Author: Corrado Minetti; Reviewer: Dimitrios Spiliotopoulos
An increasing number of reports about the capacity for the SARS-CoV-2 virus to infect domestic animals is raising concerns for the safety of people. However, it is still unclear whether domestic animals can transmit the virus to humans. Here we review and discuss what is known so far on this topic.
1. Evidence from experimental infections in the laboratory
SARS-CoV-2 was administered via intranasal inoculation to ferrets (commonly used as animal models for respiratory viruses infecting humans), cats (juvenile and subadult), dogs, pigs, chickens and ducks (1). Swabs, feces and tissues from the animals were checked regularly for the presence of viral RNA by qPCR and infectious viruses (grown in VERO cells), and their blood was checked for the presence of antibodies. Results showed that:
- viral replication and production of infectious virus occur in the upper respiratory tract (nasal turbinate, soft palate and tonsils) and possibly in the digestive tract of ferrets and cats (particularly in the juvenile animals). Low levels of viral RNA were also observed in some of the ferrets’ rectal swabs and some of the cats’ feces;
- Dogs seem to be poorly susceptible to SARS-CoV-2 infection. Viral RNA was detected in only three out of five inoculated animals, and infectious virus was never recovered from their swabs;
- No signs of viral RNA were observed in pigs and birds, suggesting there animals are not susceptible;
- Seroconversion at day 14 or onwards occurred in all exposed ferrets and cats and in half the exposed dogs, but it was not observed in the other animals;
- Infection was not associated with severe disease or death, although signs of lung and tracheal damage were observed in a limited number of ferrets and some juvenile cats.
- When naïve (not inoculated) cats were housed in cages adjacent to inoculated animals (4 cm apart, separated by a double netting, with a horizontal air flow going from the inoculated to the naive cats) limited airborne transmission was observed. In two out of six cat pairs, viral RNA and antibodies were detected in the naïve animals.
Transmission between cats was further confirmed in pairs of naïve and SARS-CoV-2-inoculated animals housed in the same cage (2). Five days post-exposure, all naïve co-housed animals were infected and all cats also seroconverted at day 24. Whether transmission occurred through droplets or a more direct contact between co-housed animals is not known.
Of particular relevance for transmission, in both studies infectious virus was never recovered from the animals’ rectal swabs or feces (1,2).
Of note, cats and ferrets had been reported to be susceptible to experimental infection of SARS coronavirus and the SARS coronavirus could be efficiently transmitted to animals living with them (3).
2. Evidence from field and epidemiological investigations
There have been reports from Hong Kong, Belgium, and New York of SARS-CoV-2-positive dogs (2 cases), cats (2 cases, one of which developed vomit, diarrhea, and breathing difficulties), tigers and lions (4 and 3, respectively, with these big felines developing dry cough and wheezing). Presumably, these animals were infected via close contact with humans with highly infective viral loads and such reports might therefore be statistical outliers. Therefore, it is not possible to predict whether SARS-CoV-2 will cause a panzootic (4).
2.1. Evidence from pets in domestic environments
A serological survey looking for virus antibodies in cats from Wuhan included 39 samples collected before the beginning of the outbreak and 102 collected afterwards. 14.7% of the samples tested positive in an ELISA test against the RBD of SARS-CoV-2 spike protein, with all of the positives found in samples collected after the start of the outbreak (5). Three out of the 15 positive cats belonged to households with confirmed COVID-19 human cases, suggesting human-to-animal transmission in domestic environments. 11 of the 15 ELISA-positive cat sera presented SARS-CoV-2 neutralizing antibodies according to the virus neutralization test (titer: 1/20 to 1/1080) with some ELISA strong positive showed weak neutralizing activities. The three cats showing the highest neutralizing titer had owners that were diagnosed as COVID-19 patients.
A study reports that two out of 15 dogs from households with confirmed human infections were infected with SARS-CoV-2 by qRT-PCR from nasal, oral and rectal swabs (6).Genome sequencing of the viral isolates from the dogs and their owners confirmed that they shared the same virus. Of note, the animals never showed any symptoms.
Evidence collected so far shows that SARS-CoV-2 can indeed infect some domestic animals (ferrets and cats in particular) and suggests that both human-to-animal and animal-to-animal transmission can occur. However, there are a few aspects we must consider:
- Experimental infections give invaluable information on the susceptibility of a host species to a particular pathogen (also leading to the development of suitable animal models for further studies), but for obvious constraints are conducted on small sample sizes and they rarely represent how transmission occurs in nature and its heterogeneity.
- Before pets are considered a significant risk for people, actual transmission of SARS-CoV-2 from an infected animal to a susceptible human must be proven.
- Molecular epidemiological evidence of transmission is needed from contexts where people and pets are in close contact (such as households, animal shelters, etc.);
- Data are lacking on the susceptibility of livestock to the virus and the risk these animals may pose.
 Shi J, Wen Z, Zhong G, Yang H, Wang C, Huang B, et al. Susceptibility of ferrets, cats, dogs, and other domesticated animals to SARS–coronavirus 2. Science. 2020.
 Halfmann PJ, Hatta M, Chiba S, Maemura T, Fan S, Takeda M, et al. Transmission of SARS-CoV-2 in Domestic Cats. N Engl J Med. 2020.
 Martina BEE, Haagmans BL, Kuiken T, Fouchier RAM, Rimmelzwaan GF, van Amerongen G, et al. SARS virus infection of cats and ferrets. Nature. 2003.
 Gollakner R, Capua I. Is COVID-19 the first pandemic that evolves into a panzootic? Vet Ital. 2020.
 Zhang Q, Zhang H, Huang K, Yang Y, Hui X, Gao J, et al. SARS-CoV-2 neutralizing serum antibodies in cats: a serological investigation. bioRxiv. 2020.
 Sit THC, Brackman CJ, Ip SM, Tam KWS, Law PYT, To EMW, et al. Infection of dogs with SARS-CoV-2. Nature. 2020.