Since its emergence in China in 2019, knowledge about the coronavirus SARSCoV-2, responsible for COVID-19 disease, has grown steadily. Many questions are being asked about the links between this virus and farm animals. But what exactly do we know today? Muriel Vayssier-Taussat, Head of the Animal Health Department at INRAE and Director of the Carnot F2E Institute, gives an overview of the state of knowledge as of May 25, 2020 based on available scientific publications.
Originally: an animal reservoir
Genetic analyses of the SARS-Cov-2 virus show that it is close to viruses infecting certain species of bats and pangolins. It could be the result of recombination (= an exchange of genetic material) between these viruses. While the viruses identified in pangolins and bats are not infectious to humans, this new SARS-CoV-2 virus has acquired the ability to infect humans, to be transmitted within the human population and to create the epidemic we know.
Which animals can be infected with SARS-CoV-2?
To answer this question, several studies have been carried out, ranging from demonstrating the presence of the virus receptor in the genome of domestic animals (and their recognition by the virus in in vitro models) to experimental animal infection tests or, more directly, to the demonstration of natural infection sites in animals.
1 – What is known about the presence of the gene coding for the virus «receptor».
The receptor of a virus can be compared to a lock, present on a cell, which allows the virus to attach itself to it thanks to a key it holds (the Spike Protein or protein S in the case of SARS-CoV-2). An article published on March 19, 2020 (Qiu et al. 2020), showed that certain animal species (such as pigs and civets) carry THE receptor that can be recognized by the SARSCoV-2 virus and are therefore susceptible to infection.
Other animal species (e.g., felids) have a different but similar receptor to that used by the virus, and others (e.g., rodents) have very different receptors and are probably not susceptible to the virus. This work has allowed predictions to be made about the susceptibility of particular animal species, but is by no means evidence of the ability of the virus to infect particular species. Indeed, the passage from one virus to another species is not only dependent on the presence of the receptor, but also on the presence of other cellular factors necessary for the reproduction of the virus.
2- What is known about the susceptibility of farm animals to the SARS-CoV-2 virus.
The available results come either from experimental infections carried out in research laboratories or from the detection of natural infections. The results published or communicated via the website of the International Society of Infectiology (https://promedmail.org/) for each animal species are summarised below:
- Pig: despite the presence of the virus receptor on pig cells, the 2 studies carried out in 2 independent laboratories showed that pigs were not susceptible to SARS-CoV-2 infection: no detection of the virus in nasopharyngeal swabs from infected animals followed for 40 days after infection, no clinical signs, no seroconversion (Beer 2020; Shi and aI 2020).
- Poultry (chickens and ducks): 4-week follow-up of animals experimentally inoculated with SARS-CoV-2 showed that poultry, like pigs, were not susceptible to SARS-CoV-2 virus (Shi et al., 2020)
- Bovine/ovine/caprine: Experimental in vivo inoculation studies are ongoing and the results are not known at this time. However, virus receptor assays in these animal species (Qiu et al., 2020), the inability of cell lines (particularly of bovine origin) to be infected in vitro with SARS-CoV-2 (Hoffmann et al., 2020) and the absence of epidemic outbreaks in these species in all the countries of the world affected by the pandemic all suggest that these species would not be susceptible to this virus.
- Horses: To date, no experimental studies have been conducted in horses and no cases of natural infection have been reported. A serological study conducted by IDDEX on thousands of animal samples, including horses, did not detect any positive serology in these animals, again suggesting that the horse is probably not susceptible to this virus.
- Mustelids (mink, ferrets…): concerning the ferret, several experimental studies show that this animal is receptive to the virus, the infection causes clinical signs and lesions observed in the respiratory tract. The possibility of animal-to-animal transmission of the virus has also been demonstrated (Kim et al., 2020; Shi et al., 2020). Very recently, in the Netherlands, cases of SARSCoV-2 infection were detected in three mink farms. These animals develop clinical signs similar to those found in humans. In one of the farms, virus (non-viable) was found in the farm’s cats (cats which are now accepted as susceptible to SARS-Cov-2 infection*) and a very probable case of human infection with mink was reported on May 20 on the Prodmed website (https://promedmail.org/promed-post/?id=7359976). At present, and since the first cases due to SARS-CoV-2 in humans have appeared, this is the only human case of COVID19 for which an animal origin is suspected.
Why are «outbreaks» of contamination in slaughterhouses being detected worldwide?
If, with the exception of certain species of mustelids, farm animals are not susceptible to viruses, one may wonder why so many outbreaks of COVID19 appear in slaughterhouses around the world. Epidemiological studies conducted in these various outbreaks all converge towards the same conclusions: the virus is introduced by an infected person and not by an animal.
In these places, employees have had to continue working in spite of the containment to ensure the food supply and even though hygiene standards are imposed, the risk of interpersonal contamination is high due to cramped working and living conditions and the high risk of virus dispersion (such as high-pressure cleaning which favours viral dispersion).
The analysis of these risks and the introduction of further appropriate protective measures, combined with the regular use of virus detection tests, should help to eliminate these outbreaks and protect slaughterhouse staff in the long term.
Is it possible to say that meat from these slaughterhouses cannot be a vector for the virus?
In theory, virus passage from an infected person to food (meat or other foods) can occur via infected droplets and, without cleaning measures, viruses of the Coronaviridae family can persist for up to 9 days (Kampf et al. 2020), especially when the temperature is low and relative humidity is low (Casanova et al. 2010). Thus, it is possible that viral particles may be projected onto meat in the case of outbreaks in slaughterhouses.
However, transmission of SARS-Cov-2 via the digestive tract has not been demonstrated to date and cooking (4 minutes at 63°C) is considered effective in inactivating coronaviruses in food. Therefore, it seems unlikely that the direct digestive transmission route can occur. For more information, an opinion of the ANSES on the potential role of food in the transmission of the virus was issued and concluded that, based on the current state of knowledge, the direct digestive transmission route of SARS-Cov-2 can be ruled out (ANSES referral No. 2020-SA-037).
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