Etiology

Table 19.1 lists various infectious agents associated with acute bronchitis. Infections with adenoviruses, influenza viruses, parainfluenza viruses, respiratory syncytial virus, and Mycoplasma pneumoniae account for most cases of acute bronchitis in children. These agents, plus many rhinoviruses, a few enteroviruses, and perhaps human metapneumovirus, human bocavirus, and the newer human coronaviruses, account for virtually all cases in the United States today.

Of the adenoviruses, type 7 has been associated most commonly with acute bronchitis in children. In military recruits, including adolescents, adenovirus types 4 and 7 cause epidemic acute respiratory disease, in which bronchitis is a usual occurrence.

Influenza A virus infection is a common cause of severe acute bronchitis, particularly at the time of an antigenic shift of influenza A virus subtype and pandemic disease. Acute bronchitis caused by influenza A virus also is a regular occurrence between pandemics in new susceptible individuals (young children) in the population. In addition, influenza b virus is an important cause of bronchitis, and it was a more common causative agent than was influenza A virus in one large longitudinal study.

All cases of measles involve the bronchi, but measles has been an uncommon occurrence since the advent of widespread use of vaccines. Of the parainfluenza viruses, type 3 is associated most commonly with acute bronchitis. Respiratory syncytial virus is a common cause of acute bronchitis, particularly in very young children. The more recently identified human metapneumovirus and human bocavirus are also causes of acute bronchitis.

Of the bacterial agents listed in Table 19.1 , only Haemophilus influenzae clearly can be incriminated. Bordetella pertussis infection involves the trachea and bronchi, but fever is an uncommon event, and the illness is outside the definition of acute bronchitis. When sought, M. pneumoniae is a common cause of bronchitis. Chlamydia pneumoniae has been found to be the cause of bronchitis in adolescents and young adults.

Epidemiology

The epidemiology of the common viruses associated with bronchitis is presented in Sections 17 and 18 of this text. Chapman and associates published the results of a study of acute bronchitis in a single private group pediatric practice in Chapel Hill, North Carolina. The study occurred during a 104-month period, during which 5489 episodes of lower respiratory tract illness occurred. Of these illnesses, 40.1% were acute bronchitis. The bronchitis attack rate was highest in children in the second year of life (6.71%), then decreased gradually to approximately 2% in teenagers. In contrast to the age-specific attack rates, the ratio of bronchitis cases to all lower respiratory illness cases increased with age. In the first year of life, the ratio was 0.29; in children 12 years old or older, it was 0.69.

During the first 6 years of life, respiratory syncytial virus and parainfluenza virus type 3 were the most common etiologic agents noted in the Chapel Hill study. During the first 2 years of life, adenoviruses also commonly were associated with bronchitis. In patients older than 6 years, M. pneumoniae and influenza A and B viruses were the most common etiologic agents. In a study of cough illnesses of 6 days’ duration or longer in university students, investigators found that 15 of 31 students with laboratory evidence of Bordetella spp. infection were considered by their primary care providers to have bronchitis.

The incidence of acute bronchitis peaks in the winter months, declines to midsummer, and increases again through the fall. Attack rates generally are higher in boys than in girls. A sex difference is most pronounced during the first 6 years of life. Darrow and associates found that primary traffic pollutants, ozone, and the organic carbon fraction of particulate matter of less than 2.5 µm in diameter exacerbate upper and lower respiratory infections in young children.

Pathophysiology and Pathology

Because acute bronchitis is an illness characterized by clinical features and one not usually associated with death, knowledge of its pathophysiology and pathology is meager. The general pathophysiology of human infections with viruses and M. pneumoniae that cause acute bronchitis is presented more completely in the sections of this book related to the individual infectious agents.

In virtually all cases of acute bronchitis, evidence of upper respiratory tract viral infection (pharyngitis, rhinitis) also is present. Tracheal and bronchial infection apparently is the result of distal spread. In bronchitis, the clinical features result from damage to the ciliated epithelium of the lower trachea and the large and medium-sized bronchi. Although the cytopathologies of the various infectious agents is different, the resulting obstruction of the air passages leads to similar symptoms. The duration of symptoms depends to some extent on the specific initial infectious agent and, in cases of prolonged illness, on secondary bacterial infection.

In acute bronchitis, the larynx and subglottic trachea are not involved prominently. Conversely, today bronchial involvement is seen only occasionally in croup.

Pathophysiology and Pathology

Because acute bronchitis is an illness characterized by clinical features and one not usually associated with death, knowledge of its pathophysiology and pathology is meager. The general pathophysiology of human infections with viruses and M. pneumoniae that cause acute bronchitis is presented more completely in the sections of this book related to the individual infectious agents.

In virtually all cases of acute bronchitis, evidence of upper respiratory tract viral infection (pharyngitis, rhinitis) also is present. Tracheal and bronchial infection apparently is the result of distal spread. In bronchitis, the clinical features result from damage to the ciliated epithelium of the lower trachea and the large and medium-sized bronchi. Although the cytopathologies of the various infectious agents is different, the resulting obstruction of the air passages leads to similar symptoms. The duration of symptoms depends to some extent on the specific initial infectious agent and, in cases of prolonged illness, on secondary bacterial infection.

In acute bronchitis, the larynx and subglottic trachea are not involved prominently. Conversely, today bronchial involvement is seen only occasionally in croup.

Clinical Presentation

Initial manifestations of acute bronchitis occur in the upper respiratory tract and, depending on the etiologic agent, are predominantly nasal, as in the common cold, or show additional objective evidence of pharyngitis, as in nasopharyngitis. Fever usually is present, and temperatures vary from 37.8°C to 39°C (100°F to 102.2°F) on most occasions. Cough is always present, and its onset can be insidious or abrupt. Initially, the cough is dry and harsh and often brassy in younger children. As the illness progresses, the cough becomes looser. In older children, purulent sputum is raised and expectorated. In younger children, the swallowing of often tenacious sputum frequently leads to gagging and vomiting. Older children may complain of chest pain resulting from coughing.

On initial physical examination, a variable degree of rhinitis usually is present; many patients have diffuse pharyngeal erythema. As the disease progresses, these upper respiratory tract signs generally decrease. Examination of the chest reveals rhonchi and referred breath sounds. Coarse, changing rales are noted frequently.

In the usual case of acute bronchitis, the illness can be separated into three phases: (1) a 1- to 2-day prodromal period when fever and upper respiratory tract symptoms predominate, (2) a 4- to 6-day period of marked tracheobronchial symptoms with some fever and general discomfort, and (3) a recovery period that may last 1 or 2 weeks and is characterized by cough and expectoration. Occasionally the recovery period is particularly distressing and is associated with a low-grade fever, suggesting secondary bacterial infection. Bronchitis caused by C. pneumoniae often is insidious in onset and frequently associated with or preceded by pharyngitis. Illness persists for several weeks but responds to appropriate antibiotic therapy.

Laboratory study in acute bronchitis is of limited use. Children in whom throat cultures reveal pathogenic bacteria in predominant growth tend to have more severe illness than do children with only viral infections. The white blood cell count usually is greater than 10,000 cells/mm ³ , and approximately one-third of the cases have a predominance of neutrophils. The chest radiograph is normal unless associated pulmonary involvement is present.