F70954-07.qxd 12/10/02 7:36 AM Page 71 Infections of the respiratory 7 tract the nasal hairs and by inertial impaction with mucus- 7.1 Pathogenesis 71 covered surfaces in the posterior nasopharynx (Fig. 11). 7.2 Diagnosis 72 The epiglottis, its closure reflex and the cough reflex all reduce the risk of microorganisms reaching the lower 7.3 Management 72 respiratory tract. Particles small enough to reach the tra- 7.4 Diseases and syndromes 73 chea and bronchi stick to the respiratory mucus lining their walls and are propelled towards the oropharynx 7.5 Organisms 79 by the action of cilia (the ‘mucociliary escalator’). Self-assessment: questions 80 Antimicrobial factors present in respiratory secretions further disable inhaled microorganisms. They include Self-assessment: answers 83 lysozyme, lactoferrin and secretory IgA. Particles in the size range 5–10 µm may penetrate further into the lungs and even reach the alveolar air Overview spaces. Here, alveolar macrophages are available to phagocytose potential pathogens, and if these are overwhelmed neutrophils can be recruited via the This chapter deals with infections of structures that constitute inflammatory response. The defences of the respira- the upper and lower respiratory tract. The general population tory tract are a reflection of its vulnerability to micro- commonly experiences upper respiratory tract infections, bial attack. Acquisition of microbial pathogens is which are often seen in general practice. Lower respiratory tract infections are less common but are more likely to cause serious illness and death. Diagnosis and specific chemotherapy of respiratory tract infections present a particular challenge to both the clinician and the laboratory staff. Successful preventive strategies are available for several respiratory infections. 7.1 Pathogenesis Learning objectives You should: G understand the mechanisms by which respiratory infections occur G know how pathogens overcome host defences G understand what factors increase vulnerability to respiratory infections. The principal function of the respiratory tract is gas exchange. It is therefore constantly exposed to the gaseous environment, including particulate organic material, such as bacteria, viruses and spores (Ch. 3). Although the entire respiratory tract is constantly exposed to air, the majority of particles are filtered out in Fig. 11 Defences of the respiratory tract. 71 F70954-07.qxd 12/10/02 7:36 AM Page 72 7 Infections of the respiratory tract primarily by inhalation, but aspiration and mucosal patient and recognising the more serious bacterial infec- and haematogenous spread also occur. Individuals tions that require specific antimicrobial chemotherapy with healthy lungs rarely have any bacteria beyond or more extensive supportive treatment. the carina. Lower respiratory tract infection should always be Respiratory pathogens have developed a range of taken seriously since it is more likely to cause serious strategies to overcome host defences. Influenza virus, morbidity or even death. for example, has specific surface antigens that adhere to mucosal epithelial cells. The virus also undergoes peri- Laboratory tests odic genetic reassortment resulting in expression of novel adhesins to which the general population has no History, physical examination, X-rays and laboratory effective immunity. Streptococcus pneumoniae and investigations focus on two issues: the degree of res- Haemophilus influenzae both produce an enzyme (IgA piratory compromise and the identity of the causal protease) capable of disabling mucosal IgA. Both these pathogen. Since a wide range of candidate pathogens species, other capsulated bacteria and mycobacteria are may have to be considered, the number of likely candi- all resistant to phagocytosis. Penetration of local tissues dates should be reduced as far as possible by searching is usually required before damage occurs, although for clues in the history, examination and preliminary viruses causing the common cold appear to be an excep- results. A history of tobacco consumption, recent travel, tion. In some lower respiratory tract infections, the host occupation, pets, and contacts with similar symptoms response is the principal cause of damage. should be sought. Human behaviour can also increase the risk of re- Diagnostic specimens can be obtained from the res- spiratory infection. Tobacco smoking has this effect by piratory tract with deceptive ease, but their value reducing the efficiency of cilial function and by causing is often limited by contamination by the indigenous the production of more viscous respiratory secretions. flora of the oral cavity. To prevent contamination Tracheal intubation for prolonged periods in the critical- of lower respiratory tract specimens, the upper res- ly ill bypasses the upper respiratory tract and provides a piratory tract must be bypassed. Chest X-rays are a conduit for microbial access directly into the lungs. fundamental part of evaluation of lower respiratory tract infections and provide evidence of the distribu- tion and extent of disease more reliably than signs 7.2 Diagnosis elicited by auscultation. Postero-anterior views are most commonly used, but a lateral view can provide valuable additional information. Learning objectives Blood gas analysis should be performed if there is any suspicion of acute respiratory compromise. The key You should: indicators of disease severity in pneumonia are raised respiratory rate (> 30 beats/min), hypoxia, hypercapnia, G know which features indicate that a specific area of the bilateral or recently enlarging radiographic opacities, respiratory tract is infected shock, renal failure and confusion. G know how to assess respiratory compromise G know how to identify the pathogen. 7.3 Management Clinical features Learning objectives The features of different respiratory tract infections You should: largely depend on the structures where inflammation is localised and the extent to which function is altered. So, G know when chemotherapy is indicated infection of the nasopharynx will result in a nasal dis- G know how to choose the most suitable drug charge, bronchitis in cough and sputum production, and pneumonia in cough and sputum, but also in increased G know how to prevent infection and the spread of infections. respiratory rate and chest radiograph changes. Most upper respiratory tract infections are caused by Chemotherapy viruses and are self-limiting. A specific aetiological diag- nosis would not alter treatment and would be costly. The antimicrobial therapy of respiratory tract infec- The role of the physician is limited to reassuring the tion depends not only on the likely microbial cause of 72 F70954-07.qxd 12/10/02 7:36 AM Page 73 Diseases and syndromes 7 infection but also on the primary site involved and the worn by staff and other visitors. At a personal level, severity of disease. The commoner upper respiratory covering the mouth when coughing or sneezing is a tract infections are rarely life threatening and in many simple but effective means of preventing the spread of cases are self-limiting. It is therefore possible to man- respiratory pathogens. age many of these infections without specific chemotherapy, thereby avoiding all the possible adverse effects. However, even apparently trivial 7.4 Diseases and syndromes infections such as pharyngitis may require specific antibiotic treatment in some cases. The problem is in knowing who and when to treat with antimicrobial Learning objectives agents. Lower respiratory infections are less of a problem You should: in this respect, since infection is much more likely to cause significant morbidity and mortality. Antibiotics G know the major infections of the respiratory tract should be used as early as possible in the course of G know the factors contributing to their occurrence infection. The problem here is in knowing which of a wide range to choose. It is often necessary to make a G understand the basis of their clinical management. ‘best guess’ or presumptive choice in severely ill patients, based on the most likely microbial agent. The The main infectious diseases of the respiratory tract are initial choice of chemotherapy may have to be sub- listed in Table 9. stantially modified in the light of laboratory results. Patients with pneumonia who are ill enough to require hospitalisation usually require parenteral antibiotics. Pharyngitis A syndrome-based choice of therapy has become the preferred approach, since antibiotic choice and deci- Pharyngitis is an inflammation of the throat, resulting in sions on the need for hospital admission and active pain on swallowing and swollen, red pharyngeal supportive care do not have to wait for a laboratory- mucosa. It is most often caused by a respiratory virus based aetiological diagnosis. (rhinovirus, coronavirus, adenovirus, influenza virus, parainfluenza viruses, respiratory syncytial virus), Epstein–Barr virus or coxsackievirus. Prevention Aetiological clues include: The ease with which respiratory infections can be spread and their associated morbidity has led to the G conjunctivitis: adenovirus development of specific preventive approaches. G constitutional symptoms (lethargy and malaise) and Influenza can be prevented by immunisation with a tonsillar exudate: Epstein–Barr virus live attenuated vaccine. The changes in epidemic G posterior palatal ulcers: coxsackievirus strains of influenza virus necessitate periodic changes G abrupt onset, ‘doughnut’ pharyngeal lesions and in vaccine composition