In April 2020, there was barely any information on COVID-19. The U.S. had just recently went into lockdown and scientists were only beginning to tease out the intricacies of SARS-CoV-2, including how it spread and why so many people were a-symptomatic.
It was at this time the first-known spillover event occurred—a Bronx Zoo tiger contracted COVID-19 from a human zookeeper. From there, more tigers and lions were diagnosed at the zoo, and—unrelated—even a few domestic house cats in New York.
Then, and still now to an extent, the full range of animal species that are susceptible to SARS-CoV-2 infection is unclear. Typically, such information is generated by experimentally infecting a large variety of animals with the virus and recording the results. In an effort to reduce and refine animal experiments, researchers at the University of Bern and the Institute of Virology and Immunology set out to answer this question in a more animal-friendly way.
Drawing upon their knowledge of advanced in vitro cell culture models for the human respiratory tract, the scientists created a large collection of similar models from various domesticated and wildlife animal species.
To do this, the team isolated airway epithelial cells (AEC) of tracheobronchial tissue from deceased animals, creating a cell biobank that could be used to determine whether different animal species can be infected with SARS-CoV-2. To date, the biobank contains primary cells from 12 different animal species—rhesus macaque, cat, ferret, dog, rabbit, pig, cattle, goat, llama, camel and two neotropical bat species.
"Our collection is unique, and thus far we are the first that have used such a large collection of advanced in vitro cell culture models from various domesticated and wildlife animal species to assess their susceptibility to SARS-CoV-2 infection," said Ronald Dijkman, a researcher at the Institute for Infectious Diseases (IFIK) at the University of Bern, and co-author of the study, recently published in the CDC’s journal Emerging Infectious Diseases.
According to the study, for both rhesus macaques and cats, the researchers observed a 4-fold increase in viral RNA load post-SARS-CoV-2 infection. For the other 10 animal species, the team detected either a continuous or declining level of viral RNA load throughout the experimental process. Additionally, using whole viral genome sequencing, the researchers observed that SARS-CoV-2 replicated in the in vitro models of rhesus macaques and cats without the need for the virus to adapt.
"[This] suggests that certain species of monkeys and cats may be particularly vulnerable to SARS-CoV-2 infection. Our findings, together with the reports from previously documented spillover events, indicate that close surveillance of [rhesus macaques and cats] and other close relatives—whether they live in the wild, captivity or households—is necessary," said Dijkman.
The researchers said their results can be used by local authorities to implement early detection surveillance programs that monitor animals for SARS-CoV-2 spillover events. Real-time monitoring like this can help prevent new variants from developing in animals and being reintroduced into the human population.
Beyond surveillance, the study showed in vitro cell cultures are a suitable alternative to traditional in vivo experiments. Because the AECs in the cell biobank were isolated from deceased animals—and the number of isolated cells could be easily increased in a petri dish—the researchers did not have to perform any experiments on live animals.
“Our study shows there is a lot of potential to replace, reduce and refine animal experimentation in the near future, and I hope that for basic fundamental research questions, our results will convince researchers, pharmaceutical companies and drug administration agencies to use advanced biologically relevant in vitro models prior to conducting animal experiments," concluded Dijkman.
Photo: Close up of AEC cultures of Rhesus macaque showing the presence of SARS-CoV-2 infected cells (green). Credit: ©IFIK/UniBE