Acute viral respiratory illnesses are among the most common of human diseases, accounting for one-half or more of all acute illnesses. The incidence of acute respiratory disease in the United States is 3–5.6 cases per person per year. The rates are highest among children <1 year old (6.1–8.3 cases per year) and remain high until age 6, when a progressive decrease begins. Adults have 3–4 cases per person per year. Morbidity from acute respiratory illnesses accounts for 30–50% of time lost from work by adults and for 60–80% of time lost from school by children. The use of antibacterial agents to treat viral respiratory infections represents a major source of abuse of that category of drugs.
It has been estimated that two-thirds to three-fourths of cases of acute respiratory illnesses are caused by viruses. More than 200 antigenically distinct viruses from 10 genera have been reported to cause acute respiratory illness, and it is likely that additional agents will be described in the future. The vast majority of these viral infections involve the upper respiratory tract, but lower respiratory tract disease can also develop, particularly in younger age groups, in the elderly, and in certain epidemiologic settings.
The illnesses caused by respiratory viruses traditionally have been divided into multiple distinct syndromes, such as the “common cold,” pharyngitis, croup (laryngotracheobronchitis), tracheitis, bronchiolitis, bronchitis, and pneumonia. Each of these general categories of illness has a certain epidemiologic and clinical profile; for example, croup occurs exclusively in very young children and has a characteristic clinical course. Some types of respiratory illness are more likely to be associated with certain viruses (e.g., the common cold with rhinoviruses), while others occupy characteristic epidemiologic niches (e.g., adenovirus infections in military recruits). The syndromes most commonly associated with infections with the major respiratory virus groups are summarized in Table 186-1. Most respiratory viruses clearly have the potential to cause more than one type of respiratory illness, and features of several types of illness may be found in the same patient. Moreover, the clinical illnesses induced by these viruses are rarely sufficiently distinctive to permit an etiologic diagnosis on clinical grounds alone, although the epidemiologic setting increases the likelihood that one group of viruses rather than another is involved. In general, laboratory methods must be relied on to establish a specific viral diagnosis.
Table 186-1 Illnesses Associated with Respiratory Viruses
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Table 186-1 Illnesses Associated with Respiratory Viruses
|Frequency of Respiratory Syndromes|
|Rhinoviruses||Common cold||Exacerbation of chronic bronchitis and asthma||Pneumonia in children|
|Coronavirusesa||Common cold||Exacerbation of chronic bronchitis and asthma||Pneumonia and bronchiolitis|
|Human respiratory syncytial virus||Pneumonia and bronchiolitis in young children||Common cold in adults||Pneumonia in elderly and immunosuppressed patients|
|Parainfluenza viruses||Croup and lower respiratory tract disease in young children||Pharyngitis and common cold||Tracheobronchitis in adults; lower respiratory tract disease in immunosuppressed patients|
|Adenoviruses||Common cold and pharyngitis in children||Outbreaks of acute respiratory disease in military recruitsb||Pneumonia in children; lower respiratory tract and disseminated disease in immunosuppressed patients|
|Influenza A viruses||Influenzac||Pneumonia and excess mortality in high-risk patients||Pneumonia in healthy individuals|
|Influenza B viruses||Influenzac||Rhinitis or pharyngitis alone||Pneumonia|
|Enteroviruses||Acute undifferentiated febrile illnessesd||Rhinitis or pharyngitis alone||Pneumonia|
|Herpes simplex viruses||Gingivostomatitis in children; pharyngotonsillitis in adults||Tracheitis and pneumonia in immunocompromised patients||Disseminated infection in immunocompromised patients|
|Human metapneumoviruses||Upper and lower respiratory tract disease in children||Upper respiratory tract illness in adults||Pneumonia in elderly and immunosuppressed patients|
This chapter reviews viral infections caused by six of the major groups of respiratory viruses: rhinoviruses, coronaviruses, respiratory syncytial viruses, metapneumoviruses, parainfluenza viruses, and adenoviruses. The extraordinary outbreaks of lower respiratory tract disease associated with coronaviruses (severe acute respiratory syndrome, or SARS) in 2002–2003 are also discussed. Influenza viruses, which are a major cause of death as well as morbidity, are reviewed in Chap. 187. Herpesviruses, which occasionally cause pharyngitis and which also cause lower respiratory tract disease in immunosuppressed patients, are reviewed in Chap. 179. Enteroviruses, which account for occasional respiratory illnesses during the summer months, are reviewed in Chap. 191.
Rhinoviruses are members of the Picornaviridae family, small (15- to 30-nm) nonenveloped viruses that contain a single-stranded RNA genome and have been divided into three genetic species: HRV-A, HRV-B, and HRV-C. In contrast to other members of the picornavirus family, such as enteroviruses, rhinoviruses are acid-labile and are almost completely inactivated at pH ≤ 3. Rhinoviruses grow preferentially at 33°–34°C (the temperature of the human nasal passages) rather than at 37°C (the temperature of the lower respiratory tract). Of the 102 recognized serotypes of rhinovirus, 91 use intercellular adhesion molecule 1 (ICAM-1) as a cellular receptor and constitute the “major” receptor group, 10 use the low-density lipoprotein receptor (LDLR) and constitute the “minor” receptor group, and 1 uses decay-accelerating factor.
Rhinoviruses are a prominent cause of the common cold and have been detected in up to 50% of common cold–like illnesses by tissue culture and polymerase chain reaction (PCR) techniques. Overall rates of rhinovirus infection are higher among infants and young children and decrease with increasing age. Rhinovirus infections occur throughout the year, with seasonal peaks in early fall and spring in temperate climates. These infections are most often introduced into families by preschool or grade-school children <6 years old. Of initial illnesses in family settings, 25–70% are followed by secondary cases, with the highest attack rates among the youngest siblings at home. Attack rates also increase with family size.
Rhinoviruses appear to spread through direct contact with infected secretions, usually respiratory droplets. In some studies of volunteers, transmission was most efficient by hand-to-hand contact, with subsequent self-inoculation of the conjunctival or nasal mucosa. Other studies demonstrated transmission by large- or small-particle aerosol. Virus can be recovered from plastic surfaces inoculated 1–3 h previously; this observation suggests that environmental surfaces contribute to transmission. In studies of married couples in which neither partner had detectable serum antibody, transmission was associated with prolonged contact (≥122 h) ...