Coronaviruses are a group of viruses known for causing the common cold. They have a halo or crown-like (corona) appearance when viewed under an electron microscope.
The common cold is a viral infection of the upper respiratory tract. The most commonly implicated virus is a rhinovirus (30%–80%), a type of picornavirus with 99 known serotypes. Other commonly implicated viruses include human coronavirus (≈15%), influenza viruses (10%–15%), adenoviruses (5%), human respiratory syncytial virus, enteroviruses other than rhinoviruses, and metapneumovirus. Frequently more than one virus is present. In total over 200 different viral types are associated with colds.
Types of cold and flu – Rhinoviruses:
Rhinoviruses (from the Greek (gen.) “nose”) are the most common viral infectious agents in humans and are the predominant cause of the common cold. Rhinovirus infection proliferates in temperatures between 33–35 °C (91–95 °F), the temperatures found in the nose. Rhinoviruses is a genus within the Picornaviridae family of viruses. Types of cold and flu.
There are 99 recognized types of human rhinoviruses that differ according to their surface proteins (serotypes). They are lytic in nature and are among the smallest viruses, with diameters of about 30 nanometers. By comparison, other viruses, such as smallpox and vaccinia, are around 10 times larger at about 300 nanometers.
The primary route of entry for human rhinoviruses is the upper respiratory tract (mouth and nose). As the virus replicates and spreads, infected cells release distress signals known as chemokines and cytokines (which in turn activate inflammatory mediators). Cell lysis occurs at the upper respiratory epithelium.
Infection occurs rapidly, with the virus adhering to surface receptors within 15 minutes of entering the respiratory tract. High risk individuals includes children and the elderly. Just over 50% of individuals will experience symptoms within 2 days of infection. Only about 5% of cases will have an incubation period of less than 20 hours, and, at the other extreme, it is expected that 5% of cases would have an incubation period of greater than four and a half days.
Human rhinoviruses preferentially grow at 32 °C (89 °F) as opposed to 37 °C (98 °F), hence infect the upper respiratory tract where respiratory airflow is in continual contact to the (colder) extrasomatic environment.
Coronaviruses are species in the genera of virus belonging to one of two subfamilies Coronavirinae and Torovirinae in the family Coronaviridae, in the order Nidovirales. Coronaviruses are enveloped viruses with a positive-sense single-stranded RNA genome and with a nucleocapsid of helical symmetry. The genomic size of coronaviruses ranges from approximately 26 to 32 kilobases, the largest for an RNA virus.
Coronaviruses primarily infect the upper respiratory and gastrointestinal tract of mammals and birds. Six different currently known strains of coronaviruses infect humans. The much publicized human coronavirus, SARS-CoV which causes SARS, has a unique pathogenesis because it causes both upper and lower respiratory tract infections.
Coronaviruses are believed to cause a significant percentage of all common colds in human adults. Coronaviruses cause colds in humans primarily in the winter and early spring seasons. The significance and economic impact of coronaviruses as causative agents of the common cold are hard to assess because, unlike rhinoviruses (another common cold virus), human coronaviruses are difficult to grow in the laboratory. Coronaviruses can cause pneumonia, either direct viral pneumonia or a secondary bacterial pneumonia, and bronchitis, either direct viral bronchitis or a secondary bacterial bronchitis.
Human respiratory syncytial virus:
Human respiratory syncytial virus (RSV) is a syncytial virus that causes respiratory tract infections. It is a major cause of lower respiratory tract infections and hospital visits during infancy and childhood. A prophylactic medication , palivizumab, can be employed to prevent RSV in preterm (under 35 weeks gestation) infants, infants with certain congenital heart defects (CHD) or bronchopulmonary dysplasia (BPD), and infants with congenital malformations of the airway. Treatment is limited to supportive care (e.g. C-PAP), including oxygen therapy.
1- In temperate climates there is an annual epidemic during the winter months. In tropical climates, infection is most common during the rainy season.
2- In the United States, 60% of infants are infected during their first RSV season, and nearly all children will have been infected with the virus by 2–3 years of age. Of those infected with RSV, 2–3% will develop bronchiolitis, necessitating hospitalization. Natural infection with RSV induces protective immunity which wanes over time—possibly more so than other respiratory viral infections—and thus people can be infected multiple times. Sometimes an infant can become symptomatically infected more than once, even within a single RSV season. Severe RSV infections have increasingly been found among elderly patients. Young adults can be re-infected every five to seven years, with symptoms looking like a sinus infection or a cold (infections can also be asymptomatic).
Flu (influenza virus)
Influenza A virus:
1- Influenza A virus causes influenza in birds and some mammals, and is the only species of influenza virus A. 2- Influenza virus A is a genus of the Orthomyxoviridae family of viruses. Strains of all subtypes of influenza A virus have been isolated from wild birds, although disease is uncommon. Some isolates of influenza A virus cause severe disease both in domestic poultry and, rarely, in humans. Occasionally, viruses are transmitted from wild aquatic birds to domestic poultry, and this may cause an outbreak or give rise to human influenza pandemics.
Influenza A viruses are negative-sense, single-stranded, segmented RNA viruses. The several subtypes are labeled according to an H number (for the type of hemagglutinin) and an N number (for the type of neuraminidase). There are 18 different known H antigens (H1 to H18) and 11 different known N antigens (N1 to N11). H17 was isolated from fruit bats in 2012. H18N11 was discovered in a Peruvian bat in 2013.
Each virus subtype has mutated into a variety of strains with differing pathogenic profiles; some are pathogenic to one species but not others, some are pathogenic to multiple species.
A filtered and purified influenza A vaccine for humans has been developed, and many countries have stockpiled it to allow a quick administration to the population in the event of an avian influenza pandemic. Avian influenza is sometimes called avian flu, and colloquially, bird flu. In 2011, researchers reported the discovery of an antibody effective against all types of the influenza A virus.
Influenza virus nomenclature (for a Fujian flu virus)
This genus has one species, influenza B virus. Influenza B almost exclusively infects humans and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the seal and the ferret. This type of influenza mutates at a rate 2–3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.
Influenza C virus
The Influenza viruses are members of the family Orthomyxoviridae. Influenza viruses A, B and C represent the three antigenic types of influenza viruses. Of the three antigenic types, influenza virus A is the most severe, influenza virus B is less severe but can still cause outbreaks, and influenza virus C is usually only associated with minor symptoms.
Influenza viris A can infect a variety of animals as well as humans and its natural host or reservoir is birds whereas influenza viruses B and C do not have animal reservoirs. Influenza virus C is not as easily isolated so less information is known of this type, but studies show that it occurs worldwide.
This virus may be spread from person to person through respiratory droplets or by fomites (non-living material) due to its ability to survive on surfaces for short durations. Influenza viruses have a relatively short incubation period (lapse of time from exposure to pathogen to the appearance of symptoms) of 18–72 hours and infect the epithelial cells of the respiratory tract. Influenza virus C tends to cause mild upper respiratory infections. Cold-like symptoms are associated with the virus including fever (38-40ᵒC), dry cough, rhinorrhea (nasal discharge), headache, muscle pain, and achiness. The virus may lead to more severe infections such as bronchitis and pneumonia.
After an individual becomes infected, the immune system develops antibodies against that infectious agent.
This is the body’s main source of protection. Most children between five and ten years old have already produced antibodies for influenza virus C. As with all influenza viruses, type C affects individuals of all ages. But is most severe in young children, the elderly and individuals with underlying health problems. Young children have less prior exposure and have not developed the antibodies and the elderly have less effective immune systems. Influenza virus infections have one of the highest preventable mortalities in many countries of the world.