Atypical pneumonia

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

The older contrast between 'typical' lobar pneumonia and an 'atypical' dry-cough syndrome is unsafe. Legionella can produce focal consolidation, pneumococcus can cause systemic symptoms and Mycoplasma can be mild or severe. The useful distinction is whether exposure, severity or epidemiology makes an intracellular pathogen sufficiently likely to change testing, treatment or public-health action.

Microbiological choice matters. Urinary antigen is rapid but narrow; respiratory molecular testing can identify a wider range of Legionella and Mycoplasma, while paired serology is often retrospective. Exposure histories should be precise enough for health-protection teams to investigate a water system, accommodation, workplace or animal source.

Key points

Atypical pneumonia commonly refers to Legionella species, Mycoplasma pneumoniae, Chlamydia psittaci and Coxiella burnetii; clinical features overlap substantially with ordinary bacterial and viral pneumonia.
Ask about hotel, hospital, spa or water-aerosol exposure for Legionella; birds for psittacosis; and sheep, cattle, goats or birthing products for Q fever.
Legionella may cause severe pneumonia with diarrhoea, confusion, hyponatraemia, renal or liver disturbance, but none of these features is sufficiently specific to diagnose it.
Legionella urinary antigen mainly detects L. pneumophila serogroup 1; a negative result does not exclude other serogroups or species, so obtain respiratory PCR/culture in severe or epidemiologically important disease.
Beta-lactams do not treat intracellular organisms. Moderate or high-severity CAP with suspected atypical infection generally needs macrolide-containing therapy through NICE/local guidance.
Doxycycline is useful for Mycoplasma, psittacosis and Q fever in appropriate adults; pregnancy, age, interactions and severe disease require specialist selection.
Legionnaires' disease is notifiable in England and Wales; identify the relevant UK jurisdiction and contact health protection promptly for suspected cases and clusters.
Do not allow an 'atypical' label to obscure severity assessment, oxygen targets, sepsis treatment, pleural complications or an alternative diagnosis.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Mycoplasma and Chlamydia

These intracellular or cell-wall-deficient bacteria spread between people and may cause community outbreaks, particularly among younger adults or close-contact groups.

Legionella

Inhalation of contaminated water aerosols can cause severe multisystem pneumonia, with risk increased by age, smoking, chronic disease and immune suppression.

Zoonotic exposure

Bird contact may indicate psittacosis, while livestock and birth-product exposure can indicate Q fever; a precise environmental history directs public-health and microbiological action.

Host vulnerability

Older age, immune suppression and comorbidity increase severity, but no reliable symptom pattern separates these pathogens from ordinary bacterial or viral pneumonia.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Inhaled pathogen
Organisms reach the lower respiratory tract through droplets or contaminated aerosols and attach to or enter respiratory cells.
2
Intracellular replication
Many implicated pathogens replicate within host cells or lack usual cell-wall targets, shaping both diagnostic yield and antimicrobial susceptibility.
3
Interstitial and alveolar inflammation
Host immune responses produce patchy pneumonitis, consolidation and impaired oxygen transfer rather than a uniquely recognisable radiographic pattern.
4
Extrapulmonary effects
Systemic inflammatory or direct infectious effects may involve liver, kidneys, nervous system, skin or electrolytes, sometimes providing useful but non-specific clues.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Legionnaires' diseaseRed flag

High fever, cough and severe pneumonia after travel, hotel, healthcare, spa or aerosolised-water exposure, sometimes with diarrhoea, confusion, hyponatraemia or renal and hepatic disturbance.

Mycoplasma pneumonia

Often a gradual febrile respiratory illness in younger people or closed-setting outbreaks, with persistent cough and possible rash, haemolysis or neurological complications.

PsittacosisRed flag

Flu-like illness, headache and pneumonia after handling parrots, poultry, pigeons or contaminated bird material; severe respiratory, hepatic, cardiac or neurological disease can occur.

Q fever

Prolonged fever, hepatitis or pneumonia after exposure to sheep, cattle, goats or birthing products; later culture-negative endocarditis risk matters in people with valvular or vascular disease.

Severe undifferentiated CAPRed flag

Shock, confusion, hypoxaemia or multilobar disease should be treated for severe CAP including credible atypical pathogens before a precise organism is known.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
Exposure and travel historyFirst step
Why
Direct organism-specific testing and identify a reportable exposure or outbreak.
Interpretation and limitations
Record accommodation and dates, water aerosols, healthcare exposure, bird species/contact and farm or parturient-animal exposure; vague 'recent travel' is insufficient for public-health investigation.
02
Legionella urinary antigen
Why
Rapidly detect common L. pneumophila serogroup 1 infection in people with defined risk factors.
Interpretation and limitations
A positive result supports diagnosis; a negative test cannot exclude non-serogroup-1 L. pneumophila or other Legionella species and should not stop therapy in severe high-suspicion disease.
03
Respiratory PCR and culture
Why
Identify Legionella beyond urinary-antigen coverage and detect Mycoplasma or other respiratory pathogens.
Interpretation and limitations
Send sputum, tracheal aspirate or appropriate respiratory material early. Legionella culture enables typing and outbreak linkage; discuss specialist/reference testing with microbiology or UKHSA.
04
Baseline severity tests
Why
Assess organ involvement and safe treatment.
Interpretation and limitations
Use chest radiography, oxygenation, FBC, renal/liver profile, sodium, CRP and blood cultures according to severity; hyponatraemia is a clue, not a diagnostic test.
05
Targeted serology or reference testing
Why
Investigate psittacosis or Q fever when exposure and syndrome fit.
Interpretation and limitations
Discuss sample timing and interpretation with microbiology or UKHSA reference services; antibodies may confirm retrospectively and should not delay empirical treatment in severe disease.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Typical bacterial pneumonia

Lobar consolidation and purulent sputum can occur in either group; culture, antigen or molecular testing and epidemiology are more reliable than phenotype labels.

Viral pneumonitis

Influenza, COVID-19 and other viruses produce similar systemic symptoms and bilateral change; respiratory molecular testing helps distinguish them.

Pulmonary embolism

Pleuritic pain, fever and hypoxaemia overlap, but venous thromboembolism risk and probability-led imaging support embolism, while chronology and directed testing distinguish the competing explanation.

Non-infective pneumonitis

Medicine timing, antigen exposure, eosinophilia or autoimmune features with negative microbiology suggest drug, hypersensitivity or inflammatory lung disease.

Pulmonary tuberculosis

Chronic symptoms, epidemiological risk, upper-zone or cavitating disease and acid-fast microbiology favour tuberculosis and require immediate infection-control consideration.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01PresentationTreat severity before taxonomyFirst stepPneumonia with a possible atypical respiratory pathogen.

1Assess ABCDE, oxygen requirement, NEWS2 and CRB65/CURB65 where appropriate; manage shock or respiratory failure immediately and obtain chest imaging.
2Take a focused exposure history and send proportionate microbiology before antibiotics when this creates no harmful delay.
3Start the current NICE/local CAP regimen with intracellular activity when moderate/high severity or exposure makes an atypical pathogen credible.

02LegionellaTest, treat and notifySevere pneumonia or compatible water/travel exposure.

1Send urinary antigen plus a respiratory sample for Legionella PCR/culture in severe or epidemiologically important illness; retain isolates or samples for reference testing.
2Use a locally approved macrolide or respiratory fluoroquinolone regimen, with infection-specialist advice for severe disease, immunocompromise, pregnancy or treatment failure.
3Notify suspected disease through the applicable UK-nation health-protection route promptly and provide detailed travel and exposure information.

03ZoonoticPursue bird or livestock exposurePneumonia linked to birds, farms or animal birthing products.

1Discuss psittacosis or Q fever testing with microbiology/reference services and assess liver, cardiac, neurological and pregnancy implications.
2AlternativeUse doxycycline in an appropriate non-pregnant adult when the local specialist pathway selects it; choose a pregnancy-safe alternative with infection/obstetric advice.
3Engage health protection or occupational-health teams when a shared source, workplace exposure or cluster is possible.

04ReviewNarrow using organism and responseResults or 48-hour clinical trajectory are available.

1Stop unnecessary beta-lactam or atypical components when a reliable diagnosis and susceptibility pattern permit, but avoid premature narrowing from a limited negative test.
2Review QT interval, liver/renal function and drug interactions before continuing macrolide or fluoroquinolone therapy.
3Investigate empyema, abscess, myocarditis, haemolysis, encephalitis or endocarditis when the organism and symptoms indicate them.

Key medicines and prescribing safety3 treatments · regimens, roles and cautions

Provides intracellular activity for suspected atypical pathogens and is combined with beta-lactam therapy in moderate or high-severity CAP pathways.

Clarithromycin

A common adult CAP regimen is 500 mg orally or IV twice daily, usually within a 5-day uncomplicated CAP course; confirmed Legionella may require a specialist-defined longer course.

Check QTc, potassium, liver function and major CYP3A4 interactions. Avoid unsafe statin or antiarrhythmic combinations and use pregnancy-specific advice.

Treats Mycoplasma and is a key option for psittacosis or Q fever in suitable adults.

Doxycycline

A typical adult regimen begins with 200 mg on day 1 then 100 mg once daily, with dose and duration adjusted to organism, severity and current local guidance.

Avoid in pregnancy and usually in children under 12 unless specialist benefits outweigh risks. Separate from iron or antacids and warn about oesophagitis and photosensitivity.

Covers common extracellular CAP bacteria while adding intracellular activity before the organism is known.

Co-amoxiclav plus clarithromycin

For high-severity CAP, use the current NICE/local oral or IV co-amoxiclav regimen together with clarithromycin 500 mg twice daily, then review at 48 hours.

This is not a universal outpatient regimen. Check allergy, renal/liver function, QT and interactions; narrow promptly when microbiology and clinical response allow.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Respiratory failure

Extensive pneumonitis can cause severe ventilation-perfusion mismatch, acute lung injury and need for ventilatory support, creating an additional need for recognition and targeted treatment.

Sepsis and organ dysfunction

A severe systemic response may cause shock, kidney injury, encephalopathy and other organ failure, particularly with Legionella.

Neurological complications

Encephalitis, neuropathy or post-infectious immune phenomena can complicate selected pathogens and may dominate the clinical picture.

Public-health transmission

Unrecognised shared environmental or zoonotic exposure can generate further cases unless the relevant incident is identified and notified.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Track oxygen requirement, haemodynamics, mental state, renal/liver function and sodium in severe Legionella or multisystem disease.
Review ECG/QTc and interacting medicines when macrolides or fluoroquinolones are used, especially with electrolyte disturbance.
At 48 hours, review microbiology, public-health information and clinical response; a negative urinary antigen alone must not overrule strong suspicion.
Follow complicated psittacosis for cardiac, hepatic or neurological involvement and Q fever for persistent symptoms or endocarditis risk.
Document that notification and exposure details reached the correct health-protection team when Legionella or a cluster is suspected.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Atypical is not a bedside diagnosis

Dry cough, modest chest signs or extrapulmonary symptoms change probability but do not reliably identify an intracellular pathogen.

Urinary antigen has a blind spot

It is fast and useful for L. pneumophila serogroup 1, but broader respiratory testing is needed for other serogroups and species.

Exposure determines the next test

A dated hotel stay, hot-tub exposure, sick bird or lambing contact is more useful than an unstructured list of symptoms.

Beta-lactams need a partner

They cover usual CAP bacteria but have no meaningful intracellular activity, so suspected atypical infection changes the regimen.

Public health is clinical care

Early Legionella notification can identify a hazardous water source and prevent additional cases; waiting for perfect confirmation loses time.

11Common pitfallsFrequent interpretation and management errors.

01
Diagnosing Legionella solely from hyponatraemia or diarrhoea.
02
Excluding Legionella because the urinary antigen is negative.
03
Using beta-lactam monotherapy when severe disease or exposure strongly suggests an intracellular pathogen.
04
Forgetting pregnancy, QT and interaction checks before doxycycline or macrolide therapy.
05
Delaying notifiable-disease communication until culture confirmation.