test exhaustedly for other potential pathogens, so we couldn’t say for sure, but the pathology was very similar to the viral pneumonia seen in Covid-19 patients.”
But are these infected pets contagious? Evidence from dogs suggests the risk of onward transmission is low because it is difficult to isolate replicating virus from them.
On cats, the jury is still out. Experimental studies have suggested they can infect other cats, but the degree to which this happens in the real world is uncertain. Cats are relatively solitary creatures – they don’t spend much time in close contact with other cats or humans (besides their owners). So if they catch Covid-19, their owner is probably the source, and any onward transmission is likely to be extremely limited.
Farmed mink, on the other hand, are forced to live in close proximity to one another, and are highly susceptible to infection with Sars-CoV-2. They can also develop pneumonia and die from it.
In November 2020, news that the virus had crossed from humans into mink, mutated and then jumped back again sounded alarm bells around the world. “That was a real wake-up call, and I think is why there’s now more emphasis on studying viruses at the human animal interface,” said Hosie.
Fortunately, although there have been other mink outbreaks, “so far [mink-related variant viruses] have not shown to be more transmissible or causing more severe impact compared to other circulating Sars-CoV-2,” a report by the European Food Safety Authority and the European Centre for Disease Prevention and Control concluded.
Of course, infected farm animals can be quarantined or culled, as millions of mink were. Farm surveillance can also be stepped up and farmworkers kitted out with protective equipment.
Vaccination is another option. In March, Russia’s state veterinary service announced that it had approved a Covid-19 vaccine called Karnivak-Kov for use on fur farms or for cats and dogs, after clinical trials on arctic foxes, cats, dogs and mink.
The US Department of Agriculture has authorised a vaccine developed by the American animal health company Zoetis for experimental use on a case-by-case basis. Early this year Zoetis agreed to supply San Diego zoo with enough doses to inoculate its great apes after the zoo’s western lowland gorilla troop became ill with Covid.
“Since then we’ve had multiple requests from different zoos and conservatories,” said Mahesh Kumar, a senior vice-president of biologics research and development at Zoetis. “I think 100 species of mammals have been vaccinated.”
More concerning than farm outbreaks would be evidence of significant transmission among wild animals. Vaccinating them would be impractical, assuming a vaccine even worked in that species.
In May the Journal of Virology reported that white-tailed deer, which are native to North, Central and South America, were capable of transmitting the virus to each other. And in August researchers at the US National Wildlife Center in Fort Collins reported that up to a third of white-tailed deer in the US north-east had antibodies to Sars-CoV-2. It is unclear how they became infected.
If white-tailed deer really are transmitting the virus to each other in a continued and significant way, that could be problematic – although if a potentially dangerous new variant arose in them, it would still need to be transmitted back to us, and humans don’t tend to spend a lot of time breathing the same air as deer.
“What we’re really concerned about is the passing back and forth between humans and animals, and to other animals, in a setting where the human population ultimately could be affected,” said Rebecca Fisher, an assistant professor of epidemiology at Texas A&M University.
Her greater fear is the virus adapting to wild animals that live in close proximity to humans, such as rodents. Fortunately, rats and mice don’t appear particularly susceptible to Sars-CoV-2 infection at the current time.
However, because of the risks involved, it could pay to be vigilant. “The current pandemic is maintained by human [to] human transmission, but we need to keep a watching brief on animals,” said Alan Radford, a professor in veterinary health informatics at the University of Liverpool.
For now, the most likely source of new variants is continued circulation of the virus in humans. With ongoing high infection rates, we still present a far greater risk to our pets than they do to us.
This is unfortunate. “When we’re sick or convalescing, what is better than a snuggle with our pets?” said Fisher. “Tough as it is, if we are sick we must try to not interact with them, and not pass anything on to them. We need to try our hardest to protect them, just as we would our human children.”
