What is influenza-like illness?

Influenza-Like Illness (ILI): Definition and Clinical Characteristics

Influenza-like illness (ILI) is defined as the presence of fever and new (or worsening) cough of acute onset in the context of influenza circulating in the community. 1 This clinical syndrome comprises a constellation of symptoms that mimic influenza but can be caused by various respiratory pathogens.

Core Definition and Key Components

• ILI is characterized by the three-fold combination of fever, cough, and acute onset, which are the most predictive clinical features for influenza infection 2, 1
• While ILI is highly suggestive of influenza during epidemic periods, it is not specific for influenza infection and can be caused by other respiratory pathogens 2, 1
• The predictive value of ILI increases significantly when influenza virus is known to be circulating in the community 1

Common Symptoms and Their Prevalence

• Fever: A paramount symptom that typically ranges between 38-40°C, peaking within 24 hours of onset and lasting 1-5 days 2, 1
• Cough: Present in approximately 85% of cases, generally dry but can be productive in up to 40% of cases 2, 1
• Additional common symptoms include:
  • Malaise (~80%) 2
  • Headache (~65%) 2, 1
  • Anorexia (~60%) 2
  • Coryzal symptoms (~60%) 2
  • Myalgia (~53%), primarily affecting the back and limbs 2, 1
  • Sore throat (~50%) 2, 1
  • Chills (~70%) 2

Diagnostic Value and Limitations

• During influenza epidemics or pandemics, the presence of both cough and fever has a positive predictive value of 79% for influenza infection 3
• The accuracy of ILI definition is higher in persons aged 60 years and above compared to younger patient groups 2, 1
• The probability of influenza infection increases with higher fever 2, 1
• In clinical practice, the diagnosis is often made clinically without laboratory confirmation 4

Differential Diagnosis

• Other respiratory pathogens that may present with ILI include:
  • Viruses: respiratory syncytial virus (RSV), adenovirus, rhinovirus, parainfluenza virus 2
  • Bacteria: Mycoplasma pneumoniae, Streptococcus pneumoniae, Chlamydia pneumoniae, Legionella species 2

Complications of ILI

• Pneumonia (either primary viral or secondary bacterial) is the most common complication in adults 2
• Secondary bacterial pneumonia typically occurs 4-5 days after illness onset 2
• Other potential complications include:
  • Acute bronchitis (common, especially in elderly and those with chronic conditions) 2
  • Cardiovascular complications (ECG abnormalities common; myocarditis and pericarditis rare) 2
  • Neurological complications (rare in adults) 2
  • Myositis (uncommon, occurs during early convalescence) 2

Clinical Course

• Typical influenza presents with abrupt onset of symptoms 5
• Gastrointestinal symptoms such as vomiting and diarrhea are uncommon (<10%) in adults with ILI 2
• The mean duration of virus shedding in immunocompetent adults is approximately 5 days but may continue for up to 10 days or more in high-risk populations 5

Clinical Implications

• When influenza is circulating in the community, patients with both cough and fever within 48 hours of symptom onset are likely to have influenza and may benefit from antiviral therapy 3
• Treatment with antivirals is most effective when started within 24 hours of symptom onset 4
• Annual influenza vaccination remains the most effective preventive measure 6