Viruses are the leading causes of acute lower respiratory tract infection in most populations. Common viral respiratory infections can be categorized by site of anatomic involvement (e.g., alveolar spaces, bronchioles, trachea, conjunctiva, middle-ear spaces), disease syndrome (e.g., rhinitis, pharyngitis, bronchiolitis, pneumonia), or etiologic agent (e.g., respiratory syncytial virus [RSV], human metapneumovirus, parainfluenza virus [PIV], adenovirus). See Chap. 59 Sore Throat, Earache, and Upper Respiratory Symptoms for additional details on viral respiratory infections.
- Age is a major determinant of risk for symptomatic disease, with children having more frequent respiratory virus infections than adults.
- Infections with most of the conventional respiratory viruses occur in winter.
- Risk factors for severe disease include underlying lung disease (especially chronic obstructive pulmonary disease), cardiovascular disease, close exposure to infected people, smoking, low socioeconomic status, and male gender.
- Respiratory viruses are generally transmitted by fomites or by large-particle aerosols of respiratory droplets that are spread by coughing or sneezing.
- Culture-based studies have isolated two or more viruses from 5% to 10% of healthy adults with acute respiratory illness.
- RSV is a single-stranded, negative-sense, nonsegmented RNA virus and a member of the family Paramyxoviridae. RSV is among the most transmissible of viruses, and infection is ubiquitous, particularly in the first few years of life. Annual epidemics typically occur between October and March in temperate regions. The very young and the elderly are at greatest risk for severe lower respiratory tract illness and hospitalization. There may be a link between RSV infection in early life and the subsequent development of asthma.
- Parainfluenza viruses are a group of four distinct serotypes (designated 1-4) of single-stranded, negative-sense RNA viruses in the family Paramyxoviridae. PIV3 most commonly causes severe disease. Primary infection in children manifests as laryngotracheitis (croup), with subsequent infections limited to the upper respiratory tract.
- Human metapneumovirus is also a member of the family Paramyxoviridae and is similar in many respects to RSV, although less virulent. Infections occur first in early childhood, and reinfections are common throughout life. The virus causes both upper and lower respiratory tract disease.
- Rhinoviruses are single-stranded, positive-sense RNA viruses in the family Picornaviridae. The more than 100 serotypes of rhinovirus are the most frequent causes of the common cold, causing ≥50% of cases. Most infected adults also have radiographic evidence of sinusitis. Rhinovirus may be able to infect the lower respiratory tract as well, although the data are less clear.
- Adenovirus is a double-stranded, nonsegmented DNA virus in the family Adenoviridae. Most human respiratory infections are caused by the B and C species and can occur throughout the year. Acute adenovirus infection is frequently associated with pharyngoconjunctival fever. Immunocompromised pts are highly susceptible to severe disease during infection with respiratory adenoviruses.
- Dozens of coronaviruses affect animals, and some have the potential to cross over and infect humans. In an outbreak caused by SARS-associated coronavirus (SARS-CoV) that occurred from November 2002 to July 2003, >8000 people were infected and the mortality rate was ∼10%. Unlike viral pneumonias, SARS lacks upper respiratory symptoms (although cough and dyspnea occur in most pts), and pts present with a nonspecific illness of fever, myalgia, malaise, and chills or rigors. The Middle East respiratory syndrome coronavirus (MERS-CoV) is associated with a 35% mortality rate in humans. This virus is thought to have emerged from bats, and humans are thought to be infected through direct or indirect contact with infected dromedary camels.
The presentation of a viral respiratory infection depends on the anatomic site affected.
- The common cold is characterized by nasal congestion, sneezing, rhinorrhea, cough, and sore throat.
- Laryngitis is accompanied by hoarseness or dysphonia.
- Acute bronchitis is characterized by a dry or productive cough of <3 weeks' duration; bacteria play a more prominent role in chronic bronchitis.
- Acute pneumonia manifests with fever, cough, sputum production, dyspnea, and chest pain.
The clinical diagnosis of a respiratory syndrome and determination of the anatomic location of infection are based on history, physical examination, and radiography. A specific viral etiology can be determined by specific diagnostic tests. Although virus isolation is the gold standard, the most sensitive tests are RT-PCR assays. Multiplex panels that assay for numerous respiratory viruses and bacterial pathogens are available. However, a positive test for a virus may indicate a recently resolved rather than an acute infection, given that the viral genome can persist in respiratory secretions for weeks.
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Acute Respiratory Virus InfectionsThe principal interventions that make a difference in the care of pts with acute respiratory virus infections are supportive and should be implemented meticulously. Antiviral treatment, for which there are limited options, generally is effective only when administered early in the course of illness. Antibiotics do not improve the outcome of uncomplicated respiratory virus infections in otherwise healthy subjects.
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Given the lack of cross-protection among serotypes for many respiratory viruses, vaccines have been difficult to develop. However, a vaccine against adenovirus serotypes 4 and 7 is used in military recruits, and live attenuated and subunit vaccine candidates against RSV are currently under development. Palivizumab, a humanized mouse monoclonal antibody to the F protein of RSV, is licensed for prevention of RSV hospitalization in high-risk infants; however, it has not been effective for treatment of immunocompetent or immunocompromised RSV-infected pts.