What is Influenza-like illness (ILI)?

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 represents a constellation of symptoms that may be caused by influenza virus infection but can also be caused by other respiratory pathogens.

Definition and Key Components

• ILI comprises a combination of fever, cough, sore throat, myalgia, and headache, though not all symptoms need to be present in every case 1
• The most predictive clinical features are the three-fold combination of fever, cough, and acute onset 1
• The CDC defines ILI as fever plus either cough or sore throat, with symptom onset within 7 days 2
• The World Health Organization's revised ILI case definition includes cough in the presence of fever (body temperature ≥38°C), which offers better specificity 3

Common Symptoms of ILI

The range of symptoms associated with uncomplicated ILI includes:

• Cough (~85% of cases) 1
• Fever (typically 38-40°C, may reach 41°C) 1
• Malaise (~80%) 1
• Headache (~65%) 1
• Chills (~70%) 1
• Coryzal symptoms (~60%) 1
• Anorexia (~60%) 1
• Myalgia (~53%), affecting mainly back and limbs 1
• Sore throat (~50%) 1

Diagnostic Value and Limitations

• ILI is not specific for influenza infection; other respiratory pathogens can present with similar symptoms 1
• Other viral causes include respiratory syncytial virus (RSV), adenovirus, rhinovirus, and parainfluenza virus 1, 4
• Bacterial pathogens such as Mycoplasma pneumoniae, Streptococcus pneumoniae, Chlamydia pneumoniae, and Legionella species can also present with ILI 1
• The predictive value of ILI increases when influenza virus is known to be circulating in the community 1
• In cohort studies, correlation of ILI with laboratory-confirmed influenza infection ranges from 25% to 45%, while in clinical trials, rates of 70% have been consistently reported 1

Factors Affecting Diagnostic Accuracy

• The accuracy of ILI definition is higher in persons aged 60 years and above compared to younger patient groups 1
• The probability of influenza infection increases with increasing level of fever 1
• During an influenza pandemic, when a pandemic strain is circulating locally in an immunologically susceptible population, ILI would be expected to be highly predictive for influenza infection 1
• However, public behavior during a pandemic (increased healthcare seeking for mild symptoms) may reduce the predictive value of ILI 1
• Including seasonality and influenza vaccination status improves the predictive value of ILI 2

Clinical Course and Complications

• Fever typically peaks within 24 hours of onset and lasts for about three days (range 1-5 days) 1
• Cough is generally dry but may be productive in up to 40% of cases, especially in those with underlying chronic lung disease 1
• Pneumonia (either primary viral or secondary bacterial) is the most common complication of influenza in adults 1
• Other complications may include acute bronchitis, myositis, ECG abnormalities, and rarely, neurological complications 1
• Gastrointestinal symptoms such as vomiting and diarrhea are uncommon (<10%) in adults 1

Practical Implications

• The symptom complex of cough, fever, and fatigue has been found to be both sensitive (43.5-75.1%) and specific (46.6-80.3%) for predicting laboratory-confirmed influenza 5
• Testing based on fever alone (without other ILI criteria) may increase case detection by approximately 34% 6
• During influenza season, the clinical overlap between influenza and other viral etiologies (particularly rhinovirus) must be considered in diagnosis and clinical management 4