H5N1 avian influenza is a zoonotic infection caused by highly pathogenic avian influenza A(H5N1) viruses. The currently circulating clade 2.3.4.4b has caused 70 U.S. human cases (March 2024–May 2025), predominantly through occupational animal exposure, with most cases presenting as mild conjunctivitis-predominant illness. [1-2] However, historic H5N1 clades have carried case fatality rates of approximately 50%, and severe pneumonia with ARDS remains possible. [2-3]
1. History
- Exposure history is paramount: Ask about contact with sick/dead poultry, dairy cattle, backyard flocks, live bird markets, raw milk, or wild birds within the prior 2–8 days [2][4]
- Occupational details: poultry depopulation activities, dairy farm work, milking, handling raw milk, defeathering [2][5]
- PPE use: gloves, eye protection, respirators/masks [2]
- Symptom characterization: onset of eye redness/irritation (most common, 89%), fever (46%), cough, sore throat, rhinorrhea, dyspnea, myalgias, headache [1-2]
- GI symptoms: diarrhea, vomiting, abdominal pain — more common in severe/historic cases [6-7]
- Timing: incubation period typically 2–5 days (up to 8 days); median symptom duration in mild clade 2.3.4.4b cases is 4 days (range 1–8) [2][7]
- Important negatives: travel to endemic regions, contact with other ill persons, laboratory exposure to H5N1 specimens [7]
2. Alarm Features
- Rapidly progressive dyspnea or pneumonia — hallmark of severe H5N1 disease [6][8]
- Bilateral pulmonary infiltrates on CXR with rapid progression to ARDS [6][9]
- Diarrhea and mucosal bleeding at presentation — associated with fatal outcomes [8]
- Altered mental status, seizures, or coma — CNS involvement reported [3][6]
- Hypoxia, tachypnea, hemodynamic instability
- Combined neutropenia + elevated ALT at admission — predicted 91% of deaths in one Vietnamese cohort [8]
- Thrombocytopenia, markedly elevated LDH, cardiac failure [9]
- Delayed presentation (>3 days from symptom onset to hospitalization) — strongly associated with mortality [10-11]
3. Medications
- Oseltamivir is the mainstay of treatment — start empirically without waiting for confirmatory testing [12-13]
- Standard dose: 75 mg PO BID × 5 days (adults/adolescents ≥13 years) [14]
- Severe/pneumonic disease: consider 150 mg PO BID × 7–10 days [4][12]
- Pediatric dosing: weight-based (30–75 mg BID) [12][14]
- Renal dose adjustment required [14]
- Zanamivir (inhaled): active against oseltamivir-resistant H5N1 variants (H274Y mutation), but suboptimal delivery in pneumonic disease [4]
- Peramivir (IV): alternative neuraminidase inhibitor [15-16]
- Baloxavir marboxil: cap-dependent endonuclease inhibitor; FDA-approved for seasonal influenza; susceptibility of H5N1 strains reported [15-16]
- Post-exposure prophylaxis: oseltamivir 75 mg daily × 7–10 days, initiated within 48 hours of exposure [14][16]
- Corticosteroids are associated with increased mortality — avoid unless otherwise indicated [8]
- Oseltamivir reduces mortality by ~49% even when started up to 6–8 days after symptom onset [17]
4. Diet
- Avoid consumption of raw or undercooked poultry, eggs, or unpasteurized dairy products (raw milk) — implicated as exposure routes [4-5]
- Adequate hydration, especially with fever, GI losses, or respiratory distress
- No specific long-term dietary modifications required post-recovery
5. Review of Systems
- Eyes: redness, tearing, discharge, photophobia (conjunctivitis in 89–93% of recent U.S. cases) [1-2]
- Respiratory: cough, sore throat, rhinorrhea, dyspnea, chest tightness
- Constitutional: fever/chills, malaise, myalgias, headache (fever in 46–60%; myalgias more common in poultry workers) [2]
- GI: diarrhea, vomiting, abdominal pain — may precede respiratory symptoms in severe cases [6-7]
- Neurologic: headache, confusion, seizures, altered consciousness [3][6]
- Hematologic: easy bruising, mucosal bleeding (associated with fatal cases) [8]
6. Collateral History and Family History
- Identify all household and occupational contacts — monitor for 10 days post-exposure [2]
- Determine if other workers on the same farm are symptomatic
- Family clusters have been reported (>90% among blood-related family members), raising the possibility of genetic susceptibility or shared exposure [4]
- No sustained human-to-human transmission has been documented with clade 2.3.4.4b [1-2]
- Social context: immigration status, language barriers, and access to healthcare may delay presentation among agricultural workers [2]
7. Risk Factors
- Occupational exposure: dairy farm workers, poultry workers (especially during depopulation), slaughterhouse workers [1-2]
- Direct contact with sick/dead poultry or their blood/secretions [4-5]
- Exposure to infected dairy cattle or raw milk [2-3]
- Visiting live bird markets [4]
- Inadequate PPE use — only 36% of workers reported using both eye protection and respirators [2]
- Swimming/bathing in potentially contaminated water [5]
- Contact with infected domestic cats or dogs (theoretical) [4]
- Backyard poultry exposure (2 U.S. cases) and unidentified exposure sources (3 U.S. cases) [1]
8. Differential Diagnosis
- Seasonal influenza A/B — most common mimic; cannot be distinguished clinically; requires subtyping [18]
- COVID-19 — overlapping respiratory symptoms; multiplex testing recommended [18]
- RSV and other viral URIs — especially in children
- Bacterial pneumonia — if lower respiratory tract involvement predominates
- Adenoviral conjunctivitis — if conjunctivitis is the sole presenting symptom
- Other novel influenza A viruses (H7N9, H9N2, variant swine influenza) [18]
- Legionella, Mycoplasma, or other atypical pneumonia — in cases with rapidly progressive bilateral infiltrates
- MERS-CoV or other emerging respiratory pathogens — in travelers from endemic regions
Distinguishing feature: Conjunctivitis as the predominant symptom with occupational animal exposure is highly suggestive of clade 2.3.4.4b H5N1. [2]
9. Past Medical History
- Most U.S. clade 2.3.4.4b cases (76%) had no underlying medical conditions [2]
- Historically, severe H5N1 disease has occurred in previously healthy young adults and children [10]
- Immunocompromised patients may have prolonged viral shedding and atypical presentations
- Pre-existing cardiac or pulmonary disease may increase risk of complications [18]
- Prior influenza vaccination status (seasonal vaccines do not protect against H5N1, but Audenz [H5N1-specific] is FDA-approved for high-risk individuals) [16]
10. Physical Exam
- Conjunctivitis: unilateral or bilateral conjunctival injection, tearing, discharge — present in ~89% of recent U.S. cases [1]
- Vital signs: fever (>38°C in 46–75%), tachypnea, hypoxia in severe cases [7-8]
- Respiratory: pharyngeal inflammation without exudates, rales/crackles on auscultation, signs of consolidation [6][18]
- Abdominal: tenderness (if GI involvement)
- Neurologic: altered mental status, meningismus (rare but reported) [6]
- Skin: rash is uncommon [18]
- Concerning findings: accessory muscle use, cyanosis, hemodynamic instability, altered sensorium
11. Lab Studies
- CBC: lymphopenia (73%), neutropenia, thrombocytopenia — all associated with severity [7-8]
- CMP/LFTs: elevated AST (69%) and ALT (61%); combined neutropenia + elevated ALT predicts fatal outcome [8]
- LDH: markedly elevated in fatal cases (median peak 1982 U/L vs 1230 U/L in survivors) [9]
- Coagulation: prolonged PT/aPTT, elevated D-dimer [7]
- CK: elevated creatine phosphokinase (myositis) [7]
- Albumin: hypoalbuminemia [7]
- Renal function: creatinine elevation in 17% (renal dysfunction) [9]
- Blood gas: assess for hypoxemia and respiratory failure
- Procalcitonin: may help distinguish from bacterial superinfection
12. Imaging
- Chest X-ray (first-line): bilateral pulmonary infiltrates (72%), lobar collapse, focal consolidation, air bronchograms; rapid progression from unilateral to bilateral involvement [6][8]
- CT chest: diffuse ground-glass opacities, consolidation consistent with ARDS in severe cases [7]
- Imaging may be normal in mild/conjunctivitis-only presentations (majority of recent U.S. cases) [2]
- Rapid radiographic progression (within days) is a hallmark of severe H5N1 pneumonia [6]
- Imaging is unnecessary in mild conjunctivitis-only cases without respiratory symptoms
13. Special Tests
- RT-PCR (gold standard): CDC Human Influenza Virus Real-Time RT-PCR Diagnostic Panel with Influenza A(H5) Subtyping Kit — results in 4–6 hours [2][4]
- Specimen collection: collect both conjunctival swabs AND nasopharyngeal swabs — conjunctival swabs were positive in 88% of cases vs. only 21–58% for NP swabs [2]
- Rapid antigen tests have poor sensitivity (0–36%) for H5N1 — a negative rapid test does NOT rule out H5N1 [4][6]
- Standard influenza A tests cannot distinguish H5 subtype — if influenza A positive but seasonal subtypes (H1, H3) negative, immediately notify public health for H5 subtyping [19]
- Throat swabs may have higher yield than nasal swabs for H5N1 (unlike seasonal influenza) [4]
- Serology (anti-H5 antibodies): seroconversion at 2–3 weeks; useful for retrospective confirmation and epidemiologic studies only [4][6]
The following figure from the 2006 Turkey outbreak illustrates the diagnostic pathway and the critical limitation of rapid tests, which were uniformly negative despite confirmed H5N1 cases by RT-PCR:
14. ECG
- ECG indicated if myocarditis or cardiac failure suspected — cardiac failure occurred in 50% of severe Chinese cases [9]
- Look for: sinus tachycardia, ST-segment changes, arrhythmias, low voltage (pericardial effusion)
- Troponin and BNP should accompany ECG if cardiac involvement is suspected
15. Assessment
Severity stratification is critical and depends heavily on the viral clade:
- Clade 2.3.4.4b (current U.S. circulating strain): predominantly mild illness — conjunctivitis ± fever ± URI symptoms; median duration 4 days; estimated case-fatality risk 0.7% (95% CI 0.02–3.9%) after adjusting for under-reporting [1][21]
- Historic clades (pre-2.3.4.4b): severe multiorgan disease with ~50% case fatality; rapid progression to ARDS, multiorgan failure, and death [2][10]
- D1.1 genotype: critically ill teenager (British Columbia) and severely ill adult (Louisiana) serve as reminders that severe disease remains possible [2]
Complications to consider: ARDS, secondary bacterial pneumonia, myocarditis/cardiac failure, encephalopathy, renal dysfunction, DIC, multiorgan failure. [3][6][9]
16. Treatment Plan
Initial stabilization (severe cases)
- Airway management, supplemental O₂, mechanical ventilation for ARDS
- Droplet + contact + airborne precautions (N95 respirator, gown, gloves, eye protection)
Antiviral therapy — initiate immediately upon clinical suspicion, do not wait for confirmatory testing: [12]
- Mild disease: Oseltamivir 75 mg PO BID × 5 days [14]
- Severe/pneumonic disease: Oseltamivir 150 mg PO BID × 7–10 days; consider combination with IV peramivir or inhaled zanamivir if oseltamivir resistance suspected [4][12]
- Benefit persists even when started up to 6–8 days after symptom onset [17]
Supportive care
- IV fluids, antipyretics, electrolyte correction
- Avoid corticosteroids (associated with increased mortality) [8]
- Lung-protective ventilation strategies for ARDS
Infection control
- Immediate notification of local/state health department and CDC [2][18]
- Isolate patient; enhanced PPE for all healthcare workers
- Monitor all close contacts for 10 days [2]
17. Disposition
Admission criteria
- Pneumonia, hypoxia, or respiratory distress
- Hemodynamic instability
- Altered mental status or neurologic symptoms
- Significant lab derangements (lymphopenia, elevated transaminases, thrombocytopenia, elevated LDH)
- Inability to tolerate oral medications
- ICU admission for ARDS, multiorgan failure, or shock
Discharge criteria (mild cases)
- Conjunctivitis-only or mild URI symptoms without hypoxia
- Able to tolerate oral oseltamivir
- Reliable follow-up and contact monitoring in place
- Public health notification completed
Specialist consultation triggers
- Infectious disease consultation for all confirmed/suspected H5N1 cases
- Pulmonology/critical care for pneumonia or ARDS
- Ophthalmology if severe or persistent conjunctivitis
- Public health authorities must be notified for every suspected case [2][18]
18. Follow Up / Return Precautions
- Follow-up within 24–48 hours for all discharged patients; daily symptom monitoring until resolution
- All household contacts monitored for 10 days post-last exposure [2]
- Return immediately for: worsening dyspnea, persistent/worsening fever, chest pain, hemoptysis, confusion, visual changes, inability to keep fluids down
- Expected recovery for mild clade 2.3.4.4b cases: 1–8 days (median 4 days) [2]
- Counsel on strict hand hygiene, avoid contact with poultry/livestock until cleared
- Emphasize that negative rapid antigen tests do not exclude H5N1 — return if symptoms worsen despite a negative rapid test [4]
- Occupational health follow-up for return-to-work clearance, especially for farm workers
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