While there are hundreds of coronaviruses that cause diseases in animals such as pigs, camels, bats and cats, till date seven different types of coronaviruses have been identified that infect humans. What are they and what kind of symptoms do they cause?
Coronaviruses are a large family of single-stranded RNA viruses that cause diseases in animals and humans. In humans, the viruses usually cause mild to moderate upper-respiratory tract illnesses such as the common cold. In the last two decades, more aggressive coronaviruses have emerged that are capable of causing serious illness and even death in humans. These include SARS-CoV, MERS and now SARS-CoV-2.
Human coronaviruses were first characterised in the mid-1960s and they are mostly considered to be responsible for causing upper respiratory tract infections in children. In 1965, scientists DJ Tyrrell and ML Bynoe were the first ones to identify a human coronavirus, which they isolated from the nasal washing of a male child who had symptoms of common cold. They termed the strain B814 and later in 1968 the term “coronavirus” was accepted. These viruses are named so because of spikes found on their surface that give them the appearance of a crown when looked through an electron microscope.
In animals, coronaviruses can cause diarrhea in cows and pigs and upper respiratory tract disease in chickens. The first coronavirus was isolated in 1937 and it was the infectious bronchitis virus (IBV) that caused respiratory disease in chickens.
Broadly, coronaviruses (CoV) are the largest group of viruses that belong to the Nidovirales order, which includes Coronaviridae among three others. Coronavirinae are one of the two subfamilies of Coronaviridea, with the other being Torovirinae. Coronavirinae can be further subdivided into alpha, beta, gamma and delta coronaviruses.
The Coronavirus Study Group of the International Committee for Taxonomy of Viruses is responsible for classifying them and roughly seven years ago they classified them into the aforementioned divisions instead of the serological groups of three. According to a paper published in the Journal of Virology, while coronaviruses from all four categories can be found in mammals, bat coronaviruses are the likely gene source of alpha and beta coronaviruses, while avian coronaviruses are the probable gene sources of gamma and delta coronaviruses.
While there are hundreds of coronaviruses, there are seven that we know can infect humans. Out of the seven, two are alpha coronaviruses (229E and NL63) and four are beta coronaviruses (OC43, HKU1, MERS and SARS-CoV). The classification of the viruses is based on their phylogeny, which is to say it reflects how these virus strains evolved from their common ancestors.
Essentially, this means whenever a virus newly emerges, its classification depends on how it relates to other known viruses and if it is distinct enough to be called a new species or if it belongs to an existing species. For instance, SARS-CoV and SARS-CoV-2 are genetically linked.
Alternatively, coronaviruses may be classified based on serology (monitoring the immune system’s antibody response to viral exposure) as per which they can be divided into three groups from I to III. Groups I and II refer to mammalian coronaviruses and Group III includes avian coronaviruses. 229E is included in Group I, which largely includes animal pathogens. Group II largely consists of pathogens of veterinary relevance and includes OC43, HKU1 and NL63. SARS coronaviruses are classified in Group II as well.
Around the world, people commonly get infected by 229E, HKU1, NL63 and OC43. Sometimes, coronaviruses that infect animals can evolve and become a human coronavirus, which include MERS, SARS-CoV-1 and SARS-CoV-2.
229E: One of the first coronaviruses strains to be described in the mid-60s, possibly by D Hamre and JJ Procknow in their 1966 paper titled, “A new virus isolated from the human respiratory tract”, published in Experimental Biology and Medicine.
OC43: Discovered in 1967 according to the Journal of Virology. However, a paper in Virology Journal has described it as the first human coronavirus to be discovered in 1965, citing a 1966 paper written by Tyrrell and Bynoe who worked with the nasal swab titled B814.
NL63 and HKU1: First identified in the Netherlands in 2004, probably after it was isolated from a seven-month-old infant showing respiratory symptoms. During this time, there was an increase in research on human coronaviruses, which led to the discovery of NL63 and HKU1 in Hong Kong in early 2005.
SARS-CoV: 2003 in China (animal source not yet known, bats thought to have given it to other animals, probably civet cats)
MERS: 2012 in Saudi Arabia (transmitted by dromedary camels)
SARS-CoV-2: 2019 in Wuhan (source not yet known, possibly bats)
Before SARS-CoV-2 and MERS, SARS-CoV was the first example of a human coronavirus that could cause serious illness in humans in the form of severe acute respiratory syndrome. Other human coronaviruses such as OC43 and 229E are known to cause the common cold, whereas NL63 is associated with serious respiratory symptoms such as upper respiratory tract infection and pneumonia.
According to a paper published in Microbiology and Molecular Biology Reviews in 2005, while NL63 is primarily associated with infections among children, it has also been detected in immuno-compromised adults with respiratory tract infections. OC43 can also cause gastroenteritis.
SARS-CoV on the other hand, was identified after the 2003 outbreaks in China. It is thought to have come from an as yet unknown animal source, probably bats. Symptoms of SARS include cough, shortness of breath, diarrhea. In severe cases, the symptoms can progress to respiratory distress, which may require intensive care.
MERS is another viral respiratory disease caused by a human coronavirus, which was first identified in Saudi Arabia in 2012. Typical symptoms include fever, cough and shortness of breath.
Further, while SARS-CoV-2 is considered to be milder than SARS-CoV and MERS, it is especially difficult to control its outbreak, given its high infectiousness.