Known Human Coronaviruses: From HCoV-OC43 to SARS-CoV-2

Known Human Coronaviruses: From HCoV-OC43 to SARS-CoV-2

There are seven strains of coronavirus known to infect humans. These viruses primarily cause respiratory tract infections, but their degree of severity range from mild to lethal. From these seven human coronaviruses, four cause mild illnesses such as the common cold, while the other three can produce medical conditions that are potentially severe and lethal.

An Overview of the Known Human Coronaviruses

There are four common human coronaviruses or HCoVs known to cause the common colds and mild lower respiratory infections in the general population. However, in some cases, they can cause severe lower respiratory infections, especially among children, the elderly, and those with compromised immune systems. These viruses are also seasonal. Hence, the disease they cause often appear during a specific period of the year.

Four Common HCoVs Known to Cause Asymptomatic and Mild Cases in the General Population

1. Human Coronavirus OC43 or HCoV-OC43

The HCoV-OC43 is a betacoronavirus that infects humans and cattle. It enters host cells by binding to the N-acetyl-9-O-acetylneuraminic acid or NANA receptor. Although it is one of the viruses responsible for the common cold, it can cause severe lower respiratory infections, especially among infants, the elderly, and immunocompromised individuals.

2. Human Coronavirus HKU1 or HCoV-HKU1

Another betacoronavirus known to cause the common cold and limited upper respiratory infections in humans is HCoV-HKU1. It also enters host cells through the NANA receptor. First discovered in 2005 in a 71-year-old man in Hong Kong, phylogenetic analysis revealed a close resemblance to the mouse hepatitis virus. Infection from this virus can advance to lower respiratory infections such as pneumonia and bronchiolitis.

3. Human Coronavirus 229E or HCoV-229E

The HCoV-229E is an alphacoronavirus that infects humans and bats. It enters the host cells by binding to the aminopeptidase N or AP-N receptor. Although infection from this virus is almost associated with asymptomatic to mild respiratory conditions, it can progress to pneumonia and bronchiolitis, especially among those with developing or weakened immune systems. Similar to other human coronaviruses, it has also been detected to exist alongside other respiratory viruses.

4. Human Coronavirus NL63 or HCoV-NL63

Another alphacoronavirus first discovered in late 2004 in a seven-month-old child in the Netherlands is HCoV-NL63. The virus primarily affects young children, the elderly, and immunocompromised individuals. The virus enters the host cells via the angiotensin-converting enzyme 2 or ACE2 receptor. Similar to other HCoVs, studies suggest a spillover event due to its origins from infected palm civets and bats.

Other Human Coronaviruses Associated with Global Pandemic and Potentially Medical Conditions

5. Severe Acute Respiratory Syndrome Coronavirus or SARS-CoV

A particular strain of coronavirus that first appeared in 2002 in China is SARS-CoV. Analyses revealed that it is a betacoronavirus that enters the host cells by binding to the ACE2 receptor. Further analyses revealed that it originated from horseshoe bats in South China and jumped to humans either directly or through intermediary animals such as palm civets held and sold in Chinese markets.

Later study conducted by researchers from Hong Kong University and the Guangzhou Centre for Disease Control and Prevention established a link between the coronaviruses appearing in civets and humans, thus confirming the SARS-civet link.

The virus causes severe acute respiratory syndrome. Initial systematic symptoms include muscle pain, headache, and fever, followed by respiratory symptoms, mainly cough, shortness of breath or dyspnea, and pneumonia.

During the outbreak in 2002 and the resulting 2002-2004 SARS Pandemic, the disease spread from Guangdong Province in China to Hong Kong and Canada, with a notable number of cases in San Francisco, Singapore, Manila, and Taiwan.

6. Middle East Respiratory Syndrome-Relative Coronavirus or MERS-CoV

MERS-CoV is another betacoronavirus that infects camels, bats, and humans. It enters the host cells by binding to the dipeptidyl peptidase-4 or DDP4 receptor. Note that it is still unknown if the virus resulted from a single zoonotic event that led to a human-to-human transmission or if there were multiple zoonotic events transpiring in different geographic locations.

Other studies suggest that the virus originated in bats after it was isolated from the animal itself, while also demonstrating genetic resemblance to the Tylonycteris bat coronavirus HKU4 and Pipistrellus bat coronavirus HKU5. Complementary studies have also shown that the virus has infected camels for at least 20 years.

Initially called the 2012 novel coronavirus or 2012-nCoV, there were MERS-CoV cases reported in over 21 countries in the Middle East, East and Southeast Asia, Europe, and North America as of 2015. There was a further outbreak affecting Saudi Arabia in 2018 and spreading to a single-digit case in South Korea, the United States, Oman, and the United Arab Emirates.

Note that MERS is primarily a viral respiratory infection. Symptoms range from mild to severe. There were also gastrointestinal symptoms, including diarrhea, vomiting, and abdominal pain. The disease can progress to pneumonia and further to acute respiratory distress syndrome.

7. Severe Acute Respiratory Syndrome Coronavirus 2 or SARS-CoV-2

The specific human coronavirus associated with the 2019-2020 COVID-19 pandemic is SARS-CoV-2. Several analyses revealed that it is a betacoronavirus of group 2B with an RNA of is approximately 30 kbp in length. It has at least 70 percent similarity with SARS-CoV and closely resembles four bat coronaviruses.

COVID-19 or the coronavirus disease 2019 is the official name of the medical condition associated with SARS-CoV-2 infection. Signs and symptoms range from asymptomatic and mild to moderate and severe. Common clinical presentation includes dry or productive cough, fatigue, and dyspnea or labored breathing.

Moderate cases include non-severe pneumonia, while severe cases include severe pneumonia complicated by acute respiratory distress syndrome or ARDS. Other signs and symptoms include gastrointestinal problems such as diarrhea, abdominal pain, nausea, and vomiting, as well as the loss of sense of smell and taste or anosmia and dysgeusia, and conjunctivitis or pink eye.

When compared with MERS-CoV, infection from SARS-CoV-2 is less fatal but highly transmissible like SARS-CoV. COVID-19 has spread from Wuhan City in mainland China to a widespread parts of the world to include Countries in East Asia, Southeast Asia and the Pacific such as the localized outbreaks in Taiwan and South Korea; South Asia, the Near East and the Middle East; Europe and the European Union,  including the severe coronavirus crisis in Italy; North America; and South America.


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