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Research comment
What can animal coronaviruses tell us about emerging human coronaviruses?
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  1. Joe Brownlie, Emeritus Professor and
  2. Dick Sibley
  1. Royal Veterinary College, Hatfield, UK
  2. West Ridge Veterinary Practice, Witheridge, UK
  1. email: JBrownlie{at}rvc.ac.uk

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Diseases are continually emerging. A conservative estimate is that there is one new human disease every eight months, with even more emerging in animals.

In 2008, the UK Government’s Foresight programme investigated the potential threat of new and emerging diseases.1,2 Of the eight categories of diseases that were considered to be particularly important, three were prescient of the present severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic. These were novel diseases, zoonotic infections and acute respiratory diseases.1

The successful control of global diseases is dependent on a number of factors. The most common natural control comes from sufficient members of the population having immunity to the infection (ie, herd immunity). However, this can break down when either the pathogen mutates, as we regularly see with influenza viruses, or the host becomes immunosuppressed.

Herd immunity can be enhanced by vaccination, but with newly emerging diseases there is insufficient time to develop, test, regulate and produce effective vaccines to influence the first ‘wave’ of an epidemic. So, as we are now with the present SARS-CoV-2 pandemic, we need to understand the pathology, epidemiology, viral shedding patterns and survival outside of the host to make informed but intuitive decisions for disease control.

Coronaviruses

Coronaviruses are enveloped, single-stranded, positive-sense RNA viruses that infect a wide variety of species, including people, livestock and companion animals. These viruses display exceptional genetic plasticity, driven by the accumulation of point mutations and recombination events. This genetic variation is responsible for the emergence of viral strains with increased virulence, different tissue tropism and/or an expanded host range.

Coronaviruses are currently classified within four genera, Alphacoronavirus, Betacoronavirus, Gammacoronavirus and Deltacoronavirus (Box 1). Many alphacoronaviruses and betacoronaviruses have their origin in bats, while gammacoronaviruses and deltacoronaviruses tend to have their origin in birds.

Box 1:

Representative viruses within the four coronavirus genera

  • Alphacoronaviruses: human coronavirus 229E, human coronavirus NL63, bat coronavirus HKU8, porcine epidemic diarrhoea virus, porcine respiratory coronavirus, transmissible gastroenteritis virus, bat coronavirus HKU2, canine [enteric] coronavirus, feline coronavirus

  • Betacoronaviruses: human coronavirus OC43, human coronavirus HKU1, murine coronavirus, bat coronavirus HKU5, bat coronavirus HKU9, severe acute respiratory syndrome-related coronaviruses (SARS-CoV and SARS-CoV-2), bat coronavirus HKU4, Middle East respiratory syndrome-related coronavirus, hedgehog coronavirus, bovine coronavirus, canine respiratory coronavirus, equine coronavirus, porcine haemagglutinating encephalomyelitis virus

  • Gammacoronaviruses: infectious bronchitis virus, cetacean coronavirus

  • Deltacoronaviruses: bulbul coronavirus HKU11, porcine coronavirus HKU15

One estimate for the first emergence of coronaviruses is about 8000 BC, although some models place the common ancestor as far back as 55 million years ago, implying long-term coevolution with bat and avian species.3 New coronaviruses have regularly emerged since then, with many emerging within the last hundred years. For example, bovine coronavirus and canine respiratory coronavirus are likely to have diverged from a common ancestor in the 1950s,4 and SARS-CoV may have diverged from a bat coronavirus in 1986.5

Canine respiratory coronavirus (CRCoV)

Like SARS-CoV-2, CRCoV is a betacoronavirus, and, to some degree, the story of its discovery has parallels with the current SARS-CoV-2 pandemic. In the early 2000s, a dog re-homing centre in London was experiencing acute, and sometimes peracute, outbreaks of respiratory diseases that resulted in a number of deaths. This led to an investigation to find out why the outbreak should be so rapid, so widespread and apparently so refractory to the established ‘kennel cough’ vaccines. During the investigation, a novel coronavirus (CRCoV) was discovered that was genetically distinct from the enteric canine coronavirus – which is an alphacoronavirus.6

Ultrastructural morphology of a severe acute respiratory syndrome coronavirus 2 virion

Image: Alissa Eckert

Rapid diagnostic tests for both the virus and antibodies to the virus were established using PCR and ELISA techniques, respectively. These tests allowed us to understand the epidemiology both in the kennel and in those entering the kennel. Furthermore, we were able to investigate other kennels and validate the occurrence and clinical importance of CRCoV.7

CRCoV was found both in air samples and on water troughs and toys left in the pens. This surface maintenance of virus was unexpected, so cleaning routines were incorporated into new biosecurity measures to reduce the infectious load of the virus within the kennels.8

The pathology of CRCoV infections in dogs shows mild inflammatory changes in both the nares and the trachea.9 There is also damage to the surface cilia, which was confirmed by the failure of latex clearance in tracheal organ cultures that were infected with CRCoV.10 This damage to the cilia is a common finding with respiratory coronavirus infections that results in mild upper respiratory disease. However, such damage does allow deeper penetration of the airways by secondary microbial infections with, in the more severe clinical cases, the development of pneumonia.

For example, studies have shown that dogs infected with CRCoV have significantly more severe clinical disease when subsequently challenged with either Bordetella bronchisepticum or canine mycoplasmas. It was these secondary infections that exacerbated the clinical disease seen in the original outbreak, with most uncomplicated CRCoV infections being mild and quickly resolved. This may also be the case with the current SARS-CoV-2 pandemic.

Other coronaviruses of animals

In addition to the contribution that research on CRCoV can make to our understanding of emerging coronaviruses such as SARS-CoV-2, research on other animal coronaviruses may also be relevant:

  • Infectious bronchitis virus of poultry was the first coronavirus to be described,11 and it has considerable genetic diversity with may strains circulating concurrently. As with other coronaviruses, it is rapidly spread by aerosol and, depending on the strain, can cause high mortality (more than 60 per cent) in unvaccinated flocks. Vaccines are available, and these may form a basis for the development of a vaccine to SARS-CoV-2.

  • It has recently become apparent that porcine haemagglutinating encephalomyelitis virus and porcine epidemic diarrhoea virus are circulating silently within Italian pig herds.12 There has even been evidence of recombination of these viruses with another coronavirus of pigs – transmissible gastroenteritis virus. This suggests that new viruses may be emerging and circulating in pig populations and that surveillance is warranted.

Diagnosing SARS-CoV-2 in people

The human coronaviruses 229E, NL63, OC43 and HKU1 are associated with mild cold-like symptoms. These viruses are constantly circulating within the population and may go largely undetected. As such, any diagnostic tests developed for SARS-CoV-2 must be able to accurately distinguish SARS-CoV-2 from these other coronaviruses. Only with accurate testing can you plan a controlled strategy to allow the free movement of the non-vulnerable people again.

The present delay in developing a specific antibody test in people with SARS-CoV-2 could be cross-reactivity to other human coronaviruses. It is also possible that these closely related viruses may give some cross-protection, which might explain some of the variability in individuals’ susceptibilities to SARS-CoV-2.

SARS-CoV-2 infection of animals

SARS-CoV-2 was only identified at the end of 2019, and information about its animal reservoirs is as yet unconfirmed. Although a number of species have been suggested, the designation of an animal as a reservoir for human infections needs both considerable caution and verifiable proof before any strategic action is taken.13

There is limited proof that pangolins may be an intermediate host but not a reservoir – the pangolin beta coronavirus has some similarities to SARS-CoV-2 but this is insufficient to make the pangolin the missing animal reservoir. However, the demonstration of a 96 per cent homology between SARS-CoV-2 and the bat SARS-like coronavirus (BAT-CoV RATG13) is convincing evidence of a bat reservoir.14

Of possibly greater significance is the recent report from the Harbin Veterinary Research Institute in China of experimental infection of cats and ferrets with SARS-CoV-2 and the onward transmission of the virus from inoculated cats to uninoculated cats.15 However, the inoculation was given intranasally at a high dose (105 plaque-forming units), and it is unclear whether similar results would be seen under natural infection conditions. Both cats and ferrets had viral replication only in their upper airways, not in the lower airways or in other organs systems.

Experimental intranasal infection of dogs showed poor establishment of the virus, and there was no evidence of susceptibility in pigs, chickens or ducks.

It is interesting to note that there have been recent reports of SARS-CoV-2 infection of a tiger and five lions in the Bronx zoo, USA. These reports highlight that the Harbin research demonstrating that cats can be infected by SARS-CoV-2 and transmit it to other cats needs to be taken seriously, not only in relation to its importance for cats but also because it raises the possibility that cats may transmit the virus to people. Therefore, further experimental and field epidemiological studies to investigate this possibility need to be supported urgently.

Conclusion

Coronaviruses circulate widely in most animal species, including people. They can transmit rapidly – mainly by aerosol transmission – and can cause severe disease. Being RNA viruses, they readily mutate and can even recombine with other coronaviruses, which can lead to the emergence of new viruses. They can also spread silently among populations, as observed with the pig coronaviruses and, to some extent, SARS-CoV-2.

Considering their long-term experience gained with animal coronaviruses, vets are in a unique position to help forge a better understanding of the origin and spread of SARS-CoV-2 and guide future research towards the development of effective vaccines and antiviral drugs.

References

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