Swine Flu (Influenza A H1N1): Clinical Features, Diagnosis, Treatment and Prevention
Clinical Medicine Year 3 • pandemic H1N1, current seasonal H1N1pdm09 illness and severity-based clinical care
What “swine flu” means today
Swine flu is the popular name used during the 2009 influenza A(H1N1) pandemic. That virus was a reassortant containing gene segments from human, avian and swine lineages. After the pandemic, A(H1N1)pdm09 became part of seasonal human influenza circulation. Separate swine-influenza viruses can infect people after direct occupational exposure to pigs and require public-health subtyping.
H1N1 illness may be mild, but it can also cause viral pneumonia, ARDS, myocarditis, encephalitis, secondary bacterial pneumonia, sepsis and death. The key clinical questions are severity, host risk and whether the illness could represent a novel animal-origin influenza infection.
Learning outcomes
- Explain influenza A structure, H1N1 nomenclature, antigenic drift and reassortment.
- Describe transmission, incubation, infectious period and high-risk groups.
- Recognise uncomplicated influenza, pneumonia, sepsis, ARDS and neurologic complications.
- Select molecular testing and understand the limits of rapid tests.
- Use oseltamivir, supportive care, isolation, vaccination and outbreak-control principles safely.
Virology and epidemiology
Influenza A is an enveloped, segmented, negative-sense RNA virus in the Orthomyxoviridae family. H1N1 identifies haemagglutinin 1 and neuraminidase 1. Haemagglutinin binds respiratory epithelial receptors and neuraminidase helps release progeny virions. Antigenic drift causes gradual seasonal change; reassortment can create a novel virus with limited population immunity.
The 2009 pandemic spread globally because many people had little pre-existing immunity. H1N1pdm09 is now included in seasonal influenza surveillance and vaccine composition, but severe illness remains possible when care is delayed or the patient has reduced cardiopulmonary/immune reserve.
Transmission, incubation and communicability
- Respiratory droplets and aerosols from coughing, sneezing, talking and clinical procedures.
- Contaminated hands and surfaces followed by touching the eyes, nose or mouth.
- Close household, school, workplace and healthcare contact.
- Rare occupational infection with swine influenza after direct pig exposure; unusual cases require notification and subtype testing.
Incubation is commonly 1–4 days, often about 2 days. Infectiousness may begin about one day before symptoms and last 5–7 days after onset, longer in young children, immunocompromised people and severe illness. Fever may be absent in older or immunosuppressed patients.
Pathophysiology
- Virus enters airway epithelial cells and replicates.
- Damage to ciliated epithelium impairs mucociliary clearance.
- Innate immune activation causes fever, malaise, myalgia and headache.
- Lower-airway infection can produce diffuse alveolar inflammation, hypoxaemia and ARDS.
- Damaged epithelium predisposes to pneumococcal or staphylococcal superinfection.
- Inflammation or direct viral invasion can affect the heart, brain, muscles and kidneys.
Clinical features
Uncomplicated influenza
- Fever, chills, malaise, headache and profound fatigue.
- Dry cough, sore throat, hoarseness and rhinorrhoea.
- Myalgia, arthralgia, backache, eye discomfort and photophobia.
- Nausea, vomiting or diarrhoea, particularly in children.
Complicated influenza
- Primary viral pneumonia with breathlessness, hypoxaemia and diffuse infiltrates.
- Secondary bacterial pneumonia with recurrent fever, purulent sputum, focal consolidation or sepsis.
- ARDS, shock, acute kidney injury, rhabdomyolysis and myocarditis.
- Encephalitis, encephalopathy, seizures or Guillain–Barré syndrome.
- Exacerbation of asthma, COPD, heart failure, diabetes or other chronic disease.
High-risk groups
| Group | Reason for increased risk |
|---|---|
| Pregnant/recently postpartum women | Respiratory and immune physiologic changes |
| Young children and older adults | Immature or reduced immune and cardiopulmonary reserve |
| Chronic lung/heart disease | Reduced reserve and decompensation risk |
| Diabetes, renal/liver/neurologic disease or obesity | Higher risk of organ complications |
| HIV, cancer, transplant or long-term steroids | Impaired immune control and prolonged shedding |
Red flags requiring urgent admission
- Shortness of breath, tachypnoea, chest pain, cyanosis or low oxygen saturation.
- Haemoptysis, persistent high fever, hypotension or sepsis.
- Confusion, drowsiness, seizures or inability to wake.
- Inability to drink, persistent vomiting, dehydration or markedly reduced urine.
- Recurrent fever or worsening cough after initial improvement.
- High-risk patient with rapidly progressive symptoms.
- In children: poor feeding, grunting, chest indrawing, convulsions or lethargy.
History and examination
| Domain | What to assess |
|---|---|
| Time course | Day of illness, sudden onset, improvement followed by deterioration |
| Exposure | Household outbreak, school/workplace, healthcare, travel and pig contact |
| Risk status | Pregnancy, age, chronic disease, HIV, immunosuppression and obesity |
| Respiratory status | Respiratory rate, work of breathing, SpO2, chest signs and ability to feed/speak |
| Systemic severity | Pulse, BP, capillary refill, hydration, mental state, glucose and urine |
| Complications | Focal chest signs, cardiac symptoms, muscle weakness, seizures or neurologic change |
Investigations
- RT-PCR/molecular respiratory panel: preferred for severe, admitted, unusual or outbreak-associated cases and for subtype information.
- Rapid antigen tests are less sensitive; a negative result does not exclude influenza in severe illness.
- Chest radiograph or ultrasound for pneumonia, oedema, effusion or ARDS.
- Full blood count, electrolytes, renal/liver tests, glucose, CRP and lactate as indicated.
- Blood/sputum cultures when bacterial superinfection or sepsis is suspected.
- ECG/troponin for myocarditis, CK for rhabdomyolysis and blood gas for respiratory failure.
- Test for malaria and other local causes of fever where appropriate; coinfection is possible.
Differential diagnosis
| Condition | Key issue |
|---|---|
| Seasonal influenza A/B | Often clinically indistinguishable; molecular testing identifies the virus. |
| COVID-19/other respiratory viruses | Overlap in symptoms and pneumonia; use local testing. |
| Bacterial pneumonia | Focal consolidation, purulent sputum, recurrent fever or sepsis. |
| Malaria | Fever, headache, anaemia, thrombocytopenia or altered mental state. |
| Typhoid, dengue and other tropical infections | Consider exposure, rash, GI disease and laboratory patterns. |
Management
Antiviral treatment
Oseltamivir
A common adult treatment dose with normal renal function is 75 mg orally twice daily for 5 days. Start promptly in severe disease, hospitalisation, pregnancy and high-risk patients; adjust for renal function, age and local guidance.
Timing
Benefit is greatest within 48 hours, but treatment can benefit severe, progressive, hospitalised or high-risk patients even when started later. Do not wait for PCR when treatment is clinically indicated.
Novel exposure
Unusual illness after pig exposure requires public-health notification, subtype testing and specialist advice about resistance and duration.
Outpatient care
- Rest, oral fluids, nutrition and safe-dose paracetamol.
- Stay home, improve ventilation and wear a mask around others while infectious.
- Give clear return precautions for breathlessness, chest pain, confusion, dehydration, persistent fever or deterioration.
- Review high-risk patients early.
Hospital and critical care
- Give oxygen and monitor respiratory effort, gas exchange and mental state.
- Use lung-protective ventilation for ARDS and avoid fluid overload.
- Investigate/treat bacterial coinfection when clinically supported; antibiotics are not routine for uncomplicated influenza.
- Manage shock, myocarditis, kidney injury, encephalitis and rhabdomyolysis.
- Use appropriate respiratory precautions; aerosol-generating procedures require higher-level PPE and ventilation controls.
- Consider prolonged antiviral therapy with specialist advice in immunocompromised patients.
Pregnancy and children
Pregnant patients should be assessed early and treated promptly when influenza is suspected; pregnancy is not a reason to delay oseltamivir. Children require weight-based dosing and monitoring for respiratory distress, encephalopathy and dehydration. Do not give aspirin to children or adolescents with influenza because of Reye syndrome risk.
Prevention and outbreak control
- Annual seasonal influenza vaccination according to national policy, prioritising healthcare workers, pregnant women, young children, older adults and chronic-disease patients.
- Hand hygiene, respiratory etiquette, masks when symptomatic, ventilation and staying home while febrile/contagious.
- Clean frequently touched surfaces and avoid sharing utensils.
- Use occupational PPE and report unusual illness after pig exposure.
- Use severity-based triage rather than outdated pandemic “category A/B/C” labels.
- Maintain surveillance, laboratory subtyping and early-warning systems for novel reassortment.
Exam-focused pearls
- The 2009 pandemic virus is A(H1N1)pdm09 and now circulates as seasonal influenza A.
- Incubation is commonly 1–4 days; transmission can begin before symptoms.
- Pregnancy, young/old age, chronic disease and immunosuppression increase risk.
- Start oseltamivir promptly in severe, hospitalised, pregnant and high-risk patients.
- Worsening after initial improvement suggests bacterial pneumonia or another complication.
- Negative rapid antigen testing does not exclude influenza in severe illness.
