At the time of writing this the number of confirmed cases of COVID-19 worldwide has touched 57,04,736 which includes 3,57,736 deaths, according to the World Health Organization’s update of May 29. Naturally, researchers are racing to find an animal model that would be suitable to study this disease. Animal models will be useful in testing out therapeutics such as vaccines and antivirals before they are tested on humans. A study published in Nature and two in Science have looked at a range of animals from this perspective. It emerges that only some of these, namely, hamsters, cats, ferrets and non-human primates, are suitable to study various aspects of the infection.
Susceptible animals
A group from China (Jiangzhong Shi and coworkers, in Science ) studied the susceptibility of ferrets, dogs, pigs, chicken, ducks and cats to infection by SARS-CoV-2 – the virus that causes the disease COVID-19. According to their study, while the virus replicates poorly in dogs, pigs, chicken and ducks, it actually replicated well in the upper respiratory tracts of ferrets and cats. They also found that cats transmitted the infection among themselves.
In the experiment with ferrets, the animals were divided into two groups one of which was inoculated with a strain of virus isolated from an environmental sample collected from Huanan Seafood Market in Wuhan and the other group was inoculated with a strain isolated from a sample taken from a human patient. Nasal and rectal swabs were examined on alternate days.
One ferret in each group developed fever and loss of appetite on days 10 and 12 after inoculation. They were sacrificed on day 13, and sample tissues from various organs were collected and tested in order to find out whether the virus had infected the lower respiratory tract and other organs. They did not find viral RNA in these organs.
“The study is useful because it indicates that from a diverse set of animals only a couple get infected in a reliable manner – ferrets and cats. It is interesting that cats can transmit the infection among themselves,” says Satyajit Rath, biologist from IISER Pune, who was not involved in these studies. He also draws attention to the fact that since none of the animals developed serious illness, it may not be immediately close to the condition of humans. “Even in humans, only a small percentage are seriously sick,” he adds, emphasising that the animal studies involved a small number only.
B. Rockx and coworkers studied cynomolugus macaques (monkeys). This study published in Science finds that when the monkeys were infected with the SARS-CoV-2 virus, they shed the virus in the upper and lower respiratory tracts. They did not show severe symptoms. A paper published in Nature by researchers from Hong Kong studies golden Syrian hamsters. The group finds that SARS-CoV-2 infection in golden Syrian hamsters was like a mild infection experienced by humans.
Right question
Which then are the animals that can be used as suitable models to study COVID-19? A ‘Perspective’ article in Science discusses this: It really depends on the question being asked – Cats and ferrets may be useful in studying how the disease is transmitted and in testing antivirals to stop disease progression. Hamsters could be used for studying efficacy of vaccines. Non-human primates could be used to assess vaccine and antiviral effectiveness.
“Animal models are approximate mirrors of the human conditions. While we can get some insights, we have to go to the human system, see whether the insights are useful and come back and test again,” says Prof. Rath.
Published - May 30, 2020 08:03 pm IST
