Acute Febrile Illness Definition
Acute febrile illness (AFI) is defined as fever (temperature ≥38.0°C or 100.4°F) of less than 14 days duration, representing a broad clinical syndrome with diverse infectious etiologies. 1, 2
Core Diagnostic Criteria
Temperature threshold:
- Fever is defined as rectal temperature >38°C (>100.4°F), which remains the gold standard for accuracy 3
- Alternative definition: temperature ≥38.3°C (101°F) as a single measurement, or ≥38.0°C (100.4°F) sustained over 1 hour 4
- In specific contexts (e.g., hantavirus surveillance), fever may be defined as temperature >101.0°F (>38.3°C) 4
Duration component:
- AFI specifically refers to acute onset with fever duration <14 days 1, 2
- This distinguishes AFI from fever of unknown origin (FUO), which typically requires ≥5 days of fever without identified source 3
- Fever without localizing signs is defined as acute onset fever (<1 week duration) with absence of localizing signs on examination 3
Clinical Spectrum
AFI encompasses multiple presentations:
- Undifferentiated fever (UF): AFI without diarrhea (≥3 loose stools in 24 hours) or lower respiratory tract symptoms (cough/difficulty breathing plus oxygen saturation <90% or chest indrawing in children <5 years) 1
- Febrile respiratory illness (FRI): new or worsening cough or shortness of breath presenting with fever or chills in the previous 24 hours 5
- AFI with organ dysfunction: fever accompanied by single or multiple organ dysfunction syndrome, representing more severe disease 6
Etiologic Considerations
The syndrome reflects diverse infectious causes:
- Bacterial, viral, protozoal, and rickettsial pathogens all cause AFI 4, 7
- Clinical discrimination between etiologies is difficult, as AFI represents a nonspecific presentation 7
- In surveillance contexts, AFI serves as a broad entry criterion for investigating multiple potential pathogens simultaneously 2
Key Clinical Pitfall
Critical distinction from prolonged fever syndromes:
- AFI's defining feature is the acute timeframe (<14 days), distinguishing it from chronic febrile conditions 1, 2
- Dengue fever, ehrlichiosis, and other tickborne febrile illnesses exemplify classic AFI presentations with acute onset and characteristic accompanying symptoms 4
- The broad definition intentionally captures a wide range of infectious diseases for epidemiologic surveillance and clinical investigation 2