Coronaviruses

Coronaviruses are a family of RNA viruses known to cause disease in both mammals and birds. In humans, they primarily spread via respiratory droplets. These viruses were first identified in the 1960s and were considered of limited clinical importance until the 2000s, when severe variants such as SARS-CoV and MERS-CoV emerged[1][2].

Epidemiologic Background

Before the 2000s, coronaviruses were viewed as “backwaters of virology” because they typically caused only mild cold-like symptoms in humans. That changed dramatically with the outbreaks of:

• SARS-CoV (2002–2003): Believed to originate from bats, with civet cats as an intermediate host[1].

• MERS-CoV (2012): Also with bats as a reservoir, but dromedary camels as the intermediate host[1].

• SARS-CoV-2 (2019): Presumed zoonotic origin, likely with bats as the primary source.

In total, seven human coronaviruses have been identified: four that have circulated in the population for a long time (hCoV-229E, hCoV-OC43, hCoV-NL63, hCoV-HKU1) and three with high mortality (SARS-CoV, MERS-CoV, SARS-CoV-2)[2].

Taxonomy

According to the International Committee on Taxonomy of Viruses, more than 40 coronaviruses are known. They are grouped into four genera:

• Alphacoronavirus and Betacoronavirus: Infect mammals, including humans, bats, pigs, and cats.

• Gammacoronavirus: Primarily birds (e.g., poultry).

• Deltacoronavirus: Birds and some mammals[3].

Transmission Mechanisms

All coronaviruses are zoonotic; they originate in animals and then jump to humans through mutation, recombination, and adaptation. This zoonotic transition explains why new variants can emerge relatively unpredictably[2].

Symptoms in Humans

Clinical presentations range from asymptomatic infection to severe disease:

• Mild forms: Cold-like symptoms (runny nose, sore throat, cough).

• Severe forms: Fever, dyspnea, cough, chest pain, and in some cases ARDS (acute respiratory distress syndrome).

Note that asymptomatic individuals can still be infectious.

Coronaviruses in Animals

Coronaviruses are also found widely in animals:

• Infectious bronchitis virus (IBV): Affects poultry and is economically significant.

• Bovine coronavirus: Common in cattle operations and routinely vaccinated against.

• Porcine epidemic diarrhea virus (PEDV): Frequent genetic changes make control difficult despite multiple vaccine attempts[4].

Animal and avian coronaviruses evolve continuously and cause new diseases, but their significance for human health became clearly evident with the SARS epidemic in 2003[5].

Vaccine Challenges

Developing effective vaccines has proven difficult. Key reasons include:

• Frequent genetic changes in the spike glycoprotein, a central target of the immune response.

• Short-lived immunity after vaccination[4].

This has hindered the development of broadly protective vaccines—even for IBV in poultry, where vaccination has been practiced for decades without achieving full protection.

Historical Perspective

Researchers in Belgium (2005) proposed that hCoV-OC43, one of today’s common cold viruses, may have jumped from cattle to humans around 1890. They suggested this could have been the cause of the pandemic attributed to influenza in 1889–1890[2]. This raises questions about how a deadly virus can, through natural adaptation, become a relatively harmless cold virus—a key theme in understanding herd immunity.

Summary

The coronavirus family encompasses both animal and human viruses, with substantial genetic diversity and zoonotic potential. The major roles of SARS-CoV, MERS-CoV, and SARS-CoV-2 in recent pandemics have underscored the need for surveillance of both animal and human viruses, along with further research on vaccines and antiviral therapies.

References

1. Ng LF, Hiscox JA. Coronaviruses in animals and humans.19.2.2020 :https://www.bmj.com/content/368/bmj.m634

2. The conversation A brief history of the coronavirus family – including one pandemic we might have missed :https://theconversation.com/a-brief-history-of-the-coronavirus-family-including-one-pandemic-we-might-have-missed-134556

3. Scient Alert Corona viruses :https://www.sciencealert.com/coronavirus

4. Tizard IR. Vaccination against coronaviruses in domestic animals. Vaccine. 2020 Jul 14;38(33):5123.:https://pubmed.ncbi.nlm.nih.gov/32563608/

5. Saif LJ. Animal coronaviruses: what can they teach us about the severe acute respiratory syndrome?. Revue scientifique et technique-Office international des épizooties. 2004 Aug 1;23(2):643-60.:https://pubmed.ncbi.nlm.nih.gov/15702725/ (