Coronaviruses

ANATOMY OR SYSTEM AFFECTED: Immune system, lungs, lymphatic system, respiratory system

DEFINITION: Enveloped RNA viruses frequently infecting the upper respiratory system and capable of producing the common cold, severe acute respiratory syndrome (SARS), COVID-19, or other severe respiratory illnesses

CAUSES: Usually contact with infected carrier animals or with surfaces or body fluids

SYMPTOMS: High fever, headache, nasal discharge, cough, sore throat, general lethargy, diarrhea, breathing difficulties; in severe cases, collapsed lungs, massively reduced white blood cell count, depleted platelet reserves, blood cells leaking into glands and major organs, dead tissue in glands and major organs

DURATION: Two to eighteen days

TREATMENTS: Fever-reducing drugs, extra oxygen, ventilator support (if needed), some antiviral drugs

Overview

Coronaviruses are spherical, enveloped virion particles sixty to two hundred nanometers in diameter with twenty-nanometer-long surface projections resembling ball-topped points of a crown. They are positive-strand ribonucleic acid (RNA) viruses with nonsegmented genomes containing roughly thirty thousand nucleotides. Coronaviruses are widespread in the environment and exist in a large range of hosts, including humans, bats, dogs, cats, mice, cattle, swine, turkeys, and chickens, and may also be present in rats and rabbits.

Most known coronaviruses belong to the subfamily Orthocoronavirinae and affect mammals or birds. Genera within the subfamily include Alphacoronavirus, Betacoronavirus, Deltacoronavirus, and Gammacoronavirus. Each genus has its natural reservoir in certain species; for example Gammacoronavirus is found mostly in birds (and is sometimes known as avian coronavirus) while Betacoronavirus originates in bats and rodents. However, what makes coronaviruses particularly notable in epidemiology is their potential to jump from one species to another. At least seven such viruses have been found to infect humans. Four of these are considered common according to the Centers for Disease Control and Prevention (CDC): 229E (an alpha coronavirus), NL63 (an alpha coronavirus), OC43 (a beta coronavirus), and HKU1 (a beta coronavirus). Other human coronaviruses emerged rapidly. These include MERS-CoV (a beta coronavirus that causes Middle East Respiratory Syndrome, or MERS); SARS-CoV (a beta coronavirus that causes severe acute respiratory syndrome, or SARS); and SARS-CoV-2 (a novel coronavirus that causes coronavirus disease 2019, or COVID-19). Individual coronaviruses may also have different strains, which are often referred to by letters of the Greek alphabet. For example, variants of SARS-CoV-2 were commonly labeled Alpha, Beta, Delta, Gamma, etc.—not to be confused with the broader genera names.

The dominant mode of coronavirus transmission is uncertain. Coronaviruses are usually transmitted through contact with infected carrier animals who act as vectors for the virus without becoming ill themselves or contact with surfaces or body fluids contaminated with the virus. Depending on environmental conditions, viruses can survive minutes to hours on exposed surfaces or within discharged fluids. In some cases airborne transmission may also be possible.

Symptoms

In humans, symptoms typically appear two to ten days after exposure. These may include fever with high temperature, headache, nasal discharge, cough and sore throat, general lethargy, diarrhea, and breathing difficulties. In severe cases, patients may experience collapsed lungs, massively reduced white blood cell count, depleted platelet reserves, blood cells leaking into glands and major organs, and dead tissue in glands and major organs. In animals, symptoms of coronavirus infection include stresses to digestive, respiratory, reproductive, renal, and central nervous systems.

It is often not possible to distinguish coronavirus infections from rhinovirus infections based on symptoms alone. Diagnosis of a coronavirus infection requires laboratory tests, which may include organ or cell cultures, electron microscopy, immunofluorescence, nucleic acid hybridization, reverse transcription-polymerase chain reaction, and serology, the most effective method being an enzyme-linked immunoassay. Though there is presently no direct line of evidence linking back to respiratory infection, coronaviruses are tenuously implicated in a number of nonrespiratory diseases, including highly infectious mononucleosis, pancreatitis, thyroiditis, nephropathy, pericarditis, and multiple sclerosis.

Certain strains of the common cold are caused by coronavirus infections. Other notable illnesses associated with coronaviruses are the highly contagious Severe acute respiratory syndrome (SARS), which produces an atypical pneumonia and a resulting high death rate, and the closely related coronavirus disease 2019 (COVID-19), which caused a global pandemic in 2020.

Coronaviruses have an incubation period of two days to two weeks. There are typically two to eighteen days of active infection, and for many virus types reinfections are common. With SARS-CoV-2, researchers also noted long-lasting symptoms in some patients, often referred to as "long COVID."

Treatment and Therapy

Like many other viruses, coronaviruses have no established cure, but symptoms can be treated. Doctors typically treat symptoms of coronavirus infection as they would the flu, prescribing rest, fluids, and occasionally medications to reduce fever and pain while the body’s immune system fights off the virus. Interventions in more serious cases may include extra oxygen or ventilator support if needed. Some antiviral drugs may be helpful in certain cases. In the case of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for COVID-19, a major international effort was launched to develop a vaccine. Early versions were approved in China and Russia, initially for limited emergency use only, and others entered widespread use in 2021. Antiviral treatments for COVID-19 include Paxlovid and molnupiravir, both of which are oral pills, and an intravenous infusion, remdesivir.

Perspective and Prospects

Human coronaviruses were first identified in 1965, and continuing epidemiological research suggests coronaviruses are prime agents of respiratory illness in humans, though the viruses can infect nerve cells and immune system cells as well. Coronaviruses are second only to rhinoviruses as causes of upper respiratory illnesses collectively known as the common cold; coronaviruses are likely responsible for 10 to 15 percent of such infections in adults. Most people have endured at least one bout of coronavirus infection before reaching adulthood. Coronaviruses may also result in severe secondary infections of the lower respiratory system if the patient has an existing health condition such as asthma.

The first identified case of SARS can be traced back to November 16, 2002, in Foshan, China. The patient was diagnosed with pneumonia and sent home to recover. Soon a large number of people in the area became ill with the same symptoms of respiratory distress. By February 2003, the disease had spread to Hong Kong, and guests at the Metropole Hotel began to fall ill. Many international guests of the hotel, showing no symptoms, boarded planes and traveled home only to become symptomatic after arriving at their destinations. As more and more people in Hong Kong, and then in distant locations, became severely ill, an intense effort was mounted to find the cause: a coronavirus.

The coronavirus responsible for SARS was traced back to specific locations in China where wild mammals were being sold in meat markets as exotic food. While the carrier animals were immune to the virus, a mutation in the virus’s genes allowed it to infect humans and then spread from person to person. The global spread of SARS was precipitated by air travel from China, initiating outbreaks in a number of countries in a matter of days. Once the source of the outbreak was identified and scientists discovered the infection was caused by a coronavirus, efforts to stop the spread of SARS were initiated worldwide. The World Health Organization (WHO) and the US Centers for Disease Control and Prevention issued global alerts. Airports and rail stations in many countries set up infrared scanners to detect travelers with fevers and quarantine them until they were screened for the virus. In some cities with identified cases of SARS, entire blocks, districts, and buildings were quarantined; sporting events, school attendance, international conferences, and religious services were canceled. Within four months of its outbreak, SARS had spread to twenty-four nations on four continents.

Eventually, the SARS epidemic was suppressed by breaking the chain of human transmission with quarantines. Globally, more than eight thousand individuals were diagnosed with SARS, and there were more than seven hundred deaths. There is no effective treatment for SARS beyond supporting the patient toward recovery. Although the international medical community was able to contain the disease in the human population, it is unknown whether the virus remains active in animals.

In 2012 researchers identified a new coronavirus behind the outbreak of Middle East respiratory syndrome (MERS), another severe respiratory illness. By May 2013 the WHO had confirmed about forty cases of the virus, more than half in Saudi Arabia. While seen as a serious public health threat, it was found to be less easily transmittable than SARS. By 2023 2,605 cases of the disease, which is spread by dromedary camels, had been recorded in twenty-seven countries.

In December 2019 another new coronavirus outbreak was reported in Wuhan, China, and identified as SARS-CoV-2. Soon the WHO designated the associated disease as coronavirus disease 2019 (COVID-19). Much like SARS, it was believed to have emerged through human contact with wild animals, particularly bats. By the end of February 2020 the virus had spread to over 80,000 people in over thirty-five countries, including the United States. On March 11, 2020, the WHO announced that COVID-19 was a pandemic, leading many governments to enact lockdown measures and promote social distancing in an effort to slow the spread, causing significant social and economic disruption around the world. In June 2020 the first COVID-19 vaccine was approved for emergency use in China, and other vaccines followed. The United States approved vaccines from the companies Pfizer-BioNTech and Moderna in December 2020, and another by Johnson & Johnson in February 2021. Yet despite vaccination campaigns, the pandemic continued into 2023 and beyond. By March 2024, there had been more than 774 million COVID-19 cases in 192 countries, resulting in more than 7 million global deaths.

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