Influenza (“the flu”), is an acute contagious respiratory illness that is caused by influenza viruses. The virus causes the infection by infecting epithelial cells in the respiratory tract. A characteristic feature is the seasonal epidemics and the occasional pandemic. These outbreaks result in a significant morbidity in the general population and increased mortality in high-risk populations.
Influenza is caused by three subtypes of the RNA orthomyxovirus (IAV, IBV & ICV). Subtypes IAV & IBV cause the seasonal flu epidemics in adults. ICV tends to only infect children. A 4th subtype (IDV) can infect humans but does not cause symptoms. Mutations frequently occur with new antigenic properties from new strains. Immunity is usually to only one strain.
Influenza is spread by through respiratory droplets produced from coughing and sneezing. Incubation is typically 1-4 days. Those with influenza are infective from 1 day prior to symptoms to 1 week after onset of symptoms.
Clinical Features:
Typically an acute onset – pyrexia, general aches and pains (myalgias), headache, nausea, harsh unproductive cough, runny nose, fatigue, loss of appetite. Less commonly may have vomiting and diarrhea.
Usually subsides in 3-8 days; can vary from mild to potentially fatal
Differential diagnosis: common cold, COVID-19, bacterial pneumonia.
Complications:
Tracheitis, bronchitis (20%), pneumonia, secondary bacterial infection, sinusitis, otitis media.
Rare complications: toxic cardiomyopathy, encephalitis, encephalopathy, myositis
The typical myalgias are common in influenza, the myositis is rare. Those with the acute myositis develop an exquisite tenderness of the muscles that are affected which are most commonly the legs – slight pressure (eg from the bedsheets) can de painful.
Management:
Bed rest, paracetamol/acetaminophen, aspirin, keeping up fluid intake, specific treatment of complications.
Antihistamines may relieve some symptoms.
Antivirals (zanamivir, oseltamivir, amantadine, rimantadine) may be used in the more seriously ill
Prevention:
The most effective prevention is annual vaccination, especially for high-risk populations. Official recommendations for this do vary by country.
Infection control measures: hand hygiene; covering mouth and nose when coughing; isolating if sick
Related Pages:
Flu Pandemics
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