1 INTRODUCTION
Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel member of the Coronaviridae family (Betacoronavirusgenus), responsible for the coronavirus disease 2019 (COVID-19) pandemic in humans, for which a spill-over from wild animals is most likely. This virus possesses a single-stranded, positive-sense RNA of ~30 kb that encodes approximately 9,860 amino-acids. Two open reading frames (ORFs), ORF1a and ORF1b, located at the 5’ terminus of the genome encode 16 non-structural proteins (NSPs). Four structural proteins [i.e. envelope (E), membrane (M), nucleocapsid (N) and spike (S) proteins], as well as several accessory proteins are produced through the expression of subgenomic mRNAs (Chan et al., 2020; Kim et al., 2020).
The S protein mediates receptor binding on target cells and determines host tropism (A. Wu et al., 2020). Similarly to SARS-CoV-1, the receptor utilized by SARS-CoV-2 to enter target cells is the angiotensin-converting enzyme 2 (ACE2) (Lu et al., 2020). It has been identified that, besides humans, the receptor-binding domain of the virus recognizes ACE2 from various animal species, including cats (Luan, Lu, Jin, & Zhang, 2020; Wan, Shang, Graham, Baric, & Li, 2020). Consequently, cats exposed to SARS-CoV-2 through contact with infected people, either asymptomatic virus carriers, or patients, can be affected, thus resulting in human-to-animal virus transmission, i.e. a reverse zoonosis (Segalés et al., 2020). It is noteworthy, meanwhile that coronaviruses have adapted species by species over relatively short to long periods of time, in human and animals. Domestic and wild cats are the hosts of feline coronavirus (FCoV), a highly prevalent virus with worldwide distribution, that is classified within theAlphacoronavirus 1 species (Alphacoronavirus genus), along with common cold-causing coronaviruses of humans (Drechsler, Alcaraz, Bossong, Collisson, & Diniz, 2011).
The capability of SARS-CoV-2 to infect cats has been indicated experimentally, as well as in clinically confirmed cases. Experimental infection of cats with a SARS-CoV-2 isolate originating from Wuhan indicated that the cats are permissive to the infection, since the virus was able to replicate in the nose and throat and caused inflammatory pathology in the lower respiratory tract (J. Shi et al., 2020). Cases of naturally infected cats with clinical signs of mild-to-moderate severity have been reported in Belgium, the USA, Spain, France and Brazil (Carlos et al., 2021; de Morais et al., 2020; Sailleau et al., 2020; Segalés et al., 2020). The clinical signs that have been described in cases involve mainly respiratory symptoms, such as breathing difficulty and cough. Gastrointestinal symptoms have also been reported (de Morais et al., 2020; Sailleau et al., 2020). In a single case the developed signs were severe, leading to euthanasia, although post-mortem examination implicated SARS-CoV-2-unrelated hypertrophic cardiomyopathy with secondary thrombosis as the major cause of death (Segalés et al., 2020).
Reports for the detection of SARS-CoV-2 or antibodies specifically directed against this virus in asymptomatic cats also exist. A serological investigation which took place in Wuhan during early 2020 revealed detection of virus-specific antibodies in naturally infected cats (Zhang et al., 2020). A more recent study which took place in Italian households indicates that cats can seroconvert under the normal conditions of pet ownership (Patterson et al., 2020). SARS-CoV-2 RNA was detected in the oral cavity, nasal and rectal swab samples obtained from a clinically healthy cat in Hong Kong (Abdel-Moneim & Abdelwhab, 2020). However, the number of reports available for natural infection of cats within house environments is limited and, albeit the airborne virus transmission between infected and susceptible cats, has been experimentally demonstrated, the role of this host species in the natural transmission of SARS-CoV-2 remains unclear.
Since data currently available for natural infection of cats are scarce, the present study aims to investigate the course of SARS-CoV-2 infection in naturally exposed cats in a small household setting, in Greece. The owner of the animals was affected by COVID-19 and was in continuous close proximity with three cats, during the days corresponding to high virus shedding and the quarantine period. Serial samplings were conducted from the cats, to determine the onset and duration of infection and viral shedding, as well as their antiviral humoral immune responses. In order to be able to detect introductions of known variants and/or any emerged and adapted variants, and to obtain a preliminary understanding of the directionality of the chain of virus transmission, whole SARS-CoV-2 genomes from the cats and their owner were sequenced and subjected to molecular characterization and quasispecies analysis.