Explained: Types of human coronaviruses and the symptoms they cause.

Human coronaviruses were first characterized in the mid-1960s and are considered primarily responsible for causing upper respiratory tract infections in children. (Illustration AP)

While there are hundreds of coronaviruses that cause disease in animals such as pigs, camels, bats, and cats, to date seven different types of coronaviruses have been identified that infect humans. What are they and what kind of symptoms do they cause?

What are coronaviruses?

Coronaviruses are a large family of single-stranded RNA viruses that cause disease in animals and humans. In humans, viruses generally cause mild to moderate diseases of the upper respiratory tract, such as the common cold. In the past 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 characterized in the mid-1960s and are considered primarily responsible for causing upper respiratory tract infections in children. In 1965, scientists DJ Tyrrell and ML Bynoe were the first to identify a human coronavirus, which they isolated from the nasal wash of a child who had symptoms of the common cold. They called the strain B814 and later, in 1968, the term “coronavirus” was accepted. These viruses are so named because of the spikes found on their surface that give them the appearance of a crown when viewed through an electron microscope.

A coronavirus disinfecting disc in Kolkata

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.

Coronavirus classification

In general, coronaviruses (CoV) are the largest group of viruses belonging to the order Nidovirales, which includes Coronaviridae among three others. Coronavirinae is one of the two subfamilies of Coronaviridea, and the other is Torovirinae. Coronavirins can be subdivided into alpha, beta, gamma and delta coronaviruses.

The Coronavirus Study Group of the International Committee on Virus Taxonomy is responsible for classifying them and approximately seven years ago they classified them in the aforementioned divisions instead of the serological groups of three. According to an article published in the Journal of Virology, while coronaviruses in all four categories can be found in mammals, bat coronaviruses are the probable genetic source of alpha and beta coronaviruses, while avian coronaviruses are the probable genetic sources of the gamma and delta coronaviruses.

While there are hundreds of coronaviruses, there are seven that we know can infect humans. Of the seven, two are alpha coronaviruses (229E and NL63) and four are beta coronaviruses (OC43, HKU1, MERS, and SARS-CoV). The classification of viruses is based on their phylogeny, that is, it reflects how these virus strains evolved from their common ancestors.

Essentially, this means that each time a virus emerges, its classification depends on how it relates to other known viruses and whether it is distinct enough to be called a new species or belongs to an existing species. For example, SARS-CoV and SARS-CoV-2 are genetically linked.

Alternatively, coronaviruses can be classified according to serology (control of the immune system’s antibody response to viral exposure) according to which they can be divided into three groups from I to III. Groups I and II refer to mammalian coronaviruses and group III includes bird coronaviruses. 229E is included in Group I, which largely includes animal pathogens. Group II is composed primarily of pathogens of veterinary relevance and includes OC43, HKU1, and NL63. SARS coronaviruses are also classified in Group II.

Around the world, people are commonly infected with 229E, HKU1, NL63, and OC43. Sometimes coronaviruses that infect animals can evolve into a human coronavirus, which includes MERS, SARS-CoV-1, and SARS-CoV-2.

When human coronaviruses were first identified

229E: One of the first coronavirus strains that was described in the mid-1960s, possibly by D Hamre and JJ Procknow in their 1966 paper, “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, an article in the Virology Journal described it as the first human coronavirus to be discovered in 1965, citing a 1966 article 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 baby who had 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 believed to have been given 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)

What are the symptoms they cause?

Before SARS-CoV-2 and MERS, SARS-CoV was the first example of a human coronavirus that could cause severe human disease in the form of severe acute respiratory syndrome. Other human coronaviruses like OC43 and 229E are known to cause the common cold, while NL63 is associated with severe respiratory symptoms such as upper respiratory tract infection and pneumonia.

According to an article published in Microbiology and Molecular Biology Reviews in 2005, although NL63 is mainly associated with infections in children, it has also been detected in immunocompromised 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 believed to come from a still unknown animal source, probably bats. The symptoms of SARS include cough, shortness of breath, diarrhea. In severe cases, symptoms can progress to respiratory distress, which may require intensive care.

MERS is another viral respiratory illness caused by a human coronavirus, which was first identified in Saudi Arabia in 2012. Typical symptoms include fever, cough, and shortness of breath.

Furthermore, 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 infectivity.

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