Purpose
Clinical Signs and Symptoms of Influenza information for health professionals.
Signs and Symptoms
Uncomplicated influenza
Uncomplicated influenza illness is typically characterized by the abrupt onset of constitutional and upper respiratory tract signs and symptoms (e.g., fever, chills, myalgia, headache, malaise, nonproductive cough, sore throat, and rhinitis). However, many people who are sick with influenza virus infection do not have a fever, especially those who are older or immunosuppressed. Atypical signs and symptoms of influenza virus infection can occur, including in frail, institutionalized older adults in long-term care facilities (e.g., mental status changes, behavioral changes, and loss of appetite). Among young children with influenza, nausea, vomiting or diarrhea may also occur with respiratory symptoms. Uncomplicated influenza signs and symptoms typically resolve after 3-7 days for the majority of people, although cough and malaise can persist for more than two weeks, especially in older adults and those with chronic lung disease.
Complications
Influenza virus infection of the respiratory tract can cause a wide range of complications that can result in severe disease. Certain people are at increased risk for complications from influenza. In young children, otitis media and respiratory complications such as croup, bronchiolitis, and tracheitis can occur. Other complications in children include cardiac (myocarditis and pericarditis), musculoskeletal (severe myositis), and neurologic (encephalopathy, encephalitis, transverse myelitis, and acute disseminated encephalomyelitis) disease. Reye syndrome is a rare neurologic complication associated with influenza (more common with influenza B than influenza A virus infections) and salicylate exposure; however, Reye syndrome with influenza is very rare since aspirin use in children with influenza or varicella has been discouraged since 1982.
In people of all ages, influenza can result in dehydration, and exacerbation of underlying medical conditions (e.g., acute decompensated heart failure, myocardial infarction, cerebrovascular accident, diabetic ketoacidosis, chronic obstructive pulmonary disease exacerbation, asthma exacerbation). Both primary influenza viral pneumonia and secondary invasive bacterial pneumonia (most commonly with Streptococcus pneumoniae, Staphylococcus aureus, methicillin-sensitive or methicillin-resistant, and group A Streptococcus) can lead to acute lung injury, respiratory failure, acute respiratory distress syndrome, sepsis, septic shock, and multi-organ failure. Invasive infection with Neisseria meningitidis resulting in meningococcemia and meningitis can follow influenza.