Respiratory Viruses

The major respiratory viruses are best organised by clinical syndrome rather than taxonomic family, because syndrome is what guides bedside reasoning. Human metapneumovirus is introduced here in full for the first time.

Genome. Negative-sense ssRNA. Envelope. Present. A member of the same broader family grouping as RSV and parainfluenza, sharing substantial clinical overlap with RSV in particular.

Transmission. Respiratory droplets and direct contact.

Tropism. Upper and lower respiratory tract epithelium, closely paralleling RSV's tropism pattern.

Major disease. A clinical picture essentially indistinguishable from RSV at the bedside in most cases: bronchiolitis and pneumonia in young children, and increasingly recognised as a significant cause of lower respiratory tract infection in elderly and immunocompromised patients. Generally considered somewhat less severe on average than RSV, though this distinction is not absolute, and specific testing is required to reliably differentiate the two given their clinical overlap.

Diagnosis. PCR respiratory panel testing, since clinical distinction from RSV is unreliable without specific virological confirmation.

A virus's specific preference for upper versus lower respiratory epithelium, itself generally reflecting receptor distribution and, for some viruses, a specific temperature preference, rhinovirus's preference for the cooler nasal mucosa over the warmer lower airway being the clearest example, directly predicts its characteristic clinical syndrome. This is the single most useful organising concept for this entire syndrome group.

Upper-tract-predominant viruses, rhinovirus and parainfluenza's specific laryngeal preference, produce syndromes centred on the nose, throat, and larynx: the common cold and croup respectively.

Lower-tract-capable viruses, RSV, human metapneumovirus, influenza, and, in severe presentations, adenovirus and coronaviruses, produce bronchiolitis and pneumonia.

Disease severity in this group generally correlates with the degree of alveolar and bronchiolar involvement and the resulting impairment of gas exchange. This is distinct from the predominantly symptomatic, non-life-threatening discomfort typical of pure upper-tract infection.

This tropism-based distinction is directly actionable at the bedside.

A young infant presenting with wheeze and respiratory distress, a lower-tract pattern, prompts specific consideration of RSV, human metapneumovirus, or, less commonly, parainfluenza or adenovirus with lower-tract extension.

A child presenting with barking cough and stridor, croup, specifically prompts consideration of parainfluenza given its laryngeal tropism.

Both are viral respiratory presentations in a similar young age group, yet the diagnostic reasoning differs entirely based on which tract is involved.

In infants and young children, RSV and human metapneumovirus (bronchiolitis), parainfluenza (croup), adenovirus (a broad range of presentations including pharyngoconjunctival fever), and rhinovirus (common cold, though also a recognised trigger of wheeze in this age group) dominate. Influenza and coronaviruses also affect this age group but are not specifically age-concentrated in the way the others listed here are.

In otherwise healthy older children and adults, rhinovirus (common cold) and influenza (acute febrile illness) dominate, with the endemic coronaviruses contributing a meaningful minority of common cold presentations across this age range as well.

In elderly and comorbid adults, influenza, RSV, and human metapneumovirus are all increasingly recognised as causes of significant morbidity and mortality.

This pattern is of growing clinical and public health emphasis, relevant to RSV vaccination programmes now being introduced specifically for older adults, alongside the long-established influenza vaccination recommendation for this age group. It reflects age-related decline in both innate and adaptive immune function generally.

Multiplexed PCR respiratory panels are now the diagnostic mainstay in most healthcare settings where clinical distinction between these viruses would otherwise be unreliable given their substantially overlapping symptom patterns.

These panels can simultaneously test a single nasopharyngeal specimen for a broad range of these viruses and, in many panels, several atypical bacterial respiratory pathogens as well.

Rapid antigen testing remains widely used for point-of-care triage specifically for influenza and RSV, and, since its emergence, SARS-CoV-2.

It offers faster but generally lower-sensitivity results compared to PCR, an accepted trade-off given the clinical value of rapid results for isolation and cohorting decisions in the immediate care setting.

Clinical diagnosis alone remains entirely adequate and appropriate for most mild, self-limited presentations in otherwise healthy individuals. A typical common cold does not require virological confirmation.

Specific testing is reserved for situations where the result will genuinely change management:

• severe disease requiring hospitalisation
• immunocompromised patients
• outbreak and infection control settings
• any presentation where a specific antiviral treatment decision depends on identifying the causative organism, most directly relevant to influenza given the availability of neuraminidase inhibitor therapy