Clinical Focus
4 AUGUST 2023 ausdoc . com . au
Therapy Update
Adult community acquired pneumonia
Respiratory
Dr Lai-Ying Zhang ( left ) is a respiratory physician currently undertaking a research fellowship and PhD in interstitial lung disease , as part of the University of Queensland / Metro North Hospital and Health Service Clinician Researcher Training Pathway .
Professor Kwun Fong ( centre ) is a respiratory physician at Metro North Hospital and Health Service , department of thoracic medicine , The Prince Charles Hospital . Dr Andrew Burke ( right ) is a respiratory physician at Metro North Hospital and Health Service , department of thoracic medicine , The Prince Charles Hospital .
Bacterial pneumonia continues to cause significant disease burden in adults and judicious choice of therapy is essential to reduce treatment side effects and resistance .
NEED TO KNOW
Streptococcus pneumoniae is the most common pathogen causing community acquired pneumonia in Australian adults .
Oral amoxicillin and doxycycline are sufficient to treat the vast majority of low-severity bacterial pneumonias managed in the community .
Broader spectrum antibiotics , such as amoxicillin-clavulanic acid or fluoroquinolones , are associated with more side effects and are a main driver for antibiotic resistance in Australia .
LOWER respiratory tract infections range from self-limiting bronchitis , which usually does not require antibiotic therapy , to pneumonia , which requires prompt diagnosis and initiation of guideline-based empirical treatment . Pneumonia can result from infection with viruses , bacteria , and less common atypical organisms such as fungi .
In Australia , bacterial community acquired pneumonia ( CAP ) continues to be associated with high rates of morbidity and mortality . Previous Australian data have suggested an annual overall incidence of CAP requiring hospital admission of 245 per 100,000 person years . 1 The incidence in the elderly is significantly higher at 1453 per 100,000 person years in those aged over 80 . 1 CAP also has a higher inpatient mortality ( 8 %) and requirement for intensive care ( 10 %) compared to other community acquired infections such as pyelonephritis and soft tissue infections . 1
Epidemiology and pathogenesis
CAP is defined as a history of new-onset purulent sputum , new or worsening cough , dyspnoea , or reduced oxygen saturation , in combination with fever and evidence of
pulmonary infiltrates on chest radiography , occurring in a patient either in the community or hospitalised for less than 48hrs .
Risk factors for CAP in the immunocompetent population include increasing age , medical comorbidities ( especially COPD , but also including congestive cardiac failure and diabetes ), active tobacco smoking , recent viral respiratory tract infection , excess alcohol consumption , and opioid use . 2-6
It was previously believed that pneumonia occurred via aspiration of pathogenic bacteria into the ‘ sterile ’ tracheobronchial tree . However , newer culture-independent techniques have established that the lower respiratory tract is a non-sterile space with its own complex microbiome , and that most cases of pneumonia occur when aspirated oropharyngeal bacteria disrupt this delicate microbial homeostasis . 7 , 8 As such , conditions which reduce the ability to protect the airway from silent aspiration — such as stroke — and conditions which alter the bacterial composition of the mouth — such as poor dental hygiene and periodontitis — are additionally associated with CAP . 5
In up to 50 % of bacterial community acquired pneumonia the causative organism may not be identified on sputum , blood or urinary testing .
Microbiology
The most common causative bacterial pathogens in Australia are Streptococcus pneumoniae , Mycoplasma pneumoniae and Haemophilus influenzae , followed by legionella and chlamydophila species . 9 It is important to note , however , that in up to 50 % of bacterial CAP cases the causative organism may not be identified on sputum , blood or urinary testing . 9 Patients with chronic suppurative lung diseases such as bronchiectasis or cystic fibrosis , immunosuppression , receiving multiple antibiotic courses or requiring frequent hospitalisations , may be more prone to pneumonia caused by less common organisms such as enteric Gram-negative bacilli , Pseudomonas aeruginosa or Staphylococcus aureus . 10 These organisms may have differing profiles of antibiotic susceptibility and are not discussed in detail in this article . Patients from tropical regions warrant assessment for Burkholderia pseudomallei infection and treated accordingly .
S . pneumoniae remains the most commonly isolated organism in CAP , as well as being the foremost organism implicated in severe CAP necessitating hospital or ICU admission . 11 Australian data have repeatedly found a bimodal distribution of invasive pneumococcal disease , with the highest incidence in the young ( aged under two ) and the elderly ( aged over 85 ), and an approximately fourfold greater risk in Indigenous Australians compared to non-Indigenous . 12 The mortality rate of bacteraemic streptococcal pneumonia has been previously reported to be up to 26 %, with differing risk factors predisposing
13 , 14 patients to early and late mortality .
In contrast , M . pneumoniae is more frequently seen in younger adult patients , presenting with flu-like symptoms preceding a dry cough and bilateral infiltrates on chest radiography . It is notable for being associated with various extrapulmonary manifestations such as autoimmune haemolytic anaemia , erythema nodosum / multiforme , acute glomerulonephritis , and rare but potentially life-threatening encephalitis . 15