As the number of human coronavirus infections continues to rise world-wide, there is some uncertainty about the role animals play in the COVID-19 virus epidemic. In this article, we try to summarize and interpret the available information in this rapidly evolving situation.
Corona viruses are common in humans and animals, some of them are zoonotic (can be transmitted to humans from animals). The novel 2019 coronavirus (SARS-CoV-2) is a new strain that is thought to originate from an animal source. Investigations on the identification of the source animal species are on-going. So far, the closest match to the human coronavirus was found in a bat in China. Sequencing data show that the human virus shares 96% of their sequence with the virus found in multiple species of bats in the genus Rhinolophus, but differs in an essential part, the receptor binding domain. This suggests that this specific bat coronavirus did not directly infect people, but could have been transmitted to people through an intermediate host. Which animal species served as intermediate host in this case is still subject to speculation.
The highest risk for COVID-19 virus infection is human-to-human transmission, but what part do animals play in the development of the current pandemic and can animals be infected at all?
There is an increasing number of reports of animals infected with SARS-CoV-2, including pets. At the moment data are missing about how easily and how often transmission occurs, and further research is needed. Probably the most common route of infection is through close contact with infected humans. According to currently available data, some animals seem more susceptible than others, cats are for example more susceptible than dogs. Whereas dogs did not show any signs of disease, cats can have respiratory and digestive symptoms. A recent study found that SARS-CoV-2 replicates efficiently in experimentally infected ferrets and cats, but poorly in dogs, pigs, chickens, and ducks. Further it was shown that infected cats can transmit the virus to other cats via respiratory droplets.
Experimental infection experiments performed by the Friedrich-Loeffler-Institute (FLI) in Germany in a study in pigs, chickens, ferrets and Nile flying foxes, also showed that especially ferrets are susceptible to the virus.
Natural infections in minks in mink farms have been reported in The Netherlands. Minks belong to the same family as ferrets, the mustelids. Infection dynamics suggest that animal-to-animal transmission likely happened. It is even plausible that employees at Dutch mink farms have caught the virus from mink. The virus the employees are infected with shows strong similarities to the virus found in the minks. These cases would be the world-wide first concrete evidence of an animal-to-human transmission.
All these findings suggest that minks, ferrets, cats and other animals susceptible to the virus could be a factor in the COVID-19 pandemic and that they should be considered in control measures. In the Netherlands, a surveillance has been initiated in 150 domestic cats owned by COVID-19 infected individuals.
The World Organization for Animal Health (OIE) has called SARS-CoV-2 an emerging disease in animals and has listed case definitions as well as guidance for sampling and testing. Animals that are found positive for the virus need to be reported to the OIE.
The OIE, the US Centers for Disease Control and Prevention (CDC) and other international organizations recommend that people sick with COVID-19 limit contact with animals to prevent a potential transmission of the virus to their animals. The WHO advices to wash hands after contact with animals to prevent contamination.
There have so far been no reports of transmission via the consumption of animal products (EFSA).
So, it is likely that animals did play a role in the origin of the epidemic. At the moment, transmission seems to be predominantly human-to-human, but animal-to-human transmissions are possible and their role in the propagation of the infection should be more thoroughly investigated.
COVID-19 is not the first disease to pass from animals to humans. As human-animal interactions are intensifying we cannot exclude future spill-over events. The occurrence of the pandemic teaches us that we need to adopt a fully integrated approach, a One Health vision, where human health, animal health, and the environment are monitored together. See also our letter to the editor published in Transboundary and Emerging Diseases.
[Updated May 29th, 2020]
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