28th July 2023

Respiratory Tract Infections

Respiratory tract infections are a major cause of morbidity and mortality worldwide. The exact microbiological aetiology is often not identified when treating patients and may include both viral and bacterial pathogens. Infection can manifest at all parts of the respiratory tract, from the nose down to the lungs. Most hospitalisation in adults is due to pneumonia (infection within the lung itself) or exacerbations of obstructive airway disease such as asthma or COPD. These will therefore be the focus of this topic.

The British Thoracic Society (BTS) provides clear guidance for managing pneumonia, which is used to inform NICE guidance. The key principles are as follows:

• Assess severity
• Conduct microbiological testing to identify the pathogen
• Initiate treatment with empirical antibiotics when bacterial pneumonia is suspected, to be narrowed if a specific pathogen is identified

Assessment and diagnosis

Severity is assessed using the CURB-65 score, with 1 point given for each of the following:

• C = confusion
• U = urea > 7 mmol/L
• R = RR ≥ 30/min
• B = BP < 90 mmHg or ≤ 60 mmHg
• 65 = age more than 65

0-1 = outpatient management, 2 = admission, 3+ = admission and consider ITU escalation.

Investigations include routine bloods such as FBC, U&Es, LTT, and CRP; blood cultures; sputum cultures, throat swab for respiratory viruses; urinary antigens; chest x-ray.

Diagnosis is made on the basis of characteristic clinical features with evidence of consolidation in the lung (i.e. auscultation findings of ‘crackles’ or radiological confirmation on chest x-ray.

The most common pathogen causing respiratory illness is Streptococcus pneumoniae. Other common causes of community acquired infection include Haemophilus influenzae and Moraxella catarrhalis, which are particularly associated with patients with COPD.

Different pathogens are implicated in different age groups, particularly paediatric respiratory tract infections, which are associated with numerous causative organisms that rarely affect adults. Children often present with upper respiratory tract infection, caused by either viruses or bacteria. This is not the focus of this topic, but a comprehensive summary can be found in the References section.

Management

The first line treatment in the community is monotherapy with amoxicillin (erythromycin if penicillin allergic). If the patient is suspected of having an infective COPD exacerbation, then doxycycline is generally the preferred first line. COPD patients should also be supported with a 5-7 day oral steroids course and may require acute nebulisers.

For those requiring hospital admission, the first line is a broad-spectrum antibiotic, such co amoxiclav or ceftriaxone, with clarithromycin generally also given as cover for atypical infections, with doxycycline or levofloxacin as alternatives.

Exact guidance varies per area, so it is important to consult local guidelines. The treatment should be appropriately tailored if sensitivities indicate a more appropriate antibiotic.

Atypical pneumonia

There are some important so-called ‘atypical’ causes of pneumonia. These may cause a different clinical syndrome or not respond to initial antibiotic treatment. These include:

• Mycoplasma pneumoniae characteristically causes bilateral pneumonia and is associated with neurological and dermatological complications.
• Legionella pneumophila, a spirochete found in water coolant systems such as air conditioners, is often associated with travel to hot countries. Legionnaires’ disease can cause transaminitis, hyponatremia and neurological features. It is difficult to diagnose as it does not grow on standard cultures; if suspected, urinary antigens should be tested as these are the most reliable investigation.
• Chlamydophila psittaci, a rare pathogen transmitted from close contact with birds, typically causing myalgia, headache and a dry cough.