Covid-19 Animal Reservoirs: Will They Take Us by Surprise?
Humans can transmit Covid-19 to other animals, and they might return the favour.
Zoonosis refers to infections that can be transmitted from animals to humans. There is also reverse zoonosis — called anthroponosis — wherein human infections get transferred to other animals. And this reverse zoonosis process can be reversed again — reverse anthroponosis.
Covid-19 Human-mink-human Transmission
Is reverse anthroponosis possible with Covid-19? Yes. In May 2020, the Dutch government announced that a few farmers contracted Covid-19 from minks (a close relative of ferrets). These minks also got Covid-19 from a human worker. Many infected minks also got sick and died. To prevent the possible animal-to-human spread, the government gassed more 500,000 minks to death with carbon monoxide.
Hence, Covid-19 can be passed from humans to animals and from animals to humans. “Concern is growing over possible anthroponosis of SARS-CoV-2, especially in light of its recent discovery and spread on mink farms in the Netherlands and in Spain,” a Lancet paper by researchers at University College London stated. “With the suggestion that there was transmission back to humans.”
Other Animals Covid-19 Affects
With direct experimental infections into animals, scientists showed that ferrets (a close relative of minks), macaques, cats, dogs, and golden hamsters could contract Covid-19. Among these, data shows cats could shed the virus in amounts close to that of human’s, and further infect other cats. “There is a public health need to recognize and further investigate the potential chain of human-cat-human transmission,” the study authors wrote. Observational evidence also found that lions and tigers could get Covid-19.
A pre-print uploaded on July reported that 3.4% of dogs and 3.9% of cats, out of 817 pets in northern Italy, had antibodies against SARS-CoV-2. “This suggests that infection in companion animals is not unusual,” the pre-print authors concluded. “Based on current knowledge, it is unlikely that infected pets play an active role in SARS-CoV-2 149 transmission to humans.”
“Where experimental data do not exist…modelling the spike–ACE2 interactions…provides further evidence for the potential of infection,” the Lancet paper also said when reviewing these animal studies. “The results of these computational studies suggest attention should be paid to rabbits, sheep, goats, cattle, and horses because of the implications of infection.”
Computational methods are useful to find things we usually missed or would not have otherwise expected. Who would have thought rabbits, sheep, goats, cattle, and horses could be potential reservoirs for Covid-19. But note that these are possibilities, which await experimental confirmation.
Animals Covid-19 Doesn’t (or Unlikely to) Affect
In contrast, experiments to infect pigs, ducks, and chickens with Covid-19 did not work. Modelling studies [ref 1, 2, 3] further ruled out reptiles, fishes, amphibians, platypus, elephant, hedgehog, meerkat, rat and mouse as possible animal reservoirs for Covid-19. Indeed, to use mice as an animal model to study Covid-19, scientists have to genetically engineer the mice to express the human ACE2 receptor.
Using cells in a laboratory dish, a study in Nature showed that SARS-CoV-2 could interact with the ACE2 receptor of horseshoe bats, civets, and pigs, but not of mice. Yet an animal infection study does not support pigs as a host for SARS-CoV-2. Modelling studies discussed above are also inconclusive if pigs could be a host reservoir for Covid-19.
Influenza Virus Human-animals-human Spread
“If SARS-CoV-2 becomes established in wildlife or other species that have close contact with livestock, then this increases the possibility for interspecies transmission,” Linda Saif, a distinguished professor of virology at the Ohio State University in Wooster, said in Nature.
The 2009 influenza virus exemplifies this where “the flu can spread from pigs to people and from people to pigs,” the CDC stated. To make things worse, the influenza virus mutated as it circulated in the animals. Specifically, the virus combined with other flu viruses to create novel virus variants, which later jumped back to humans.
Furthermore, in 2016, an influenza outbreak occurred in a cat shelter in New York. Isolating these cat influenza virus revealed its close resemblance to that of an infected human’s, suggesting a human-cat or cat-human transmission. “The circulation of an influenza A(H7N2) virus at the animal-human interface, especially among common companion animals such as domestic cats, is of public health concern,” the study authors wrote.
Animal Surveillance, Just in Case
While the human-to-human is the predominant mode of transmission of Covid-19, animal-to-human or human-to-animal could happen. To date, science has not emphasized on how Covid-19 can circulate among various animal species. Covid-19 might even stay hidden in some animals that remain symptomless, only to jump to humans again.
“It makes sense to do research on it now,” Arjan Stegeman, professor of Farm Animal Health and Epidemiology of Infectious Diseases at Utrecht University The Netherlands, told The Washington Post in June. “Once we have very limited human-to-human transmission … you could get again a jump from an animal reservoir to humans. Things would start all over again.”
Fortunately, a few research teams have begun evaluating the susceptibility of animals to Covid-19 — particularly cats, dogs, ferrets, and hamsters with close contacts with humans are. While animals might not play a substantial part presently in the Covid-19 pandemic, they might in the future.
In response to this, Joanne Santini, a Professor of Microbiology at University College London, and lead author of the Lancet paper commented: “We need to develop surveillance strategies to ensure we don’t get taken by surprise by a large outbreak in animals, which could pose a threat not just to animal health but to human health as well.”