Respiratory Viral Disease
Viral respiratory diseases are infectious diseases caused by a variety of viruses involved in the respiratory tract. Respiratory infections have a significant impact on health worldwide, mainly in children and older adults. Causes of severe viral respiratory illnesses include measles virus, influenza, parainfluenza viruses, and respiratory syncytial viruses. Although recovery is usually rapid, some infections with extensive bronchiolitis develop that may be severe enough to threaten life. Respiratory viruses (RVs) not only are important in terms of clinical morbidity but also impose a significant economic burden in the world. For example, the current COVID-19 pandemic still looms largest as a risk to economic growth in the world.
A wide range of different and common viruses can cause respiratory diseases. One of the most common viruses is the influenza virus, also known as the ‘flu’. The flu virus leads to symptoms of the common cold, with coryza, cough, hoarseness, and even serious lung infections, such as pneumonia. Other viruses include adenovirus, picornaviruses (enteroviruses and parechoviruses), human metapneumovirus, coronaviruses, respiratory syncytial virus, rhinovirus, and parainfluenza virus. Although specific pathogens commonly cause characteristic clinical manifestations, these clinical syndromes include common cold, pharyngitis, acute laryngotracheobronchitis, tracheitis and tracheobronchitis, bronchiolitis, and viral pneumonia. Since most symptoms are mild and self-limited, medications are generally not used in treatment. Antiviral medication may be used for severe or prolonged viral respiratory infections.
Symptoms and Classification
The respiratory system consists of a set of organs divided into two groups: the upper respiratory tract (nasal cavity, pharynx, larynx, trachea) and lower respiratory tract (bronchi, bronchioles, and lungs). RVs are the most frequent causative agents of disease in humans, with a significant impact on morbidity and mortality worldwide. Most RVs cause cold-like symptoms or flu-like illnesses. The symptoms vary relied on disease severity. Viral respiratory infection mainly includes:
- Upper respiratory tract infection (URTI). Large particles are usually trapped in the turbinates and sinuses and could cause URTIs. URTIs usually present acutely and are the most common infections in humans, arise throughout the year but the incidence is higher in winter, is generally of low severity. Viruses originated 70-90% of these episodes and viruses that are associated with URTIs are: respiratory syncytial virus, rhinovirus, parainfluenza, influenza A, adenovirus, and coronavirus. All individuals can be infected by these viruses, however, it has been observed that children are the most affected. A virus can cause several syndromes, also too a syndrome may be caused by different viruses such that the clinical manifestations are variable. The most common syndromes in the upper airways are nasopharyngitis, adenoiditis, pharyngitis, sinusitis, laryngitis, and croup.
- Lower respiratory tract infection (LRTI). Smaller particles can reach the alveolar spaces and cause LRTI. LRTIs occupy a smaller percentage, but with high mortality rates. LRTI symptoms include tachypnea, wheeze, severe cough, breathlessness, and respiratory distress. The disease is increased by several factors including anatomical disorders, immunological, metabolic, or other diseases such as acquired immunodeficiency syndrome (AIDS), asthma, or chronic obstructive pulmonary disease.
Fig.1 Different RVs that Infect Particular Parts of the Upper and Lower Respiratory Tract. (Subbarao, 2020)
- Pandemic influenza virus (A and B)
- H5N1 and H7N9 avian influenza A viruses
- Parainfluenza virus
- Respiratory syncytial virus (A and B)
- Human metapneumovirus
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- Subbarao, K.; Mahanty, S. Respiratory Virus Infections: Understanding COVID-19. Immunity. 2020. 52(6): 905-909.
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