Flu Burung

01Pathophysiology

Avian influenza A (H5N1) is one of the most virulent influenza subtypes affecting humans. The virus belongs to the Orthomyxoviridae family and is classified by two surface glycoproteins: hemagglutinin (HA) and neuraminidase (NA). There are 16 known HA subtypes and 9 NA subtypes circulating in birds. Of these, the H5 and H7 subtypes carry the potential for high pathogenicity. Subtypes confirmed to cause human infection include H5N1, H7N9, H5N6, H10N8, H9N2, H7N7, H7N2, and H7N3, but H5N1 stands out for its strong infectivity and high case fatality rate.

The key to understanding why H5N1 causes such severe disease lies in receptor tropism and cytokine dysregulation. Human seasonal influenza viruses preferentially bind to sialic acid alpha-2,6-galactose receptors found in the upper respiratory tract. H5N1, by contrast, preferentially binds to sialic acid alpha-2,3-galactose receptors, which are concentrated on type II pneumocytes and alveolar macrophages in the lower respiratory tract. This explains two critical exam-testable facts. First, H5N1 causes severe lower respiratory tract disease (pneumonia and ARDS) rather than typical upper respiratory symptoms. Second, throat swabs and lower respiratory samples offer more sensitive diagnostic yields than nasal swabs, which is the opposite of seasonal influenza.

Once the virus infects alveolar epithelial cells and macrophages, it triggers a profound pro-inflammatory cytokine response commonly termed the "cytokine storm." This dysregulated immune response involves excessive levels of TNF-alpha, IL-6, IL-8, MCP-1, and other mediators, leading to diffuse alveolar damage, capillary leak, and multi-organ failure. Apoptosis of alveolar epithelial cells destroys the respiratory epithelium and is considered a primary mechanism of H5N1 pneumonia. Additionally, apoptosis of leukocytes in the lungs, coupled with systemic cytokine-mediated destruction of lymphocytes in lymphoid organs, produces the characteristic leukopenia and lymphopenia observed on laboratory testing.

H5N1 infection is a systemic process, not purely a respiratory one. Autopsy studies have demonstrated viral RNA in the lungs, brain, intestines, kidneys, and liver. This explains why patients develop gastrointestinal symptoms (diarrhea, vomiting, abdominal pain), encephalopathy, elevated transaminases, and renal failure in addition to respiratory distress.

The reason sustained human-to-human transmission has not yet occurred is that the alpha-2,3 receptors needed for H5N1 binding are deep in the lower airways, making aerosolization and efficient person-to-person spread far less likely than with seasonal influenza. The virus does not currently transmit easily between humans, and sustained human-to-human transmission has not been documented. This is a central point tested in public health questions on the topic.

02Classification and Clinical Manifestation

Classification of Avian Influenza Viruses Relevant to Human Disease

Clinical Spectrum of H5N1 in Humans

Key Clinical Features by Organ System

03Diagnostic Workup

The diagnostic approach begins with clinical suspicion. The most critical step is the exposure history. Clinicians should consider H5N1 in any patient with acute respiratory illness or conjunctivitis and a relevant epidemiologic exposure, including work on poultry or dairy farms with confirmed or suspected infection. Without this exposure link, the diagnosis is nearly impossible to make on clinical grounds alone, because the initial symptoms mimic ordinary influenza or community-acquired pneumonia.

When clinical suspicion is raised, the best initial test in most clinical settings is a standard influenza A RT-PCR targeting the conserved matrix (M) gene. This will confirm whether the patient has influenza A at all. The critical clue is when the specimen tests positive for influenza A but cannot be subtyped to H1 or H3 by routine assays. This "unsubtypeable influenza A" result is the laboratory red flag that mandates immediate referral of the specimen to a public health reference laboratory for H5-subtype-targeted RT-PCR.

For patients with conjunctivitis and acute respiratory illness, conjunctival, nasopharyngeal, and combination nasal-oropharyngeal swabs should be collected. For patients with only conjunctivitis, conjunctival and nasopharyngeal swabs are recommended. Recall that throat swabs are preferred over nasal swabs for H5N1 because viral titers are higher in the lower respiratory tract.

A confirmed case requires laboratory evidence of H5N1 by viral isolation, by RT-PCR from respiratory specimens, or by a four-fold or greater rise in H5N1-specific antibody titer between paired sera.

A crucial testing pearl: rapid antigen tests (RIDTs) have poor sensitivity for H5N1 and should never be used to rule out the diagnosis. Confirmatory diagnosis requires sophisticated laboratory support, and commercial rapid antigen tests are insensitive. A negative RIDT in a patient with exposure history and compatible symptoms should lead to immediate RT-PCR testing and empiric antiviral treatment.

On imaging, findings progress rapidly. Early CXR may show unilateral focal consolidation or patchy infiltrates, but within days the disease typically evolves into bilateral diffuse opacities with air bronchograms and ground-glass changes, reflecting ARDS. CT chest may show bilateral ground-glass opacities, consolidation, and air bronchograms.

04Management and Treatment

Treatment with oseltamivir should begin as soon as possible. An observational registry study showed that oseltamivir initiated within 2 days of symptom onset in patients without respiratory failure was significantly associated with survival. The principle is: treat empirically before confirmation. Antiviral treatment should never be delayed while awaiting laboratory results.

For outpatients with suspected H5N1, the standard dose of oseltamivir for adolescents and adults is 75 mg twice daily for 5 days. Pediatric dosing is weight-based per standard guidelines.

For hospitalized patients, particularly those with severe pneumonia or respiratory failure, three modifications apply. First, treatment duration may be extended to 10 days for severely ill hospitalized patients. Second, combination antiviral therapy with oseltamivir plus baloxavir can be considered because H5N1 strains may develop resistance to neuraminidase inhibitors when used alone. Third, in patients who cannot tolerate oral medication, IV peramivir at 600 mg once daily for a minimum of 5 days is recommended, which differs from the single-dose regimen used for uncomplicated seasonal influenza.

Most H5N1, H5N6, and H7N9 viruses are susceptible to neuraminidase inhibitors (oseltamivir, peramivir, zanamivir) and baloxavir, but are often resistant to adamantanes (amantadine and rimantadine). This means adamantanes are NOT recommended under any circumstance.

For post-exposure prophylaxis (PEP), the dosing is notably different from standard seasonal flu PEP. Oseltamivir PEP is given at treatment-frequency dosing (twice daily), not the once-daily prophylaxis regimen used for seasonal influenza. Duration is 5 days for time-limited exposures or 10 days if exposure is ongoing.

Supportive care for severe cases includes ICU admission, mechanical ventilation with lung-protective strategies for ARDS ($V_{T}V\_T$ 6 mL/kg predicted body weight, plateau pressure <30 cmH2O), prone positioning, ECMO when available, and management of multi-organ failure. Corticosteroids are generally avoided or used with caution, as they have not been shown to improve outcomes in H5N1 and may prolong viral shedding.

Infection control is an important management consideration: patients should be placed on airborne precautions (negative pressure room, N95 respirator) plus contact precautions, in contrast to standard seasonal influenza which requires only droplet precautions.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common trap with avian influenza questions is failing to connect the exposure history to the diagnosis. A vignette will describe a patient with severe, rapidly progressive bilateral pneumonia and lymphopenia. The clinical picture alone overlaps heavily with COVID-19, SARS, or severe bacterial pneumonia. The test-writer places the answer in the stem: a single sentence about the patient working on a poultry farm, visiting a live bird market, or handling sick chickens. If you skip over that exposure detail, you will pick the wrong diagnosis.

A second frequent trap involves the rapid antigen test result. A vignette may describe a patient with textbook H5N1 features whose rapid influenza test returns negative. The wrong answer is to stop the workup. The correct next step is to order RT-PCR and start empiric oseltamivir immediately. Remember: rapid antigen tests have poor sensitivity for H5N1, and a negative result does not exclude the diagnosis.

Third, students frequently confuse PEP dosing for H5N1 with PEP dosing for seasonal influenza. For seasonal flu, oseltamivir prophylaxis is once daily. For H5N1, PEP uses treatment-frequency dosing (twice daily). This distinction is a favorite tested detail.

Fourth, understand that adamantanes are always wrong for avian influenza. If amantadine or rimantadine appears as an answer choice for treatment of H5N1, it is a distractor.

Fifth, remember the specimen collection nuance: for H5N1, throat swabs and lower respiratory samples outperform nasal swabs. This is the opposite of seasonal influenza, where nasal specimens are standard.

Finally, the core competency being tested in avian influenza questions is your ability to integrate epidemiologic context (exposure to poultry, farm work, travel to endemic regions) with atypical clinical features (GI symptoms preceding respiratory failure, conjunctivitis, lymphopenia out of proportion to the rest of the CBC) and then execute the correct empiric management (oseltamivir without waiting for confirmatory results). The vignette tests whether you can recognize a rare but high-stakes diagnosis that hides behind an ordinary-looking respiratory illness.