Prospective cohort, Ecological, Other
Study population and setting
The authors perform a series of experiments wherein groups of animals were inoculated with the SARS-CoV-2 virus and then monitored for the presence of viral RNA and infectious virus in various tissue samples. This included two different isolates of SARS-CoV-2, F13-E from the Huanan Seafood Market in Wuhan and CTan-H isolated from a human patient. Experiments were performed on a total of 18 ferrets, 14 cats (7 subadult and 7 juvenile), 7 dogs, 8 pigs, 8 chickens, and 8 ducks.
Summary of Main Findings
All inoculated ferrets tested positive for viral RNA and/or infectious virus in upper respiratory tract samples but not lungs or other tissue. In inoculated ferrets that were monitored over two weeks after inoculation, virus was detectable in nasal swabs for up to 8 days and in some rectal swabs, indicating some possible low-level viral replication in the digestive system of ferrets; tested ferrets also showed antibodies to SARS-CoV-2 after infection. Four cats inoculated with the virus tested positive for viral RNA and infectious virus in multiple tissue samples on days 3 and 6 post-inoculation, including from the upper respiratory tract, lungs, and small intestine. Three other cats were inoculated with the virus and then placed in paired cages with uninfected cats: all of the inoculated cats and one of the exposed cats tested positive for viral RNA in feces and upper respiratory tract samples, indicating that transmission between cats had occurred through respiratory droplets. Five dogs were inoculated with the virus and housed with three uninfected dogs: viral RNA was detected in rectal swabs from three inoculated dogs and antibodies were detected in two inoculated dogs, but none had detectable infectious viruses. Pigs, chickens, or ducks were evaluated similarly to dogs, but none of the animals tested positive for viral RNA or antibodies after inoculation, indicating that these animals may not be susceptible to infection.
The study uses multiple experiments to investigate whether animals can be infected with the virus and might be capable of transmitting the virus to uninfected animals. The researchers tested multiple tissue types in infected animals to determine if the virus infects similar areas of the body as it does in humans. Additionally, the researchers tested for the presence of both viral RNA and infectious virus in samples. This is an important advantage because detection of viral RNA does not necessarily mean that the individual is currently infected, a distinction that is not always made in other studies of viral prevalence.
The study includes only a small number of animals, especially in the transmission experiments. Preliminary experimental studies of a novel infection like this one (especially ones that must be done in a high biosafety level laboratory) might be excused for having a small sample size. Follow-up work that might look at the effects of treatments on viral transmission or clinical outcomes in an animal model would need much larger sample sizes. Also, the study did not attempt to establish an infectious model of COVID-19 in ferrets where symptoms and viral dynamics are tracked simultaneously, so it cannot replicate differences in disease severity between males and females seen in humans. The study also does not thoroughly investigate why virus does not replicate in the lungs of ferrets as it does in humans.
This study establishes that ferrets and cats are highly susceptible to infection with SARS-CoV-2 in an experimental setting, dogs have lower susceptibility with limited viral replication, and pigs, chickens, and ducks are not susceptible to SARS-CoV-2 infection.