1 Introduction

The COVID-19 pandemic is a stark reminder that we live in a time where pathogens spread faster than we can track them. The incidence of other diseases in humans such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS),1 H1N1 pandemic influenza,2 H5N1 avian influenza,3 Ebola haemorrhagic fever,4 etc. in just the last two to three decades has also highlighted that globalization and animal–human interface are major factors in the transmission of pathogens from wild and domestic animals to humans.

Recent advances, especially in genomics, have spurned the understanding of diseases in terms of their aetiology, origin, identification of index case, routes of spread and potential transmission, evolution of the pathogen, minutest details of host–pathogen interaction and pathogenesis, and vaccine and/or drug discovery, facilitating disease mitigation strategies. At least one new infectious disease is thought to emerge or re-emerge every year.5,6 Between 1940 and 2004, there were 335 emerging infectious diseases, about 60% of them originating from animals, with not much change in decade-wise proportion of zoonotic diseases.7 About three-fourths of all emerging and re-emerging human diseases can be traced back to animals, particularly wild animals,7 and it has been propounded, probably rather radically, that every species of Kingdom Animalia may be a potential carrier of human pathogens.8

2 SARS-CoV-2 and Its Origin

The SARS-CoV-2 belongs to the family Coronaviridae and has one of the largest single-stranded RNA genomes (29.9 kilo bases) for a virus. The major structural proteins encoded by SARS-CoV-2 are spike (S), nucleoprotein, envelope and membrane proteins. Among these structural proteins, the S protein is critical for the receptor-mediated entry of the virus into susceptible cells. The receptor binding domain (RBD) of the S protein binds the cellular receptor, the angiotensin converting enzyme-2 (ACE2),9 facilitating entry of the virion particle. Further, transmembrane protease serine 2 (TMPRSS2) helps in the priming of the S protein, where the S protein is cleaved so as to allow the fusion of viral envelope with cellular membrane.9 In vitro, ACE2 from as many as 44 mammals has been shown to bind to the RBD.10,11 On the other hand, modelling studies have returned supporting as well as conflicting results on the interaction between S and ACE2.12,13,14,15 Together, these studies only suggest interactions while receptor expression and other host factors may dictate infection, replication and pathogenesis as well as transmission to other hosts.

The first cases of COVID-19 are thought to have originated from live animal markets in Wuhan, China. Analysis of the genomes of SARS-CoV-2 has suggested that the virus may have originated from the Asian bats belonging to the Rhinolopus species16,17,18,19 and that the transmission to humans may have involved an intermediate host, possibly the pangolin.16,17,20,21,22,23 However, both the precise origin and the exact intermediate host are conjectures at best. On the other hand, higher sequence similarities of SARS-CoV-2 with bat coronavirus-RaTG13 (96.2%) and Guangdong Pangolin-CoV (92.4%)22,24 support the zoonotic origin of the virus. However, the SARS-CoV-2 has a unique “PRRA” motif at the junction of the S1 and S2 subunits of the S protein, which makes it distinct from RaTG13 or the Guangdong Pangolin-CoV. Further, the binding affinity of pangolin ACE2 receptor with RBD has been shown to be low.25 Recent genetic analysis has revealed that three viruses in bats from the limestone karstic terrain in North Laos, BANAL-52, BANAL-103 and BANAL-236 are very close to SARS-CoV-2 with only one or two mutations in the RBD and the absence of a cleavage site for the cellular enzyme furin. These viruses are therefore considered to be the closest match to SARS-CoV-2 in nature.26

3 Animal Models for SARS-CoV-2 Infection

Experimental reproduction of COVID-19 in mice requires adaptation of SARS-CoV-2 through serial passage27 as wild-type mice are resistant to infection.28 However, mouse adaptation also leads to changes in SARS-CoV-2, which is not ideal for challenge studies. When mutations relevant to mouse adaptation were introduced through reverse genetics, the resultant virus replicated in the upper and the lower respiratory tract of BALB/c mice when the virus was administered intranasally.29 The main reason for the species restriction appears to be the receptor, since SARS-CoV-2 not adapted to mice or adapted to grow in cell culture can efficiently infect transgenic mice expressing human ACE2 (hACE2),28,29,30,31 and such mice are considered suitable for challenge studies. Some transgenic mice such as HFH4-hACE2 mice express the receptor in the respiratory tract and the central nervous system, resulting in lethal encephalitis upon infection through the respiratory route.29,30,32 A sole study has also reported that when administered intragastrically, the virus could potentially be transmitted by the faecal–oral route33; however, the implications of this to transmission in humans are not clear, although SARS-CoV-2 causes gastroenteritis and is excreted in the faeces in humans, but faecal–oral transmission needs further evidence.34

Transient expression of hACE2 has also been employed for mouse infection studies. This method has the advantage of utilizing mice from diverse genetic backgrounds. Adenovirus or adeno-associated virus-mediated transduction has been used to express hACE2 in the respiratory tract of mice.35,36 When mice were transduced with human adenovirus 5 expressing hACE2 and subsequently infected with SARS-CoV-2 through intranasal or intranasal and intravenous route, only the peripheral and not the systemic route resulted in lung infection in mice.35 In mice transduced with adeno-associated virus 9 expressing hACE2, the transcriptome profile mimicked those in COVID-19 patients, in that interferon (IFN)-stimulated genes and inflammatory cytokines were up-regulated during SARS-CoV-2 infection. It was also observed that type I IFNs were responsible for pathological changes rather than controlling SARS-CoV-2 replication.36

Unlike mice, hamsters are susceptible to SARS-CoV-2 infection without the need for adaptation of the virus.30,37,38 Upon infection, hamsters lose weight initially but regain it by 14 days post-infection. Highest viral loads are detected in the nasal turbinates, lungs and trachea, facilitating the transmission of the virus to in-contact hamsters.37 Similar to humans, aerosol and droplet transmission between hamsters was higher with the D614G mutant of the virus.30

Two mustelids, which are used for commercial (minks) or experimental (ferrets) purposes, are highly susceptible to SARS-CoV-2. Minks undergo efficient upper and lower respiratory tract infection, with viral RNA being detected in the nasal turbinates, soft palates, tonsils, all lung lobes and submaxillary lymph nodes and infectious virus being detected in most of these samples. Severe pathological conditions are observed in infected minks with extensive and diffuse consolidation of the lungs. Nasal mucosa and submucosa of the vestibular, respiratory and olfactory regions show inflammatory infiltrates, epithelial degeneration and necrosis. Notably, the SARS-CoV-2-mediated respiratory system damage is similar to what is observed in severe human COVID-19 cases. Transmission of SARS-CoV-2 between minks through droplets has also been demonstrated.39

Ferrets are also suitable animal models. SARS-CoV-2-infected ferrets show infectious virus in the upper respiratory tract40 and virus is shed in saliva, urine and faeces.41 Infected ferrets transmit the virus through respiratory droplets to direct and indirect contact ferrets.42 Similar to ferrets, racoon dogs infected with SARS-CoV-2 shed the virus through nasal route, leading to infection of in-contact animals. However, the racoon dogs do not show any clinical signs.43

Experimentally, SARS-CoV-2 has also been shown to replicate in cats and be excreted in respiratory secretions, and infected cats can transmit the virus to co-housed cats.44 On the other hand, virus replication is poor in bank voles, cattle, dogs, pigs, chicken, ducks and tree shrews.40,45,46,47,48.

Experimental studies have also been carried out to study the SARS-CoV-2 infection in non-human primates (NHP). Infection produces acute respiratory distress and interstitial pneumonia in rhesus macaques, and severe pneumonia in baboons,49 but no clinical signs in cynomolgus monkeys.50 Irrespective of the clinical signs, SARS-CoV-2-infected NHPs show highest viral load in the upper and lower respiratory tract, indicating that NHP’s are a good model to study COVID-19 pathogenesis and protective efficacy of vaccines.

White-tailed deer produced subclinical infection upon intranasal administration of a SARS-CoV-2 isolate from a tiger, with the virus excreted in nasal secretions.51 Pregnant deer could transmit the virus through direct contact and vertically from doe to the foetus, with similar genome proportions in the tissues of the primary and in-contact animals and foetuses.52 Experimental infection of sheep with SARS-CoV-2 resulted in a mild infection and the viral RNA could be detected in nasal and oral swabs one day post-infection. However, the transmission of the virus from the infected sheep to naïve sheep was limited. Similar to white-tailed deer,52 when sheep were co-infected with two lineages of SARS-CoV-2, the ancestral lineage A appears to have less advantage over the α variant. However, these experiments have used 1:10 ratio of lineage A and α variant virus for the infection, making appropriate interpretation difficult.53

Details of the experimental infection of SARS-CoV-2 in animals are compiled in Table 1.

Table 1: Experimental infection of SARS-CoV-2 in animals.

4 Natural Infection of Animals with SARS-CoV-2

Till date, 31 countries have reported SARS-CoV-2 infection in more than ten animal species (Table 2). While most of the infections are reported from pet animals, outbreaks have also been reported from organized mink farms, and zoo and wild animals.

Table 2: Natural infection of SARS-CoV-2 in animals.

Several observations have established that pet animals can be infected by SARS-CoV-2 (Table 2). Tests on 17 dogs and 8 cats from COVID-19 households in Hong Kong confirmed positivity in two dogs by molecular and serological assays, with virus being isolated in one case54,55; there has also been a press report of a dog showing mild respiratory symptoms and testing positive for SARS-CoV-2 in North Carolina, USA, but a cat and another dog from the same COVID-19 household tested negative.56

From a Belgian COVID-19 household, a cat showed respiratory and gastrointestinal symptoms and shed the virus in faeces and gastric fluid, but recovered.57 Two cats from two independent households in New York state also tested positive by reverse transcription—polymerase chain reaction (RT-PCR), one from a COVID-19 household and the other within an affected neighbourhood and allowed to go outdoors, with the latter reportedly showing respiratory symptoms.58,59 A cat from Hong Kong also showed positivity without symptoms.60 Further, 15 (14.7%) of 102 cats tested after the outbreak started in Wuhan were found to carry antibodies to SARS-CoV-2 by enzyme-linked immunosorbent assay (ELISA), and 11 samples showed neutralization of SARS-CoV-2 and no cross-reactivity with feline coronavirus; these samples had been obtained from stray cats, at a veterinary hospital and from animals owned by patients.61 Thus, it appears that while cats and other felids can be infected, the infection rate may be low, and a symptomatic outcome is rare. In support of this, a study has reported no infection of 9 cats and 12 dogs which were in close contact with COVID-19 patients in a veterinary campus in France,62 and only one asymptomatic (but positive by RT-PCR) cat among 12 dogs, 8 cats, 2 rabbits and one guinea pig from 17 confirmed COVID-19 households in Spain.63 In addition, a serological survey of preserved sera from 487 dogs and 87 cats from among 1914 samples from 35 species of animals from China showed no positivity, although the samples probably predated COVID-19.64 Studies on pets which belong to COVID-19 patients in Brazil also indicated that the pet animals were susceptible to SARS-CoV-2 infection. Nine out of 29 dogs and four out of 10 cats were seropositive for SARS-CoV-2. Partial genome sequence of SARS-CoV-2 was obtained from the samples, but no virus isolation was attempted.65 In most of the documented cases of SARS-CoV-2 infection in dogs and cats, the symptoms were variable; while most of the animals were asymptomatic, some of the animals had mild respiratory distress. Recent studies have also linked myocarditis as one possible outcome with SAR-CoV-2 B.1.1.7 variant infection of dogs and cats.66

Direct or indirect evidence of SARS-CoV-2 infection in pet animals during the COVID-19 pandemic exists from Germany, Italy, Peru, the USA, France and several other countries.65,67,68,69,70,71 In multiple cases, genome sequences of SARS-CoV-2 obtained from pet animals were similar to that of the circulating human strains. For example, the SARS-CoV-2 genome sequence from a cat had D614G mutation specific to the phylogenetic clade A2 observed in the French SARS-CoV-2 genome sequences.72 Studies in France also showed that the SARS-CoV-2 genome sequences from a pet dog and its COVID-19 affected owners were of B.1.160 lineage with 99–100% identity.73 The SARS-CoV-2 genome from a domestic cat was close to the human B.1.1.39 lineage in Switzerland.74 Similar observations have been made in Argentina, where the SARS-CoV-2 genome sequence from domestic cats belonged to the B.1.499 lineage, which circulated in humans in that region.75 In one instance, the SARS-CoV-2 genome sequence of the owner and the pet animals (dog and cat) from the same household were found to be identical and belonged to the B.1.575 lineage.76

In a seroprevalence study on samples collected from cats (2160 samples) from Germany, the UK, Italy and Spain, 96 samples (4.4%) were positive by virus neutralization test and 92 samples (4.3%) were positive by an ELISA for the RBD. Since the samples were collected for some other ailments and not for SARS-CoV-2 infection, the presence of antibodies against SARS-CoV-2 during the COVID-19 pandemic in the samples from cats indicates possible human-to-cat transmission.77 In Slovenia, a pet ferret of a COVID-19-affected owner was positive for SARS-CoV-2 by RT-PCR,78 and this supports the findings of the experimental studies where ferrets were documented to be highly susceptible for SARS-CoV-2 infection.40,41,42

5 SARS-CoV-2 Infection in Captive Animals

Apart from pet animals, SARS-CoV-2 can also infect captive and farm animals. Mild respiratory symptoms were reported in five tigers and three lions in the Bronx zoo, New York, supposedly from being in contact with a COVID-19 asymptomatic zookeeper; SARS-CoV-2 was confirmed by RT-PCR,79 the virus was isolated, and complete viral genome sequence was elucidated in the case of a Malayan tiger.80 Virus isolation was not attempted from other zoo animals. Interestingly, SARS-CoV-2 genome sequence from tigers and lions was of different genotypes, which indicates two independent sources of infection.

Other zoo and captive animals that can be affected include puma, mink (farmed and wild), lion, tiger, ferret, snow leopard, gorilla, otter, cougar, coatimundi, binturong, Canada lynx and hyena.78,81 While most of the animals showed mild symptoms of cough and respiratory distress with or without nasal discharge and recovered later, four snow leopards succumbed to SARS-CoV-2 infection at the Great Plains Zoo, South Dakota, USA.82,83

Fur industry across various countries has been affected severely due to the high susceptibility of minks to SARS-CoV-2. Netherlands reported the first outbreak of SARS-CoV-2 in a mink farm in April 2020. Since then, the disease has been reported from Denmark, the USA, France, Greece, Italy, Poland, Lithuania, Canada, Spain and Sweden. Minks are housed in confined spaces, and the infection could spread rapidly among them through droplets. Severity of respiratory sickness is pronounced in minks with high mortality and post-mortem findings of acute interstitial pneumonia.84 Considering the severity of the disease, Denmark, Spain, the Netherlands and France culled entire affected mink colonies and the Netherlands has banned mink farming permanently.

In the Netherlands, one mink farm worker was found positive for the virus and clear evidence of mink-to-mink transmission was established.85 Three out of 11 tested cats in one of the farms were also positive for anti-SARS-CoV-2 antibodies, but no viable virus could be detected,86 although the direction of transmission is not clear in this case. In one of the outbreaks, a probable mink-to-feral cat transmission was recorded.87 Mink-to-human transmission was also established based on the sequence data from the Netherlands,111 Denmark88 and the USA.89 SARS-CoV-2 virus isolates obtained from minks with Spike Y453F/D614G mutation have higher affinity to hACE2.90 Whole-genome sequencing of the SARS-CoV-2 from minks revealed 170 mutations, and the mink-specific mutations of SARS-CoV-2 were also found in 300 samples collected from humans indicating mink-to-human transmission.91 By contrast, the mink SARS-CoV-2 genome from Greece lacked the Y453F mutation in the S protein,92 suggesting differential adaptability or simply randomness in the emergence of the variants.

Recently, transmission of SARS-CoV-2 was recorded from Syrian hamsters to humans in a pet shop at Hong Kong. Further investigation revealed that Syrian hamsters from both pet shop (50%) and warehouse (58%) were positive for SARS-CoV-2 with RT-qPCR which resulted in the culling of about 2000 hamsters across Hong Kong. The SARS-CoV-2 genome sequence from affected humans and Syrian hamsters belonged to the delta variant (AY.127), and analyses pointed to multiple hamster-to-human transmission events. Interestingly, other animals from the pet shop such as dwarf hamsters, rabbits, guinea pigs, chinchillas and mice tested negative for SARS-CoV-2. This incident is the only documented evidence of Syrian hamsters acquiring SARS-CoV-2 infection in natural settings.93

About 40% of the serum samples from white-tailed deer collected by the United States Department of Agriculture between January 2020 and March 2021 were positive for SARS-CoV-2 antibodies.94 Based on the initial finding, further study was initiated to understand the SARS-CoV-2 strains circulating in deer. Retropharyngeal lymph nodes were collected from white-tailed deer from Iowa between April 2020 and January 2021. Whole-genome sequence of SARS-CoV-2 from white-tailed deer indicated the presence of 12 lineages of SARS-CoV-2, with the predominance of B.1.2 (54.5%), B.1.311 (20%), B.1 (7%) and B.1.234 (6%) lineages. Surprisingly, the B.1.2 lineage was also the most abundant lineage circulating in humans in Iowa during that period, suggesting multiple human-to-deer and deer-to deer transmissions; however, deer-to-human transmission could not be interpreted from this study.95

Brazil has reported the presence of SARS-CoV-2 nucleic acid in wild animals such as Giant anteaters, black-tailed marmoset and West Indian manatee.96 Since these reports were based on random surveillance, the source of infection could not be determined. However, these findings have further widened the list of animals susceptible to SARS-CoV-2 infection.

6 The Veterinary and One Health Perspective

SARS-CoV-2 infection has been confirmed so far in canids (dogs and raccoon dogs), felids (cats, tigers, lions, puma/cougar, lynx and snow leopard), mustelids (ferrets and minks), viverrids (binturongs), procyonids (coatimundis), hyaenids (hyenas), cervids (white-tailed deer, mule deer), cricetids (Syrian hamster), callitrichids (black-tailed marmoset), myrmecophagids (giant anteater), trichechids (West India manatee), castorids (beaver) and primates (gorilla). With such a wide range of hosts available for the virus, it is possible that the virus can mutate, adapt and be transmitted widely.97 The close similarity of the SARS-CoV-2 genome sequence between humans and pet animals and the higher incidence of SARS-CoV-2 infection in pet animals during the human pandemic strongly suggest that humans can transmit SARS-CoV-2 to closely in-contact pets such as cats and dogs. On the other hand, transmission of SARS-CoV-2 from cats or dogs to humans is yet to be established and needs further investigation. Though there is no documented evidence of transmission of SARS-CoV-2 from animals to humans, except from minks and Syrian hamsters, animal owners and veterinarians must be vigilant while handling animals with respiratory illness to minimize the potential risk of transmission from pets.

One way to mitigate transmission from animals is to vaccinate the animals. In addition, this could also save endangered animals. Since SARS-CoV-2 has broad host range, it may be practically impossible to vaccinate all the animals. However, susceptible pet animals and captive animals that are in close proximity with humans should be vaccinated in order to avoid any animal-to-human transmission of SARS-CoV-2. However, very few vaccines have been tested or are available for use in animals. An aluminium-adjuvanted subunit vaccine based on modified S protein was demonstrated to protect minks from SARS-CoV-2 challenge. Upon challenge, the virus was not detected in any of the organs that were tested in the vaccinated group while high titre of the virus was detected in all the organs of control animals.39 Similarly, black-footed ferrets, which are endangered in North America, could produce neutralizing antibodies when immunized with commercially available purified SARS-CoV-2 Spike protein adsorbed to aluminium oxyhydroxide (alhydrogel® 2%). More than 100 captive animals were then vaccinated in a similar way, and the detailed scientific data are awaited.98

Early this year, Russia announced the development and testing of an inactivated SARS-CoV-2-based vaccine named Carnivac-Cov for animals. As per the available information, it has been tested in dogs, cats, minks and foxes and found to be safe and immunogenic.99 In September 2021, the Finnish Breeders' Association (FIFUR) announced in collaboration with the University of Helsinki the development of Furcovac, a vaccine for minks. However, scientific details of the vaccine are not in the public domain.100 Currently, zoo animals and minks in the USA are vaccinated with an adjuvanted SARS-CoV-2 trimeric spike protein-based subunit vaccine.101 Further scientific data are awaited.

Veterinary professionals can not only contribute to control of animal infectious diseases and play a critical role in contributing to the gross domestic product (GDP) of any nation through improving animal health and farmer’s welfare (to quote Mohandas Gandhi, “the greatness of a nation can be judged by the way its animals are treated”), but also are an important cog in the public health system. Along with other disciplines, the veterinary profession has an equal part in approaching zoonotic diseases holistically, and this includes COVID-19.

Indeed, One Health has its beginnings in veterinary medicine. When the cattle plague rinderpest was decimating bovine populations and disrupting the human food supply, Pope Clement XI is reported to have instructed the papal physician Giovanni Lancisi to provide a solution. This led to Lancisi in 1914, followed by Thomas Bates in England, to recommend culling of affected animals and burying their carcasses and implement restrictions in animal movement.102 The contributions of Rudolf Virchow also cannot be ignored. Son of a butcher by profession, he experimented on the lifecycle of the parasite Trichinella in the pig muscle as well as cysticercosis and tuberculosis in cattle. He stated that “between animal and human medicine, there are no dividing lines, nor should there be,” and coined the word ‘zoonosis’.103 In recent times, veterinary public health specialists James Steele and Calvin Schwabe pioneered the holistic approach to infectious disease medicine. The word One Medicine was coined by Calvin Schwabe in the 1970s. As per this concept, both human and veterinary medicine are considered indifferent and contribute to the development of each other. Later, the concept of “One Medicine” was extended to “One Health” through practical application.104

International organizations such as the World Organization for Animal Health (WOAH), the Food and Agriculture Organization (FAO), the World Health Organization (WHO) and the One Health Commission have come together to promote “One Health” and its implementation at various levels.105 In March 2022, the United Nations Environment Programme (UNEP) joined this One Health alliance. As human, animal, plant and environmental health are intricately intertwined, there is need for concerted efforts for cross-sectoral dialogue and cooperation at all levels in order to promote the well-being of humans and animals. Ultimately, we should move towards Planetary Health. However, the risk of zoonosis or their reporting is not uniform worldwide. Tropical, mostly low-to-middle income countries tend to be at greatest risk of zoonoses, and they need to evolve locally feasible solutions, including developing infrastructure, indigenous low cost diagnostics such as lateral flow assays, strengthening of reporting systems and flow of information, improving care systems in the case of disease outbreaks and implementation of preventive measures, including development of locally deployable vaccines and immunization programs.