Lung infections bullet  4: Pathogenesis of lower respiratory tract infections due to Chlamydia, Mycoplasma, Legionella and viruses

Acute infection of the lower respiratory tract comprises bronchitis, bronchiolitis, and pneumonia. From a clinical point of view it may be difficult to distinguish these disease entities and one infection may progress into another. The most common pathogens causing these infections are the primary respiratory viruses (respiratory syncytial virus (RSV), influenza virus, adenoviruses, parainfluenza virus, and rhinovirus1 2), Mycoplasma pneumoniae ,3 and Chlamydia species.4-6 Legionella may cause pneumonia and non-pneumonic upper respiratory tract infection and approximately 85% of c ases are caused by L pneumophila . Long lasting sequelae such as bronchiectasis, lung fibrosis, and decreased lung function are seen after lower respiratory tract infections7-10 and it has been debated whether respiratory tract infection can cause chronic bronchial asthma.6 11-13 Acute bronchitis is an inflammatory condition of the bronchi often caused by infectious agents, although in many cases no aetiology can be established.14 15 It shares many pathological and clinical features with bronchiolitis, and the same agents may induce both conditions. Most cases of acute bronchitis of known aetiology are due to respiratory viruses such as influenza virus, adenovirus, RSV, rhinovirus, and coronavirus,1 2 16 and a few are caused by M pneumoniae , Bordetella pertussis and C pneumoniae .4 5 15 17 Legionella infections limited to the bronchial tree are not described. ### ADHERENCE OF PATHOGENS The pathogenesis has not been studied for all agents, but the transmission of disease is thought to occur through droplet spread from an infected person. After inhalation the infectious agent may adhere to different receptors such as acid containing glycoproteins or the adhesion molecule ICAM-1 on respiratory epithelial cells.18 19 The infectious chlamydial particle is the elementary body that attaches itself to and enters a susceptible cell where it changes to the larger metabolically active reticulate body.20 …