2. Requirements of a SARS-CoV-2 animal model
A perfect animal model should be permissive to infection and be able to
mimic the clinical pathology of disease as observed in humans (Sutton &
Subbarao, 2015). These models play a crucial role in studying host-virus
interactions that contribute to disease pathogenesis and transmission. A
protocol for the development of animal models is mentioned by the ‘FDA
animal efficacy rule’(Product Development Under the Animal Rule ,
2015). Accordingly, these models should have the same receptors as those
present in humans that help viruses in the attachment and entry process,
and the outcome of the infection should be as severe as in humans. In
many emerging disease studies, in vitro studies cannot completely
simulate human pathophysiology. Also, the immunological components are
very complex in humans, which cannot be proven in the in-vitro
experiments. However, despite the differences in animal model
translation to humans, a lot of critical information related to the
pathogenesis, prevention and treatment of newly emerging infectious
diseases can still be discovered.
Animal models can be categorized as small or large. Small animal models
include the use of mice, rats, ferrets, hamsters and rabbits, which are
relatively smaller in size and require limited space. Small animal
models offer several advantages because the animals and reagents
specific to the animals are easily available, the animals can be handled
with less effort and cost, and allows the use of a large number of
animals to provide data for strong statistical analyses (Sutton &
Subbarao, 2015). The small animal model is limited by the significant
intrinsic biological differences between humans and rodents or small
mammals, which have led to the requirement that viruses must be adapted
to the animals, or the animals, must be genetically manipulated to
recapitulate the human system (Subbarao et al., 2004). In addition, the
animals’ short life span hampers the ability to monitor the long-term
prognosis of the disease (Gretebeck & Subbarao, 2015). Large animal
models such as non-human primates (NHP) are more reliable models to
replicate human disease pathogenesis as they are physiologically,
immunologically and genetically more closely related to humans (Fujiyama
et al., 2002; Lu et al., 2008). The major limitation of using NHP models
are the high cost and resources involved in the study, which limits the
number of animals that may be included in a study and thus adversely
affecting the statistical power of the outcome. In addition, most NHPs
are outbred animals and have a wide variability in genetic backgrounds,
which sometimes make it difficult to interpret the outcome of a study
due to variability in results among individual animals (Lu et al.,
2008).
For the ongoing COVID-19 pandemic, direct human clinical trials are
complicated by the mild to severe forms of the disease due to genetic
diversities, age of the host, comorbidities, multiple infections along
with other preexisting diseases (P. K. Chan & Chan, 2013; Gold et al.,
2020; ”What explains Covid-19’s lethality for the elderly? Scientists
look to ‘twilight’ of the immune system,” 2020). The virus uses the
ACE-2 receptor that is expressed in cells found in the heart, lungs,
gastrointestinal tract, and renal tract (Hamming et al., 2004; Hoffmann
et al., 2020). After entry, viral replication kinetics in most of the
target cells remains unknown. Air droplets and aerosols are critical
aspects of the global spread of SARS-CoV-2 (Hamid, Mir, & Rohela, 2020;
Meselson, 2020). Regardless, there is an urgent need to understand the
risks factors of transmission for SARS-CoV-2 (Figure 2 ). The
symptoms of COVID-19 with other specific and non-specific upper
respiratory tract symptoms are similar in nature, which make it
difficult to distinguish from other diseases (Bhatraju et al., 2020). A
case report from Eiju General Hospital Tokyo, Japan, showed the
existence of the Influenza virus along with SARS-CoV-2 showing similar
types of symptoms that posed a difficulty in the differential diagnosis
of the two diseases (Azekawa, Namkoong, Mitamura, Kawaoka, & Saito,
2020). Another case in Rhode Island showed the co-infection of
SARS-CoV-2 with Human metapneumovirus, where a patient tested positive
for human metapneumovirus but failed to test for SARS-CoV-2. The
symptoms did not subside even after treatment for metapneumovirus.
Nevertheless, when the patient was tested for SARS-CoV-2 by PCR and the
results came back as positive (Touzard-Romo, Tape, & Lonks, 2020). A
travel-related case of an 80-year old male from Japan, with a history of
diabetes, showed coinfection of SARS-CoV-2 and Legionella with
respiratory distress and gastrointestinal symptoms. The patient passed
away after 13 days but was later confirmed to be infected with COVID-19
(Arashiro et al., 2020). These individual case reports compel the need
for suitable animal studies of SARS-CoV-2 co-infection with other
pathogens with similar disease manifestations for better clinical
outcomes.
COVID-19 has a variety of clinical outcomes (J. F.-W. Chan et al., 2020;
Huang et al., 2020). Most of the patients who are admitted to hospitals
with severe clinical manifestations have other comorbid conditions such
as diabetes, cardiovascular disease, gastrointestinal disease, or
hypertension (Gold et al., 2020). In the case of Influenza, one
publication showed that the risk of the Acute Respiratory Distress
increases 3.4-fold in the H7N9 infected person with similar comorbidity
(H. N. Gao et al., 2013). Age-related co-morbidity has mostly affected
the transmission cycle of disease (Sun et al., 2020). The exact
mechanism of how these comorbidities deteriorate SARS-CoV-2 patient
conditions remains unknown. These comorbid conditions that lead to
severe to fatal outcomes are another aspect of COVID-19 pathogenesis
that needs a suitable animal model for study.
There is a concerted effort in the research community to develop
therapeutics and vaccines against COVID-19. For example, Remdesivir is
an antiviral drug that failed the clinical trial for Ebola a decade ago
(Mulangu et al., 2019) and now has been proposed for COVID-19 treatment
(Beigel et al., 2020) (ClinicalTrials.gov: NCT04257656, NCT04252664,
NCT04280705). It was repurposed for COVID-19 treatment because it
directly blocked RNA synthesis (Agostini et al., 2018). Preliminary
reports for Remdesivir in a clinical trial of 1063 adult patients with
lower respiratory tract infection showed a shortening in recovery time
in 538 patients given Remdesivir intravenously (median 11 days),
compared to the 521 patients given a placebo (median 15 days) (Beigel et
al., 2020). However, there were still serious adverse effects reported
in COVID-19 patients given Remdesivir, but Remdesivir worked better in
comparison to the placebo. To test the efficacy of Remdesivir in disease
outcome, a rhesus macaque model of SARS-CoV-2 infection was developed,
where infected macaques developed mild to moderate clinical
symptoms(Brandi N. Williamson, 2020). The macaques treated with
Remdesivir did not show any respiratory distress, and viral titers in
bronchoalveolar lavage was reduced after 12 hours of treatment,
suggesting that Remdesivir may have some beneficial effect in the
treatment of COVID-19 (Brandi N. Williamson, 2020). A study using
Lopinavir, a protease inhibitor and Ritonavir, that primes the action of
Lopinavir widely used to treat HIV infection, are in clinical trials for
COVID-19 (Bhatnagar et al., 2020). The combination of Lopinavir and
Ritonavir had shown efficacy against SARS-CoV in mice (Chu et al., 2004)
and MERS-CoV in NHPs (J. F. Chan et al., 2015). ACE-2 inhibitors as well
as some fusion inhibitors such as Arbidol are also in clinical trials
(Chinese Government Clinical Trials: ChiCTR2000029573)(Wang et al.,
2020). Clinical trials using convalescent serum as treatment for
COVID-19 has already begun at the Icahn School of Medicine at Mount
Sinai, New York, USA (Sean T. H. Liu, 2020) . Similarly, polyclonal
antibodies have also been employed in clinical trials because polyclonal
human immunoglobulin G (IgG) had shown effectiveness against
MERS-CoVs.(Luke et al., 2016). Vaccine development is important to the
control of disease and protection against the transmission of the virus
to immunized individuals. Many groups are working to develop potential
vaccines for SARS-CoV-2, out of which some candidates are already in
clinical trials (Mukherjee, 2020). Moderna, a vaccine manufacturer, has
started phase II clinical trials of their vaccine candidate mRNA- 1273,
which passed phase I trial recently by generating desired immune
response by natural infection (ClinicalTrials.gov Identifier:
NCT04283461) (”Moderna Moderna announces funding award from CEPI to
accelerate development of messenger RNA (mRNA) vaccine against novel
coronavirus; 2020. [accessed 2020 February15]
,”). A purified inactivated SARS-CoV-2 virus candidate (PiCoVacc) tested
in mice, rats and non-human primates have also shown protective immune
response suggesting neutralizing activity against SARS-CoV-2 (Q. Gao et
al., 2020). Recombinant protein-based vaccines by University of
Queensland and viral-vector based vaccines by University of Oxford,
England are also being actively tested for vaccine efficacy (”Developing
Therapeutics and Vaccines for Coronaviruses,”). Another study has
suggested that polio vaccine could be used to prevent SARS-CoV-2 (Yeh et
al., 2020).
In order to study various unanswered questions about the disease
pathogenesis, suitable animal models are essential. Several studies are
in progress to find suitable animal models to study the transmission,
disease pathogenesis, and pre-clinical trials of potential therapeutics
for the management of COVID-19. However, to successfully end the
COVID-19 pandemic, efforts to develop vaccines to prevent the virus
spread should synchronize with studies to uncover disease pathogenic
mechanisms in comorbid and high-risk co-infection conditions. Although
difficult, identifying a model that adequately mimics the human disease
of SARS-CoV-2 is bolstered by several studies of SARS-CoV and MERS-CoV,
which has provided some preliminary insights on the models. All
laboratory animal models, like mice to hamsters then ferret to NHP’s,
are equally valuable to dissect out various questions to better
understand disease mechanisms.