Coronaviruses are actually viruses and belong to the largest family of viruses which is Coronaviridae. All coronaviruses are enveloped, positive-sense, single-stranded RNA viruses that look like crowns under the microscope due to spike-like glycoproteins protruding from the viral envelope. They cause disease in wild and domestic mammals and birds. The severity and symptoms of the diseases caused vary according to the type of coronavirus and the infected host species. In humans, such coronaviruses are responsible for causing respiratory tract infections resulting in symptoms ranging from mild common cold caused by some human coronaviruses (229E, NL63, OC43, HKU1) to severe forms of the disease caused by previously undetected strains. Animals, such as severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and most recently called severe acute respiratory syndrome syndrome 2 (SARS-CoV-2), which suddenly emerged in late 2019 and was identified as the causative agent of the COVID-19 pandemic.
Coronaviruses belong to various classifications such as Order Nidovirales, Family Coronaviridae and Subfamily Orthocoronavirinae, which are further divided into four genera: Alphacoronavirus (formerly coronaviruses found in carnivores), Betacoronavirus, Gammacoronavirus and Deltacoronavirus. From the different species, there is a distinctive character and specific host. Generally, it is said that the first two infect mammals; which includes humans, bats and pigs; while the last two mainly infect birds and some mammals. Their Genetic Structure: Corona viruses have very large genomes, ranging from about 26 to 32 kb in length, making it the largest among all RNA viruses, which are known to encode for structural proteins spike (S), envelope (E), membrane (M), nucleocapsid (N) and non-structural and accessory proteins important for viral replication and evasion of host immune responses.
Coronaviruses are generally of zoonotic origin and are characterized by their ability to pass from animals to humans and vice versa. Emergence in the human population is usually related to contact with infected animals such as bats, civet cats, camels and pangolins, which are called the natural reservoirs or intermediate hosts of these viruses. Zoonotic spillover of coronaviruses is defined by the presence of certain genetic mutations or recombination, which enable the virus to replicate in human cells. This replication is facilitated by the spike (S) protein binding to various receptors specific to host cells such as the angiotensin-converting enzyme 2 (ACE2) receptor for SARS-CoV and SARS-CoV-2 or the dipeptidyl peptidase 4 (DPP4) receptor for MERS-CoV. The ability of coronaviruses to attach themselves to these receptors makes a major contribution to their host range, transmissibility and pathogenicity.
The pathogenesis of infection with coronavirus in humans begins with the virus entering its target cells via receptor-mediated endocytosis or membrane fusion. The virion subsequently releases a viral RNA genome into the cytoplasm, where it is translated and copied, using the cell's machinery to do so. The viral progeny is then assembled into a mature virion that emerges from the infected cell to infect other cells and tissues. In mild cases, this process may cause the infection to remain localised, with symptoms such as fever, cough and sore throat. However, it can develop into a severe condition, sometimes known as a 'cytokine storm' in which there is a severe immune response capable of causing systemic inflammation, acute respiratory distress syndrome (ARDS), multi-organ failure and ultimately death.
The global consequences of the coronavirus became evident with the COVID-19 pandemic, which originated from a virus called SARS-CoV-2, which was first identified in Wuhan, China, in December of 2019. Within a short period of time, this new virus came to spread across the world, courtesy of its high rate of transmission wherein humans spread the agent through respiratory droplets, aerosols, and fomites and causing symptomatic and asymptomatic infections. The pandemic raised several public health issues related to the coronavirus, demanding rapid diagnosis for treatment purposes, as well as vaccine preparation, to prevent viral spread and severe consequences from the virus. Notably, studies on coronavirus have started to spread and bear fruits in the form of mRNA-based vaccines, antiviral drugs, monoclonal antibody therapy, and other knowledge gained to understand the molecular mechanisms behind the infection and replication processes by coronaviruses as well as how they evade immunological responses.
Corona viruses have a significant history of causing both epidemics and pandemics. For example, the SARS crisis between 2002 and 2003 was caused by SARS-CoV, which passed to humans via civet cats after originating from bats. Then in the year 2012, came the turn of MERS-CoV, attributed to dromedary camels acting as intermediate hosts, which was responsible for the outbreak known as MERS. These outbreaks have made clear the need for surveillance and early detection of zoonotic viruses and the implementation of preventive measures such as quarantine, travel restrictions and public health education that will prevent infectious diseases from spreading. This recurrence of novel corona viruses is a reflection of the complex and dynamic interactions between human activity, wildlife habitats and viral evolution. This issue emphasises the need for a One Health perspective in terms of human, animal and environmental health to tackle zoonotic disease risks.
The high mutation rate in coronaviruses allows their adaptability and the generation of new variants with altered transmissibility, virulence, or immune escape potential. Such evolutionary plasticity is posing challenges for vaccine development and management of coronavirus outbreaks, as revealed by the emergence of multiple SARS-CoV-2 variants such as Alpha, Beta, Delta, and Omicron during the COVID-19 pandemic, all associated with different transmissibility, severity, and neutralizing antibody escape. Therefore, in genomics surveillance, monitoring the genetic diversity of coronaviruses will play a vital role in understanding the evolution of such viruses to inform public health interventions.
They cause diseases with economic impact apart from human health, diseases such as avian infectious bronchitis in poultry, porcine epidemic diarrhea in pigs, and feline infectious peritonitis in cats are economically important. Such diseases can threaten large-scale agriculture and livestock production as they affect animals far more than humans. Research on veterinary coronaviruses also gives good insights into the biology and control of the viruses and their possible future crosses across species that may lead to zoonotic spillover due to species interference between wild and domestic populations.
Research on coronaviruses has evolved since they were first discovered in the 1960s. It has revolutionised the methods of public health intervention against infection with the virus; designed measures against the spread of the virus; and improved antiviral therapies targeting specific viral proteins or host factors that are essential for viral replication. The benefits of this have been the development of broad-spectrum vaccines that will provide cross-protection across different strains of coronavirus and which have defined the underlying science on host immune responses towards determining potential biomarkers for disease severity and identification of therapeutic targets. The field has progressed rapidly by bringing in molecular biology information on virology, immunology and epidemiology, leading to an impressive understanding of the pathogenesis and immune mechanisms by which coronavirus achieves its effect, thus incorporating new innovative designs in combating coronavirus infection.
New promise lies in the prevention and prediction of future disasters resulting from the advent of the coronavirus. Initiatives such as global surveillance networks, biosafety protocols and improved multidisciplinary collaboration between scientists, health workers, policymakers and communities have all shaped efforts in preparedness and response capacities on the development of new infectious diseases with interrelated human-animal health consequences and environmental impacts such as the emergence of zoonotic viruses and as transmission vectors.
Coronaviruses comprise a wide range of RNA viruses, have complex biology, and greatly impact human and animal health, being able to cause both endemic and pandemic diseases, zoonosis, and their adaptability or evolutionary capacity to keep abreast of changing environmental-host conditions. Coronaviruses are today under intense scientific focus and investigation, resulting in beneficial insights on the ongoing molecular and ecological dynamics of viral infections, and a constant reminder of what an emerging infectious disease is, in a closer world than ever before.
