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.