Fortnightly Review: Management of community acquired lower respiratory tract infection

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Abstract
Epidemiology Infections of the lower respiratory tract are responsible for 6% of all general practitioner consultations and form 4.4% of all hospital admissions.1 They account for 3-5% of deaths in adults up to the age of 60. The best estimate from available data suggests that around 25 million prescriptions for antibiotics are written each year by general practitioners to treat respiratory infections.2 As with most diseases, the less severe types predominate. Acute bronchitis is common and usually self limiting, although the associated cough and sleep disturbance can be distressing. Children who develop croup or stridor during a viral respiratory infection may need to be admitted to hospital. Community acquired pneumonia is potentially more serious, with elderly people and those with pre-existing illness at greatest risk of dying. Community acquired pneumonia is distinct from other types (box 1) in terms of epidemiology, causative organisms, management, and outcome. It occurs more commonly in those over 65, smokers, and those with other non-respiratory illnesses (particularly alcoholism), and the incidence doubles during the winter months. Most cases are managed by primary care teams - 78-95% of adults with pneumonia are treated at home by general practitioners.1 The remainder of patients with more severe illness will require hospital admission, including all those with adverse prognostic factors, severe pain, or breathlessness. Box 1: Classification of pneumonia Community acquired Hospital acquired aesocomial) Geographical pneumonia Aspiration pneumonia Pneumonia in the immunocompromised host Organisms Viruses account for most cases of acute bronchitis and up to a fifth of cases of pneumonia. Viral lower respiratory tract infection is most prevalent in autumn and winter. Common viruses causing respiratory disease include the influenza viruses, parainfluenza viruses, respiratory syncytial virus, and adenoviruses. A hospital survey by the British Thoracic Society of community acquired pneumonia in adults found influenza A virus in 7% of cases, the third most commonly isolated pathogen.3 Viral infections are the commonest cause of acute bronchitis, bronchiolitis, and pneumonia in children, with parainfluenza viruses and respiratory syncytial virus predominating.3,4 Outbreaks of lower respiratory tract infection caused by respiratory syncytial virus have been recorded in elderly people in nursing homes.5 Viruses damage the respiratory epithelium, predisposing to bacterial superinfection, and impairment of mucociliary function may increase the severity of subsequent bacterial infection. Staphylococcal pneumonia during an influenza epidemic is the most common and severe example. Streptococcus pneumoniae remains the most commonly identified organism in community acquired pneumonia (fig 1), isolated in 20-30% cases.*RF 3,7-9* It probably also accounts for most of the third of cases in which no causative organism is found.10 View larger version: In this window In a new window FIG 1 Streptococcus pneumoniae in a Gram stained film of sputum Other agents commonly found are Mycoplasma pneumoniae, Haemophilus influenzae, Legionella spp and Staphylococcus aureus (fig 2). Mycoplasma infection occurs in three to four year cycles in temperate climates and often affects children and young adults.11 In children over 4 years of age pneumococcal and mycoplasma infections account for most non-viral cases of lower respiratory tract infection.4 H influenzae is the commonest cause of bacterial exacerbation of chronic obstructive pulmonary disease and accounts for 5-10% of cases of pneumonia. View larger version: In this window In a new window FIG 2 Pathogens in community acquired pneumonia isolated in four large studies Legionella infection remains rare in the United Kingdom; half of cases are acquired abroad. It occasionally occurs in clusters of patients with no history of foreign travel - a source of infection (such as cooling towers) should be sought in such circumstances. Other organisms may be isolated from the sputum but their causative role in lower respiratory tract infection is unclear. Moraxella (previously Branhamella) catarrhalis is often found in those with underlying chronic obstructive pulmonary disease but its significance is disputed; in the context of an infective exacerbation of chronic obstructive pulmonary disease it should be treated.12,13 It is occasionally identified as the likely cause of acute pneumonia.7,9 The role of Chlamydia pneumoniae is unclear.14 It has been identified as the cause of up to 6% of cases of community acquired pneumonia, but limited laboratory diagnostic tests make assessment of pathogenicity difficult. Recent work suggests that it may be a more common respiratory pathogen than previously thought - positive serological results initially thought to suggest infection with C trachom(???) and C psittaci may in fact be due to cross reaction with antibodies due to C pneumoniae.15 There seems to be a small but increasing number of patients presenting with lower respiratory tract infection in whom Pneumocystis carinii is identified, prompting the diagnosis of unsuspected HIV infection. Recent studies show that pneumonia due to Klebsiella pneumoniae is much less common than older textbooks have suggested.16 The patient's history may give clues to the cause of the pneumonia. Mycoplasma is commonly associated with cough, sore throat, nausea, diarrhoea, headache, chills, and myalgia. It is occasionally associated with neurological features.11 The knowledge that an epidemic is under way may raise the level of suspicion. Staphylococcal pneumonia may be suspected during an influenza epidemic, especially if the patient is unwell and has radiological evidence of cavitation. Injecting drug users have a greater risk of staphylococcal lung infection and abscess formation, often associated with right sided endocarditis. Contact with birds (typically a sick parrot) might suggest...