Coronaviruses are a family of viruses known for containing strains that cause potentially deadly diseases in mammals and birds. In humans they're typically spread via airborne droplets of fluid produced by infected individuals. Scientists have known of the human coronavirus since the 1960s. But only rarely has it garnered wider recognition over the past half a century.
- During the mid-1990s these viruses were described as the backwater of virology, since none caused serious disease in humans. However, this changed in 2002-03 with the emergence of a coronavirus causing severe acute respiratory syndrome (SARS-CoV), and then in 2012 the Middle East respiratory syndrome coronavirus (MERS-CoV) in Saudi Arabia. The origin of both viruses was thought to be in bats, with civet cats and dromedary camels being the confirmed zoonotic reservoirs for SARS-CoV and MERS-CoV, respectively.
- All coronaviruses are zoonotic. They start off in animals and can then, following mutation, recombination and adaptation, be passed on to humans.
The International Committee for the Taxonomy of Viruses has approved the naming of more than 40 coronaviruses. The vast majority of these infect animals.
- The COVID-19 outbreak has brought the number of identified coronaviruses that infect humans to seven.
- Four of these are community acquired and have circulated through the human population continually for a very long time. The four community-acquired human coronaviruses typically cause mild cold-like symptoms in humans. Two of them, hCoV-OC43 and hCoV-229E, have been responsible for between 10% and 30% of all common colds since about the 1960s.
- Three of these (SARS-CoV, MERS-CoV and SARS-CoV-2) appear to have jumped to the human population more recently. Worryingly, these three result in a high mortality rate.
First described in detail in the 1960s, the coronavirus gets its name from a distinctive corona or 'crown' of sugary-proteins that projects from the envelope surrounding the particle.
- Encoding the virus's make-up is the longest genome of any RNA-based virus – a single strand of nucleic acid roughly 26,000 to 32,000 bases long.
There are four known genuses in the family
- Alphacoronavirus, Betacoronavirus: only infect mammals, including bats, pigs, cats, and humans
- Gammacoronavirus: mostly infects birds such as poultry
- Deltacoronavirus: can infect both birds and mammals.
Coronaviruses can give rise to a variety of symptoms in different animals.
- In a high percentage of cases, infection causes no symptoms at all. These individuals can still easily spread the virus without knowing they're infected.
- While some strains cause diarrhoea in pigs and in turkeys, in humans infections are usually compared to a bad cold, causing mild to moderate upper respiratory problems such as a runny nose and sore throat.
- There are a handful of lethal exceptions, which have had a devastating impact on livestock and human health around the globe. Symptoms in these cases tend to start with a sore throat and fever.
Coronaviruses have been around for many years, including a variety of animal and avian coronaviruses, such as infectious bronchitis virus (IBV), which infects poultry. Animal and avian coronaviruses can have high mortality rates among infected animals and illustrate the difficulties in developing vaccines. Similar to influenza viruses, despite many decades of research there is no vaccine that protects against all strains of infectious bronchitis virus (IBV) coronavirus.
- This is due in part to the continuously shifting diversity in the virus spike glycoprotein, a major immunogenic target and hence a good vaccine candidate for animal and human infections.
- Issues that have been encountered in developing these animal vaccines include a relatively short duration of protective immunity, and a lack of effectiveness of inactivated vaccines (On the other hand, they have been relatively cheap to make and lend themselves to mass vaccination procedures).
- Scientists were aware of compelling evidence pointing to the emergence of new CoV strains and the mutation of existing strains resulting in new disease syndromes in animals, but the evolution and disease impact of CoVs was not widely appreciated before SARS.
Diverse coronaviruses can infect the domestic species from dogs and cats, to cattle and pigs to poultry. Many of these infections are controlled by routine vaccination. eg:
- Vaccines against bovine coronavirus are widely employed in cattle where they protect against enteric and respiratory disease in young calves.
- Pigs may be infected with six different coronaviruses, one of which, porcine epidemic diarrhea, has proven difficult to control despite the development of several innovative vaccines. Porcine epidemic diarrhea virus undergoes frequent genetic changes.
- Infectious bronchitis coronavirus causes an economically devastating disease of chickens. It too undergoes frequent genetic shifts and as a result, can only be controlled by extensive and repeated vaccination.
Food for Thought
In 2005, a research group from Belgium postulated that the ancestor of hCoV-OC43 – one of the current human coronaviruses responsible for a common cold – jumped from cattle to humans, leading to what they described as a pandemic of respiratory disease recorded around 1890 in human history.
They propose that the influenza virus was not the virus responsible for the 1889-1890 pandemic and the another culprit could be a hCoV-043. Since the most recent ancestor of the bovine coronavirus and hCoV-O43 was also traced to about 1890, could the 1889-1890 pandemic have been the result of the jump of cow coronaviruses to humans?
If so the next question is this:
- Could the adaptation and co-evolution in the 130 years since the pandemic explain why CoV-OC43 currently causes only mild cold-like symptoms in humans? How is it that the selfsame virus that killed one million people in the 1890s now causes no more than a few uncomfortable sniffles?
- This could possibly be a key case study in herd immunity.
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